MOLECULAR SPECTROSCOPY WITH OPTOGALVANIC DETECTION

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MOLECULAR SPECTROSCOPY WITH OPTOGALVANIC DETECTION

C. Collins

To cite this version:

C. Collins. MOLECULAR SPECTROSCOPY WITH OPTOGALVANIC DETECTION. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-395-C7-409. �10.1051/jphyscol:1983738�. �jpa-00223295�

JOURNAIL DE PHYSIQUE

Colloque C7, suppldment au noll, Tome 44, novembre 1983 page C7-395

MOLECULAR SPECTROSCOPY WITH OPTOGALVANIC DETECTION

C.B. Collins

Center for Quantum EZectronics, University of Texas at DaZZas, P.O. Box 688, Richardson, TX 75080, U . S.A.

Rcsumd - Dix anndes d'utilisation en spectroscopie des moldcules de la tech- nique optogalvanique avec laser sont passdes en revue dans ce rapport. Tout au dcbut, capable seulement de reproduire les rdsultats des experiences clas- siques d'absorption, la mdthode optogalvanique laser est devenue depuis la plus puissante et versatile des nouvelles mdthodes qui bouleversent maintenant la spectroscopie mol6culaire. La plupart de ces mdthodes sont ddcrites en db- tails dans les exposds suivants. Nous passerons en revue ici, celle dans la- quelle notre laboratoire s'est spdcialisd : l'utilisation de la spectroscopie optogalvanique laser pour 6tudier La photolyse de mol6cules simples dans des dtats choisis.

Abstract - The past ten years of accomplishment in the use of laser optogal- vanic techniques for the spectroscopy of molecules is reviewed in this report.

From a curiousity at first only able to reproduce the results of classical absorption experiments, the laser optogalvanic (LOG) methods have emerged as the most powerful and versatile of the new procedures that are now revolution- izing molecular spectroscopy. Host of these are described in more detail in the following manuscripts. The one in which our laboratory has specialized is reviewed here. In this work the use of optogalvanic techniques in the study of the state-selective photolysis of simple molecules is discussed.

1 . Introduction and review.

-

In the times before lasers were available, it had been the conventional techniques of absorption spectroscopy that had contributed most heavily to the understanding of the structure of simple molecules. However, the development of the tunable dye laser initiated a modernization of almost all facets of the classi- cal fields of absorption and emission spectroscopy. The high intensities which became available made it generally impractical to search for an occurrence of low probability in a slight attenuation of such large intensities.

The introduction of the powerful new techniques of Laser Induced Fluorescence (LIF), Laser Optoacoustic Detection (LOAD) and the method at the focus of our conference here, all met the need for a technique which detected the effects under investigation against a relatively low background. However, the latter two shared an advantage of operating in situ. This most often resulted in a superior effici- ency for the collection of the agent communicating the occurrence of the physical event of interest which was usually, of course, a transition from one quantum state of an atom or molecule to another. While they also share other advantages, including an extraordinary sensitivity, the technique of interest to us emerged as the superior choice whenever the experimental systems such as discharges and flamcs generated acoustical noise. Even in quiet systems, LOAD techniques deve- loped difficulties in double resonance experiments because they tended to record the logical sum of the component transitions while the competing methods produced the product and thus, a lower backgound. These intrinsic advantages have been

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983738

C7-396 JOURNAL DE PHYSIQUE

beautifully exploited in an ever-growing number of profound and illuminating experiments stretching over the past decade.

Possibly the largest difficulty that had to be overcome in this new field of molecular optogalvanic spectroscopy resulted from the three name changes that tended to retard a unification of the underlying concepts. A posteriori it seems clear that space-charge detection, optogalvanic detection and optical impedance detection were implementations of the more general concept which might be termed simply, impedance spectroscopy. Unfortunately, that acronym has already found other usage. So, to continue current terminology, Laser Optogalvanic (LOG) spec- troscopy of molecules will be used throughout this review; but in the most inclu- sive sense. In contrast to classical techniques of spectroscopy, it concerns the detection of perturbations of the electromagnetic impedance of a medium produced by the absorption or emission of photons by the constituent molecules. Whether these impedance changes are detected through modulation of a space-charge limited current flowing through a thermionic diode, through modification of a quasista- tionary discharge current flowing between biased electrodes or through changes in the loading of a circuit resonant at radio or microwave frequencies is merely a matter of experimental style, most probably dictated by the questions of reactivity and system survivability. There is an underlying unity of the basic concepts.

What seems to be the first LOG spectrum of a molecule showing identifiable l + 1 structure was reported [l] in 1973. It was the spectrum of the X Ig -t C nu band of CS reproduced in Fig. 1. Through a "coin-toss," I became first author on that article and it is now a pleasure to be able to insert into this retrospective a comment about the international aspects of that project. My co-workers at our Center and I had the rewarding priviledge of collaborating in this and subsequent studies with Professor IOVITZU POPESCU, DENISA POPESCU and their colleagues at the Central Institute of Physics of Bucharest within the framework of a cooperative program sanctioned by our respective national science agencies. This arrangement supported most of our work in this field over an extended period of time that overlaped most of the history of LOG spectroscopy.

The first data shown in Fig. 1 were obtained as a measure of the current flowing between a heated filament and an anode arranged to contain the absorber between them. A bias was provided by the contact potential between the dissimilar materials used in the filament and anode. The light source was a tunable dye laser operated with cresyl violet dye and pumped with a pulsed nitrogen laser.

Both lasers had been constructed in the laboratory and outputs were quite low by current standards, being less than 50 pW average power.

The subsequent applications of the techniques of impedance spectroscopy in their diverse forms to an increasing variety of molecules is recorded in Fig. 2a.

Some of the more notable events in this history are charted in Fig. 2b. The acceleration of the pace of research can be clearly seen. What is perhaps more difficult to appreciate from these figures is the depth of study undertaken. The resurgence of interest starting, perhaps, in 1981 results from the fruition of powerful and profound studies in which the LOG techniques have made possible important new results in addition to new methods.

Particularly exciting to me are the recent extensions by WEBSTER and MENZIES [20] of LOG spectroscopy into the infrared region spanned by tunable lead salt diode lasers, the uses of LOG spectroscopy in photodetachment studies of MCDERMID and colleagues 1211 and BETEROV'S [22] work in negative ion formation. Detailed investigations with LOG techniques of the lifetimes for the quenching of individual rotational components of OH radicals have been reported by MCDERMID and LAUDEN- SLAGER [23] and of the lifetimes against predissociation of rotational lines of the HCO radical have been given by VASUDEV and ZARE [24]. The first detection of molecular ions is being reported at this conference by DREYFUS et al.

Perhaps, the most promising new techniques currently building momentum are the double resonance techniques of VIDAL and co-workers [25] and the microwave methods of SUZUKI et al. Even at radiofrequencies remarkable potential had been found [18,26]. In those studies the medium to be studied had been placed in a coil or capacitor comprising a part of a resonant circuit. The impedance of the medium was varied by the resonant absorption of optical radiation as in the usual LOG circumstance, but those changes appeared as variations in the loading of the resonant circuit. Typical signals are reproduced [l81 in Fig. 3. The upper two

WAVELENGTH

( a

FIG. la. Conductivity spectra as a function of laser wavelength for the

region corresponding to the 6250-8 band system of Cs2. .Short intervals of neglig- ible signal result from removal of excitation for basellne verification.

6460 6470 6480 6490

WAVELENGTH

( a )

FIG. lb. Photoionization current as a function of laser wavelength for the three consecutive 34-8 segments indicated. The ratio of signal less dark current to the square of the laser intensity is shown. Band heads of the transitions in the 6250-8 system to the intermediate state are identified by the corresponding vibrational quantum numbers shown according to the scheme v", v. The shape of the brack- ets identifies the red degraded band heads as shown in Ref. 1 with ( ) brackets for heads in the Q branch, < > brackets for heads in the R branch, and

[ ] brackets for heads in the P branch. The vibra- tional numbers without brackets correspond to the violet degraded band heads.

The last two curves have been drawn in a vertical scale three times larger than the previous one.

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FIG. 2a. Schematic indication of the number of new molecules studied in each of the past ten years with one of the optogalvanic techniques. Asterisks indicate molecules first studied with LOG techniques used to excite dissociative transitions and reference numbers are enclosed in brackets.

0 z a

3

a W F-m 0 a u 5 -

S* W W

zo"

LLr 0 K W m

1978 1979 1980 1981 1982 1983 Y E A R

FIG. 2b. Summary of some of the notable events in the adaption of LOG techniques to molecular spectroscopy and the year in which they occurred.

- - -

N2

" 2

"4"'

NO2 [51 Li,

[6]

CsKr CsAr [ 7 l

- -

-

I

1 9 7 3 1976 1978 1979 1981 1982 1983

Y E A R Yb2*

[81

-

In;

[41

rr) m E TI

---

NH3

[l31 CN [l41 He, 1151 12

191 Cs,Kr

[l01

,

CO [I 11

HCO [l21

RED E MODE

WAVELENGTH 610 605 600 595 nm

FIG. 3. Dispersion curves of the impedance of a sample of 0.1 Torr of cesium vapor determined from measurements of the loading of a circuit reso- nant at a radiofrequency as functions of the wavelength of the illuminating radia- tion from a 200 mW dye laser under two conditions of excitation and two conditions of detection as follows:

Uppermost panel: No electrical discharge in sample. Lowest rf powers used to detect impedance changes in thermalized vapor caused by absorption

of dye laser radiation.

Resonances occur at wave- length for which there is an accidental coincidence between a transition in CS

to a molecular state disso- 2 ciating to Cs(6P) and a transition in that product from the 6P state to a Rydberg state sufficiently excited to cause a substan- tial change of impedance.

Second panel from top:

No electrical discharge.

Same conditions as upper panel except that dissocia- tion of CS to a molecular

2 . state dissociating to Cs(5D) was enhanced by the supplemental illumination of the sample with 50 mW of argon ion laser output.

Third panel: Weak rf discharge in sample caused by higher power in the resonant circuit used to detect impedance changes. In this case the high resulting electron temperature was sufficient to excite a population of Cs(5D) atoms by electron impact and only dye laser illumination was used to produce the resonances shown.

Bottom panel: Strong rf discharge caused by power substantially in excess of that in the panel immediately above. Other conditions were the same as for the third panel.

panels show the data obtained when the resonant circuit contained so little power that no discharge occurred and the spectra reflect thermal energies of excitation.

The lower panels record data taken when the power which oscillated in the

detection circuit was sufficiently high that it caused a radiofrequency discharge.

Under those conditions the spectra shown are completely equivalent to LOG spectra, but with the advantage that no contact of the measuring circuitry with the test medium was required. The impedance changes were detected by inductively or capaci- tively coupling through the walls of the vessel confining the medium which could have been aggressively reactive. Parenthetically, it is notable that the term

C7-400 JOURNAL DE PHYSIQUE

"-galvanicH is particularly inappropriate under such cases in which no dc current flows in the medium, but the underlying principle remains the same.

We are extremely fortunate in having most of the principals involved in this forward thrust which is now building in this field, as participants in this con- ference. Their invited papers which follow immediately remove any need to provide a further summary of their stimulating results. Rather, it is perhaps interesting to return to Fig. 2 and to note that the few works published prior to 1977, and hence prior to the introduction of the term "optogalvanic spectroscopy" comprise a type of prehistory, often overlooked in reviews [27,28]. Indeed, perhaps, the only value in extending retrospectives back to earlier and earlier experiments is to identify the instances in which significantly new results were made possible by the technique being introduced. The 1973 data of Fig. 1 does not meet this criter- ion because it simply reproduced that part of the spectrum of Cs2 which had been obtained at higher contrast and resolution with conventional absorption techniques.

However, in 1974 a double resonance LOG technique was introduced that is still being used to study dissociation processes in molecules. It will be reviewed in the material immediately following.

2. State-Selective photolysis of molecules. - ^A rather general double resonance method for the correlation of photolysis bands observed in the spectra of simple molecules has evolved from early [l61 LOG experiments in which lines appeared in the spectrum of Cs2 molecules that resulted from accidental coincidences in wave- length of resonances for the absorption to a dissociative state followed by absorp- tion from an excited state of a product CS atom. These resonances were termed hybrid by the authors and they were subsequently found in Rb [ 2 ] , In [4], and Yb

[8]. In appearance they took the typical forms of Rydberg series of discrete absorption lines from particular excited states of the dissociation products that were modulated in intensity by the probabilities for the molecular part of the absorption that had led to the dissociation of the parent dimer. Figure 4 shows a typical example for Rb reproduced from Ref. 2. In some cases the Rydberg series spectra were observed $0 be anamolously terminated when wavelengths were reached at which molecular dissociation did not occur. However, the span of wavelengths over which hybrid resonances could be found, even in the first dispersion curves obtained, [l61 suggested that selective photolysis could be accomplished over fairly generous bandwidths of excitation, at least for a few types of simple molecules.

Because of these early experiments, it seemed reasonable to hope that such electronic transitions might occur over sufficiently broad bands of wavelengths so that relatively inexpensive photons, from filtered incoherent radiation, for example, might be able to produce appreciable populations of discrete product states. This provided the underlying motivation for this line of investigation.

The photolytic pumping of the iodine laser is a good example of the application of the selective nature of this type of process. However, a more general application of such techniques to other systems would require detailed knowledge of the disso- ciating state to be excited, of the states of the products with which it were correlated and of the dynamics of the dissociation process which would determine the extent to which the product channels might become mixed. Unfortunately even now practically nothing is known about the dissociative electronic states of most molecules.

This review describes a rather general application of impedance spectroscopy for the study of such electronically excited, dissociative states of simple mole- cules that has been found to be extremely effective. It was applied in our research center to a study of the photolysis of Cs2 dimers and CsKr excimers in order to resolve two general questions: (1) whether relatively broad dissociation ba~fds exist in the spectra that might populate selected product states, and (2) whether such photolytic spectra could lead to useful estimates of the potential curves describing the dissociative states involved [29-371.

This technique was first used in a study of the photolysis of Cs2 with ex- tremely encouraging results. [29-321. A relatively 9road band w3s found which selectively inverted the product populations in the 5 D and 6 P states to such an extent that even continuous laser action should3&! sustainea'gy those products with adequate optical pumping at wavelengths within that band. However,

4850A 4800H

I

W A V E L E N G T H ( H )

FIG. 4. Optogalvanic spectrum of rubidium as a function

05

hyjrid resonances corresponding to parts of the diffuse (5 P +n D) aqd sharp (5 P -tn S) series of atomic rubidium. No resonances invol%g the 5 P level are Z6Zerved. Data plotted are the changes in current flowing through t8(2rubidium diode divided by the square of the simultaneously measured laser intensities.

Irregularities seen in the wavelength scale are artifacts from the drive mechanism and have been calibrated as shown.

beyond the satisfaction of practical motivations associated with the identifica- tion of new media for photolytically pumped lasers, that work provided the bases for several general conclusions about state-selective photolysis of simple mole- cules. First, at least for Cs2 the existence of broad spectral bands for photo- lysis that produced remarkably selective distributions of product state popula- tions was found to be a fairly common occurrence. Second, the actual dissociation process seemed to occur adiabatically, and hence selectively, if the transition energy absorbed by the parent molecule was not too much in excess of the actual threshold for the process. Finally, the converse appeared to hold as well, in that for transitions which excited a considerable excess of energy in the unstable state of the parent, dissociation occurred non-adiabatically, and hence nonselec- tively.

In principle the double-resonance method introduced in this series of work is straightforward. Spectra such as shown in Fig. 4 suggested that the requirement for an accidental coincidence in the wavelengths needed to induce photolysis and to detect the products by a subsequent absorption could be relaxed through the use of two independently tunable lasers. For example, the lower trace of Fig. 5, taken from Ref. 2, shows two suc9 accidental coincidences found in the LOG spectrum of Rb2. No absorption from Rb(5 P ) pryducts could be found at those wavelengths.

The upper trace shows the appearan;L20f 5 PIL2 dissociation products that resulted from the supplementary illumination of the v por with filtered red light from an incandescent lamp. Clearly the longer waveJengths were able to excite unstable states of Rb that could dissociate to Rb(5 P ) while the shorter wavelengths needed to exgite the products to states suffi;f$ntly excited to alter the medium impedance could not produce those particular dissociations. It is readily appre- ciated that if one laser were fixed in wavelength to correspond to the peak in Fig. 5 marked 5P * + l l D , another source could be tuned through longer wavelengths with the result kLat the impedance of the medium would be affected in a manner depending upon the effi5iency with which the second laser selectively dissociated the Rb2 to produce Rb(5

JOURNAL DE PHYSIQUE

FIG.5. Relative current detected from the rubidium diode plotted as a function of wavelength. Resonances corresponding to components of the diffuse and sharp serioes of atomic rubidium are identified. Upper curve: data obtained from the absorp tion of the pulsed laser radiation in the vapor illuminated with an

auxiliary continuous source of colli- mated but unfocused radiation fil- tered with a red filter. Lower curve: comparison data to that of the upper curve obtained under the same conditions but without the auxiliary illumination of the vapor.

When finally implemented, two independently tunable dye lasers were synchron- ously pulsed and used to illuminate a vapor of the molecules to be studied. The first pulse dissociated the molecules of the sample along the various possible channels. Then a delayed pulse identified the products by exciting them indivi- dually into easily detected Rydberg states in a time too short to permit colli- sional mixing or radiative cascading of the particular populations produced by the first pulse. The distinctive features of this technique is this rapid detection of the direct products of photolysis, whose relative populations are unperturbed by subsequent relaxation chemistry. In contrast, LIF studies typically use cw dye lasers and so include the collisional mixing of product populations to an undeter- mined degree.

While the photolysis spectrum of CS proved to be an intrinsically interesting demonstration of this new photolysis tec&nique and what it could accomplish, the quantitative analyses of the resulting spectra were impeded by the paucity of theoretical estimates for the potential curves of Cs2. In contrast, the alkali- metal-inert-gas excimers offered a completely different perspective. Being essen- tially one electron systems, they were computationally tractable and at the same time experimentally accessible.

3. Method.

-

COLLISION

T

>-

W K W Z W

production of CS" by the photolysis of CsKr. In the Born-Oppenheimer approximation, the relative importance of channels (la) and (lb), together with their dependences upon the wavelength of the source inducing the transitlon would depend upon the matrix elements for the transition between the electronic states, the Franck-Condon factors depending upon the inltial and final states of the nuclear motion, the Honl-London factors describing the probability for any changes in rotational motion, and upon the probabilities for spontaneous dissociation of the excited state formed. In principle, except for the last, these are well-known quantities whose product is the transition probability for that particular absorption band of CsKr. When multiplied by the last quantity, and with an adjustment of numerical cogstants it becomes the cross section for the photolysis of CsKr into the products CS + Kr. It is the measurement of this cross section that lay at the focus of the type of work reviewed here.

In the LIF studies undertaken with conventional techniques the isolation of particular channels for photolysis was typically accomplished by experimentally correlating absorption data from transitions such as (la) and (lb) in Fig. 1 with fluorescence radiation from a transition, such as (2') emitted from populations of atoms produced from the direct products of dissociation by cascade or by colli- sional processes. To be successful such a technique required: (1) that the prob- ability for the emission of the fluorescence be high in comparison to the prob- ability that the state would be quenched nonradiatively, (2) that the emission occur before collisional prosesses would transfer the excitation from some other product state into either CS or into the upper statg radiat~ng (2'), or the reverse, and (3) that the necessary relaxation of CS" into the upper state radiating (2') not be precluded by steps taken to satisfy the first two require- ment. Generally, the degree to which the experimental situations conformed to these idealizations could not be unequivocally demonstrated.

The double resonance technique for impedance spectroscopy overcame these difficulties and demonstrated a very high sensitivity for detection. The products of dissociation were detected through the absorption of a second photon in a transitlon such as (2) in Fig. 6. However, as usual in LOG studies instead of attempting to detect the slight attenuation of an illuminating beam of wavelength corresponding to transition (21, the strategy of ug&ng the second transition to populate a relatively long-lived Rydberg state, CS , was employed. Then, as had

( 2 )

CS*+ Kr

-J

Q ^COLLISION

b Z

OR I CASCADE

LL

(2')

y C s + K r I N T E R N U C L E A R S E P A R A T I O N

JOURNAL DE PHYSIQUE

---J

m

C L - C I L I N O R I C A L LENS OH-OIELECTRIC MIRROR MIRROR $ L - S P W E R I C I L LENS

NDF-NEUTRAL O E N S l T I FlCTER CELL

FIG. 7: Schematic representation of the experimental apparatus. The portion common to that used in the precursive studies of Cs2 is enclosed by the dotted lines.

in energy to the ionization limit, it would have+essentialfy unit probability for undergoing collisional ionization, either to Is or to CS + Cs , through associa- tive ionization as a component step, or to CS

gy

[ 7 ] . The resulting change of impedance was considerable and could be readily detected.

The apparatus used in this type of photolysis work is shown schematically in Fig. 7. The primary photolysis system enclosed in dotted lines is the same as used in previous work with CS Basically, it consisted of two independently

2' .

tunable dye lasers pumped by a single pulsed nitrogen laser. The output beams from the dye lasers were aligned to be spatially collinear, but were temporally dispersed so that the one used to dissociate the population of parent molecules arrived first at the absorption cell by an amount equal to the duration of the pulse. Provisions were available for the adjustment of the temporal separation of the pulses, but for the examination of the spectrum the interpulse delay was generally set to the minimum practicable value of 3 nsec in order to reduce the possibility for collisionally mixing the product populations. As expected, with a reversed temporal sequence the effect was completely absent. The linewidth of the output from the dye lasers was of the order of 0.01 nm and corresponded approxi- mately to the resolution with which the wavelength could be set.

The additional dye laser system shown outside the dotted enclosure in Fig. 7 was pumped by a separate N2 laser, electrically synchronized through a variable delay with the N2 laser pumping the primary photolysis system. The beam from this additional source was combined collinearly with the beams for photolysis and detection. However, since the electrical delay was arranged for the trigger pulse initiating the sequence of photolysis and detection, the pulse of supplementary illumination preceeded the normal sequence by a time which could be set to a value between 20 nsec and 10 psec. It was generally used to extend the range of times over which the delay between photolysis and detection of the products could be

adjusted. With this modification the stability of the distribution of product state populations could be examined.

4 . Typical results. - For wavelengths longer than about 450 nm only one of four possible excited products can result from the photolysis of molecules formed from combinations of cesium and krypton. In order to simplify the kinetics tending to mix product state populations, to date the majority of at&ent$on was zocused upon she photolytic production of these four states of CS , ^{6 PlI2,} 6 P 5 2 ~

Both in pure cesium vapor and in mixtures of cesium and k&g6n, t2k2

~ ~ ~ t ? l ~ ( ~ . c h a n n e l s appear to be concentrated into three spectral regions correla- ting reasonably well with the red, yellow and blue absorption bands of both CS and CsKr that had been reported [39] as a result of classical studies conducte8 at orders of magnitude h i g p r inert gas density. In fact, excypt for the red band for the production of 5 D atoms at energies near 15,800 cm , the similarity between the photolysis bands for CsKr and Cs2 was remarkable to an extent raising questions about the identity of the actual parent molecules. This similarity can be appreciated more clearly from Fig. 8 where some of the data has been shown on a scale of wavelengths, together with the corresponding data for Cs2.

FIG. 8: Relative cross sections as fgnctions of wavelength for selective photo- lysis leading to the production of Cs2 atoms having $=3/2. Solid and dashed curves record cross sections for Cs(6 P and Cs(5 D ) products, respectively.

lightly drawn curves identify the resul?d2:or Cs2 whilq/8eavy curves describe the results finally attributed to CS Kr Only cross sections pertaining to the same parent molecule were plotted on ghe'same relative scale.

C7-406 JOURNAL DE PHYSIQUE

The band in the yellow is an excellent example of the additional insight into molecular structure provided by this technique. The molecular state resulting from Cs(7 2 S ) + Kr is a single, nondegenerate 7so sta e from rhich predjssocia-

5

tion into si68es adiabatically separating into either 6 P 5 D or 5 D

seem precluded by available theoretical estimates [40,41]~6~'this~6~1ecular~&stem.

Separation into all three seems completely forbidden, adiabatically. Diabatic separation seems to be excluded by the strong dependence upon transition energy of the proportions of the products. Rather it appears that most of the features of Fig. 8 could be the result of the photolysis of a previously unsuspected trimer, Cs2Kr, principally along the CS-CS bond. In such a system it would be reasonable to expect that the spectrum would resemble that of CS , red-shifted by the in- creasing potential found in the polarization energy

01

The small red band, reproduced in Fig. 8 is the feature most unequivo~6$ly attributable to photolysis of a parent CsKr molecule. It warrants several observa- tions. Only the channel producing 5D is clearly the result of the photolysis of CsKr. The other channel seen in Fig. 8 giving 6P at the red wavelengths is also most probably due to CS Kr for the reasons discuZAgd above, as adiabatic separation of CsKr into 6P woul8 be completely inexplicable at these transition energies.

As in the case 2i27so, theory indicates that the 5do state to which this transition is g nerally attributed E [ 4 2 ] is correlated adiabatically with a single limit, Cs(5 D ) + Kr. The relatively small cross section shown in Fig. 8 for the compon2hg of photolysis producing Cs(5 2 D ) actually has the same structure as found in the major channel producing Cs(5

342

542

To obtain values of delay between photolysis and detect-~on that were compar- able to the times which might be required for collisional mixing of the dissocia- tion products, the photolysis step had to be excited by the auxilliary beam from the laser shown outside the dotted enclosure in Fig. 7. In this case the normal photolysis pulse was blocked and the detection step was delayed electronically as shown in Fig. 7. T p result is seen in Fig. 9 . The initially strong increase in the production of 5 D3/2 atoms with inc eased time of delay seems conclusive in

5

indicating a secondary orlgln of that 5 D popylation. Moreover, the data of Fig. 9 implies there to be no prompt sour!b20f 5 D3,2 atoms.

I I I I I ^I I

0 500

T I M E (nsec.)

FIG. 9: Relative yields of the photolysis of CsKr at 632.5 nm as functions of the time elapsed between the arrival of the pulse used to induce photolysis and the pulse used to excite result- ing products to Rydberg states for detection. Data points plot the expe imental measurements of

i

the Cs(5 D) product populations in each of the two fine struc- 2 t2re states; + - 5 D , ^o -

5 DgI2. Solid curves56fot the bes comparable estimates obtained from modeling the kinetics of the product state populations using rate coeffici- ents described in the text.

Dotted curves show sensitivity of the model to changes of a factor of two in the rate coeffic- ient used to describe the mixing of the fine structure components.

Superficially, the functional form of the dependence upon time appears to consist of the sum of two exponentials. This is to be expected if the populations of the two fine structure components were tightly coupled by collisional mixing, while being quenched more slowly at a common rate. Assuming transport to h a ~ e been either negligible or identical for the populations , ^{N5 and N3} of the 5 D

,, 512

and 5L~312 levels, respectively, the kinetics modeling the following system,

can be solved analytically and the important coefficients, Kd, and Q fit so that the analytical predictions agree with the measured results.

Shown in Fig. 9 by the solid lines are the computed populations which were in the best agreement with the data. The two adjustable parameters, Q and K, were

6 -1

given the values Q = 2.1 X 10 sec and Kd = 2.2 X 10' sec-'. The effect of changes in Q were to change the late-time slope proportionally, while the sensi- tivity of the computations to changes in Kd of a factor of two have been explicitly shown in the figure. Using the tabulated values for the radiative lifetimes of

the 5D states the rate coefficient for the collisional qu chi g of-Ihe ~ ~ ( 5 ~ ~ 1 population by Kr was estimated to be K5D = 3.7 k .7 X 10

-"

rate coefficient for mixing of the fine struct e p pulafjons produced by colli- sions with Kr was estimated to be Kd = 7 X 10 cmg sec to within a factor of 2. The latter value is not inconsistent with mixing rates reported in the litera- ture for selected states of various alkalis.

The above considerations seemed to confirm both qualitatively and quantita- tively the interpretation tqat the photolysis band found in the red region of the spec rum of CsKr yielding 5 D products is the single 5d02band correlated with CS(~'D,~~) + Kr. The smaller56fnd shown in Fig. 3 giving

3

2 5. Conclusions. - The major conclusion of this particular study was that the occurrence in the spectra of Cs2, Cs2Kr and CsKr of relatively broad photolysis bands leading to extremely well-selected product states is a fairly common event.

In general, it appears that photolysis occurring at a transition energy not too much in excess of the threshold for the process leads to adiabatic and hence state selective dissociation.

Finally, from the observations of the stability of the populations of the dissociation products agalnst collisional mixing, conclusions can be drawn con- cerning the characteristic times o er which state selective photolysis could be

Y

maintained. At least for the Cs(5 D) products, populations could be expected to remain in the same quantum state for tens of nanoseconds and in the same energy level, neglecting fine structure splitting, for a time of the order of 5gY2nse5.

Qualitatively these ti1y3 co5respyd to rate coefficients of Kd = 7 X 10 cm sec an$ Q = 3.7 X 10 cm sec for fine structure mixing and for quenching of the Cs(5 D) states, respectively. The practical importance of this conclusion is that the time scale over which product populations could be photolytically pumped is defined. For example, at this density corresponding to a filling pressure for Kr of 100 Torr the maximum fluence which could be applied to the production of populations of a single state would be equal to the illuminating intensity inte- grated over a few tens of nanoseconds. The fluence which could be used to popu- late a single energy level would be over an order of magnitude larger.

C7-408 JOURNAL DE PHYSIQUE

Qualitatively, such results demonstrating both highly selective but broad bands for photolysis, together with reasonable stability of product populations appear very encouraging from the perspective of laser chemistry.

In general the sensitivity and resolution achieved in this application of impedance spectroscopy to the studies of selective photolysis reviewed in some detail above attest to the promise held by this particular double resonance techni- que. It appears ready to serve for the detailed characterization of the repulsive states of simple molecules that are becoming of critical interest in a variety of applications. The following papers will provide more detailed perceptions of other exciting results on the frontiers of optogalvanic methodology that hold the clear promise of precipitating a revolutionary advance in molecular spectroscopy.

Acknowledgments.

-

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