Journal of Electron Spectroscopy and Related Phenomena, 51 (1990) 383-396 EleevierSciencePublishersB.V.,Amsterdam-PrintedinTheNetberlands

383

APPLICATION OF MPI PHOTOELECTRON SPECTROSCOPY TO EXCITED MOLECULES.

USE OF TUNABLE VUV LASER RADIATION

K. Kimura, M. Takahashi, K. Okuyama and I. Plazibat'

Institute for Molecular Science, Okazaki, 444 Japan

('Present address: Department of Natural and Mathematical Science, University of Split, Nikole Tesle 12, 58000 Split, Yugoslavia)

SUMMAEY

In this paper we demonstrate the applicability of VW pulsed laser radiation to photoelectron spectroscopy of electronically excited states as well as ground states. The VW lasers used here are based on: 1) four-wave sum mixing in Mg vapor giving continuous tunability from 140 to 160 nm; 2) third harmonic generation in Xe and Mg vapor (118.2 and 143.6 nm, respectively); and 3) an ArF excimer laser (193 nm). Two-color MPI experiments with these VW lasers were successfully carried out for a few test samples, such as NO, aniline, and pyrazine, clearly indicating that these VW pulsed lasers are promising future light sources.

1. INTRODUCTION

1.1 Photoelectron Spectroscopy

Since the pioneering work of Siegbahn et al.[l] and Turner et

a1.[21 in the 1960's, photoelectron spectroscopy has been

developed as one of main spectroscopic techniques of chemical

analysis in molecular and material sciences. During the last two

decades, much information about electronic structure of atoms,

molecules and materials in the ground state as well as about ionic

states has been obtained using VUV and X-ray photoelectron

spectroscopy.

Ionization potential data obtained from ordinary ground-state

photoelectron spectra have been very useful in interpreting

excited-state photoelectron spectra in the most laser multiphoton

photoelectron experiments. For example, a handbook published by

Kimura 131, which contains He1 photoelectron spectra and

ionization potentials of many fundamental organic molecules and

their ab initio assignments, has provided basic information on

low-lying ionic states necessary for interpretation MPI

photoelectron spectra.

036%2048/90/%03.50 0 199OElsevierSciencePublishersB.V.

384

1.2 MPI Photoelectron Spectroscopy

In atoms and molecules, multiphoton ionization (MPI) is

remarkably enhanced at special laser wavelengths where optical

resonances take place. Such resonance photoionization provides

high-resolution molecular MPI (ion-current) spectroscopy, as

developed by Johnson [4]. Since 1980, photoelectron spectroscopy

of resonant MPI has been developed with nanosecond W/visible

lasers to detect photoelectron spectra of excited-state molecules

in this laboratory [5,6] and others [7,8]. Studies so far

published on excited-state photoelectron spectroscopy have been

reviewed by Kimura [9,10], by Compton and Miller [ll], and by

Pratt et al. 1121. Furthermore, laser photoelectron spectroscopy

has been developed also for single-photon ionization of negative

ions [13].

1.3 Advantages of VW Lasers

Most photoelectron studies of resonant MPI have so far been

carried out with visible/UV pulse lasers [g-12]. In measurements

of excited-state photoelectron spectra, it is important to ionize

any excited states by single photons. If two or more photons are

absorbed by a specific excited state to be studied, further

resonance may occur with higher excited states, making spectral

interpretation difficult.

Therefore, single-photon excitation and ionization are

essential in the following cases, as shown in Fig. 1: Namely, (a)

single-photon ionization of a resonant low-lying excited state;

u+

n

. . . . .:. . . . ‘. . . :: . .

. .

ii

1 %I

(a) (b)

.::.. . . . . . . a.

. .

. .

. . . .

,. . . k . . . .

L % (4

Fig. 1. Schematic energy level diagram relevant to one- and two-photon ionization processes using VW laser radiation in photoelectron spectroscopy: (a) ionization from a low-lying excited state, (b) excitation to a highly excited state followed by ionization, (c) ionization to a higher ionic state, and (d) one-photon ionization from the ground state.

335

(b) single-photon excitation to a resonant highly excited state,

followed by ionization; (c) single-photon ionization producing a

high ionic state from a resonant excited state; and (d) single-

photon ionization of a ground state molecule. Use of VUV laser

radiation therefore has a unique advantage in laser MPI

photoelectron spectroscopy.

Photoelectron spectra of resonant ionization give direct

information about ionization transitions between resonant excited

states and the final ionic states. In other words, excited states

and ionic states are correlated through ionization transitions.

Such excited-state photoelectron spectroscopy can be applied to

various non-radiative excited states in addition to radiative

excited states. Furthermore, photoionization from an excited

state is always in competition with deactivation processes such as

relaxation and dissociation, so that new information about dynamic

behavior of excited states may also be deduced from time-dependent

photoelectron measurements.

1.4 Tunable VUV Lasers

Since pioneering experiments of Harris and Miles 1141 and

Hodgson et al. [151, four-wave sum mixing in metal vapors has been

used to generate tunable coherent VUV laser radiation over broad

regions. The mechanism of four-wave sum mixing is schematically

shown in Fig. 2 in the case of Mg vapor. Metal vapors such as Mg,

Cd, Hg, etc. are efficient nonlinear media to produce coherent VUV

Fig. 2. Energy-level diagram of Mg atom, showing the level of two-photon resonance at which the Wt of four-wave sum mixing takes place. The energy region of ionization continuun and broad autoionizing level associated with four-wavemixingisindicatedbythedottedarea.

386

radiation by four-wave sum mixing (%UV = 201 + @2)1 with high

conversion efficiency 114-161. In Fig. 2, the two-photon

resonance level (3s3d 'D) of Mg atom is indicated as an example.

A technique for generating four-wave sum mixing of metal vapors

has been well developed [17], and many applications to molecular

spectroscopy have been published [18-201.

So far, in MPI photoelectron studies of excited states,

UV/visible lasers have been mostly used, and essentially no VUV

lasers have been used, except for a few cases mentioned in this

paper. Hence, the use of tunable coherent VUV radiation will

further develop MPI photoelectron spectroscopy of excited states.

In the present paper, we want to demonstrate several new

applications of VUV laser radiation to MPI photoelectron

spectroscopy, as well as to illustrate that tunable VUV pulse

lasers should be very important for future studies of molecular

photoelectrongpectroscopy.

2. RXPERIMRNTAL

2.1 Tunable VUV Laser by Four-Wave Sum Mixing in Mg Vapor

A heat pipe oven for Mg vapor was designed and constructed to

generate tunable VUV radiation in the present work. Figure 3

shows a schematic drawing of our heat pipe, which consists of a

horizontal tube containing Mg vapor and a vertical tube containing

Na vapor. The concept and the design of a heat pipe and its use

High-r water

Fig. 3. A heat-pipe oven for Mg vapor to prod- tunable VW laser radiation, consisting of two pipes; the horizontal one containing Mg vapor and the vertical one containing Na vapor. A metal mesh is used as a wick for metal transport.

387

Plate

Plate

Fig. 4. Schematic diagram of the overall laser system producing tunable VW radiation. A dye laser pumped by a XeCl excimer laser is used for @I, and the Nd-YAG pumped dye laser is used for I, both lasers being synchronized.

in spectroscopy has been discussed by Vidal [21,22].

Helium was mixed with Mg vapor in the horizontal tube (He:

200 Torr; Mg: 20 Torr) to attain phase-matching conditions in Mg-

He. In this heat pipe, Na was initially heated at about 800°C,

and then Mg was heated by heat transfer from Na vapor.

Our laser system for generating tunable VUV radiation by

four-wave mixing in metal vapor is schematically shown in Fig. 4.

A Nd-YAG laser pumped dye laser system (Quanta-Ray DCR-lA, PDL-1,

WEX; 10Hz) was used for m2, while a XeCl excimer laser (Lambda

Physik EMG50; 10 Hz) pumped dye laser (Lambda Physik FL2002) was

employed for 'P'I (430.82 nm; Coumarine 480). The two lasers "I

and u2 were synchronized.

Using the above laser conditions, we obtained tunable VUV

radiation with continuous tunability in the region 120-174 nm (10

ns, 10 Hz). The resulting photon flux was evaluated to be 8~10~~

photons/pulse from the intensity measurements with a solar blind

photomultiplier, assuming that the reflectivities of the MgF2

plate and the Cu surface used are 3 % and 100 %, respectively.

From the photon flux, the conversion efficiency was estimated to

be larger than 0.2 %. The resolution of the tunable VUV laser

radiation is 0.1-1.0 cm -1 , depending on incident lasers.

2.2 Third Harmonic Generation in Xenon

Xenon gas is also known as a nonlinear medium for generating

VUV laser radiation by third harmonic generation (THG), 'P'VUV = 3@I

1231. In the present work, we used a gas cell of Xe (100 mTorr;

20 cm long) to generate VUV laser radiation of LVUV = 118.2 nm

388

eVUV (118.2 m)

Pulse Nozzle

Xe Gas Cell f? I‘ : : : :

II 01 (354.7 In) Fig. 5. Schematic drawing of a Xe gas cell (100 mTorr), which produces the 118.2~nm THG radiation, and a molecular beam crossing with the WV laser radiation. Quartz and LiF lenses (f = 100 urn) are used for the windows, and the Xe gas is irradiated with the third harmonic of the Nd-YAG fundamental.

(10.49eV) by tripling the third harmonic (l.1 = 354.7 nm) of the

Nd-YAG fundamental (1064 nm); that is, the 9th harmonic generation

of the 1064 nm fundamental. This method is similar to that

reported by McCann et al. [24].

A schematic drawing of our Xe cell and VUV laser crossing a

molecular beam is shown in Fig 5. The visible laser beam was

introduced through a quartz lens (f = 100 mm) which is the input

window of the Xe cell, while the VUV laser radiation thus

generated was taken out through a LiF lens (f = 100 mm) which is

the output window.

2.3 Photoelectron Apparatus and Measurements

Our molecular-beam photoelectron apparatus has been described

in detail elsewhere [5,6,91. A gas sample was introduced through

a pulsed nozzle into the ionization region. Ion-current

measurements were first carried out for a given gas sample as a

function of laser wavelength, showing a series of peaks which

correspond to electronically excited states (vibrationally and

rotationally resolved). Then, at individual ion-current peaks,

photoelectron kinetic energy measurements were carried out with a

time-of-flight (TOF) electron analyzer (28 cm in length).

Photoelectron spectra were recorded by accumulating photoelectron

signals for at least a few thousand laser shots (10 Hz), using a

transient recorder (Biomation 6500), and then transferred into a

microcomputer data acquisition system.

389

3. APPLICATIONS

3.1 Single-Photon Ionization of Aniline by THG in Mg Vapor

The ionization potential of aniline, Iv = 8.00 eV [25], is

lower than the single-photon energy of the THG of Mg (kvUV q 143.6

nm; 8.63 eV) which is generated at 11 = 431 nm. Figure 6 shows a

photoelectron spectrum of aniline which was obtained by the

nanosecond VUV radiation of the THG of Mg, indicating several

vibrational peaks (shown by arrows) on the first ionization band.

The vibrational structure of the first band in Fig. 6 is

essentially the same as that of an available He1 photoelectron

spectrum of aniline [25]. In Fig. 6, a sharp peak due to

scattered VUV light is also shown on the left side of the band.

The important point to be noted here is that photoelectron

spectra of ground-state molecules can be obtained by such VUV

pulse laser radiation. Therefore, this technique is applicable to

short-lived transient species or photofragment species, by

combining with a photolysis laser. There are many He1

photoelectron studies for simple free radicals, as earlier

reviewed by Dyke et al. [261. However, additional information

about transient species will be obtained from the laser

photoelectron experiments of a flash-photolysis type with VUV

laser radiation.

scattered VW Light

I

II, I

1.0 0.6 0.4 0.2

Photoelectron Energy' (eV)

..-.

@VW (143.6 nm)

Aniline

Fig. 6. The photoelectron spsctrun of aniline , obtained by the 143.6-nm pulsed laser (THG of Mg vapor), showing vibrational structure. A peak due to scattered VW light also appears.

3.2 Ionization of Rydberg F2A State of NO by Tunable VUV

In the present work, we selected the Rydberg F2A state (v'=l)

of NO as the first example of our application of four-wave mixing

in the wavelength region lVUV = 155.2-155.6 nm, using kl = 430.82

nm and L2 = 555.4 - 560.7 nm. As a result, we were able to detect

a two-color ion-current spectrum which shows several rotational

peaks of the v '=l vibrational level of the Rydberg F2A state [271,

as shown in Fig. 7(a).

In order to confirm the nature of resonance of the Rydberg

F2A state (v'=l), we measured a photoelectron spectrum at the main

ion-current peak. The resulting photoelectron spectrum is shown

in Fig. 7(b), indicating a sharp ionization peak at an energy of

0.67 eV, which can be assigned to the v+=l level of the NO+ ion.

This clearly indicates that Av=O ionization takes place from the

v'=l level of the Rydberg F2A state of NO to the v+=l level of the

NO+ XlE+ ion. (Here, the Rydberg F2A state was ,.ionized by 02

rather than @I, because u2 was much stronger.)

The rotational structure in Fig. 7(a) was analyzed in terms

of the rotational constant, the assignments of the rotational

peaks being also shown in Fig. 7(a). The resulting rotational

constant is essentially the same as that determined from an

available high-resolution absorption spectrum 1281. It should be

mentioned here that tunable four-wave mixing in Mg vapor has been

used for the first time in the present work, and it is a very

promising VUV laser source for photoelectron spectroscopy.

(a)

(b)

v+=1

Pll Rll

155.5 155.4 155.3 155.2

Wavelength (a) 5 2 1 0.5 0.1

Fhotoelectron&ergy (eV)

Fig. 7. (a) A two-color ion-current spectrum of NO in the Rydberg F2A state (v'=l), obtained by tunable four-wave mixing radiation in Mg vapor. (b) A photoelectron spectnsn observed at the main ion-current peak, indicating a single peak due to Av=O ionization transition.

3.3 Ionization of the Rydberg N2A State of NO by Tunable VUV

Strong electronic interaction between the Rydberg N2A v'=O

and valence Bt2A (v'- -7) states of NO has been studied in detail by

Dressler and Miescher [291 and by Jungen [301 with high-resolution

absorption and emission spectroscopy. In the present experiments,

391

the NO molecule was excited to the Rydberg N'A (v'=O) state by VUV

radiation from four-wave mixing in Mg (%Uv = 2q + @3), and the

excited state was then ionized by c+. The resulting two-color

ion-current spectrum of the Rydberg N2A v'=O state of NO is shown

in Fig. 8, observed in the wavelength region 147.88 - 147.97 nm.

This spectrum indicates several rotational peaks which can be

explained in terms of rotational progressions Ql and Rl.

RI I

j-112

A=--

147.95 147.90

Wavelength (nm)

No

Fig. 8. A two-color ion-current spectm of the Rydberg N2A v'=O state of obtainedusingtunableVWrad.iation fromMgvapor. Theupperourve shows intensity of the WV radiation.

No, the

3.4 Ionization of the Valence B"A State of NO by Tunable VUV

A two-color ion-current spectrum associated with the v'=7

level of the valence B12A state was obtained here in the

wavelength region 147.67-147.75 nm with tunable VUV from Mg vapor,

as is shown in Fig. 9, indicating several rotational peaks. As

mentioned before, it is known that the valence B"A state at the

v'=7 level interacts to a considerable extent with the Rydberg N2A

state at the v'=O level 1303. (Here again, the excited state B"A

was ionized by p2). These rotational peaks have been interpreted

in terms of two rotational progressions Q21 and RZlr as indicated

on the peaks in Fig. 9.

The results deduced from rotational analysis of the spectra

in Fig. 9 are consistent with those of Jungen [3Ol. Time-resolved

fluorescence measurements at several vibrational levels (v'nl-8)

of the valence B"A state of NO have been carried out by Banic et

392

Fig. 9. A two-color ion-current spectm of No, obtained by WV excitation to the valence H'2A v'=7 state by light fran four-wave mixing in Mg vapor. The upper curve she? the intensity of the VW radiation.

al. [28], using four-wave mixing in Mg vapor. Such a tunable VUV

laser should be an important light source for MPI spectroscopy as

well as for absorption and fluorescence spectroscopy. One of the

advantages of tunable VUV laser radiation is that highly excited

states can be produced by single photons from the ground state

(see Fig. lb), thus simplifying the excitation process compared

with multiphoton processes.

3.5 Single-Photon Ionization of NO by THG in Xe

Third harmonic generation in Xe gas produces coherent VUV

radiation at 118.2 nm (10.48 eV). Its single photon energy is

greater than the adiabatic ionization potential of NO molecule, I,

= 9.26 eV [313. Therefore, in the present work we decided to

measure a photoelectron spectrum of NO by 118.2-nm VUV radiation,

with the experimental arrangements mentioned before (Fig. 5).

Figure 10 shows a photoelectron spectrum of NO thus obtained,

indicating four vibrational peaks (v+=O-3).

Although the photoelectron vibrational structure in Fig. 10

is essentially the same as that of available He1 photoelectron

spectra [2,31], the important point to be noted is that such a

simple Xe VUV laser source makes it possible to measure

photoelectron spectra of ground-state molecules. This suggests

that even for short-lived transient species it may be possible to

detect photoelectron spectra with the 118.2-nm radiation, It

should also be mentioned that observation of such photoelectron

393

v+=2

v+=1 I <. . . .

NO+ ::::

v+=3

I

NO - 5 2 1 0.5 0.2

Photoelectron energy (eV)

:. ‘.‘. ._. . . . . . c

@WV

(118.2 run)

Fig. 10. A photoelectron spectm of NC in the ground state, obtained by 118.2~nm laser radiation, showing well-resolved vibrational peaks of the first ionization bend.

vibrational structure of NO provides useful energy calibraticn.

3.6 Photoelectron

Photoelectron measurements of organic molecules in their

excited triplet states are of considerable interest, since no

photoelectron studies have been reported. Laser MPI may be a new

technique to explore triplet manifolds of organic molecules.

Recently, the triplet pyrazine molecule has been studied by means

of a one-color (lt2) MPI technique by Villa et al. [321 and by

Turner et al. [333. However, in one-color MPI experiments, there

would be another possibility of resonance at a higher excited

state, making spectral interpretation confusing.

In the present work, we were able to obtain a photoelectron

spectrum originating from the triplet Tl 0' state for the first

time from two-color experiments [34], as is shown in Fig. 11.

Combining a XeCl excimer laser pumped dye laser (al) and a 193-nm

ArF excimer laser (02), we directly excited jet-cooled pyrazine

molecule to Tl 0' by ml, and then ionized by @2. The spectrum in

Fig. 11 has been interpreted in terms of the vibrational

frequencies of the ion produced by photoionization from Tl:

3B3u(nz*). Vibrational assignments shown in Fig. 11 have been

made on the basis of the available information on the ion [351.

394

5 2 1 0.5 0.2 0.1

Pfi0tOde0tron Eherg~ (ev)

Tl oo -r

+ u1 (372.86 nm)

I

Pyrazine

Fig. 11. A photoelectron spectrum of pyrazine in the lowest triplet state (Tl OO). Pyrazine molecules were directly excited to T1 from the ground state by the first laser radiation (372.86 nm), then ionized by the second, ,193-nm ArF excimer laser radiation.

4, Conclusion

As demonstrated in the present work, THG and four-wave sum

mixing in rare gases and metal vapors are promising coherent VUV

sources for future application of excited-state photoelectron

spectroscopy. Use of the VUV laser radiation in photoelectron

spectroscopy is necessary, especially for studying highly excited

states, low-lying excited states, higher ionic states (including

core excitation), and even for ground-state transient species.

Several applications of MPI photoelectron spectroscopy to

photophysical and photochemical processes have been developed in

this laboratory [lo]; 1) selective photoionization, 2) one-photon

forbidden, two-photon allowed excited states, 3) autoionization,

4) intramolecular electronic and vibrational relaxation, 5)

electronic states of photofragments, and 6) excited triplet

states. In order to further extend this technique to these

subjects, we need various VUV laser sources including those

mentioned in this paper.

The Xe gas cell to generate VUV radiation by tripling the

output of a YAG-pumped dye laser is easy to handle, and should be

useful for photoelectron spectroscopy, although its tunability is

only within a bandwidth ofwl cm-' [36]. A high pressure H2 Raman

shifter to generate far-VW radiation is also useful for MPI

spectroscopy. For example, in our previous MPI studies [37,38],

we have used 192.2-nm radiation (the 8th anti-Stokes line of the

395

2nd harmonic of Nd-YAG) and 199.8-nm radiation (the 3rd anti-

Stokes line of the 4th harmonic of Nd-YAG laser).

Threshold photoelectron measurements in MPI experiments are

especially important for studying ionic states in very high

resolution spectroscopy [391. Use of a tunable VUV laser such as

four-wave mixing of Mg vapor in threshold photoelectron

spectroscopy will make it possible to study higher ionic states at

high resolution.

ACKNOWLEDGEMENTS

We would like to thank Professor K. Shobatake for letting us

use the XeCl excimer laser for the generation of four-wave mixing

in Mg vapor. We would also like to thank Professor J. H. D. Eland

and Professor E. I. von Nagy-Felsobuki, IMS Visiting Professors,

for reading the manuscript and their kind discussion and advice.

REFERENCES

1.

2r

3.

4. 5.

6.

7.

8. 9. 10. 11.

12.

13.

14.

K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hsmrin, U. Gelius, T. Bergmark, L. 0. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North-Holland, Amsterdam, London, 1969. D. W. Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Interscience, London, 1970. K. Kimura, S. Katslnnata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He1 Photoelectron Spectra of Fundamental Organic Molecules, Halsted Press, New York, 1981. P. M. Johnson, Act. Cbem. Rev., 13 (1980) 20. (a) Y. Achiba, K. Shobatake and K. Kimura, Construction of an Apparatus for Studying MPI Electron and Mass Spectroscopy, in Annual Review, IMS, Ckazaki, 1980, p.100; (b) Y. Achiba, K. Sato, Y. Shindo and K. Kimura, Construction of a TGF Electron Analyzer for Laser Photoelectron Spectroscopy, in Annual Review, IMS, Ckazski, 1980, p.105; (c) Y. Achiba, K. Sato, K. Shobatake and K. Kimura, J. Photochem., 17 (1981) 53. (a) Y. Achiba, K. Sato, K. Shobatake and K. Kimura, J. Cbem. Phys., 77 (1982) 2709. R. N. Compton, J. C. Miller, A. E. Carter and P. Kruit, Cbem. Phys. L&t., 71 (1980) 87. J. T. Meek, R. K. Jones and J. P. Reilly, J. Chem. Phys., 73 (1980) 3503. K. Kimura, Advsn. Chem. Phys., 60 (1985) 161. K. Kimura, Intern. Rev. Phys. &em., 6 (1987) 195. R. N. Compton and John C. Miller, Multiphoton Ionic&ion Photoelectron Spectroscopy: MPI-PES, in Laser Applications in Physcial Chemistry, ed. by D. K. Evans, Marcel Dekker, New York, 1988. S. T. Pratt, P. M. Dehmer and J. L. Dehmer, Resonantly Enhanced Multiphoton Ionization - Photoelectron Spectroscopy as a Probe of Molecular Photophysics and Photochemistry, in Advances in Multiphoton Process and Spectroscopy, Vol. 4, ed. S. H. Lin, World Scientific, Singapore, 1988, p.69. R. R. Corderman and W. C. Lineberger, Ann. Rev. F’hys. Chem., 30 (1979) 347. (a) S. E. Harris and R. B. Miles, Appl. Phys. Lett., 19 (1971) 385. (b) R. B. Miles snd S. E. Harris, IEEE J. Quantun Electron., GE-9 (1973) 470.

396

15.

16. 17.

18.

19.

20.

21.

22. 23. 24.

25. T. P. Debies and J. W. Rabalais, J. Electron Spectrosc., 1 (1972/1973) 355.

26. J. M. Dyke, N. Jonathan and A. Morris, Intern. Rev. Phys. Chem., 2 (1982)

27. 28.

29. 30. 31. 32. 33.

34.

35. 36.

37. 38.

39.

R. T. Hodgson, P. P. Sorokin and J. J. Wynne, Phys. Rev. L&t., 32 (1974) 343. S. C. Wallace snd G. Zdasiuk, Appl. Phys. L&t., 28 (1976) 449. W. Jamroz and B. P. Stoicheff, Generation of Tunable Coherent Vacuum Ultraviolet Radiation, in Progress in Optics, Ed. by E. Wolf, North Holland, Amsterdam, 1983, Vol. 20, p. 326. C. R. Vidal, Vacutm~ Ultraviolet Laser Spectroscopy of Small Molecules, in Advances in Atomic and Molecular Physics, 1987, Vol. 22. I. Burak, J. W. Hepburn, N. Sivakumar, G. E. Hall, G. Chawla, P. L. Houston, J. Chem. Phys., 86 (1987) 1258. P. R. Herman, P. E. LaRocque, R. H. Lipson, W. Jamroz and B. P. Stoicheff, Can. J. Phys., 63 (1985) 1581. C. R. Vidal, Four-Wave Frequency Mixing in Gases, in Tunable Lasers, edited by L. F. Mollenauer and J. C. White, Springer, Berlin, 1987, p. 57. C. R. Vidal and J. Cooper, J. Appl. Phys., 40 (1969) 3370. A. H. Kung, Opt. L&t., 8 (1983) 24. M. P. McCann, C. H. Chen and M. G. Payne, Chem. Phys. Lett., 138 (1987) 250.

n

ii: Takahas’ 1, K. Gkuyama and K. Kimura, to be published. J. R. Banic, R. H. Lipson and B. P. Stoicheff, Can J. Phys., 62 (1984) 1629. K. Dressler snd E. Miescher, Astrophys. J., 141 (1965) 1266. Cb. Jungen, Can. J. Phys., 44 (1966) 3197. C. R. Brundle, Chem. Phys. Lett., 5 (1970) 410. E. Villa, M. Terazima and E. C. Lim, Chem. Phys. L&t., 129 (1986) 336. R. E. Turner, V. Vaida, C. A. Molini, J. 0. Berg and D. H. Parker, Chem. Phys., 28 (1978) 47. K. Gkuysma, M. Takahashi, S. Nagaoka, K. Kimura, T. Ksmei, S. Yamauchi, M. Hirota, to be published. A. Goto, M. Fuji and M. Ito, J. Phys. Chem., 91 (1987) 2268. P. Chen, J. B. Parix, W. A. Chupka snd S. D. Colson, J. Chem. Phys., 86 (1987) 516. K. Sato, Y. Achiba and K. Kimura, Chem. Phys. L&t., 126 (1986) 306. H. Shinohara, K. Sato, Y. Achiba, N. Nishi and K. Kimura, Chem. Phys. L&t., 130 (1986) 231. K. Muller-Dethlefs, M. Sander and E. W. Schlag, Z. Naturforsch., 39a (1984) 1089.