Experimental setup for multistep photoexcitation with onevacuum-ultraviolet photon as the first step
Eric Audouard, Pierre Laporte, and Nicole Damany
Equipe de Spectroscopie, Centre National de la Recherche Scientifique UA 171), Universit6s de Saint-Etienne etde Lyon I, 158 bis Cours Fauriel, 42023 Saint-Etienne Cedex, France
Received January 3, 1989; accepted March 6, 1989
A simple experimental arrangement allowing multistep excitation studies including one VUV photon as the firstexcitation step is described. The system is based on the simultaneous use of a monochromatized VUV-laser plasmasource with a tunable dye laser. The same pump laser is used for both excitation sources, thus allowing, if required,a well-defined time delay between the two excitation steps. The system permits both photoionization and time-resolved fluorescence measurement. Its capability is illustrated by experiments on rare gases. This demonstrationshows that such a device allows one to obtain spectroscopic and kinetic information.
INTRODUCTION
Because of unique properties of lasers, many basic physicalproperties have become amenable to experimental investi-gation. In particular, the use of multiphoton and/or multi-color excitation has permitted major progress in atomic andmolecular spectroscopy, mainly using pulsed lasers. Never-theless, if one considers atoms or molecules with low-lyingexcited states situated in the vacuum-ultraviolet (VUV)range, such as rare gases and small molecules, the parame-ters describing the electronic properties are less accessible.Most work in progress deals with two or more nonresonantphotoexcitations as the first step. This technique, in princi-ple, permits the atom or molecule to undergo selective exci-tation. The different excitation pathways are often comple-mentary because of different selection rules. In practice, awell-defined first excitation step is achieved rather well withtwo-photon excitation when the peak power is limited.1'2But in the case of three (or more) -photon excitation, reso-nantly enhanced multiphoton ionization (REMPI) or otherprocesses3 may dominate, which complicate multicolor spec-troscopy of high-energy excited states, even though they canlead to many interesting results.
The simplest multicolor-spectroscopy scheme involves afirst one-photon excitation step. Wherever VUV energylevels are considered, several experimental methods may inprinciple be applied. One of them consists in coupling alaser with synchrotron radiation,4' 5 with the drawback thatthe measurement is done at the laser repetition frequency sothat most of VUV photons are unused. Thanks to progressin coherent VUV generation, multistep excitation spectros-copy has been performed in this range. 6' 7 Such techniques,although promising, still remain rather delicate to set, andpulse-to-pulse reproducibility is rather poor. It should bementioned that these techniques differ considerably fromrecently reported research that includes a visible cw laser forthe first excitation step.8' 9
Recently some of us successfully performed optical-opti-cal double-resonance (OODR) experiments including oneVUV photon as the first excitation (VUVODR).10 In theseexperiments a N2 laser was coupled to a Ballofet-Romand-
Vodar-type spark source,"' and ionization cross sections at337 nm of relaxed rare-gas excimers were obtained from thedepletion of VUV excimer fluorescence. Despite the high-VUV photon flux and recent improvement in that source, 2
use of the source remains limited because of the large timewidth (150 nsec), residual jitter (50 nsec), and low repeti-tion rate (<5 Hz).
In the present paper we describe a system that is based onthe use of a single Nd:YAG laser as the primary source forsequential excitation. It is easy to set and handle and in-cludes parts that are currently available in most laboratoriesworking in the field of spectroscopy.
EXPERIMENTAL SETUP
The experimental setup is sketched in Fig. 1. The lasersystem is the Datachrom 5000 from Quantel, consisting of aNd:YAG laser and a dye laser. The Nd:YAG laser delivers<800-mJ, 11-nsec, 20-Hz light pulses at 1064 nm. The dyelaser is pumped by the doubled (or tripled) Nd:YAG laserbeam and gives <80 mJ of energy at 590 nm in a bandwidthof 0.08 cm-'. Associated with commercially available acces-sories, the dye-laser spectral range extends from the VUV tothe IR, which is important since spectroscopy between excit-ed states involves a wide range of energy.
After doubling (or tripling), the residual part of the funda-mental beam at 1064 nm (400 mJ) is extracted and focusedinto a rare-gas cell, giving a laser plasma, the capability ofwhich as a VUV source was recently described. 13 In thepresent setup the plasma itself acts as an entrance slit of a40-cm Seya-type monochromator equipped with a 3600-groove/mm Jobin-Yvon holographic grating. The plasmadimensions are roughly those of a cylinder with a typicalwidth of 0.7 mm and a height of 7 mm. The dispersed VUVbeam enters the experimental cell (G) at its center through afixed 1-mm-width slit and a MgF2 window so that a typically0.7-nm spectral width of VUV light at a wavelength above115 nm is selected. This cell is simply composed of a com-mercial stainless-steel six-way cross connected to other partsusing metallic O-ring type gaskets (Helicoflex from Cefilac-France), so that the system is ultrahigh vacuum and pressure
Fig. 1. Experimental setup: Y Quantel Nd:YAG laser; D, dye laser; Ml-M 4 , mirrors; L1, focusing lens; L2 , optical element for laser peak-
power adjustment in the cell; S, plasma source cell; M, monochromator; G, excitation gas cell; MCP, microchannel plates; HV, high voltage; TD,
fast transient digitizer; V, charge-collection battery (18 V); E, electrometer; C, microcomputer; P, plotter; OG, Ne hollow-cathode lamp for
optogalvanic detection; A, amplifier; At, optical delay line.
compatible. The six-way-cross-shaped cell allows one towork with either counterpropagating or perpendicularlycrossed VUV/dye-laser beams. The best excitation condi-tions depend on both the focusing and penetration depth ofVUV light into the gas. The cell is connected to a gas-
handling system that includes a turbomolecular pump andvarious pressure gauges. The fluorescence signal is detectedwith microchannel plates (MCP's) equipped with an addi-tional cesium iodide photocathode' 4 for solar-blind VUV
detection. A BaF2 filter prevents detection of all parasiticlight possibly scattered from the excitation beam. TheMCP's are coupled with a fast transient digitizer (Tektronix7912 AD) with a rise time of 0.7 nsec. Detection in other
spectral ranges is possible with systems blind for both exci-
tation beams, which would require the use of a large-aper-
ture secondary monochromator. The cell is also equippedwith two electrodes: one is grounded; the other is coupled to
an electrometer through an 18-V battery. The noise of thissystem is in the 10-14 -A range. The second electrode is thin
and positively polarized in order to reduce the photoelectriceffect arising directly from VUV light-metal interaction. Inpractice this effect is found to be almost negligible andconstant, also in the 10-14-A range, in the recorded photo-ionization spectra.
The signals detected by either the electrometer or the fasttransient digitizer are sent to the personal-computer-com-patible microcomputer. No simultaneous computing is per-formed since the digitizer is dedicated primarily to averagingat a fixed excitation wavelength, in contrast with the elec-trometer, which is devoted to photoionization spectra result-ing from scanning the dye laser. During scanning, part of
the dye-laser beam is sent to an optogalvanic detector cou-pled to the computer through an amplifier, providing accu-rate wavelength calibration. For the VUV light that pro-vides the first excitation step, the wavelength calibration iseasily controlled from the slight minimum in fluorescenceintensity observed when exciting at the line center of theabsorption band where strong absorption occurs (well-known surface quenching).
Finally an optical delay line may introduce a delay At
between the VUV and the dye-laser beams entering the cell,when excited-state relaxation is aimed at either reachingkinetic information or populating metastable intermediatestates, for example. In practice, we use as much as 100-nsecdelay (optical line of 30 m). This method of time delay hasthe obvious advantage of eliminating jitter between the twopulses of light. The lens L2 before the entrance of thedelayed beam may be a convergent or divergent sphericalone or may be convergent cylindrical one, depending onexperimental conditions: VUV depth of penetration, beamsize, front or side dye-laser excitation. The only care is toavoid tight focusing onto optical surfaces, particularly thegrating. It was found that, in most cases, REMPI allows oneto work at a moderate peak power (_1 MW/cm 2).
ILLUSTRATION OF THE SETUP CAPABILITY
We will illustrate the capability of the system by resultsobtained with krypton.
The first resonance lines of rare gases are situated at awavelength below 150 nm, and the ionization limits are inthe XUV region at X < 100 nm. The kinetics and spectros-copy of rare gases have attracted considerable interest in thepast 15 years because of their value as a laser medium.' 5
The Kr2* excimer potential curves (according to Refs. 16and 17) are sketched in Fig. 2. One-photon excitation takesplace in the neighborhood of atomic resonances because ofcollisional broadening or weakly bound excimer absorption.Relaxed excimer formation takes place only through colli-sions, since direct formation from the ground state is notpossible because of the ground state's repulsive potential.Depending on excitation wavelength, pressure, and delay,various atomic and/or molecular states are transiently popu-lated and may therefore be probed by laser excitation, pro-viding both spectroscopic and kinetic information.
Figure 3 reproduces the photoionization spectrum whenthe VUV excitation is to the 'P, (5s'[1/2fl) krypton level andwhen the dye-laser excitation is near 558 nm and delayed 100nsec. The photoionization has to be understood as a reso-
nant two-photon process as schematically represented in
Audouard et al.
1286 J. Opt. Soc. Am. B/Vol. 6, No. 7/July 1989 Audouard et al.
Kr~
II og 4~ ~~P1Internuclear distance [ A]
Fig. 2. Atomic energy levels, molecular potential-energy curvesand transitions involved in present experiments. The VUV ex-cimer emission band is centered at 145 nm.
Fig. 2. In Fig. 3, curve A, the krypton pressure is 19 kPa,and, because of the long kinetic times involved at such apressure,' 8 the photoionization spectrum originates mostlyfrom the 'PI level. At increasing pressure (Fig. 3, curves Band C) collisional decay times are shortened, and two mainfeatures develop: (i) the red wing associated with the 5s'[1/2]l (1P1) - 6 P[1/2]o line (a), (ii) the 5s[3/2]; (3P2) - 5p'[1/2],contribution ().
The first decay step after 'P, excitation is the 0 ('P,)excimer formation'8 and relaxation. Actually the 0 state isknown to be attractive, 6"19 and the extended red wing of the5s'[1/2]l ('PI) - 6 p[1/2]o line demonstrates that the uppermolecular state involved in that transition is still more at-tractive than the 0+ state, at least at a large distance. It hasto be noted that the blue edge of the line remains steep,excluding transitions to another less attractive, or repulsive,state. As for the 5s[3/21; (3P2) - 5p'[1/2], line, its rise withpressure unambiguously reveals that passage through the3P2 atomic level is an effective step that has to be taken intoaccount in the decay sequence.
In Fig. 4 we illustrate the effect of a delay At, which leadsto a dramatic simplification of the spectrum. The simpli-fied spectrum (curve B) corresponds to transitions arisingfrom the 5s[3/2]; (3P2), which is, as in the case of Fig. 3,transiently populated. Actually the final decay step is therelaxed lu (3P2) excimer state,18 but its REMPI spectrumcorresponds to the spectral range of other laser dyes, as
studied recently by Killeen and Eden.2 0 The rich spectrumobtained at At 0 (curve C) exhibits lines for which therelaxed 0+ molecular state plays a major role in addition tothe lines originating from 3P2. The radiative decay of the 0+state is very fast (o = 3.4 nsec)' 8 so that stringent conditionsare necessary for information about this level to be obtained,i.e.:
* Experimental conditions (pressure, delay) must be fa-vorable to allow a 0+ vibrational relaxation time < To, so thata population reservoir can be built.* 5s[3/2]° (3P,) red-wing excitation must be used becauseit shortens the relaxation time' 8 and so favors the relaxed 0+state formation. Both selective excitation and sufficientpressure are necessary to have absorption and relaxationwithout dramatic radiative losses in the Kr2* first continu-um.
The relaxed 0 state contributes to the features in spec-trum C of Fig. 4 through two possibly different schemes (Fig.2):
(i) a bound-bound process according to
Kr2* (0+) - Kr2** (bound) Kr2+ + e, (1)
560 558 X (nm)Fig. 3. Pressure effect on the REMPI spectrum. X is the dye-laserreexcitation wavelength, and the VUV first excitation is near thesecond resonance line ('So - 1P at 116.5 nm), At = 100 nsec. A, PKr
= 19 kPa; B, PKr = 60 kPa; C, PKr = 80 kPa. a, 5s'[1/2]1 ('PI) -6 p[1/2]o. 3, 5s[3/2]; (3P2) -5 p'[1/ 2 ],. a, 5s[3/2] (3P 2) -5 p'[3/212-
.A_0
0)
C0
0N_C
0-.
I
I
I
Vol. 6, No. 7/July 1989/J. Opt. Soc. Am. B 1287
570 565 560 l (n m)
Fig. 4. Optical delay (At) effect on the REMPI spectrum: firstVUV excitation at 124.6 nm, reexcitation wavelength at X. A, Neoptogalvanic spectrum for wavelength calibration. B, PKr = 20 kPa.At = 100 nsec. Only atomic transitions (s) starting from the 5st3/2];(3P2) state are observed. C, PKr 30 kPa, At = O nsec. New featurescorrespond to transitions implying 0+ relaxed molecular-level tran-sient formation (see text). The lines labeled p correspond to transi-tions originating from 5p states to higher atomic Rydberg states.
t (50 nsec/div.)
Fig. 5. Kr2 * excimer emission versus time. PKr = 150 kPa. CurveA, VUV excitation only. Curve B, VUV excitation and laser excita-tion at X = 355 nm (At = 45 nsec) illustrating the LRF phenomenon.
(ii) a bound-free excimer transition to a dissociativestate according to
Kr2* (U) - Kr2** (dissociative) - Kr + Kr*
and followed by
Kr* - Kr** - Kr+ + e-.
(2a)
Actually the clear observation of atomic lines originating
from the 5p manifold (Fig. 4) demonstrates the existence ofat least the indirect excitation scheme (ii). Indeed, no direct5p excitation can be imagined because of selection rules andof MgF2 optical cutoff at 87 X 103 cm-1 . It is important tonote that scheme (ii) is compatible with the kinetic results' 8
and with energetic considerations.Nevertheless, owing to the pressure-dependent line over-
lap and to the entirely new character of the spectrum imply-ing a relaxed 0' state, a final assignment must be carried outwith care. In particular, only a more extensive set of experi-mental data and a comparison with ab initio calculationswill permit us to separate the features resulting from (i) or(ii) processes and then to establish clearly the atomic ormolecular character of the observed bands. Such a detailedstudy is still under way.
Finally, the system described allows the observation oflaser reduced fluorescence (LRF), which was extended re-cently to the VUV.6,1o The VUV relaxed excimer fluores-cence at 145 nm (the BaF2 filter prevents first continuumdetection at 125 nm) can be abruptly reduced by depopulat-ing the 1, relaxed level if enough laser power is directed intothe VUV-excited volume at a resonance wavelength. TheLRF effect is illustrated in Fig. 5, in which one-photon ion-ization occurs. The method could be extended to obtain theabsorption spectrum of excited molecules by scanning thedye laser. Furthermore, useful information about the kinet-ic behavior can be obtained when higher-energy transientspecies are excited. In particular, as shown in Fig. 5, theoccurrence of depletion is synchronous with the laser pulserise (a few nanoseconds), which proves that the dye laserdirectly excites the radiating 1, relaxed level, in agreementwith the kinetic study.' 8
CONCLUSION
In the present paper we describe a rather simple arrange-ment well adapted to VUVODR experiments. The use ofthe same Nd:YAG laser for both VUV generation and dye-laser pumping avoids any time jitter between the two beams.The tunability over a wide VUV spectral range is obtainedwith excellent pulse-to-pulse reproducibility. In the rare-gas experiments presented here a rather wide VUV spectralwidth (0.7 nm) is used. Higher resolution could be obtainedeasily in the case of LRF detection, to which single-photontechniques can be applied. Photoionization detection islimited by electrometer performance. The VUV-source in-tensity may also be improved.'3 The ability to obtain spec-troscopic and/or kinetic information by this method is dem-onstrated. Further extensions of the technique are readilyimagined, such as the use of a solid target laser plasma forXUV generation. Experiments on molecular beams withdirect ion detection and mass analysis are envisaged. Be-cause of the high sensitivity of mass spectroscopy, theseexperiments would be compatible with high resolution in theVUV/XUV spectral range for the first excitation.
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