LASER INDUCED PROCESSES AND GENERATION OF COHERENT VACUUM ULTRAVIOLET RADIATION IN MOLECULES

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LASER INDUCED PROCESSES AND GENERATION OF COHERENT VACUUM ULTRAVIOLET

RADIATION IN MOLECULES

F. Vallée, L. Hellner, J. Lukasik

To cite this version:

F. Vallée, L. Hellner, J. Lukasik. LASER INDUCED PROCESSES AND GENERATION OF CO-

HERENT VACUUM ULTRAVIOLET RADIATION IN MOLECULES. Journal de Physique Collo-

ques, 1985, 46 (C1), pp.C1-135-C1-145. �10.1051/jphyscol:1985113�. �jpa-00224484�

JOURNAL DE PHYSIQUE

Colloque CI, suppl6rnent au nOl, T o m e 46, janvier 1985 page CI-135

LASER INDUCED PROCESSES AND GENERATION OF COHERENT VACUUM ULTRAVIOLET RADIATION IN MOLECULES

F. Vallse, L. Hellner and J. Lukasik

Laboratoire d 'Optique Quantique du C . N.R.S., EcoZe PoZytechnique, 91 128 PaZaiseau, France

RQsumC - Les techniques d'excitation multiphotonique sont utilisdes pour mettre en Gvidence les processus induits par laser inpliquant les Gtats glectroniques d16nergie 6levGe (E > IleV) dans le nonoxyde de carbone. La mgthode emploie deux lasers B colorant d'une puissance crste dlevGe. Un des lasers est doubls en frdquence.

Les processus de mdlange de frgquences 2 quatre ondes exalt6s par des rgso- nances permettent de produire le rayonnement coherent, accordable et d'une faible largeur de raie dans l'ultraviolet lointain dans les rdgions de 1400 et 1150 A. Ces exp6riences dgterminent 6galement les sections efficaces d'absorption et les indices de rsfraction du CO dans l'ultraviolet lointain.

La photodissociation multiphotonique du CO a GtG observge et confirmge par l'absorption dans 1'UVL du carbone atomique. Les rGsultats pr6liminaires concernant la gdn6ration du rayonnement cohdrent dans l'ultraviolet extrzme autour de 940-950 A sont aussi prGsentGs.

Abstract - f4ultiphoton excitation techniques are used to demonstrate laser induced processes involving high energy (E > 1 1 eV) electronic states in carbon monoxide. The method utilizes two high peak power, tunable dye lasers, one of which is frequency doubled.

Resonantly enhanced four-wave sum frequency mixing processes lead to the ge- neration of coherent, continuously tunable and narrow - bandwidth vacuum ultraviolet (VUV) radiation in 1400 and 1150 ranges. These experiments allow to determine the VUV absorption cross sections and indices of refrac- tion of CO. Multiphoton photolysis of CO has been observed and confirmed by the VUV absorption in atomic carbon. Preliminary results on coherent ex- treme ultraviolet generation in the 940 - ⁹⁵⁰ A range in CO are also repor- ted.

I - INTRODUCTION

The field of laser induced effects in molecular systems has been,for years, a center of great research activity. For various reasons, a particular interest has been recently focused on the vacuum ultraviolet (VW) region. For example, molecules seem to be attractive candidates for VUV radiation sources. They exist in a great varie- ty of species, they have a rich spectrum of high energy states, they are easily available in large densities and their experimental handling is, in principle, quite simple. Furthermore, laser field processes in the VUV play an important part in understanding models of molecular dynanics and provide a useful tool for studying such key research areas of molecular physics and chemistry as photofragmentation or photoionization.

This paper summarizes certain results obtained in our laboratory on experimental demonstration of processes involving high energy valence or Rydberg states of carbon monoxide using multiphoton transition technique. These processes lead to the genera- tion of coherent and tunable VUV or XUV (extreme ultraviolet extending below 1000 A)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985113

JOURNAL DE PHYSIQUE

light.

A simplified energy level diagram of carbon monoxide illustrating schematically two- and three-photon resonant enhancement effects is s p w n in fig.1. Our experiments exploit the presence of electronic states A ~ I I , and a few valence states around

106 000 cm-I. The process involved in the generation of coherent and tunable VUV light is resonantly enhanced sum frequency mixing technique and regions around 1400, 1150 and 940 d have been studied.

Fig.1 - Various sum frequency mixing processes studied and pertinent energy levels of carbon monoxide.

I1 - EXPERIMENTAL

The experiments are carried out with an excitation system consisting of two indepen- dently tunable and synchronized in time 5320 d - pumped visible (rhodamine) dye la- sers and a VUV detection setup. The dye lasers operate in a usual oscillator - ampli- fier mode, use transversely pumped capillary cells, work at a repetition rate of 5 Hz and deliver 30-40 mJ of energy in about 10 ns. Tuning is accomplished in the oscillator by a system consisting of a fixed-grazing incidence grating and a movable

100 % reylecting mirror. A prism beam ex ander inserted in the dye oscillator provi- des a linewidth of the order of 0.08 cmbP. One of the lasers is frequency doubled in a KD*P crystal to create a tunable 8 ns UV source with 5 mJ of energy. These pulses can be spatially overlapped and focused into a gas cell mounted in front of a 1-m vacuum Seya-Namioka monochromator to a measured area of about cm2. The monochro- mator is followed by a solar blind photomultiplier with a CsI cathode and an ampli- fier. A boxcar and an X-Y recorder allow for signal processing and data analysis.

A modification of the setup is necessary in order to study the XUV region. The UV

dye laser light is focused by a lens (f = 100 mm) in front of the center of a 300um

diameter aperture which separates the conversion gas cell from a small vacuum cham-

ber. This vacuum chamber is mounted in front of the entrance slit of a 0.2 m Mc Pherson 234 VUV monochromator and evacuated by a Root Zivy RSV 350 pump. The XUV light is detected by a windowless Cu-Be photomultiplier (EMR model 510 W-00-16) mounted to the exit slit of the monochromator behind a thin indium filter (thick- ness < 1500 A ) . The detector is enclosed in a vacuum chamber evacuated by a small diffusion pump to pressures lower than torr while the gas cell is operated at pressures of the order of a few torr. The pressures in the conversion cell are mo- nitored with a precision Baratron gauge.

I11 - COHERENT AND TUNABLE VUV LIGHT GENERATION

Coherent, tunable and narrowband radiation in the VUV has been efficiently produced over the past few years through a powerful optical frequency mixing which uses in- tense dye lasers [I-21. The tuning range of the sum frequency process in a focused laser light configuration is restricted to spectral regions in which the difference between the wave vectors of the generated radiation and the driving polarization

(phase mismatch Ak) is negative. On the other hand, the frequency difference can be produced in a medium with Ak either negative, zero or positive [ 3 ] .

a) Region of 1400 a - In our experiments, coherent and tunable emissions are produ- ced in a few regions around 1400 d when the sum frequency w3 = wl + 2w2 is tuned to the vicinity of v=4 and v=5 levels of the A ~ I I state (fig. I ) . The generated si- gnal around 1420 A at p=3 torr of pure CO is shown in fig.2. w l is fixed and w2 is scanned in this experiment. The VUV signal is greatly enhanced on the blue side of the A-X (4-0) bandhead, in the negatively dispersive region of carbon monoxide spec- trum. Increasing the CO pressure in this region leads to a considerable extension of the tunability range which exceeds 350 cm-l (fig.3). Its blue shift at higher pres- sures, consistent with the vector phase matching requirements, confirms similar ob- servations of Glownia and Sander [4]. In fig.2 resonant enhancement of the generated signal due to P,Q and R branches of the A-X transition is clearly seen and demons- trates the useful spectroscopic application of the sum frequency mixing method. It should be noted that exploiting various vibrational levels from 0 to 12 of the A'II state and combining with c13016, one can efficiently produce coherent and tunable radiation in all the range between 1550 and 1250 8 [4].

Total

Fig.2 - Generation of w3 = wl + 2w2 showing enhancements due to P, Q and R branches

of the A-X (4-0) transition. CO pressure is 3 torr.

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Total energy (cm-l)

Fig.3 - Generation of w3 = wl+ 2w2 light around 1400 1. CO pressure is 32 torr.

b) Region of 1150 A - Fig.4 shows a portion of our results from a double resonance type experiment where the wl dye laser is tuned to the bandhead of the S branch of the A-X (3,9 - 0,7) transiiion and w2 is chosen such that the sum frequency 2wl+w2 probes the v'=O of the B*C state. Tuning the wl frequency to the R or Q branches bandheads results in different and less intense spectra due to the fact that diffe- rent selection rules on rotational transitions must be considered. The figure indi- cates that the experimentally determined sum frequency 2 w l + w2 is in good agree- ment with B-X (0-0) known transitions through a conventional absorption spectros- copy [ 5 ] . Additional to the B-X branch structure, clearly and reproducibly observed

Total energy

Fig.4 - Portion of double resonant enhancement spectrum showing the R branch of the B-X (0-0) transition in CO. The 2011 frequency is fixed and set to the S(7) two- photon A-X transition at 69119 cm- while the w2 dye laser is scanned. CO pressure

is 1.8 torr.

B - X ( 0 . 0 )

I .

86979 86980 86981 86982

*

Total energy (cm-')

as in fig.5, indicates possible influence of various rovibronic levels belonging to dif- ferent singlet and triplet states in the vicinity of v'=O of the B state. It may be pointed out a likely contribution of the A-X (19-0) transition which in this region, at least partially, overlaps B-X(0-0) [6].

Bjorklund [ 3 ] showed that the intensity of the VUV radiation created in a focused Gaussian beam configuration for a sum fre- quency mixing process is

where N is the number density, I and I

0 1 *2

are the intensities of the incident beams, X(3) is the third order nonlinear suscepti- bility responsible for the sum mixing pro- cess and F(Ak) is a constant depending on Fig.5 - Higher resolution recording of the R(14) line in the B-X(0-0) transition the degree of wave vector mismatch Ak = k g - 2 k l - kg. The variation of the VUV in- tensity is found to be proportional to I ' ^{and I} as predicted by the theory and

"'

1 "'2

isdisplayed in fig.6.

I -

1 10 10

l o 2

W2 Intensity (a,".) W1 Intensity (a,".)

We have also investigated the pure CO pressure dependence on the w3 intensity and the results are shown in fig.7. The initial slope of 2 confirms that a coherent pro- cess is taking place and the deviation from a quadratic behavior for CO pressures exceeding 3 torr suggests either a reabsorption of the emitted radiation or it may demonstrate a phase-matching destruction of the process.

Fig.6 - Variation of the V W intensity as a function of the intensity of the inci- dent w l and 0 2 beams.

1o2-

--?

A

r' di

P f

,

-Slope

1

JI lixS'Ope

I

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With I1 = 109w/cm2 and I2 = 10l0 w/cm2, measured conversion efficiencies of about lead to approximately 10 W of coherent radiation around 1150 &. This value is in reasonable agreement with theoretical calculation of X(3) (2wl+ w2) yielding 4 x esu and 20 W at pure CO pres- sure of 3 torr. The linewidth has not been di- rectly measured. From the convolution of the incident dye laser linewidths and the transi- tion linewidth in CO it is estimated to be of the order of 0.3 cm-'. This corresponds to less than 4 d at 1150 8.

Proper phase matching can greatly enhance the VUV signal intensity and increase its tunabi- lity range 17-91. Xenon, which is negatively dispersive between 1135 and 1170 & [lo], can be mixed with carbon monoxide to extend the tuning range over 1200 cm-' with peak powers estimated

1

I . .

. . ^{. . . .} . ^*

0.1 1 10

Fig.7 - Variation of the VUV intensity as a function of carbon monoxide pressure

C O

to be of the order of 20 W. The tuning range of w3 can thus be extended between 1142 and 1159 i? and is displayed in fig.8. It is, at present limited only by the

a w -

C

? e

3 ^30.

- _,,

-

.- g

- - sco-

O r n o 86M)O 85900 87200 87500

-

2w, +w2 Total

Fig.8 - Tuning ranges and relative intensity of VUV radiation in different mixtures

of C0:Xe. The shaded curves F,G and H represent symbolically enhancements due to

P and R branches of the B-X transition. Atomic carbon absorption is designed by

c ( a ) .

tunability of the w2 dye laser. Each partial tuning curve (A-K) in fig.8 corres- ponds to a different, experimentally optimized at each peak, pressure ratio of C0:Xe.

The shaded curves F-H represent symbolically enhancements due to P and R branches of the B-X transition with G(b) and G(c) corresponding to their respective bandheads and are obtained with pure CO. Adding Xe in this central region (curves G-H) decrea- sed the VUV signal which confirms that CO must be negatively dispersif there. The addition of positively dispersif argon provokes only slight enhancement of the VUV intensity. Also, further increase in the CO pressure leads to a strong reabsorption of the w3 signal.

IV - ABSORPTION CROSS SECTION OF CO IN THE VUV

The reabsorption of the generated signal under optimized phase-matching conditions permits, in fact, to evaluate o(w3), the absorption cross section at the VUV fre- quency of CO. Taking into account reabsorption effects eq.(l) should be replaced by

with L being the distance between the focus and the exit window of the cell. This eq. is valid only if one can neglect : 1) xenon absorption at 03, 2) third order nonlinear susceptibility of xenon in comparison to carbon monoxide.

Evaluating u CO at, for example, 0 3 = 87170 cm-l we note that the closest absorbing level of xenon is more than 1000 cm-I away and calculating the susceptibilities we find X(3) = 2 x esu >> X(3) = 10-~6 esu. This is confirmed in our experiments

CO Xe

through the observation of I (CO) > Iw?(Xe) and justifies eq.(2). Using this eq.

W?

we can experimentally determine the absorption cross section of CO at w3 by measu- ring I at different pressures of CO in C0:Xe mixtures and plotting log(1 /N' )

w3 w co

as a function of N At 87170 cm-I (h;ac = 1147.2 1) we find oCO = 6 x cm2 CO'

in good agreement with the value obtained in synchrotron photoabsorption measure- ments by Lee and Guest [ll] who estimate oCO< 8 x cm2 in this spectral region.

V - INDEX OF REFRACTION OF CO IN THE VUV

From the same experiment we can also derive the k vector mismatch per atom CCO, between the generated radiation at w g and the driving polarizations which is defi- ned by [lo]

The optimisation of the phase matching factor F imposing bAk

- 2[3] (b is the confocal lens parameter), we have

where B = p/N - ^{2.8 x 10-l7} torr at 20°C. Furthermore, from the slope of the fig.9 we obtain C Xe/CCO = - 4.66 at 1147.2 1 . The value of CXe was calculated at this wavelength for the case of third harmonic generation [lo]. Knowing the indices of refraction for xenon at w l and w2 [12], C , _ is found in our experiment to be

A S

- 1.53 x 10-~'crn~ and C = +3.3 x 10-l8 cm2 at the above wavelength. This last va- lue easily allows the ggtermination of the index of refraction of CO at us. Using the experimentally obtained value of nl at wl [I31 and n2 derived from the

Sellmeier equation at 02 [I21 we find for CO (n3 -1) = 5.2 x at 1147.2 8.

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VI - MULTIPHOTON PHOTOLYSIS OF CO IN THE VUV

2 5 0 .

-

- C g

2

8

50

0

Fig.8 (Ca) and 1 0 show an interesting feature of absorption of the VUV signal at 86519 cm-l. Atomic carbon energy levels [14] identify this frequency as correspon- ding to the 2p23~J=0 - 5d 3 ~ j = 1 transition.

I

-

-

-

Fig.9 - CO pressure as afunction of Xe pressure for different phase-matched C0:Xe

25 50 75

- mixtures at 1142 A.

The atomic carbon is most probably produced in our experiments through a multiphoton photolysis of CO involving three w l pho- tons of the total energy of 12.86 eV :

Xe Pressure (Torr)

0 1

-

Total energy (cm')

Fig.10 - Atomic carbon absorption profile

at 86519 cm-l.pCO= 80 torr.

A similar process was observed by Bokor et al. [15] using two quanta of an ArF*

laser. The region of 12.8 eV is rich in numerous valence and Rydberg states 1161 which can significantly contribute to the photodissociation of the CO molecule.

Such photodissociation is facilitated in our experiments through the two-photon resonant enhancement of the v=3 level in the A'II state. The slow buildup of photo- lytic products (black powder due to C2 molecules) noticeable in our cell is another confirmation of the process. It is interesting to note that no other absorptions originating from other than J=O levels in the 2p2 p 3 state where observed. This seems to indicate that our photolysis process in CO selectively creates the atomic carbon in its fundamental state with J=O only.

The efficiency of atomic carbon production can be estimated from the absorption linewidth using the curve of growth method [17] . In an experiment carried out at pCO = 80 torr, we estimate the density of atomic carbon to be of the order of

loi3 cm-3 which corresponds to its production efficiency of 3 x

VII - GENERATION OF COHERENT XUV RADIATION

An extension of the VUV tuning range to wavelengths in the extreme ultraviolet (XUV : X < 1000 &) is possible by frequency tripling or sum frequency mixing of intense UV radiation [2,18-191. A modification of the experimental system necessa- ry for detection of the XUV light was described in section I1 of this paper.

Fig.11 - Absorption coefficients of CO between 900 - ¹⁰⁰⁰ & [20].

The results we are presenting here are only preliminary. We are, at present, explo- ring 940 - 950 ft region of CO by a simple third harmonic generation from a single, frequency doubled visible dye laser. The process is, however, resonantly enhanced by a two-photon transition in the vicinity of the v=4 level of the A'II state.

The absorption coefficients of carbon monoxide in the 895 - 1006 ft wavelength re-

gion obtained with a helium continuum lamp [20] are shown in fig.11. The spectrum

between 105500 and 105750 cm-l obtained in our experiments is presented in Fig.12.

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3w, : XuV GENERATED FREQUENCY (cm*)

Fig.12 - XUV intensity as a function of the third harmonic frequency. CO pressure is 10 torr.

It is thought that the XUV signal above 105700 cm-' corresponds to a negatively dis- persive region of CO and is produced just above the 946 W transition bandhead. The structure of this high lying electronic state can be finely explored by tuning the UV laser toward lower frequencies but certain enhancements may arise from the two- photon A-X(4-0) resonant transitions.

It is clear that a considerable improvement will be achieved using two tunable UV lasers. One of them will be tuned close to a two-photon A-X transition. The other will be scanned across the rich spectrum of high energy states. Such experiments are presently under preparation in our laboratory. It is obvious that high resolu- tion nonlinear spectroscopy through X ( 3 ) can become a powerful tool in clarifying

W ?

many details of electronic structure in those high energy levels of molecular sys- tems,

VIII - CONCLUSIONS

We have shown that molecular gases such as carbon monoxide or nitrogen (similar, preliminary experiments on this last system are currently under way) can become an extremely useful source of continuously tunable coherent VUV/XUV radiation. The rich spectrun of rovibronic levels in a dense electronic singlet - triplet manifold com- bined with the simple frequency mixing technique can produce a very high brightness radiation useful and competitive for many different applications. In our experiments it was successfully utilized to measure in CO VUV absorption cross section, index of refraction and to study multiphoton photodissociation of the system.

Acknowledgements - The authors express their thanks to M. Bierry, D. Cldment,

M. Chateau and.P. Chauffournier for their expert technical assistance in solving

numerous mechanical, optical and vacuum problems.

REFERENCES

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