Laboratory Astrophysics: Gas Phase Experiments
Outline
- a few words about Vacuum and Number Density - Modern Vacuum Systems
Vacuum in Space and on Earth
Gas Phase Spectroscopy
- The difficulties with spectroscopy of interstellar molecules - Beer’s law and ways to reach high sensitivities - Gas phase spectroscopy with Ions and Radicals
Reaction Rates and Dynamics - Laboratory Experiments of Neutral-Neutral Collisions - Laboratory experiments of Ion-Neutral Collisions - Electron Recombination Measurements
Gas Phase Spectroscopy: Basics
Beer’s law: 𝑇 = 𝐼
𝐼0
= 𝑒−𝛼 𝐿
transmission incoming intensity
transmitted intensity
absorption coefficient
path length
incoming light
transmitted light
gas cell
Light source
Monochromator (wavelength filter)
detector
Simple Experiment
Challenges in Laboratory Spectroscopy of Interstellar Molecules
- Many interstellar molecules are radicals or molecular ions, they are difficult to produce in large quantities under terrestrial conditions. - The enormous pahtlength that is typically sampled in interstellar observations can not be reproduced in the laboratory - Interstellar clouds have very low temperatures, therefore absorption spectra involve only lines originating from the lowest levels of the molecules. Laboratory preparation of gas samples at 10-100 K can be difficult. Measurements at higher temperatures are difficult to analyze because lines may overlap: spectral congestion. - To determine accurate line frequencies, narrow linewidths are essential, the Doppler width due to the molecular motion at high temperatures can be a problem.
Ways to beat Beer’s law
1) try to get the sample density as high as possible (difficult with ions and radicals)
2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy
3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion
Discharge for Spectroscopy of Molecular Ions (H3+, CH5
+, etc …)
Group of Takeshi Oka, University of Chicago
Discharge for Spectroscopy of Molecular Ions (H3+, CH5
+, etc …)
Group of Takeshi Oka, University of Chicago
Discharge length: 1m Ion density: 1013 cm-3
Neutral/ion ratio 106
Ways to beat Beer’s law
1) try to get the sample density as high as possible (difficult with ions and radicals)
2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy
3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion
Discharge for Spectroscopy of Molecular Ions (H3+, CH5
+, etc …)
Thesis B.J. McCall, Group of Takeshi Oka, University of Chicago
Cavity Ringdown Spectroscopy
without gas in the cell
with gas in the cell
With absorbant: τ = 𝑡𝑟
2[ 1 − 𝑅 + α 𝐿] cavity length
absorption coefficient
τ = 𝑡𝑟
2[ 1 − 𝑅 ]
round trip time of the light in th cavity
mirror reflectivity (0.9999)
decay constant (33 μs)
𝐼 𝑡 = 𝐼0𝑒( −𝑡τ ) Empty cavity:
Ways to beat Beer’s law
1) try to get the sample density as high as possible (difficult with ions and radicals)
2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy
3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion
E1
E2
hν
B1
2 ρ
B2
1 ρ
A2
1
ρ: energy density of the radiation field
Absorption Spontaneous
emission
Induced emission
Einstein Coefficients and Light Amplification Condition
𝐵12= 𝑔2
𝑔1𝐵21
𝐴21= 8𝜋ℎν3
𝑐3 𝐵21
absorption Induced emission
Spontaneous emission
R12 = N1 B12 R21 = N2 B21
Rate of absorption Rate of induced emission
For laser/maser action: R21 > R12
Since B21 = B12
Assume (g1=g2)
N2 > N1
Population inversion is a condition for light amplification through stimulated emission
Laser Principle (3-Level)
Exte
rnal
exc
itat
ion
Spontaneous emission
Short lived
Long lived N2
N1
Stimulated Emission Laser action
Population Inversion N2 > N1
- Stimulated emission (coherent) has to compete with spontaneous emission (incoherent). - Since 𝐴21~ ν3 , light amplification much easier to realize at longer wave length (Masers were realized before lasers).
𝐴21= 8𝜋ℎν3
𝑐3 𝐵21
Laser Principle (4-Level)
Exte
rnal
exc
itat
ion
Spontaneous emission
Short lived
Long lived N2
N1
Stimulated Emission Laser action
Population Inversion N2 > N1
Spontaneous emission
Short lived
Laser Scheme
External Excitation Source
mirror
semi-transparent mirror
Laser medium
Coherent Light beam
Example: Ruby Laser
Tunable Light Sources: Dye Laser, Titanium:Sapphire Laser
Dye Laser Titanium:Sapphire Laser
Output power up to 2W
Dye Laser Coverage
Laser Diode Coverage
But: Laser diodes typically have a very narrow tuning range (30nm), one has to buy a new one if one goes beyond that range
Modern Tunable Laser Systems in the Infrared
OPO - Optic Parametric Oscillator System (2-5 μm)
Splits one pump photon into 2!
Ways to beat Beer’s law
1) try to get the sample density as high as possible (difficult with ions and radicals)
2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy
3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion
Discharge for Spectroscopy of Molecular Ions (H3+, CH5
+, etc …)
Group of Takeshi Oka, University of Chicago
Liqiud nitrogen cooled discharge (77K)
The Benefit of Low Temperatures
H3+ Spectrum
2000-3500 cm-1 Tvib = Trot = 3000 K
H3+ Spectrum
2000-3500 cm-1 Tvib = Trot = 100 K
Supersonic Expansion
- High pressure gas is relaxed through a small nozzle - Molecular velocities exceed the local speed of sound - Heat energy is converted into translational energy in collisions - Rotational temperatures between 0.5 – 30 K can be reached
Cavity Ringdown Spectroscopy with Supersonic Expansion
Ways to beat Beer’s law
1) try to get the sample density as high as possible (difficult with ions and radicals)
2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy
3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion
Adler et al. , Annual Review of Analytical Chemsitry 3, 175 (2010)
Precision Spectroscopy in Discharges
From webpage: Menlo Systems
Revolution in Precision: Frequency Combs
Source: NIST webpage
Revolution in Precision: Frequency Combs
Beat frequency in the microwave domain, Can be counted with standard electronics
NICE OHMS Ion Beam Spectroscopy
Mills et al., JCP 135, 224201 (2011)
PAUSE
Vacuum in Space and on Earth
Ideal Gas law: 𝑝𝑉 = 𝑛𝑅𝑇 𝑛 = 𝑝𝑉
𝑅𝑇
T = 273.15 K V = 1 cm3 = 1 x 10-6 m3
R = 8.314472 J mol-1 K-1
1 mol = 6.02 x 1023
Environment Number density [cm-3] Pressure [mbar]
Earth’s atmosphere 2.7 x 1019 1013.25
Dense interstellar clouds 1 x 104 3.8 x 10-13
Diffuse interstellar clouds 100 3.8 x 10-15
Average Iinterstellar medium 1 3.8 x 10-17
(Very) Approximate density of the Universe
1 x 10-6 3.8 x 10-23
Best laboratory pressure
Collision Time Scales
Collision rate = Rate coefficient x number density
𝑅 = 𝛼 𝑛
Neutral-neutral collision in Earth’s atmosphere: α = 10-13 cm3 s-1 , n = 3 x 1019 cm-3
Example 1)
R = 3 x 106 s-1
(3 million collisions per second)
Example 2)
ion-neutral collision in diffuse ISM: α = 10-9 cm3 s-1 , n = 100 cm-3
R = 1 x 10-7 s-1
(3 collisions per year)
Different Vacuum Regimes
Parameter to judge the pressure regime (or quality) of a vacuum system:
𝐾𝑛 = λ
𝐿 Knudsen number:
λ : mean free path 𝐿 : characteristic dimension
𝐾𝑛 > 0.5 molecular regime (statistical mechanics)
0.5 >𝐾𝑛 > 0.01 transition regime
𝐾𝑛 < 0.01 viscose flow regime (fluid mechanics)
Is Interstellar Space a Good Vacuum?
Consider a diffuse molecular cloud: - The number density is very low for terrestrial standards (100 cm-3)
- On the other hand: the extension of the “vacuum system” is huge!
(on the order of parsec). Therefore the dynamics of a particle is largely determined by collisions with other particles in the system.
The mean free path may be on the order of millions of kilometers, but not as large as the cloud dimensions
𝑲𝒏 ≪ 𝟎. 𝟎𝟏 Interstellar Space is a “Bad Vacuum” !
The small Knudsen number means that Interstellar Clouds are dynamical environments, with paticles undergoing many collisions before they leave the system. Otherwise the dynamics in the ISM would be very boring, and no reactions would occur.
Vacuum generation: Now and Then
Magdeburg hemispheres (Otto von Guericke 1656)
Equipment on exhibition at the Deutsche Museum (Munich)
Pumping techniques (somewhat obsolete)
Oil diffusion Pump Ultimate pressure 10-9 mbar
Rotary Vane Pump Ultimate pressure: 10-3 mbar)
Modern Pump Systems
Scroll pump (rough vacuum 10-2 mbar)
Turbo-Molecular Pump (ultra-high vacuum 10-9 mbar or better)
The Test Storage Ring at the Max-Planck Institut, Heidelberg
55.4 m circumference P ≈ 5 x 10-11 mbar
Ion Pump / Titanium Sublimation Pump
The Cryogenic Storage Ring CSR at MPIK
Cryogenic Storage Ring CSR at MPIK, Heidelberg An Ultracold Storage Ring for Molecular Ions Temperature < 10 K Pressure < 10-13 mbar Electrostatic deflection
no mass limit ideal for molecular ions
The CSR Cryostat Design
Neutral-Neutral Experiments: Crossed Beams
Group of Ralf I. Kaiser Department of Chemistry, University of Hawai’i at Manoa
(Y. T. Lee: Nobel prize in Chemistry 1986)
Examples: HCCCCH on CN (molecular clouds, Titan) B on C2H2, C2H4, CH3CCH, etc …
The Cresu Technique
I. Smith, Annu. Rev. Astron. Astrophys. 49, 29 (2011)
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2 e
OH+ H2O+
H3O+ H2O
OH e
O
H2
H2
HCO+ CO
HCN CH3NH2
CH3CN
C2H5CN
CH CH2CO
CH3OH
CH3OCH3
CH3+
C2H5+ e
C2H4
e
C3H2 e
C3H
e
C2H
McCall, PhD thesis, Chicago 2001
Ion – Neutral Reactions: Engines of Interstellar Chemistry
Molecular abundances largely determined by the competition between ion-neutral reactions and dissociative electron recombination.
Ion – Neutral Reactions: Engines of Interstellar Chemistry
I. Smith, Annu. Rev. Astron. Astrophys. 49, 29 (2011)
The Flowing Afterglow / Selected Ion Flow Tube (SIFT)Technique
SIFT Instrument In Boulder/Colorado Snow & Bierbaum, Annu. Rev. Anal. Chem. 1, 229 (2008)
5 cm
Cold Ion-Neutral reactions: Radiofrequency Ion Traps
Gerlich, Physica Scripta, T59, 256, (1995)
Quadrupole 22-Pole
22-Pole field Geometry
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
0
0.5
1
1.5
2
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
0
0.5
1
1.5
2
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
Example: CH4+ + H2 CH5
+ + H measured in a 22-pole ion trap at 15K
Asvany, Chem. Phys. 298, 97 (2004)
Merged Beams
Bruhns et al, Rev. Sci. Instrum. 81, 013112 (2010)
Interaction region H2 molecule formation
e- ejected
H2- complex
H2 molecule
H- + H H2 + e-
H2+ detector
Laser Helium gas cell
H- ion source 10 kV
Detection region H2 stripping in helium
H2+ detection by energy analyzers
H2 + He H2+ + [He, e-]
e- ejected
H2+ ion
1 m Beam profile monitors
Photodetachment region Partial neutralization of the H- beam
inside a drift tube at variable voltages -Uf
H- + νIR H + e-
-Uf
Example: H- + H H2+e associative detachment
H+ detector
H- dump
cw Nd: YAG Laser cavity / 1-2 kW
H0 dump
demerge magnet
1 m 0
Merged Beams: Neutral-Ion Collisions
spherical deflector
H- beam 9 keV
Ion beam from ECR (20-270 keV)
Neutral H beam
(Oak Ridge National Lab, TN, USA / courtesy C.C. Havener)
Charged products detctor
An Advanced Ion-Atom Collision Setup at CSR
neutral beam (C, 40 keV)
single particle detector
reaction product (CH+, 43.3 keV) stored molecular
ion beam (H3+ , 10 keV)
AIACS at CSR
Example: H3
+ + C CH+ + H2
Ion-Atom Reactions: State-of-the-Art
Flowing Afterglow/ Selected Ion Flow Tube
AIACS
Atoms studied
Ions studied
Absolute measurements
Temperature 300 K
H, O, N
H3+, O2
+, C6H6+, C2
-, … ions stable under plasma conditions
Often only relative calibration possible
40-40000 K
H, D, C, O
H2+, HD+, H3
+, O2+, C6H6
+, H-, HC2
-, C2-, … ,C10
-, ions stable under interstellar conditions
Electron Recombination: Flowing Afterglow
Flowing Afterglow Setup, Charles University Prague
M. Tichy, AIP Conf. Proc. Ser. 669, 60 (2003)
Electron Recombination: Stationary Afterglow
Advanced Stationary Afterglow (AISA), Charles University Prague
J. Glosik, J. Phys: Conf. Ser. 4, 104 (2005)
Electron-Ion Dissociative Recombination Measurements in Heavy-Ion Storage Rings
• radiative relaxation (rotations, vibrations) • direct measurement
• 100% detection efficiency
• high resolution
Advantages
Storage Rings: ASTRID (Aarhus, Denmark) CRYRING (Stockholm, Sweden) TARN II (Tokio, Japan) TSR (Heidelberg, Germany)
no longer active
Examples: H2
+, H3+, DCO+, HCO+, ….
Example: Storage Ring Rate Coefficient Measurement
CRYRING 2003 (n-H2)
TSR 2009 (n-H2:Ar 1:5)
H3+ + e- H2 + H
H + H + H
Kreckel et al., PRA 82, 042715 (2010)
Literature
General: Ian W.M. Smith: “Laboratory Astrochemistry: Gas-Phase Processes” Annu. Rev. Astron. Astrophys. 49, 29-66 (2011)
Electron Recombination:
Mats Larsson and Ann E. Orel “Dissociative Recombination of Molecular Ions” Cambridge University Press. 2008
Ion-Atom Collisions:
T. Snow & V. Bierbaum “Ion Chemistry in the Interstella Medium” Annu. Rev. Anal. Chem. 1, 229 (20-8)