Universiteit van Amsterdam

Faculty: Anton Pannekoek Instituut

On the carriers of Diffuse Interstellar Bands

A search for their identity with recently obtained IR spectra

Bachelor project (Chemistry)

07/01/2013 – 29/03/2013

By Abel Schootemeijer (5669146)

Supervisors: prof. dr. Lex Kaper (UvA), prof. dr. Harold LinnzartzSecond reviewer: dr. Steen Ingemann Jorgensen

Table of ContentsPopulairwetenschappelijke samenvatting (dutch)………………………………………………………………….. 2Abstract…………………………………………………………………………………………………………………………………... 3Introduction…………………………………………………………………………………………………………………………….. 3

Diffuse interstellar bands (DIBs)…………………………………………………………………………………. 3 The interstellar medium.…………………………………………………………………………………………….. 4 Reddening …………………………………………………………………………………………………………………… 4 Potential DIB-carriers…………………………………………………………………………………………………… 5 How are DIBs identified?................................................................................................. 7 Very Large Telescope/X-shooter…………………….…………………………………………………………… 7 DIBs in IR region…………………………………………………………………………………………………………… 8 How to match a DIB from star’s spectrum

and a DIB-candidate’s absorption spectrum………………………………………………………………. 8 Research in this project………………………………………………………………………………………………. 8

Experiment part I….…………………………………………………………………………………………………………………. 9 IBBCEAS………………………………………………………………………………………………………………………. 9 MIS………………………………………………………………………………………………………………………………. 10

Experiment part II…………………………………………………………………………………………………………………….. 11 NIR spectra………………………………………………………………………………………………………………….. 11 The 485 μm DIB…………………………………………………………………………………………………………… 12

Results, …………………………………………………………………………………………………………………………………… 13 NIR spectra…..…………………………………………………………………………………………………………….. 13 The 485 μm DIB & DIBs in the optical range……………………………………………………………….. 20

Discussion……………………………………………………………………………………………………………………………….. 22 NIR spectra…………………………………………………………………………………………………………………. 22 The 485 μm DIB & DIBs in the optical range………………………………………………………………. 22

Conclusions……………………………………………………………………………………………………………………………… 24Future research……………………………………………………………………………………………………………………….. 24Acknowledgements…………………………………………………………………………………………………………………. 25References………………………………………………………………………………………………………………………………. 25Appendix A………………………………………………………………………………………………………………………………. 27Appendix B ……………………………………………………………………………………………………………………………… 28

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Populairwetenschappelijke samenvattingNiet iedereen weet dat het heelal niet alleen sterren en planeten bevat; een deel van de materie die in het heelal aanwezig is, bevindt zich in interstellaire gaswolken. Deze gaswolken zijn heel koud, met typische temperaturen van 50 K tot 100 K (-220 °C tot -170 °C). Ook hebben deze gaswolken een hele lage dichtheid. Deze gaswolken zenden zeer weinig licht uit, en zeker als ze ver weg staan zijn voor ons daardoor zeer moeilijk waarneembaar. We kunnen ze wel op een andere manier zien, namelijk door naar sterren te kijken die zich achter die gaswolken bevinden. De moleculen in de gaswolken absorberen licht dat door die sterren wordt uitgezonden: zo weten we dat ze er zijn. De verschillende moleculen in die gaswolken absorberen licht bij verschillende golflengtes. Dit komt doordat de moleculen in staat zijn om fotonen met een bepaalde hoeveelheid energie (daarbij hoort een bepaalde golflengte van het foton) te absorberen en daarbij in een hogere energietoestand geraken. Aan de hand van de golflengtes van fotonen waarbij bepaalde moleculen absorberen, kan bepaald worden welke moleculen zich in de gaswolken bevinden. Dit is echter niet altijd even makkelijk: in het gebied van het zichtbare licht (golflente tussen 400 nanometer en 1000 nanometer) worden ongeveer 400 absorptiebanden waargenomen van interstellaire gaswolken waarvan het onbekend is welke moleculen ze veroorzaken. Deze absorptiebanden worden de ‘Diffuse Interstellar Bands’ (DIBs) genoemd. Met een telescoop die ook nauwkeurig het licht in het nabije infraroodgebied kan meten (dit was vroeger moeilijk), zijn in het infrarood ook recentelijk DIBs waargenomen. In dit project is onderzocht of er moleculen geïdentificeerd konden worden die deze absorptiebanden in het nabije infrarood konden verklaren. Hiervoor zijn de absorptiespectra van bepaalde polycyclische aromatische koolwaterstoffen (PAKs, dit zijn grote moleculen die uit meerdere benzeenringen bestaan) en negatief geladen koolstofketens (C12

-, C14-, C16

-, C18-, C20

-) vergeleken met de absorptiebanden in het nabije infrarood. Het resultaat is dat beide types moleculen de absorptiebanden in het nabije infrarood niet kunnen verklaren.Verder is er nog een pas ontdekte absorptieband met een lange golflengte van 485 μm (ver infrarood) onderzocht. Het vermoeden is dat deze absorptieband veroorzaakt kan worden doordat een bepaald molecuul in een hogere vibratie-energietoestand raakt door een foton met deze lange golflengte (=lage energie) te absorberen. Als die aanname klopt, zou het kunnen dat er andere overgangen te zien zijn in energietoestanden waarbij de vibratietoestand ook verandert. Stel dat die andere overgangen licht in het zichtbare deel van het spectrum zouden absorberen (pas op,dit is weer een aanname!). In dat geval zouden er DIBs waargenomen kunnen worden die vlakbij elkaar liggen, op een bekende afstand uit elkaar(namelijk: het energieverschil tussen de vibratietoestanden). Door de bekende DIBs te analyseren zijn er drie DIBs gevonden die inderdaad de juiste posities ten opzichte van elkaar hadden qua golflengte.Door het grote aantal DIBs in het zichtbare deel van het spectrum kan dit echter ook toeval zijn. In een volgend onderzoek zou dit onderzocht kunnen worden door langs andere gezichtslijnen naar sterren te kijken; als die DIBs in een andere verhouding zichtbaar zijn in die andere gezichtslijnen, dan kunnen deze DIBs niet door hetzelfde molecuul worden veroorzaakt. Als de DIBs wel in dezelfde verhouding aanwezig zijn, zouden ze inderdaad veroorzaakt kunnen worden door energie-overgangen binnen hetzelfde molecuul. De volgende stap zou dan zijn om erachter te komen welk molecuul dit zou kunnen zijn.

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AbstractIn this research, recently obtained infrared spectra of stars have been used to search for the identity of Diffuse Interstellar Bands (DIBs). With VLT/X-shooter, DIBs have been identified in the near infrared region (1000 nm – 2500 nm). Spectra of medium to large sized PAHs and carbon chain anions have been compared to DIBs in the NIR spectrum of the O-supergiant in the high-mass binary 4U 1907+07. None of the molecules showed an interesting match with one or more DIBs.Furthermore, a possible far infrared DIB at 485 μm has recently been reported. The assumption has been tested that this absorption band is caused by a vibrational transition. The DIBs at λair = 769,6 nm, 770,8 nm and 772,0 nm had energy separations that match the energy of a 485 μm photon, and they also show the absorption line characteristics that are expected if the assumption is true. In future research, this assumption can be tested further by determining whether the three DIBs are correlated: if they are not, they cannot have the same carrier.

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IntroductionDiffuse Interstellar BandsDiffuse Interstellar Bands (DIBs) were first observed over 90 years ago, in 1922, by Mary Lea Heger (Heger, 1922). When observing heavily reddened binary stars, she noticed that the positions of some of the absorption lines in the stellar spectra were varying with time, while other lines remained stationary. The fact that some of the lines had their position changing with time can be explained by the Doppler effect; wavelengths are shortened when a source is moving towards us and stretched when the source is moving away from us.

Δ λλ

= vc (Doppler shift)

Since stars in binaries are moving in elliptical orbits around each other, their velocity towards us is varying with time and therefore, the positions of the centers of the absorption lines are varying with time. But how could the non-moving lines be explained? The answer is that a stationary cloud that is causing the absorption must be in the Interstellar Medium (ISM). It could consist of either dust particles or particles in the gas phase (or both); they are called the DIB-carriers.DIBs are mostly observed between λ = 400 nm and λ = 1000 nm; this includes the near

ultraviolet (near UV), the visible and the near infrared (NIR) part of the electromagnetic spectrum (Herbig, 1995). See figure 1. They are characterized by their broad, diffuseprofile, with a FWHM (Full Width at Half Maximum) ranging from 0,05 nm to 3 nm (Cox, 2011).DIBs are best observed in the spectra of hot O-type and B-type stars. One reason is that these stars are much brighter than stars of a colder

spectral type, so that a longer sightline can be probed. The second reason is that these stars are preferred when researching DIBs, is that they have most of their stellar spectral absorption lines in the far UV, near the peak of the energy distribution; therefore they have relatively few absorption lines in the DIB-range that would contaminate DIB-spectra.Figure 1: The diffuse interstellar bands in the near UV, visible and NIR. Image was created by the McCall group.

Currently, over 400 DIBs are known (Hobbs et al, 2009). Most of these DIBs are relatively

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narrow and shallow, while fewer bands are very broad and/or deep. DIB-spectra are not identical for different stars. For example, in an earlier research (Hobbs, 2008) along another line of sight, a part of the observed bands (around 30%) showed that were not found in the research in 2009 and vice versa.

The ISMThe fact that different clouds do not have exactly matching DIB-spectra is not surprising: we do not expect these different clouds in the ISM to have exactly the same excitation and ionization conditions since the local conditions are not the same everywhere. There are variations in cloud densities, cloud temperatures and exposure to radiation. Especially UV-photon intensities are an important factor: UV-photons have energies high enough to break covalent bonds in molecules. If a strong UV-radiation field is present in a cloud in the ISM, hydrogen is mainly present as unbound H-atoms since the covalent bond in molecular hydrogen, H2

(bond dissociation energy: 4,52 eV) can be destroyed by UV -photons with an equal or larger energy, i. e. λ ≤ 275 nm (Herbig, 1995). In dense clouds, hydrogen and other atoms or molecules in the outer layers of the cloud absorbs the UV-photons, protecting hydrogen in the core of the cloud and reducing the rate at which hydrogen molecules are destroyed. Among sightlines of dense clouds, molecular hydrogen fractions can be as high as fH2 = 0,6, while fH2 ≤ 0,1 for diffuse clouds (Vos et al., 2011). This shielding effect is called the ‘skin effect’.Although no DIBs have been assigned to any molecule with certainty, a large number of molecules has been found in the ISM by identifying absorption lines at other wavelengths than the DIB-region (i.e. 400 nm – 1000 nm). H2 is by far the most abundant molecule in the ISM. Other molecules that are relatively abundant are small molecules like CO, CO2, H2O and NH3 (Winnewisser & Kramer, 1999, Cheung et al., 1968).In the mm - μm region, rotational transitions of diverse small molecules and also of medium sized carbon chains have been found (Ehrenfreund & Charnley, 2000). This method has it flaws: only polar molecules can be

detected, since their dipole moment changes while they are rotating. Non-polar molecules without dipole moment are therefore not observable. (Winnewisser & Kramer, 1999)By mid-IR spectroscopy, polycyclic aromatic hydrocarbons have been detected in diffuse interstellar clouds. In the wavelength region between 3 μm and 15 μm, vibrational and vibrational bending transitions were identified that arose from C-H, C-C and C=C bonds (Hudgins & Allamandola, 2004). Although this method did not allow for the identification of specific PAHs, the authors were able to conclude that PAHs are present in the ISM.Other complex molecules that were found in the ISM are carbon chains: e. g., spectroscopy in the microwave region resulted in the detection of low column densities of HC11N (Bell et al, 1997).

ReddeningThe spectral energy distribution of a blackbody depends only on its effective temperature. Thus, the intrinsic ratio of the photon intensities at different wavelengths is known if the spectral type of a star is identified (Fitzgerald, 1970). This information is used to calculate the flux ratio of light around λ=400 nm (blue) called ‘B’ and around λ=650 nm (green, or ‘visible’) called ‘V’. This ratio is called the intrinsic (B-V), or intrinsic color.Dust in the ISM scatters blue light with λ = 400 nm more than visible light with λ = 650 nm, so ‘reddening’ is observed in stars behind clouds that contain dust. The amount of reddening, E(B-V), is expressed by subtracting the intrinsic (B-V) from the observed (B-V):

E(B-V) = (B-V)observed - (B-V)intrinsic Reddening is proportional to the column density of interstellar dust. Reddening and column density are also proportional to DIB-strengths, although the correlation is not perfect (Friedman et al., 2011). This means that the presence of interstellar dust is associated with the presence of DIB-carriers, or that the interstellar dust itself is the DIB-carrier.

Potential DIB carriers

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A variety of DIB-carriers has been proposed. The most important classes of potential DIB-carriers are listed below.

FullerenesFullerenes are molecules that are entirely (if spherical or ellipsoidal) or almost entirely (if tubular, with hydrogen atoms at the side ends) composited of carbon atoms. The best known fullerene is C60, or buckminsterfullerene, which has a football-like structure (see figure 2).

Fullerenes are candidates DIB-carriers because they are large molecules, and the presence of large molecules in the ISM is a good explanation for the broad rotational bands. Especially C60, is a popular candidate because it is known for its stability (Kroto et al, 1985). C60

and C70 molecules have been found in planetary nebula TC1 (Cami et al., 2010). Unfortunately, no absorption lines in the optical (DIB) range were detected in that planetary nebula for C60 (Garcia-Hernandez & Diaz-Luis, 2013), meaning that the column density is below the detection limit. A reason for the column density being under the detection limit might have been that most of these molecules are ionized and are present as C60

+. In the article by Garcia-Hernandez & Diaz-Luis, the authors suggested that bucky-onions may be responsible for the DIBs instead. Bucky-onions are relatively small spherical fullerenes like C60 contained in larger spherical fullerenes like C240, hence the name ‘onions’. The absorption spectra of bucky-onions are hard to test experimentally; they

are difficult to synthesize due to their bad solubility. The ionized C60

+ is tentatively assigned to the DIBs at 957,7 nm and 963,2 nm by matrix isolation spectroscopy (Foing and Ehrenfreund, 1994), but assignment has still to be confirmed by gas phase spectroscopy (Cami et al., 2010), which is a more accurate method (see chapter 2: experiment).Tubular fullerenes are also proposed as DIB-carriers. These molecules have been investigated by electronic structure calculations, and a mechanism for the synthesis of these tubular fullerenes was proposed (Zhou et al., 2006). However, they concluded that the error margin of their calculations was too large to be able to assign them to known DIBs.

PAHsPolycyclic Aromatic Hydrocarbons (PAHs) are molecules consisting of multiple benzene rings, with 2n+1 double bonds in the molecule (definition of ‘aromatic’). The simplest PAH is naphthalene, C10H8, which contains two rings (see figure 3).

Figure 3: Structures of the small PAHs naphtalene and pyrene. Image was created with ChemDraw.

PAHs are considered possible DIB carriers because they are resistant to destruction by UV-photons, because of the high abundance of C-atoms in the ISM and because their transitions are expected to be seen in the optical part of the spectrum (Cox, 2011). The absorption spectra of small PAHs like naphthalene and pyrene (four rings in a lozenge-like arrangement) were investigated, but no DIBs could be assigned to their spectra (Herbig, 1995). Also, their absorption bands were too narrow to be DIBs. In another research, Steglich et al., 2011 found no individual medium sized PAHs that matched either any DIB in the 400 nm-1000 nm range or the 217,5 nm UV bump. They suggested that the UV-bump might, however, be caused by a variety of medium to large sized PAHs.

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Figure 2: C60. Image was taken from www.nature.com

In a research by Iglesias-Groth et al, 2008, absorption lines of the naphthalene cation, C10H8

+ seemed to match two previously unknown DIBs. These DIBs did not appear along other lines of sight and corresponded to only a low column density. Still, this finding makes the case of the PAH hypothesis stronger.

Unsaturated carbon chainsCarbon chains are known to have strong transitions in the visible part of the spectrum, and polar C-chains have been found in interstellar molecular clouds by millimeter spectroscopy (Schmidt & Sharp, 2005). For these reasons, C-chains are logical candidates as DIB-carriers. Remember that non-polar molecules cannot be found in by mm-spectroscopy, as mentioned earlier in the introduction. Polar carbon chains are for example molecules with radicals containing one H-atom, C2nH, or chains containing a N-atom.Linnartz etc al, 1998 and Motylewski et al., 2000, studied the spectra of carbon chain radicals. Molecules they took spectra of included C6H, C8H and C10H, HC4H+, HC6H+, and also C-chain radicals with one or more nitrogen atoms (NC4N+, NC6N+, HC5N+). They were unable to convincingly match any of the absorption spectra of these molecules to DIBs. HC5N+, however, showed a correlation with the weak 581,9 nm DIB and it cannot be excluded that it causes this DIB. C-chains with a hydrogen atom at both ends, HC(2n+1)H with 3≤n≤6 have been studied, but did also not match any DIBs (Ball et al, 2000).Because of the large variety of DIBs, coincidental (near) matches occur from time to time. For example, Güthe et al. (2001) claimed that l-C3H2

- was found in the ISM, but higher resolution laboratory data proved that the wavelengths did not match the assigned DIBs (McCall et al, 2002). The spectrum of C7

- showed striking similarities with several DIBs (Tulej et al. 1998), but later discrepancies in line shape and line position (of one of the lines) were found (McCall et al, 2000), so C7

- was discarded as a DIB-carrier.

Molecular hydrogen

It has been suggested that molecular hydrogen, H2, is responsible for DIBs in the 768 nm – 788 nm interval (Sorokin & Glownia, 1995). They suggested a mechanism where the vibrational energy levels of molecular hydrogen in the vicinity of hot O-type stars (T = 45000 K) were occupied by electrons excited by Ly-α photons combined with other photons (two-photon absorption). In a critique, Snow (1995) argued that the H2-spectrum did not completely match their assigned DIBs. Furthermore, the general belief is that DIBs are not (only) caused by gas clouds near hot stars; DIBs have also been observed in the spectra of much colder (T = 10000 K) stars. In response, Sorokin & Glownia (1996) adjusted their theory, but concluded that even though there seem to be similarities between DIBs and the H2-spectrum, they were not able to proof their H2-hypothesis. For that, more research would be required. This H2-hypothesis is not the most popular explanation for DIBs because it assumes the absorption lines originate from clouds near O-stars. Also, the fact that the theory requires a lot of tuning is a bad sign. Still, it is not ruled out and in the research described in this paper, H2 is one of the investigated DIB-candidates.There is another theory suggesting H2 as DIB-carrier. It is estimated that H2 is roughly 106 times more abundant in the ISM than the most abundant large carbon based molecules (Schmidt & Sharp, 2005).Due to the high abundance of molecular hydrogen, H2, in the interstellar gas clouds, lines in the DIB-spectrum are possibly caused by indistinct absorption lines of H2 with low oscillator strength. However, this theory is also disputable since DIB-strengths seems to correlate well with neutral atomic hydrogen column densities and poorly with H2 column densities (Friedman et al., 2011)

Dust grainsInterstellar dust grains are small condensed dust particles. Their sizes varies, but most stay smaller than 400 nm: this is known because they scatter blue light ( λ ≈ 450 nm) more efficiently than red light (λ = ≈ 650 nm). Grains were once a popular candidate for DIB-carriers because, as mentioned, DIB-strengths

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correlate with reddening and reddening correlates with the column density of grains (therefore, DIB-strength correlates with grain column density), but now they are out of fashion because of lack of evidence of grains as DIB-carriers (Herbig, 1995), and counter-evidence has been proposed: grains are known to align to a magnetic field, which should cause the DIBs to be polarized if these DIB-carrying grains are situated in a magnetic field. This effect, however, has never been observed (Cox et al, 2011).How are DIBs identified?To be able to identify the DIB-carriers, their spectra should be compared to the observed DIBs. Obtaining the spectrum of a DIB-candidate is often challenging, since the circumstances in the ISM are different from the circumstances here on Earth; low densities of particles, low temperatures and usually a high exposure to UV light. These circumstances can result in the presence of molecules that are not stable in circumstances here on Earth, like radicals. Since radical molecules are very reactive and consequently have a short lifetime, it is not possible to create a sample of say, HC7H and measure its spectrum by conventional methods like IR-spectroscopy.Instead, several other methods are available in which candidate DIB-carriers are stable so their spectra can be measured. Three methods are discussed here:-Matrix Identification Spectroscopy-Cavity Ringdown Spectroscopy-Computational methodsIn Matrix Identification Spectroscopy (MIS), possible DIB-carrier molecules are caught in a very cold (temperatures as low as 5K) matrix of a noble gas, most often neon (Freivogel et al., 1994). Then, the absorption of the matrix is measured to obtain the spectra of the molecules. The advantage of this method is that high column densities of target molecules can be achieved. A major downside, however, is that interaction of the molecules with their environment (the neon atoms) distorts their absorption spectra. Therefore, this method is inaccurate and it has partly been taken over by gas-phase spectroscopy in the past few years.

Cavity Ring-Down spectroscopy (CRD) is a method in which molecules in the gas phase are released in a low-pressure cavity at high (usually supersonic) velocities. In the cavity, light is reflected between two mirrors before it will reach the detector: this is done to increase the path length of the photons, so the absorption can be measured more accurately.In computational methods, simulations are used to calculate spectra of molecules. These simulations are not perfectly accurate, but are able to give a good approximation of the real absorption spectra (Zhou et al., 2006).VLT/X-shooterThe Very Large Telescope (VLT) and X-shooter have been built recently by ESO (European Southern Observatory) . The VLT (figure 4)is a reflective telescope with a 8,2 m main mirror.

Figure 4: ESO’s Very Large Telescope on the mountain Cerro Paranal in, Chile. Photo was taken by ESO.

To translate the images taken by this telescope into spectra, the ‘X-shooter’ instrument was used. X-shooter consists of three arms; beam splitters split the incident light into them, separating Near Infrared (NIR), visible and Ultraviolet (UV) light (Vernet et al., 2011). After the beams are split, they all pass through a grating, separating the photons with different wavelengths. A Charge Coupled Device (CCD) measures the photon intensities at different wavelengths.This design of X-shooter allows it to measure a very wide range of wavelengths: from the UV with λ = 300 nm to the NIR part of the spectrum with λ = 2500 nm. This wide range allows a reliable study of the whole DIB-range, and also the possible detection of DIBs in a region of which no good data was available in the past, i. e. the λ = 1000 nm to λ = 2500 nm

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range. A detailed description of X-shooter is given in Vernet et al., 2011.The NIR-spectrum of 4U 1907+07, which is used in this research, was taken with VLT/X-shooter.DIBs in the (N)IR regionUntil recently, DIBs were only observed in the region between λ = 400 nm and λ = 1000 nm. Two DIBs were discovered at λ=1179 nm and λ=1317 nm (Joblin et al., 1990) and with the VLT/X-shooter, thirteen DIBs were found between λ=1500 nm and λ=1800 nm (Geballe et al., 2011). Very recently, a DIB in the far IR region has been found at 486 μm (Müller et al, not yet published). These IR DIBs have not yet been studied intensely and might, if they can be identified, hint scientists in which direction they have to search for the identity of the DIBs in the optical range. In this research, available spectra from times when no near IR-DIBs were known are compared to these newly obtained DIBs.The wavelength of the far IR 486 μm DIB corresponds to a wavenumber of 20,599 cm-1,

or an energy of E=h cλ = 2,56 . 10-3 eV. This

energy value might also be the energy difference between two rotational states of a DIB carrier. In that case, two DIBs should be found spaced 20,599 cm-1 apart more on this in the ‘experiment’ chapter).

How to match a DIB from star’s spectrum and a DIB-candidate’s absorption spectrumTo be able to assign an absorption lines of an investigated molecule to a DIB, the first criterion is that the wavelengths of both lines have to match. Secondly, the shapes of the lines have to be similar. The third criterion is that, if a molecule has two or more absorption lines, all of them have to show in a line of sight, and not just one. For example: a suspected DIB-carrier has absorption lines at 500 nm and 550 nm, and a DIB is seen at 500 nm but not at 550 nm, there is no match.Because of the large number DIBs (over 400) in the relatively small range between 400 nm and 1000 nm, a large chance exists that an absorption line and a DIB will coincidentally seem to match. Therefore, a lot of care should be taken to assign a line to a DIB; various false

assignments have been made in the past (see chapter ‘potential DIB-carriers’). To prevent wrong DIB identifications (and to identify a DIB with enough certainty), accurate methods with small error margins are required.

Why do we want to identify DIBs?-To obtain information about the constitution of the interstellar medium. A problem is that by rotational spectroscopy, only polar molecules can be detected. If DIBs can be identified, more accurate information will be available.This research projectIn this research, a description is given of two instruments that use the two most important methods to obtain spectra of candidate DIB-carriers; Cavity Ring-Down Spectroscopy (CRDS) and Matrix Isolation Spectroscopy (MIS). In the remainder of the study, the newly obtained IR-spectra are investigated. NIR-DIBs are compared to absorption spectra of possible DIB-carriers. Finally, a DIB-catalog was used to search for DIBs that had difference in wavenumber that matched the wavenumber of the 485 μm DIB.To summarize, there are two goals in this research project: 1) Answer the question: What are DIBs and how can we identify them? 2) Use newly obtained IR DIB-data to search for the identity of DIB-carriers.

Experiment part IIn the first part of this chapter, two examples are described of methods that can be used to obtain spectra of molecules of interest.

IBBCEASIn Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS), the absorption is measured of a plasma that is periodically released in a cavity. In that cavity, incoherent light (i. e. photons have different phase and wavelengths) is trapped between two mirrors. Trapping the light in a cavity increases the path length of the photons and as a result, the absorption. The absorption of the plasma is measured only when the plasma is present in the cavity; this is achieved using an optomechanical shutter.

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The plasmaAs mentioned in the introduction, candidate DIB-carriers like C-chains with radicals are unstable under normal terrestrial conditions. Therefore, they have to be created in situ. The reactant is acetylene(H-C≡C-H) gas, 0,5% diluted in 1:1 He/Ar gas. This gas is stored behind the nozzle at a pressure of 12 bar.

When the gas runs through the nozzle, a voltage of -1200 V is applied between the jaws

of the nozzle and the grounded metallic plate in the slit. As a result, a current will flow through the gas (depending on its resistivity) and a variety of carbon chain molecules is expected to be produced. After passing the nozzle the gas enters the cavity, which is kept at a very low pressure: ±10-5 bar. Because of the large difference in pressure between the gas chamber and the cavity, the gas will expand at a very high speed (v > vs, hence the ‘supersonical’ expansion). This rapid expansion results in a rapid cooling of the gas to a rovibrational temperature of

roughly 15 K, which is in the range of the temperatures of diffuse interstellar clouds. The nozzle releases the plasma in pulses rather than constantly. This is because with pulsed release, the pressure in the cavity can be kept at a lower value than if the release were constant. This low pressure in the cavity results in a fast expansion of the plasma when it is released; the result is a cold plasma with a

temperature that correlates well with the targeted temperature.Carbon chain molecules are expected to form in the plasma, but the structure of the products is not exactly known. If the produced carbon molecules do not match DIB absorption lines, no further research takes place. If one of the lines of the obtained spectrum is a possible match, further research is done to obtain the identity of the absorber. This can be done with MIS and computational

methods.

The setupA beam of incoherent light sent by a 300 W Xe-Arc lamp is directed at a cavity of two mirrors. A Xe-arc is a lamp with a potential different which causes an arc of current to flow in the gas in the lamp, which is constituted of xenon and other gasses; it is designed to emit a continuum spectrum. Both mirrors have a reflectivity of roughly 0,99995 (i. e. a fraction of 5*10-5 of the light passes through the mirrors). This means that only a

small fraction of the light sent by the lamp actually enters the cavity: most of it gets reflected by the first mirror. The light that passes the first mirror then gets trapped in the cavity. Half of it will eventually pass through the first mirror and half of it will pass through the second mirror, behind which the detector is situated. The path of the light is directed by the telescope, the lenses and the apertures.

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Figure 5: The IBBCEAS setup displayed schematically. The abbreviations have the following meaning: TS=telescope, M 1 & M2=high reflectivity mirror, PDN=pulsed discharge nozzle, A1 & A2=aperture, L1 & L2=lens with an f of 7,5 cm, OS=optimechenical shutter, BS=beam splitter, L3= lens with an f of 5,0 cm, PD=photodiode, F=interference filter, OF=optic fibre bundle, CCD=charge coupled device. Image was taken from Walsh et al., 2013.

A pulsed slit discharge nozzle sends plasma in the cavity at a rate of 4 Hz, with pulses lasting roughly 1000 μs. Measurements are only done when plasma is present in the cavity to achieve a high S/N ratio. Therefore, an optomechanical shutter is used. This optomechanical shutter is activated just after the plasma enters the cavity (using delays) and it is designed to stay open for 500 μs, shutting just before the plasma leaves the cavity. It is open for a shorter time than the plasma stays in the cavity, to make sure that the measurements are only done when the plasma is present. Behind the cavity, a beam splitter is used to split the beam of light; it reflects 10% of the light to a photodiode, to measure whether the optomechanical shutter is letting light through the cavity or not (i.e.: is the measured absorption of importance or not?). The other 90% of the light is let through to pass the interference filter (or not, if the light does not have a wavelength of interest). Then, the light is guided by the lens and the fiber bundle to the spectrograph and the CCD, where its spectrum is taken.

Measuring the absorptionThe absorbance of a sample is defined by the following formula. I0 is the light’s intensity without a sample present in the cavity; I is the light’s intensity with sample present.

A = ln(I0 / I)Which can be approximated by:

A = I0 / I - 1 Because the light in the cavity is trapped between two mirrors, its path length is greatly increased. Therefore, an (1 – R) term has to be introduced in the equation to prevent the absorbance from being overestimated by a large margin. After correcting for this, equation then becomes:

A = ((I0 / I )– 1)(1 – R)In this instrument, the emission effects of the plasma have to be considered. Also, there is some ‘dark current’, a signal that is always detected by the detector.Four scenarios are discerned:I: ‘Normal’ run, plasma is present and

the Xe/Arc lamp is onI0: Plasma is not present, lamp is on

BGplasma: Background measurement: plasma is running, but the lamp is turned onBG: Background measurement with both the plasma not present and the light turned offBy subtracting BG (dark current) from I0, the light’s intensity of the experiment with empty cavity is obtained; by subtracting BGplasma (dark current & emission effects) from I, the light’s intensity in a cavity with plasma is obtained.Thus, the equation for absorption becomes:

A = ((I0 – BG) / (I - BGplasma) – 1)(1 – R)

Matrix Isolation Spectroscopy: OASISOASIS is the name of the Matrix Identification Spectroscopy (MIS) setup used in this experiment. Solid phase DIB-candidates are studied. In a low-pressure chamber, a matrix and the molecules of interest are deposited on a glass plate. The pressure in the chamber is extraordinary low, around 10-10 bar. Also, the temperature in the chamber is very low: it can be varied for different purposes, but a minimum of 12 K is attainable. Various choices are possible for the matrix compound: noble gases like neon and argon can be used to minimize shifts in absorption lines, which allows the most accurate measurements for compounds compared to their gas phase absorption spectrum. OASIS is still not as accurate as direct gas-phase measurements like IBBCEAS, but it has its advantages: 1) it is easier to perform and 2) large molecules, which are hard or impossible (depending on their size) to put in the gas phase, can be studied.MIS can also be applied for researching solid phase DIB-candidates, although these solid phase DIB-carriers are not the most popular DIB-candidates. Among the most likely environment for solid phase DIB-carriers are H2O and NH3, which are relatively abundant molecules in the ISM.

11

Figure 6interference (lower graph). Image was created with Mathematica.

H2O and NH3, being polar molecules, cause a large shift of the spectra of matrix-solved molecules.

The setupIn a cooled low pressure chamber, a glass plate is situated at which the matrix compound and the molecules of interest are deposited. A Xe/Arc lamp directs a beam of light with a continuum spectrum through the glass plate into a light detector. This detector uses a grating to disperse the light and a CCD to measure the light’s intensity at different wavelengths. To measure the thickness of the matrix, a laser beam is directed towards it. The intensity of this beam is measured; the intensity varies because destructive or constructive interference occur when the light is reflected towards the laser detector. In figure 6, it is shown that if the matrix surface moves 0,25 λ to the left, the interference becomes constructive rather than destructive. So the time interval between two nodes is the time it takes for the matrix to grow a distance of 0,5 λ of the incident light beam (see illustration). Combining the duration and the speed of the matrix growth, the thickness of the matrix can be calculated.

Experiment part IIIn the second part of this chapter, it is described how carbon chain anions and PAH molecules were sought in the ISM by comparing the NIR spectra of these molecules with the spectrum of an O-star.

NIR-DIBsA spectrum of the O 9.5 type star 4U 1907+09, taken by VLT/X-shooter, was used to compare near infrared (NIR) DIBs to absorption spectra of various molecules. The spectrum of this star was normalized in a previous bachelor project by Suzanne van Hooff (2012). Eight of the thirteen NIR-DIBs found by Geballe et al. (2011) showed up in this spectrum. Also, the two NIR-DIBs found by Joblin et al. (1990) at 1178 nm and 1317 nm were visible.

NIR DIB-spectra of carbon chain anionsBy matrix identification spectroscopy, absorption lines were measured of various carbon chains, in a research by J. P. Maier

(1997). The chains in this research that are of interest for this paper are negatively charged bare carbon chains with 12, 14, 16, 18 or 20 carbon atoms (C12

-, C14-, C16

-, C18-, C20-); these

were the only molecules that had absorption lines in the NIR range.In this research, interactions with the matrix compound causedshift of the absorption lines: the maximal Δλ/λ was 10-2. These shifts could be in the direction of either longer or shorter wavelengths. It is assumed that Δλ/λ is constant. Thus, the shifts should not be larger than 20 nm for absorption lines at lower wavelength than 2000 nm; a shift of Δλ≤20 nm was assumed for the absorption lines at 1269 nm, 1460 nm and 1729 nm. For the absorption lines at 2069 nm and 2440 nm, a shift of Δλ≤30 nm was assumed.

NIR DIB-spectra of medium to large sized PAHsBy matrix identification spectroscopy, spectra of cations and anions of 27 medium to large sized PAH molecules have been taken in the near infrared range (Mattioda et al. 2005). Because the matrix in MIS can cause the centers of absorption lines to shift, this method is not perfectly accurate.A maximum line shift of 30 nm was assumed; lines further away from DIBs were considered to be too far away to be a possible match. This value was chosen because it is known that matrices can cause a shift of Δλ/λ≈0,01 for carbon chains (Maier, 1997), which would correspond to 10-18 nm for the DIBs in the NIR range; to be on the safe side, and because the author noted that larger molecules tended to show larger shifts, 30 nm was chosen for medium to large sized PAHs.Furthermore, only absorption lines with absolute intensities larger than 10*103 km mol-1 were taken into account; absorption lines with intensities lower than this value are relatively small absorption lines in the spectra, which are not expected to cause DIBs. If they would cause the DIBs, much larger absorption lines should be visible from transitions with higher oscillator strengths. Spectra without absorption lines matching these two criteria will not be discussed.

The 485 μm DIB

12

DIBs are typically located in the UV/VIS and NIR, energy ranges that are associated with electronic transitions. In parallel, also vibrational and/or rotational energy levels can be involved, and therefore DIBs are likely due to unresolved rovibronic bands, i.e. transitions involving composed electronic, vibrational and rotational bands. The amount of energy needed to excite vibrational and rotational modes is substantially smaller than needed for an electronic excitation. A vibrational transition is a transition between two vibrational states (indicated by v). A pure vibrational transition involves a transition between vibrational states within one electronic state, a vibronic transition is a transition between vibrational levels in two different electronic states. The goal of this part is to link the UV/VIS spectra of DIBs to a new FIR feature around 485 m (Müller et al., not yet published) that has been linked to the diffuse interstellar medium and that is typical for a purely vibrational mode. This recently observed 485 μm DIB-analog might occur from a purely vibrational transition (in the electronic ground state or an excited state) of a DIB-carrier. In that case, there might be observable transitions in which the electronic and the vibrational energy levels change (see figure 7). In the sketch, the three transitions are sketched that are most likely to occur. The sketched transition are from the electronic ground state and the first excited state:

1) from v’’=0 to v’=1 (blue arrow in figure 5)

2) from v’’=0 to v’=0 (purple arrow)3) from v’’=1 to v’=0 (red arrow)

(Note: figure 7 describes a simplified situation, more energy levels exist in a more realistic scenario).The energy difference that we are looking for between transition 1) and transition 2), and between transition 2) and transition 3) is

E=h cλ = 0.00256 eV; an energy difference

that corresponds to a difference in

Figure 7: Transitions between different electronic (E=0, E=1) and vibrational (v=1,2) energy levels.

wavenumbers of 20.599 cm-1 and the energy of a 485 μm photon.Thus, if two or more DIBs are spaced each 20.599 cm-1 apart, they might arise from the same carrier that has mentioned electronic and vibrational transitions.The transition from v’’=0 to v’=0 (purple arrow in figure 7) is expected to occur most often, since most electrons are expected to be in the ground state in molecules under interstellar conditions (i. e. low temperatures). The absorption line caused by this transition is often referred to as the ‘origin band’. The transition represented by the blue arrow from v’’=0 to v’=1 , also from the ground state, is expected to cause an absorption line of nearly the same strength. In some cases, this line can even be stronger than the origin band.The v’’=1 to v’=0 transition (red arrow) would generally have the lowest line intensity; this is because it cannot occur unless an electron is in the first excited vibriational state, which is less populated than the ground state under those conditions. In figure 8, an impression of the expected intensities of the three mentioned transitions is displayed. The dashed parts of the lines indicate that there is an uncertainty in the magnitude of the intensities.

A downside of this method is that a molecule might be distorted when it’s in the first excited electronic state, causing the vibrational energy levels to change a bit. Then, the energy difference between v’’=0 and v’’=1 is not equal to the energy difference between v’=0 and v’=1. The result is that that the ‘blue’ transition might have a different wavenumber

13

separation from the ‘purple’ transition than the ‘red’ transition has.The DIB catalogue of Hobbs et al., 2009, was used to obtain the wavelengths of 488 DIB

Figure 8: Relative expected line intensities for transitions at different wavelengths

lines. 71 of these lines were unconfirmed DIBs. The wavelengths from this catalogue, which are measured in air, were converted to wavelengths in vacuum with a method described by Morton in 1991 (see references). Then, the wavelengths were converted to reciprocal centimeters. An uncertainty of ±1,00 cm-1 was assumed.

ResultsNIR DIB-spectra of carbon chain anions

Figure 9: The absorption spectra of five carbon chain anions obtained by MIS. This image was taken from J. P. Maiers paper (1997), see references.

Molecule λ (in nm)C12

- 1249C14

- 1460C16

- 1729C18

- 2069C20

- 2440

Table 1: Centers of the main absorption lines of carbon chain anions within the NIR-range

DIBS in 4U 1907+07 ( λ in nm)1179,81317,51522,2; 1526,7; 1561,1; 1565,8; 1566,71656,8; 1658,01780,0Table 2: Observed DIBs along the line of sight of 4U 1907+07.

Of the absorption lines of the carbon chain anions (see figure 9 and table 1), none is within the assumed error margin (20 nm for C12

-, C14- , C16

-; 30 nm for C18-, C20

-) of one of the DIBs observable among the line of sight of 4U 1907+07, which are listed in table 2.

NIR DIB-spectra of medium to large sized PAHs moleculesTen molecules with absorption lines close to NIR DIBs are listed below. In the figures, the VLT/X-shooter spectrum of 4U 1907+09 is shown; the location of the absorption lines of the potential DIB-carriers is indicated with a black line, the location of the DIBs is indicated with a dashed line.The structural formulae of these PAH molecules can be found in appendix A.

Cations:Molecule 1: Chrysene, C18H12

+

Chrysene has two transitions in the NIR range, one of them is 11,8 nm away from the 1179,8 DIB.

λ (in nm) A (in 103 km mol-1)1001 371168 120Table 3: Wavelengths and absolute intensities of the NIR absorption lines of chrysene.

14

Figure 10: The second NIR absorption line of chrysene (solid line) and the 1180 nm DIB (dashed line).

If it is assumed that the absorption line of chrysene at 1168 nm (λ0) would cause the 1180 nm DIB, it’s expected that its other absorption line at 1001 nm (λ1) should have a shift of Δλ1=(Δλ0/λ0)*λ1, towards a longer wavelength as well (since Δλ/λ is supposed to to be constant). Thus, an absorption line is expected at 1011,3 nm. This absorption line should have an absolute intensity with a value around 0,31 times the absolute intensity of the 1179,8 nm DIB, and its location should be at the dotted line in figure 11.

Figure 11: Location of the first NIR absorption line of chrysene (solid line) and its expected location if chrysene would be the carrier of the 1180 nm DIB (dashed line).

The absorption line that would be expected if chrysene were the carrier of the 1179,8 nm DIB does not show in figure 11.

Molecule 2: 3,4;5,6;10,11;12,13-tetrabenzo-peropyrene, C36H16

+

Of the NIR absorption lines of molecule 2, the 1340 nm absorption line is the only one in the vicinity of a NIR DIB; it is at 22,5 nm from the 1317,5 nm DIB.

λ (in nm) A (in 103 km mol-1)1108 131210 601340 20Table 4: Wavelengths and absolute intensities of the NIR absorption lines of molecule 2.

Shifts of Δλ1=(Δλ0/λ0)*λ1 would result in absorption lines at 1088,6 nm and 1188,9 nm. The absorption line at 1088,6 nm should have 0,65 times the value of the absolute intensity of the 1317,5 nm DIB; the absolute intensity of the 1188,9 nm line should be 3,0 times larger than the absolute intensity of the 1317,5 nm DIB.

Figure 9: The third NIR absorption line of molecule 2 (solid line) and the 1317 nm DIB (dashed line).

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Figure 10: Location of the first NIR absorption line of molecule 2 (solid line) and its expected location if molecule 2 would be the carrier of the 1317 nm DIB (dashed line).

Figure 11: Location of the second NIR absorption line of molecule 2 (solid line) and its expected location if molecule 2 would be the carrier of the 1317 nm DIB (dashed line).

In figure 13, no absorption line is observed at 1088,6 nm with a value of 0,65 times the absolute intensity of the 1317,5 nm NIR DIB. The large absorption line that would be expected at 1188,9 nm does not appear either in figure 14.

Molecule 3: dipyreno-(1’,3’;10,2),(1’’,3’’;5,7)-pyrene, C40H18

+

This molecules has only one absorption line in the NIR range at 1301 nm, 16,5 nm away from the 1317,5 nm DIB. It has an absolute intensity of 140 . 103 km mol-1.Comparing the 1317,5 nm DIB (see figure 12) and the absorption line at 1301 nm from this

molecule from the research by Mattioda et al. (2005), it can be seen that the 1317 nm DIB is much less wide.

Molecule 4: difluoranthen-(3’,5’;4,6), (4’’,6’’; 9,11)-coronene, C48H20

+

This molecule has, just like molecule 3, only one significant absorption line in the NIR range (with absolute intensity of 330 . 103 km mol-1). This line is at 1291 nm, 26,5 nm away from the 1317,5 nm DIB. It seems to be too broad to match the 1317 nm DIB.

Molecule 5: 12-13-o-phenylene-1,2;3,4;5,6; 7,8;9,10-pentabenzoperopyrene, C48H22

+

In the spectrum of this molecule, the 1154 nm absorption line is within the error margin range of a DIB; it is 25,8 nm away from the 1179,8 nm DIB.

λ (in nm) A (in 103 km mol-1)1056 1601154 251393 14Table 5: Wavelengths and absolute intensities of the absorption lines of molecule 5.

If the 1056 nm and 1393 nm absorption lines are shifted with Δλ1=(Δλ0/λ0)*λ1, their centers should be at 1079,1 nm and 1423,5 nm, respectively.The absorption line that would be expected at 1079,1 nm should have an absolute intensity that is 6,4 times larger than the absolute intensity of the absorption line that causes the DIB at 1179,8 nm: a rather large line should be seen near the dotted line in figure 16.The absorption line at 1423,5 nm (dotted line in figure 17) would have an intensity of 0.56 times the absolute intensity of the 1179,8 nm DIB.

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Figure 12: The second NIR absorption line of molecule 5 (solid line) and the 1180 nm DIB (dashed line)

Figure 13: Location of the first NIR absorption line of molecule 5 (solid line) and its expected location if molecule 5 would be the carrier of the 1317 nm DIB (dashed line).

An absorption line shows within 1 nm of the dotted line in figure 16. However, it is comparable in size to the 1179,8 nm DIB, while it should be over six times larger if it were caused by molecule 5.The wavelength interval near the third expected line of molecule 5 (figure 17) contains too much noise to identify a line that would match.

Figure 14: Location of the third NIR absorption line of molecule 5 (solid line) and its expected location if molecule 5 would be the carrier of the 1317 nm DIB (dashed line).

AnionsMolecule 6: 3,4;5,6;7,8-tribenzoperopyrene, C34H16

-

This molecule has, just like molecule 3 and 4, only one significant absorption line in the NIR range (with absolute intensity of 130 . 103 km mol-1). This line is at 1160 nm, 19,8 nm away from the 1179,8 nm DIB.

Molecule 7: 3,4;5,6;10,11;12,13-tetrabenzo-peropyrene, C36H16

-

In the spectrum of this molecule, the 1153 nm absorption line is close to a DIB: 26,8 nm away from the 1179,8 nm DIB.

λ (in nm) A (in 103 km mol-1)1055 5,81153 11Table 6: Wavelengths and absolute intensities of the absorption lines of molecule 6.

If the 1055 nm absorption line is shifted with Δλ1=(Δλ0/λ0)*λ1, its center should be at 1079,1 nm. Also, that 1079,1 nm absorption line should have 0,53 times the absolute intensity of the 1179,8 nm DIB.

17

Figure 15: The second NIR absorption line of molecule 7 (solid line) and the 1180 nm DIB (dashed line).

Figure 19: Location of the first NIR absorption line of molecule 7 (solid line) and its expected location if molecule 7 would be the carrier of the 1180 nm DIB (dashed line).

In figure 19, a narrow, deep line shows within 1 nm left of the location of the expected absorption line. This line is not smaller than the 1179,8 nm DIB, while its intensity should be nearly two times smaller. To the right, there is also a broad, shallow line. This line’s intensity is also larger than it should be if it were a match. It can be concluded that there is no matching line for the first NIR absorption line of molecule 7.

Molecule 8: 3,4;4,5;10,11;12,13-tetrabenzo-peropyrene, C36H16

-

λ (in nm) A (in 103 km mol-1)1353 161519 21Table 7: Wavelengths and absolute intensities of the absorption lines of molecule 8.

In the spectrum of this molecule, the 1519 nm absorption line is close to two DIBs; 3,2 nm away from the 1522,2 nm DIB and 7,8 nm away from the 1526,8 nm DIB.If the 1353 nm absorption line is shifted with Δλ1=(Δλ0/λ0)*λ1, its center should be at 1355,9 nm if it corresponds to the first DIB, or 1360,0 nm if it corresponds to the second DIB. The absolute intensity of the line that would be expected is a factor 0,76 of the DIB it would match with.

Figure 20: The second NIR absorption line of molecule 8 (solid line) and the 1522 nm and 1526 nm DIBs (dashed lines).

Figure 21: Location of the first NIR absorption line of molecule 8 (solid line) and its expected locations line 1 and line 2 if molecule 8 would be the carrier of the 1522 nm or 1526 nm DIB, respectively (dashed line).

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The spectrum of 4U 1907+07 contains a lot of noise in the wavelength region of the 1353 nm absorption line (figure 21). At 1355,9 nm, an absorption line is visible. However, this line is narrower and deeper than the 1522,2 nm DIB. The total intensity of the absorption line is larger than the total intensity of the 1522,2 DIB, while it is expected to have a value of 0,76 times the value of the 1522,2 nm DIB’s absolute intensity. Because of the large amount of absorption lines in this region, the chance of coincidentally having an absorption line there is considerable.At 1360,0 nm, no absorption line with 0,76 times the absolute intensity of the 1526,8 nm DIB is observed.

Molecule 9: difluoranthen-(3’,5’;4,6), (4’’,6’’; 9,11)-coronene, C48H20

-

λ (in nm) A (in 103 km mol-1)1313 1601468 1601797 46Table 8: Wavelengths and absolute intensities of the absorption lines of molecule 9.

In the spectrum of this molecule, two absorption lines are close to a DIB: the 1313 nm absorption line is 4,5 nm away from the 1317,5 nm DIB, and the 1797 nm absorption line is 16,9 nm away from the 1780,1 nm DIB.If it is assumed that the 1313 nm absorption line is causing the DIB, an absorption line with identical absolute intensity as the 1317,5 nm DIB would be expected at 1473,0 nm if a shift of Δλ1=(Δλ0/λ0)*λ1 is assumed. An absorption line with an absolute intensity a factor 0,29 times of that of the 1317,5 nm DIB would be expected at 1803,1 nm.The absorption line that would be expected does not show in figure 23. It should be noted that no data are available in the nearby interval between 1473,7 nm and 1474,9 nm.In figure 24, an absorption line is visible around 1.5 nm short of the expected line’s location. This interval is crowded with absorption lines, which

Figure 22: The first NIR absorption line of molecule 9 (solid line) and the 1317 nm DIBs (dashed lines).

Figure 23: Location of the second NIR absorption line of molecule 9 (solid line) and its expected location if molecule 9 would be the carrier of the 1317 nm DIB (dashed line).

Figure 24: Location of the third NIR absorption line of molecule 9 (solid line) and its expected location if molecule 9 would be the carrier of the 1317 nm DIB (dashed line).

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Secondly, the case if the 1797 nm absorption line were the DIB–carrier is examined. If Δλ1=(Δλ0/λ0)*λ1 is assumed, absorption lines would be expected at 1300,5 nm and 1454,1 nm. Both absorption lines should have an absolute intensity a factor 3,5 times larger than the 1780,1 nm DIB.The expected line at 1300,5 nm does not show in figure 26. There is an absorption line that would have roughly the same size at λ=1297 nm, however.

Figure 25: he third NIR absorption line of molecule 9 (solid line) and the 1780 nm DIB (dashed lines).

Figure 26: Location of the first NIR absorption line of molecule 9 (solid line) and its expected location if molecule 9 would be the carrier of the 1780 nm DIB (dashed line).

Figure 27: Location of the second NIR absorption line of molecule 9 (solid line) and its expected location if molecule 9 would be the carrier of the 1780 nm DIB (dashed line).

In figure 27, no absorption line is found with the intensity that would be expected if molecule 9 were the carrier of the 1780.1 nm DIB.

Molecule 10: 1,14-benzodiphenanthreno-(1’’,9’’;2,4),(9’’’,1’’’;11,13)-bisanthene, C50H22

-

λ (in nm) A (in 103 km mol-1)1120 401333 73Table 9: Wavelengths and absolute intensities of the absorption lines of molecule 10.

Figure 28: The first NIR absorption line of molecule 10 (solid line) and the 1317 nm DIB (dashed lines).

In the spectrum of this molecule, the 1333 nm absorption line is close to a DIB: 15,5 nm away from the 1317,5 nm DIB.

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If the 1120 nm absorption line is shifted with Δλ1=(Δλ0/λ0)*λ1, its center should be at 1106,8 nm. Also, that 1106,8 nm absorption line should have 0,55 times the absolute intensity of the 1179,8 nm DIB.In figure 29, no absorption line is found with the intensity that would be expected if molecule 9 were the carrier of the 1317,5 nm DIB.

Figure 29: Location of the first NIR absorption line of molecule 10 (solid line) and its expected location if molecule 10 would be the carrier of the 1317 nm DIB (dashed line).

The 485 μm DIBAssuming an error margin of Δν=1,00 cm-1, 77 pairs were found that are separated between 19.599 cm-1 and 21.599 cm-1 apart (see tables 11, 12 and 13 in appendix B). Fourteen of these pairs included an unconfirmed DIB. The DIB-pairs which had separations closest to 20,599 cm-1 (Δν=0,25 cm-1) are listed in table 11. Nineteen of such pairs were found, with four including an unconfirmed DIB.

In Table 10, subsequent DIB-pairs are listed. Eight cases were found where three subsequent DIBs were all spaced 20,6 ± 1,0 cm-1 apart. In four cases, four DIBs were subsequently spaced with a separation of 20,6 ± 1,0 cm-1. In the DIB-rich region between DIB-numbers 300 (670,8 nm) and 350 (683,6 nm), 38 of 51 DIBs were part of a of 20,6 ± 1,0 cm-1 DIB pair. In this interval, five of the twelve subsequent DIB-pairs are found.

DiscussionNIR spectra: carbon chain anionsNone of the absorption lines of the main transition of any carbon chain anion is within the error margin of a DIB. Therefore, these molecules are unlikely to cause any of the NIR DIBs in 4U 1907+07.

NIR spectra: PAHsMolecules 1, 2 and 5 had one or more other absorption lines in the NIR, apart from the one near a DIB. It was assumed that the other absorption lines shift with Δλ1=(Δλ0/λ0)*λ1.The spectrum of 4U 1907+07 showed no absorption lines that would correspond to these predicted absorption lines. Thus, if our assumption that the other absorption lines shift with Δλ1=(Δλ0/λ0)*λ1 is correct, it can be concluded that none of these molecules is causing the NIR-DIBs.Molecules 3 and 4 have only one absorption line in the NIR range. Therefore, it is hard to draw any conclusion about whether or not this molecule is a DIB-carrier; the fact that a line occurs within 30 nm of a DIB might very well be coincidence. The fact that the absorption lines are much broader in the DIBs than in the spectrum of 4U 1907+07 implies that these molecules do not cause the NIR DIBs, but it has to be taken into account that broadening is caused by matrix interactions and far higher column densities in MIS. The absorption lines are with a Δλ of 16,5 nm for molecule 3 and a Δλ of 26,5 nm for molecule 4 quite far from the DIBs; these separations do no rule them out as DIB-carriers since this separation is within the error margin, but it does make them less likely (especially molecule 4).For molecule 6, the same goes as for molecules 3 and 4. A large Δλ of 20 nm makes it less likely for molecule be a DIB carrierThe spectrum of 4U 1907+07 has an interesting looking absorption line close the wavelength of the expected absorption line from molecule 7, but a closer look at this line showed that it cannot be from molecule 7. Molecule 8 could not be examined due to the noise in the part of the spectrum that was of interest. No absorption lines were found in the spectrum of 4U 1907+07 that were likely

21

to come from absorption lines of molecules 9 and 10 that were not close to a DIB.Of the ten medium to large sized PAHs discussed above, not a single one showed any evidence that it is likely to cause a NIR DIB. The same goes for the other 17 molecules from the paper of Mattioda et al. (2005) that had no absorption lines in the vicinity of DIBs. For the three molecules that had only one absorption line in the NIR range, it is hard to prove they don’t cause the DIBs, because it is hard to draw any conclusions from a single absorption line if there are as many uncertainties as in this research (e.g. in absorption line width, and location of the absorption line center).If we do not take these molecules with only one NIR absorption line into consideration, it can be concluded that every medium to large sized PAH from Mattioda et al.’s research has an absorption spectrum that does not match with the NIR DIBs. Therefore, it is highly unlikely that these medium to large sized PAH are the carriers of the NIR DIBs. The next question to answer is: ‘do these medium/large PAHs not cause the NIR DIBs, or

can we say that no medium or large PAHs cause the NIR DIBs? It is of course hard to prove that the PAHs that are not investigated do not cause the NIR DIBs, but the PAHs that were investigated have quite ‘basic’ structures (see appendix A for the structures with absorption lines near DIBs, and Mattioda’s paper for all structural formulae). Of course, it cannot be proven in this research that in the ISM, no slightly different PAH structures are formed that do cause the NIR DIBs. As a general conclusion: it cannot be proven by the method used in this research that medium to large sized PAHs do not cause the NIR DIBs, but based on the results, it seems unlikely that they do.

The 485 μm DIBWhat do we expect to see? As explained in the chapter ‘Experiment’, we are looking for pairs of transitions separated 20,6 cm-1. It is interesting to look at sequences of DIB-pairs spaced 20,6 ± 1,0 cm-1 away rather than individual pairs, because a lot of pairs can be expected to be spaced that amount of wavenumbers apart by coincidence; if three or

Subsequent pairs 1st line 2nd lineRatio

line1line2 Ratio

line1line2DIBs Δν EW FWHM EW FWHM

cm-1 mÅ Å mÅ Åin EQWEQW in

FWHMFWHM

189 & 192 20,2 151,3 2,47 7,8 0,64 19,40 3,86192 & 194 20,7 7,8 0,64 8,1 0,88 0,96 0,73

241 & 243 19,8 2 0,45 10,1 0,84 0,20 0,54243 & 247 19,8 10,1 0,84 10,1 0,92 1,00 0,91247 & 249 19,8 10,1 0,92 3 0,67 3,37 1,37

268 & 271 21,0 341,6 1,08 17,1 0,71 19,98 1,52271 & 275 20,0 17,1 0,71 4,6 0,48 3,72 1,48

282 & 286 19,9 3,9 0,69 59,7 0,67 0,07 1,03286 & 290 19,8 59,7 0,67 9,2 1,17 6,49 0,57

301 & 303 19,6 18,7 0,96 5,3 0,62 3,53 1,55303 & 306 20,6 5,3 0,62 7,6 0,71 0,70 0,87306 & 312 21,4 7,6 0,71 10,9 0,81 0,70 0,88

301 & 304 21,5 18,7 0,96 7,7 0,71 2,43 1,35304 & 307 20,6 7,7 0,71 7,7 0,7 1,00 1,01

22

305 & 310 20,4 2,6 0,67 10,7 1,24 0,24 0,54310 & 314 20,3 10,7 1,24 4,3 0,89 2,49 1,39314 & 317 21,5 11,3 0,89 13,4 1,13 0,84 0,79

323 & 328 21,1 5,1 0,71 7,9 1,2 0,65 0,59328 & 332 19,9 7,9 1,2 6,2 0,86 1,27 1,40332 & 337 20,8 6,2 0,86 7,1 1,19 0,87 0,72

326 & 331 19,8 4,3 0,74 2 0,54 2,15 1,37331 & 335 19,8 2 0,54 16,2 0,8 0,12 0,68

448 & 450 21,5 7 0,98 9,5 1,12 0,74 0,88450 & 451 20,0 9,5 1,12 19,7 2,58 0,48 0,43

453 & 458 19,9 21,3 1,86 41,3 5,61 0,52 0,33458 & 462 20,2 41,3 5,61 52,7 1,5 0,78 3,74

464 & 467 20,5 17,8 0,86 11,1 0,72 1,60 1,19467 & 468 20,5 11,1 0,72 5,9 0,68 1,88 1,06

Table 10: All subsequent DIB-pairs with a separation in wavenumbers of 19,6 < Δν < 21,6. The DIBs are from Hobbs’ DIB-catalogue (Hobbs et al., 2009).

more DIBs have wavenumber separations of 20,6 ± 1,0 cm-1, this is much less likely to be coincidental.

The transitions we are looking for are assumed to be from the electronic ground state to the first excited electronic state, of which at least one transition should also have a change in the vibrational energy level (see figure 7). In section ‘Experiment part II’, it was explained that the most transitions with the highest intensity (and thus, highest EW) are expected to be the ones with the shortest wavelength and the lowest DIB number (see figure 8). This is because the ground state is expected to have the highest occupation under interstellar conditions. Also, we expect the absorption lines to have similar shapes and therefore, the FWHMs of the absorption lines should be comparable.

In short, for the transitions we are looking for, we expect four features to be observed in these subsequent DIB-pairs:

1) the EWs of the two absorption lines with the shortest wavelengths are the largest, the EW of the smallest energy transition (and thus, the longest wavelength) is the smallest

2) comparable FWHMs3) equal separations in wavenumbers

between absorption lines4) a value in wavenumber separations

close to 20,6 cm-1

Due to a very high number of DIBs in the relatively small interval between 670,8 nm and 683,6 nm (DIBs 300-350), a lot of coincidental matches can be expected there. When the five subsequent DIB-pairs in this interval were examined, they all seemed to show random values of wavenumbers between lines, line EWs, and line FWHMs. Therefore, they were judged to be most likely coincidental. These are pairs (301 & 303 + 303 & 306 + 306 & 312), (301 & 304 + 304 & 307),

23

(305 & 310 + 310 & 314 + 314 & 317), (323 & 328 + 328 & 332 + 332 & 337) and (326 & 331 + 331 & 335). The other subsequent DIBs are discussed below.

189 & 192 + 192 & 194These pairs are not very interesting because the first absorption line has a very high EW, while this line is not expected to be (much) larger than the origin band. Furthermore, the FWHMs are not very similar and the third line does not have a smaller EW than the second, contrary to what would be expected. The wavenumber separations between the lines are not similar. All in all, no convincing evidence is found that these lines originate from different vibrational states.

241 & 243 + 243 & 247 + 247 & 249The similarity in separations in wavenumbers between these lines is striking: all three have a value of 19,8 cm-1. However, the EWs did not show a pattern of EW decreasing with wavelength: the DIB with the lowest wavelength is very small (EW=2mÅ), and the second and third DIB have the same, larger EW. Therefore, we cannot assume that these subsequent DIB-pairs originate from the transitions of different vibrational states that we are looking for.

268 & 271 + 271 & 275The separations in wavenumbers from these DIBs are, with 21,0 cm-1 and 20,0 cm-1, not very similar. Also, the EW of the first line is almost 20 times larger than the EW of the origin band, which is not what we expect to see if these DIBs are absorption lines from the transitions we are looking for.

282 & 286 + 286 & 290These subsequent DIB-pairs can be discarded, since the EWs and FWHMs of the absorption lines show do not match the criteria at all (although the wavenumber separations are similar).

448 & 450 450 & 451These DIB-pairs do not match any of the three criteria that are set in this research for subsequent DIB-pairs; there is no reason to

assume these pairs occur from different vibrational states of the same carrier.

453 & 458 + 458 & 462The ratio in EW between the first transition: origin band is 0,52, which is a reasonable value. However, the EW of the last transition is too large. Also, the FWHMs are not similar, so it can be concluded that there is not enough reason to assume that these pairs occur from different vibrational states of the same carrier (although the wavenumber separations match reasonably).

464 & 467 +467 & 468These two DIB-pairs are very interesting since they have equal wavenumber separations and comparable FWHM values. The only downside of these pairs is that the first transition has a larger EW than the origin band (ratio=1,60), while the origin band is expected to be the strongest transition. This does not rule out these pairs as being transitions with different vibrational energy levels, since the origin band is not always the strongest transition.It should be noted that DIB 468 is an unconfirmed DIB.

Of the twelve subsequent DIB-pairs found, most are most likely coincidental; they showed no (or not enough) consistency in the values of their EWs and FWHMs, or in their separation in wavenumbers. Although multiple subsequent DIB-pairs look interesting, the only ones that match all the criteria that were set are the 464-467-468 DIBs; the last transition had the smaller EW while the first two were comparable in magnitude, they had equal wavenumber separations, and a value of the wavenumber separation very close to the value 20,6 cm-1

(being 20,5 cm-1). This are the DIBs at λair = 769,6 nm, 770,8 nm and 772,0 nm. Therefore, the 464-467-468 DIBs are the most likely DIBs to be associated with the energy of the 485 μm DIB, if the assumption that the 485 μm DIB occurs from a vibrational transition is correct.Of the other subsequent DIB-pairs, the 241-243-247-249 DIBs stand out the most because of their similar separations in wavenumbers of three DIB-pairs. However, we are not able to

24

explain the pattern in absorption line EWs, which are highest for the two lines in the middle. Also, with their separation of 19,8 cm-

1, they are only just within the error margin. Therefore, these subsequent DIB-pairs do not seem to be the most interesting topic for future research in this area.

Conclusions- The absorption lines of carbon chain anions C12

-, C14-, C16

-, C18-, C20

- do not match with any of the NIR DIBs. Anion chains with a different number of carbon atoms had no NIR absorption lines. It is concluded that no relation is found between carbon chain anions and NIR DIBs.

- Of the 27 medium to large sized PAH spectra taken by Mattioda et al., 24 did not match with the NIR DIBs observed in 4U 1907+07. The other three PAH spectra had only one absorption line in the NIR range, so it was hard to draw any conclusions for these PAHs; the assumed error margin Δλ/λ was relatively large. In conclusion, there is no reason to assume that these medium to large sized PAHs are the carriers of the NIR DIBs, but the method used in this research is not accurate enough to completely rule them out.

- Of the sets of subsequent DIB-pairs, there was one set of three DIBs that matched all the criteria that were set for absorption lines associated with transitions from different vibrational energy levels. This were the DIBs with indicated in this research with number 464, 467 and 468, with λair = 769,6 nm, 770,8 nm and 772,0 nm. Some other subsequent pairs also showed interesting features, but did not match the criteria. Therefore, they were considered to be most likely coincidental.

Future researchNIR spectraThe results of this research indicated no relation between the NIR DIBs and both carbon chain anions or medium to large sized PAHs. Therefore, I do not recommend further research projects to identify NIR DIBs to focus on these classes of molecules.

The 485 μm DIBIt was found in this research that the λair = 769,6 nm, 770,8 nm and 772,0 nm DIBs are the most likely DIBs to be associated with different vibrational energy states, with energy difference corresponding to the energy of a 485 μm photon. The first step to test our hypothesis, is to investigate if these DIBs are correlated. Correlated means, in this context, that the DIBs have the same ratios of EWs along other lines of sight. If that’s not the case, the DIBs are very unlikely to have the same carrier. Also, because the 485 μm DIB is thought to occur due to a vibrational transition of the same carrier as the 464-467-468 DIBs, the EW of the 485 μm DIB should correlate with the EW of these DIBs.If research would prove that the DIBs are correlated, the next step would be to search for the identity of the carrier.

AcknowledgementsI would like to thank Lex Kaper for helping me find a project where I could combine chemistry and astrophysics, and being helpful and thinking with me while I was working on it at the Anton Pannekoek Instituut. I would like to thank everyone at the API for having me here for three months. I would like to think Harold Linnartz and various other people at the Sterrewacht Leiden for their hospitality and for how they have helped me with my project.

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Appendix A

Figure 30: Structures of the ten PAH molecules with absorption lines near NIR DIBs. Molecules 1-5 are cations, molecules 6-10 are anions. Molecule 4 and 9 are actually the same molecule, but with a different charge. Image was created with ChemDraw.

28

Appendix B

1st line 2nd lineDIBs Possible Δν Λ Ν EW FWHM λ ν EW FWHM

DIB? cm-1 Å cm-1 mÅ Å Å cm-1 mÅ Å20,35 ˂ Δν ˂ 20,8511 & 12 both 20,7 4679,81 21368,4 4,8 0,74 4684,35 21347,7 3,8 0,5464 & 65 20,7 5537,36 18059,2 139 3,91 5543,71 18038,5 14,1 1,26102 & 104 20,4 5813,52 17201,3 16,8 1,06 5820,42 17180,9 7,7 0,65169 & 171 20,7 6129,9 16313,5 11,3 2,18 6137,68 16292,8 6,1 1,41192 & 194 20,7 6214,72 16090,8 7,8 0,64 6222,74 16070,1 8,1 0,88210 & 211 20,7 6318,45 15826,7 45,7 2,23 6326,73 15806 19,8 0,84256 & 258 20,5 6536,35 15299,1 329 12,7 6545,1 15278,6 13,5 0,83270 & 274 20,7 6623,54 15097,7 5,5 0,67 6632,65 15076,9 5,3 0,56303 & 306 20,6 6720,23 14880,4 5,3 0,62 6729,54 14859,9 7,6 0,71304 & 307 20,6 6721,11 14878,9 7,7 0,71 6730,41 14857,9 7,7 0,7305 & 310 305 20,4 6726 14867,7 2,6 0,67 6735,24 14847,3 10,7 1,24318 & 322 20,5 6757,85 14797,6 7,9 1,06 6767,25 14777,1 5,2 1,13332 & 337 20,8 6788,42 14731 6,2 0,86 6798,01 14710,2 7,1 1,19345 & 349 20,4 6823,57 14655,1 13,8 1,32 6833,1 14634,7 1,8 0,6346 & 350 350 20,5 6825,48 14651 6,5 0,56 6835,03 14630,5 2,2 0,83426 & 431 20,7 7356,97 13592,6 14,5 0,96 7368,19 13571,9 25,3 0,93453 & 459 20,7 7560,52 13226,6 21,3 1,86 7572,37 13205,9 5,8 0,75464 & 467 20,5 7698,2 12990,1 17,8 0,86 7710,37 12969,5 11,1 0,72467 & 468 468 20,5 7710,37 12969,5 11,1 0,72 7722,57 12949,1 5,9 0,68

Table 11: DIB-pairs with a separation in wavenumbers close to the wavenumber of the 485 μm DIB (20,35 ˂ Δν ˂ 20,85).

29

1st line 2nd lineDIBs Possible Δν Λ ν EW FWHM Λ ν EW FWHM

DIB? cm-1 Å cm-1 mÅ Å Å cm-1 mÅ Å19,6 ˂ Δν ˂ 20,35

74 & 75 19,7 5638,94 17733,8 4,5 1,235645,2

2 11714,1 2 0,65

107 & 110 20,1 5839,71 17124,1 1,9 0,465846,5

8 17104 11,6 0,63

122 & 127 127 19,8 5948,96 16809,7 14,6 0,785955,9

8 16789,9 5,9 0,6

131 & 132 131 19,8 5977,4 16729,7 3,4 0,45984,4

7 16709,9 12,1 0,85

141 & 143 20,2 6022,04 16605,7 11,9 0,766029,3

7 16585,5 57,8 2,08

161 & 163 20,2 6104,12 16382,4 3,5 0,646111,6

4 16362,2 7,5 0,72

166 & 168 20,3 6118,53 16343,8 13,8 0,916126,1

3 16323,5 4,2 1,08

189 &192 20,2 6206,92 16111,1 151 2,476214,7

2 16090,8 7,8 0,64

197 & 198 20,1 6228,01 16056,5 5,2 0,666235,8

3 16036,4 18,9 0,65

204 & 205 20,2 6271,66 15944,7 256 1,326279,0

9 15924,6 27,6 1,35

212 & 213 19,9 6331,8 15793,3 17,9 0,736339,7

7 15773,4 2,8 0,5

229 & 232 20,3 6412,12 15595,5 11,1 0,986420,4

8 15575,2 5,8 0,67

241 & 243 19,8 6454 15494,3 2 0,456462,2

8 15474,4 10,1 0,84

243 & 247 19,8 6462,28 15474,4 10,1 0,846470,5

4 15454,7 10,1 0,92

247 & 249 249 19,8 6470,54 15454,7 10,1 0,926478,8

4 15434,9 3 0,67

271 & 275 20 6624,75 15094,9 17,1 0,716633,5

4 15074,9 4,6 0,48

282 & 286 282 19,9 6653,74 15029,2 3,9 0,696662,5

7 15009,2 59,7 0,67

286 & 290 19,8 6662,57 15009,2 59,7 0,676671,3

5 14989,5 9,2 1,17

301 & 303 19,6 6711,39 14900 18,7 0,966720,2

3 14880,4 5,3 0,62

310 & 314 20,3 6735,24 14847,3 10,7 1,246744,4

4 14827 4,3 0,89

316 & 319 20,3 6749,77 14815,3 12,3 1,196759,0

5 14795 6,5 0,9

326 & 331 Both 19,8 6776,22 14757,5 4,3 0,746785,3

1 14737,7 2 0,54328 & 332 328 19,9 6779,25 14750,9 7,9 1,2 6788,4 14731 6,2 0,86

30

2

331 & 335 331 19,8 6785,31 14737,7 2 0,546794,4

2 14718 16,2 0,8

336 & 340 20,3 6797,17 14712 10,6 0,626806,5

6 14691,7 6,8 1,38

339 & 343 20,3 6805,25 14694,5 9,1 0,66814,6

4 14674,3 25,1 1,41

344 & 348 19,6 6820,19 14662,4 9,5 1,496829,3

2 14642,7 29,7 1,04

394 & 395 20,1 7108,27 14068,1 63,4 3,997118,4

4 14048 8,3 0,71

425 & 429 19,9 7351,88 13602 16,5 0,817362,6

5 13582,1 12,2 0,68

450 & 451 20 7534,8 13271,8 9,5 1,127546,1

9 13251,7 19,7 2,58

453 & 458 19,9 7560,52 13226,6 21,3 1,867571,9

1 13206,7 41,3 5,61

457 & 461 19,8 7570,3 13209,5 11,7 0,877581,6

8 13189,7 24,6 1,14458 & 462 20,2 7571,91 13206,7 41,3 5,61 7583,5 13186,5 52,7 1,5

Table 12: DIB-pairs with a separation in wavenumbers smaller than the wavenumber of the 485 μm DIB but still within the error margin: 19,6 ˂ Δν ˂ 20,35.

1st line 2nd lineDIBs Possible Δν λ ν EW FWHM Λ ν EW FWHM

DIB? cm-1 Å cm-1 mÅ Å Å cm-1 mÅ Å20,85 ˂ Δν ˂ 21,616 & 17 16 20,9 4759,22 21011,9 176 14,5 4763,95 20991 127 2,5

25 & 26 20,4 4965,37 20139,5 26,4 0,72 4970,6620118,

1 7,5 0,89

54 & 56 21,4 5489,22 18217,5 236 3,38 5495,6918196,

1 31,2 0,69

134 & 135 21,5 5989,88 16694,8 10,9 0,87 5997,5916673,

4 9,3 0,83

147 & 149 21,3 6035,26 16520 5,4 0,98 6061,0916498,

7 10,6 0,91

148 & 151 21 6059,29 16503,6 5,1 0,68 6066,0916482,

6 13,9 0,6

154 & 155 155 21,4 6072,9 16466,6 8,8 1,16 6080,7916445,

2 14,9 3,51

164 & 167 21,3 6112,48 16360 2,6 0,57 6120,4316338,

7 4,4 0,65

193 & 195 21,3 6217,15 16084,5 10,4 1,62 6225,3816063,

3 10,3 0,51

31

199 & 200 21,1 6238,59 16029,3 10,1 0,7 6246,8216008,

2 18,8 1,36

245 & 248 20,9 6467,29 15462,4 5,1 0,64 6476,0515441,

5 12,4 1,25

268 & 271 21 6615,53 15116 342 1,08 6624,7515094,

9 17,1 0,71

283 & 288 21,2 6656,47 15023 11,5 1,12 6656,8915001,

8 7,6 1,07

297 & 300 21,3 6698,91 14927,8 5,9 0,73 6708,4614906,

6 6,5 0,96

301 & 304 21,5 6711,39 14900 18,7 0,96 6721,1114878,

5 7,7 0,71

306 & 312 21,4 6729,54 14859,9 7,6 0,71 6739,2614838,

4 10,9 0,81

314 & 317 21,5 6744,44 14827 11,3 0,89 6754,2214805,

6 13,4 1,13

323 & 328 328 21,1 6769,56 14772 5,1 0,71 6779,2514750,

9 7,9 1,2329 & 333 21,3 6780,92 14747,3 5,2 0,64 6790,71 14726 14,3 1,02

338 & 342 21,1 6803,42 14698,5 17,4 0,8 6813,1914677,

4 27,6 0,93

353 & 359 21,4 6839,64 14620,7 8,4 0,72 6849,6714599,

3 6,5 0,76

361 & 363 21,4 6854,43 14589,1 15,7 0,8 6864,514567,

7 11,6 0,65

402 & 406 21,3 7154,22 13977,8 15 2,26 7165,1313956,

5 15,9 0,84

413 & 415 21 7259,44 13775,2 22,5 1,36 7270,5313754,

2 19,2 1,29

448 & 450 21,5 7722,6 13293,3 7 0,98 7534,813271,

8 9,5 1,12Table 13: DIB-pairs with a separation in wavenumbers larger than the wavenumber of the 485 μm DIB, but still within the error margin: 20,85 ˂ Δν ˂ 21,6.

32