Collision-free coherent anti-stokes raman spectroscopy (cars) of molecular photofragments

CHEMICAL

PHYSICS

15 October

LEJIERS

1981

COLLJSION-FREE COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (CARS) OF MOLECULAR PROTOFRAGMENTS James J. VALENTINI

*, D.S. h!OORE ** and David S. BOMSE *

Lor Alamos Ichnonal Laboratory, Umen-~ty of CMfomur. Los Alamos. New hfexrco 87545.

USA

Received 18 August 1981

Colbsion-free CARS spectra of molecular oxygen produced in the vlnble photodtssociafion of ozone have been obmed. The -cent rotat.~onaland vibrational state dkhiiunons of the oxygen have been ektraaed from these spectra.

1. Introduction Determinatron of the internal energy drstributions of molecular photofragments is an important endeavor rn photochemistry, for both fundamental and practical reasons. These energy drstnbutions reveal directly the nature of the dissocrative electromc states and the dynamics of the motions and interactions of the separating fragments. Thus, detailed measurement of the drstributions is important in the development of theories of molecular dissocration. The distnbutron of the dissociation energy over the fragment degrees of freedom also determines the rates and mechanisms of the chemical reactions which take place after photodissociation. Hence, knowledge of tbe energy distributions ISof considerable practical significance to the understandmg of chemical systems in which photodissociation plays an important role, such as atmosphenc chemistry, laser isotope separation, and chemical lasers. Despite their importance, photofragment internal energy distributions have been detemuned for only a few molecular dissociation processes. Since the photofragments generally will relax collisionally or react very rapidly, collision-free measurements are necessary * Los Alamos National Laboratory J. Robert Oppenheimer Fellow, 1978-1980: author to whom correspondence should be addressed. l * Los Alamos National Laborafory Postdoctoral Fellow, 1980-1981. * LOS Akunos National Laboratory J. Robert Oppenheimer Fellow.

ensure that the nascent drstributions are observed. The requirement of collision-free measurement greatly restricts the use of spectroscopic technique-s in the determination of photofragment quantum-state distributions. The applicatron of coherent Raman spectroscopic techniques [ 1.21 to the determination of photofrag ment energy distributions seems particularly promising. The generality of Raman spectroscopy and the very high temporal resolution possible with this Eightscattering phenomenon make it potentially very useN as a method for obtaining collision-free spectra for a large number of photofragments. Moreover, the successful application of inverse Raman spectroscopy ]3] and CARS [4] to molecular beams has demonstrated the sensitivity of coherent Raman spectroscopies, and the applicability of such methods to the hig&esolution spectroscopic mvestigation of low-density media. In addition, work by Nibler [S], and more recently Fisenthal [6], has shown how CARS can be used to identify transient species following laser photolysis. in this letter we report the first use of a coherent Raman technique, CARS, in the determination of nascent photofragment internal energy distributions. Rotationally and vibrationally resolved spectra of molecular oxygen, produced in the visible (532-580 MI) photodissociation of ozone, to

were obtained by effecting photodissociation photofiagment spectroscopy simultaneously

and during a 217

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15 O c t o b e r 1981

6 ns duration laser pulse in a gas cell conmlnirlg ozone at 2 Torr. Since the average time between collisions in the cell, ~ 1 0 0 ns, is much greater than the photolysis/ spectroscopic probe period collision-free sicectxa were obtained, and the nascent rotational, vibrational, and electronic state distributions could be determined.

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2. Experimental

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In a CARS experiment two laser beams, a pump laser beam at frequency COp, and a lower frequency Stokes beam at COs are focused into a sample. A signal at a third frequency, the anti-Stokes frequency, COaS, ~ven by COaS = 2COp -- COS '

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is generated by four-wave mixing [l ]. When the frequency difference between the pump and Stokes beams, COp -- coS, is equal to corn, the frequency o f some Raman-active molecular transition, there is a very strong resonant enhancement o f the anti-Stokes signal. Thus, a Raman spectrum in a CARS experiment can be obtained by scanning the frequency difference. COp -- COS, while momtoring the anti-Stokes signal intensity. The experimental apparatus we use to obtain CARS spectra is shown schematically in fig. 1. A Nd:YAGpumped pulsed dye laser system * is used to generate the 10 Hz, 6 ns fwhrn pump and Stokes CARS beants. The Nd:YAG second harmonic at 532 nm serves as the t-Lxed-frequency pump beam, COp, and the output o f the dye laser provides the tunable Stokes beam, cosThese two laser beams axe expanded to a diameter o f ~ 1 8 mm, combined on a dichroie mirror, and collinearly focused into the gas cell u~ng a 1 m focal length lens. A very small fraction o f the beams is picked o f f by a beam splitter beyond the focusing lens, directed through a pinhole at the focus, and imaged on a viewing screen. This feature allows continuous monitoring o f beam overlap. After exciting the gas cell, the pump and Stokes laser beams are speetrally and spatially filtered from the anti-Stokes signal beam, which is detected by a photomuidplier and processed by a boxcar integrator.

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Fig. 1. Schematic diagram o f the CARS apparatus used in this experiment.

CARS spectra are obtained by scanning the Stokes (dye laser) frequency and recording tile anti-Stokes signal intensity as a function o f frequency on a stripchart recorder. The output o f a photodiode which monitors the dye laser etalon fringes is also recorded, providing frequency and intensity calibration o f the scan.

In general, as shown in fig. 1, a pulsed UV laser would be time-synchronized with the CARS laser beams and focused into the gas cell to effect photodissociation. However, for the experiment described here we have used the pump and Stokes CARS laser beams to both photo~ssociate ozone and record the oxygen photofragment spectra. Ozone has a weak absorption, the Chappuis bands, extending from 4 4 0 to 850 nm, with a peak near 600 nm [8]. Ozone is prepared by ac electric discharge in oxygen, and stored in a silica gel trap at 195 IC During the photolysis it is flowed rapidly (~0.2 Tort £ s - l ) from the trap through the gas sample cell at a pressure o f ~ Tort.

3. Remits and discussion Fig, 2 shows a typical collision-free CARS spectrum of molecular oxygen formed in the visible photodissociatron of ozone. The strong feature between 1545 and 1560 cm-l is due to ground vibrational state oxygen with a rotational temperature of 300 K. This signal arises from the molecular oxygen unpurity m the ozone, and also from atmospheric oxygen outside the gas cell. The latter contnbution is due to CARS signal generated in the region between the drchro~c mirror and the entrance to the gas cell, over which the CARS beams are spatially overlapped. At smaller Raman shrfts, however, we observe molecular oxygen Raman transitions which arise from rotatronal and vibrational states which are not thermally accessible- These tram&ions are due to “hot” mo lecular photofragments, and are labelled in fig. 2. As indrcated, we see transrtions Rom high 3 states of the IJ= 0, 1,2,3, and 4 vibrational levels of 0,. AU of these Raman lines can be assigned to rotational and vibrational states of the ground (3Zg) electronic state of oxygen. We see no evidence for formation of 0, (l a,), which is energetically allowed at these wavelengths, but only by a spin-forbidden processes which also forms 0(3P)_ lbe spectrum shown m fig. 2 was obtained with laser pulse energies of a mJ in both the pump and Stokes beams, with baridwldths of 0.1 and 0-g cm-l.

Fig. 2. ColhsionXree CARS spectrum of the Q branch of oxygen formed in the visiile photodksociation of ozone. Spectral features between 1545 and 1560 cm-t are due to oxygen impurity. Dye her etAon fringes (2.392 an-’ free spectd range) arc shown at the top of the figurealongwith vibrational and rotational state identification of the Raman transtionn

respectively. However, we have also recorded the o-xygen CARS spectrum at several lower laser pulse energies, down to ~7 mJ pump and 22 mJ Stokes, where the signal level is reduced by a factor of ~500. In all these spectra the relative peak intensities and line-shapes are identical to those in the spectrum shown in fig. 2, indicating that saturation [9] and ac Stark broadening [IO] effects are negligible. The peak intensity of a CARS line is proportional to (NV,, - NUe,/,)2, I.e. the square of the population difference between the lower and upper molecular states, characterized by vibrational and rotational quantum numbers u,J and v’,f, respectively, connected by the CARS transition- Therefore, in order to extract the state populations, NV,,, more information than just the CARS spectral intensities is needed. The sign of the population difference (N, , Nun,r) can be determined from an examin i&on of the CARS line&ape- it can be shown [i ] that due to mixing of non-resonant and resonant contributions to the CARS signal the lineshape is asymmetric, with a “tail” on the low-frequency side for NV,, - N$,f > 0, and on the high-frequency side for ND,, - N6.r < O_ Fig- 3 shows an expanded frequency scale plot of the CARS lines from a few rotational levels in the 0+1,1+2,2+3,and3+4vibrationalbands_Note that the 2 + 3 transitions “tail off” toward high frequency, while the lines in a0 other bands are skewed toward low frequency. l-his indicates that the lower

Fig_ 3. Expanded frequency scale plot of sekcfed rotational lmes from the CARS spectrum of mokcuk oxygen photofagment~owninfie.2.LinesLiomtheO~1.1-,2.2--C3, and 3 + 4 viiratronal bands are sbowa Dashed lines show r hneshape to emphasize asymmetryin CARS line-

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v i b r a t i o n a l s t a t e has higher p o p u l a t i o n for all b u t t h e v = 2 -> v = 3 t r a n s i t i o n s , for w h i c h t h e p o p u l a t i o n is inverted. T h e relative p o p u l a t i o n s can b e o b t a i n e d f r o m t h e p o p u l a t i o n d i f f e r e n c e s d e r i v e d in thi~ w a y b y a p p l i c a t i o n o f t h e b o u n d a r y c o n d i t i o n N s , j = 0 f o r all ./. We have n o t b e e n a b l e t o o b s e r v e a n y t r a n s i t i o n s f r o m v = 5, even w h e n using h i g h e r pressures ( u p t o 8 T o r t ) o f o z o n e in t h e cell. F r o m a close e x a m i n a t i o n o f t h e s p e c t r a w e can set a n u p p e r l i m i t o n t h e p o p u l a t i o n in v = -~ o f 3 0 , o o f t h e p o p u l a t i o n in v = 4 , a n d for simp l i c i t y will a s s u m e NS, j = 0 f o r all Jr. Since t h e p o p u l a t i o n in v = 4 is s u c h a s m a l l f r a c t i o n (~-~6%) o f t h e t o t a l p o p u l a t i o n , this a s s u m p t i o n has negligible e f f e c t o n t h e relative p o p u l a t i o n s t h u s d e r i v e d . F i n a l l y , these p o p u l a t i o n s are c o r r e c t e d for t h e (v + l ) d e p e n d e n c e o f t h e R a m a n cross s e c t i o n o n v i b r a t i o n a l s t a t e [ 11 ]. The rotational and vibrational state distributions o b t a i n e d in this w a y are s h o w n in fig. 4. T h e vibrat i o n a l d i s t r i b u t i o n is n o n - B o l t z m a n n a n d non-statistical, w i t h t h e a f o r e m e n t i o n e d inversion b e t w e e n o = 2 a n d o = 3. T h e r o t a t i o n a l d i s t r i b u t i o n , w h i c h is also n o n - B o l t z m a n n a n d n o n - s t a t i s t i c a l , is p e a k e d r.z J = 3 3 , 3 5 , 3 3 , 3 1 , a n d 25 for o = O, 1, 2, 3, a n d 4, respectively. These o b s e r v a t i o n s s e e m t o i n d i c a t e a d i r e c t dissoc i a t i o n process in o z o n e , f r o m a s t a t e w i t h a non-linear g e o m e t r y . A m o r e c o m p l e t e discussion o f t h e dissoc i a t i o n d y n a m i c s will b e given in a f u t u r e p u b l i c a t i o n . We m u s t m e n t i o n n o w , h o w e v e r , t h a t t h e r e is s o m e a m b i g u i t y in t h e interpretation o f the v i b r a t i o n a l s t a t e d i s t r i b u t i o n o b t a i n e d h e r e , since b o t h the p u m p and Stokes frequencies lead to dissociation of ozone,

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a n d w e have f o u n d s h a r p d i f f e r e n c e s in t h e v i b r a t i o n a l state d i s t r i b u t i o n w h e n t h e d i s s o c i a t i o n is e f f e c t e d e i t h e r at o n l y t h e S t o k e s o r o n l y t h e p u m p f r e q u e n c y . T h e s t a t e p o p u l a t i o n s s h o w n in fig. 4 w e r e c o m p u t e d f r o m an average o f t h r e e s e p a r a t e s p e c t r a l scans like the o n e s h o w n in fig. 2, a n d t h e u n c e r t a i n t i e s in t h e relative p o p u l a t i o n s s h o u l d b e less t h a n 5%. This c l a i m is s u p p o r t e d b y c o m p a r i s o n o f c a l c u l a t e d a n d m e a s u r e d relative p o p u l a t i o n s for t h e . / = 19, 2 1 , 2 3 , a n d 25 levels ( l o w e r . / l i n e s are o v e r l a p p e d ) o f o x y g e n at 3 0 0 K, for w h i c h t h e m e a s u r e d p o p u l a t i o n r a t i o s w e r e w i t h i n 1.5% o f t h e c o m p u t e d values.

4. Conclusion O u r initial results i n d i c a t e t h a t the s e n s i t i v i t y a n d t e m p o r a l r e s o l u t i o n o f C A R S c a n be e x p l o i t e d t o o b tain n a s c e n t r o t a t i o n a l , v i b r a t i o n a l , a n d e l e c t r o n i c s t a t e d i s t r i b u t i o n s for m o l e c u l a r p h o t o f r a ~ n e n t s . T h e s e results are p a r t i c u l a r l y e n c o u r a g i n g in view o f t h e fact t h a t t h e visible p h o t o d i ~ o c i a t i o n o f o z o n e s t u d i e d here has a very small cross s e c t i o n , ~ X l O - 2 1 c m 2 [ 8 ] , o r d e r s o f m a g n i t u d e smaller t h a n m o s t d i s s o c i a t i o n processes. It s h o u l d also be p o s s ~ l e t o e x t e n d these C A R S e x p e r i m e n t s t o t h e s t u d y o f c o l l i ~ o n a l processes following photolysis. By time delaying the CARS probe w i t h r e s p e c t t o p h o t o l y s i s s t a t e - t o - s t a t e c o l l i s i o n a l relaxation and chemical reaction of atomic or molecular photofragments could be investigated.

Acknowledgement We wish t o t h a n k Drs. Del O w y o u n g a n d P e t e r E s h e r i c k f o r v e r y h e l p f u l discussions. We also t h a n k Messrs. F r a n k A r c h u l e t a a n d A n t h o n y G a r c i a f o r exp e r t t e c h n i c a l assistance.

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CHEMICAL PHY$ICS LETTERS

[41 M.D. Duxxan, P. O&terlin and R&. Byer. Opt. Letters 6 (1981) 90. [Sl KS. Gross, DX Guthals and J-W. Nibler, J. Chem. Phys.

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15 October 1981

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P.L. Nattem, Phys. Rev_ Letters 45 (1980) 620. Taran. J. Tail&t, MFBawl and A.M. Bnmeteau, J. A-mt Phys. 52 (1981) 2687.

[ 10) M. PeaIat. J96.

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