J. Phys. Chem. 1984, 88, 1287-1293
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FEATURE ARTICLE Photodissociation: A Critical Survey J. P. Simons Chemistry Department, University of Nottingham. University Park, Nottingham NG7 ZRD, United Kingdom (Received: November 3, 1983)
The profitable interaction between high-resolution spectroscopists, laser photochemists, molecular reaction dynamicists, and quantum theorists has introduced an era of “high-resolution” photodissociation dynamics. Experiments and theory have reached a level of sophistication which prompts an appraisal of what experimentalists should be most anxious to measure (and also the corollary, what experimental measurements are no longer of informative value). A list of recommendations is followed by a survey of some of the most exciting new experiments in the field of small molecule photodissociation.
I. Introduction Some reviews tend to be collections, some to be clarifications. This one will attempt a little of both, dealing with matters that have preoccupied the author’s thinking since his youth-the nature of molecular photodissociation processes and the interface between dynamics and spectroscopy. The technological changes that have overtaken photochemistry laboratories allow experiments to be conducted at higher and higher levels of state and of time resolution. The result is a flood of increasingly specific and ingenious experiments to expose the microscopic detail of the elementary photodissociation process. There is a strong interplay between the new data and the theoretical advances that follow in their wake-or which occasionally precede them-and this review will attempt to point to some directions in which that interplay seems most likely to be fruitful. The study of vector correlations, the use of multiple-photon excitation techniques, and in particular the interaction between molecular dynamic and spectroscopic approaches to the study of molecular photodissociation are all acting in concert to usher in a new era of “high-resolution photochemistry”. The reviewer’s aim is to transmit some of this excitement to the reader. 11. A Theoretical Prelude and an Experimental Challenge
Theoretical developments in the analysis of molecular photodissociation have been reviewed very recently by Shapiro and Bersohn Alternative descriptions include classical “half-collision” trajectory models, Franck-Condon and/or forced oscillator models, statistical RRKM or phase space models, and, finally, formal quantum treatments of the total scattering process. Early developments focussed attention principally on calculations of energy disposal into vibration and translation since this was the kind of information which contemporary experiments could generate. At first, it was enough to restrict discussion to collinear systems and to direct photodissociation. The separation of intrafragment changes (associated with photon absorption and governed by generalized Franck-Condon factors) from interfragment effects (associated with the subsequent half-collision under a repulsive potential) was soon recognized as an enticing and physically transparent simplification.’-* The prbbability of generating
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(1) Shapiro, M.; Bersohn, R. Annu. Rev. Phys. Chem. 1982, 33, 409. (2) Mitchell, R. C.; Simons, J. P. Faraday Discuss. Chem. Soc. 1967, 44, 208. (3) Berry, M. J. Chem. Phys. Lett. 1974, 27, 73. 1974, 2, 329. (4) Simons, J. P.; Tasker, P. W. Mol. Phys. 1973, 26, 1267. 1974, 27, 1691. (5) Band, Y. B.; Freed, K. F. Chem. Phys. Len. 1974, 28, 328.
0022-3654/84/2088-1287$01.50/0
fragments in the final state If) could be expressed simply as the weighted sum
where 17) is an intermediate state in the photoexcited molecule, SF,is the scattering matrix for the interfragmment half-collision which transfers the system into the final state If) and VIfis the perturbation coupling the electronic states in the parent molecule, eg., the radiation field in direct dissociation, vibronic or some other coupling mechanism in predissociation.6 Later developments reacted to the increasing sophistication of experiments which allowed resolution of angular and rotational distributions9 in the dissociation fragments and subsequently, the alignment of their rotational angular momenta.‘0 Classical and quantum treatments of photofragment alignment and orientation (the firs! and second Legendre moments of the distribution function f(J’.Z) for the rotational angular momentum directions) have been developed and expertly reviewed by Greene and Zare.lo The new developments led first to a relaxation of the constraint of collinearity and eventually to the formulation of detailed and “exact” three-dimensional quantum scattering treatments’,’ which accommodated photon absorption and the coupling between alternative exit channels in a unifiid way. The “exact” treatments incorporate a set of amplitudes from which individual or partially averaged state-to-state photodissociation cross sections, both integral (total) and differential (angle-resolved), can, in principle, be determined. The qualification “in principle” has to be added, first of all, because of ignorance of the details of the relevant molecular and interfragment potentials. Secondly, the number of coupled exit channels is likely to be prohibitively large for all molecules more complex than triatomic and solutions for the “exact” theory are therefore intractable. Approximations can be introduced of course; for example, the dissociation
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CH3-I
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CH3 + I
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can be treated as a three-body process by taking the W3grouping (6) Freed, K. F.; Band, Y. B. In “Excited States”; E. C. Lim, Ed.; Academic Press: New York, 1977; Vol 3, p 109, Morse, M. D.; Freed, K. F. J . Chem. Phys. 1981, 74, 4395. (7) Simons, J. P. In “Gas Kinetics and Energy Transfer”, Ashmore, P. G.; Donovan, R. J.; Ed; The Chemical Society: London, 1977; Chem. Soc. Spec. Period. Rep., Vol. 2, p 56. (8) Gelbart, W. M. Annu. Rev. Pkys. Chem. 1977, 28, 323. (9) Leone, S . I(.Adv. Chem. Phys. 1982, 50, 255. (10) Greene, C. H.; Zare, R. N. Annu. Rev. Phys. Chem. 1982,33, 119. (11) Band, Y.B.; Freed, K. F.; Kouri, D. J. J. Chem. Phys. 1981, 74,4380.
0 1984 American Chemical Society
1288 The Journal of Physical Chemistry, Vol. 88, NO. 7, 1984
as a single mass unit.‘ This may simplify the theoretical analysis but the results may yet fall short of detailed experimental observation because of ignorance or tacit assumptions regarding the relevant potential(s). In the example cited, the ‘E and ,E states associated with the 5pn u* transition in CH31 are transformed by spin-orbit coupling into five components, AI, A2, E, E, and E. Single-photon excitation at any given wavelength in the associated UV continuum (-220-300 nm) could transport the molecule into all states except A2 (which is symmetry forbidden for a one-photon transition). Are their potentials all very similar? Almost certainly not. The full three-dimensional models all indicate strong sensitivity to the quantum states selected by the absorbed photon in both the ground and electronically excited states. This puts the onus on the experimentalist to generate the levels of state-to-state resolution implicit in the formal theory, not only for triatomic molecules but for polyatomic ones as well. Then the “really good data which deserve an exact calculation to test how well we can rely on simpler mode1s)”l canm be provided. So, what should the experimentalist be measuring? Already some guidelines are clear: (i) Because rotational energy disposal is also subject to the constraints of angular momentum conservation and disposal, it is this experimental observable that is the most sensitive to the dynamics of the photodissociation process and therefore merits the most detailed attention. (ii) For similar reasons, measurements of the anisotropy of vector properties such as the photofragment angular distributions, rotational alignment, and orientation provide very sensitive indicators of the dissociation dynamics. These measurements have been placed on a sound theoretical footing through advances in formulating both the classical and quantal theories of photofragment alignment and orientation-see the inspiring review of Greene and Zare.‘O Measurements of the polarization of resolved photofragment fluorescence spectra (spontaneous or laser induced) will be particularly valuable in view of their sensitivity to any delay between photon absorption and subsequent dissociation.1z At wavelengths where two (or more) electronic transitions overlap and/or where the measured rotational distributions include contributions from alternative dissociation pathways, the simultaneous measurement of energy (scalar) and angular momentum (vector) distributions may help in their resolution. (iii) In general, there is little point in achieving high levels of state resolution when monitoring the products of photodissociation unless this is matched by similar levels of initial parent molecular state selection. The most important factor, from the point of view of comparison with theory, will be the choice of states of known angular momentum IJ,K): experiments conducted in jet-cooled 0 provide the most simple selection nozzle beams, where J method when dissociation proceeds via a direct mechanism. In many cases, however, the mechanisms are predissociative, especially in the vacuum UV region where Rydberg state potentials may support resolved rotational as well as vibronic structure in the parent molecular absorption spectrum. Fine-tuned rotational or initial vibronic state selection can then be achieved simply by varying the wavelength of the incident photolysis beam; the increasing availability of tunable vacuum UV laser sources and the use of two photon near-UV excitation techniques provide alternative methods for selective excitation in this region of the spqctrum. Multiple-photon excitation techniques can also be employed to extend the range of electronic state selection, accessing states otherwise forbidden by the operation of single-photon dipole selection rules.13 (iv) With the development of picosecond laser techniques, the direct monitoring of time-resolved growth of photofragment p o p ~ l a t i o n is s ~becoming ~ a practical proposition. (An early report of the slow growth of C O absorption following UV photodisso-
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(12) Nagata,T.; Kondow, T.; Kuchitsu, K.; Loge,G . W.; Zare, R. N . Mol. Phys. 1983, 50, 49. (13) Donovan, R. J. In ‘Gas Kinetics and Energy Transfer”,The Chemical Society: London, 1981; Chem. SOC.Spec. Period. Rep., Vol. 4, p 117. (14) Craig, B. B.; Faust, W. L.; Weiss, R. G . J . Chem. Phys. 1983, 79, 1286.
Simons ciation of HzCO, originallySascribed to slow evolution into the molecular fragment species, is known now to be associated with slow relaxation from the high rotation levels of CO initially populated.16) The competition between molecular photodissociation proceeding via a quasi-bound state (Le., predissociation) and resonance-enhanced multiple-photon ionization (REMPI) which uses the intermediate state as a “stepping stone” can provide an alternative indirect method for estimating predissociation rates. l7 (v) Next, there is the problem of the interfragment potentials. An exciting new experimental entry into this unknown territory is the discovery of structured (and predicted’*) free-bound luminescence spectra, from repulsive electronically excited polyatomic molecular states.19 The laser-excited luminescence of 0, for example, promoted by absorption into the Hartley continuumi9 generates a banded emission spectrum with vibrational progressions running out to the ground molecular dissociation limit. Their intensities reflect the contours of the excited- and ground-state potentials at geometries far away from the localized FranckCondon region probed in absorption. A knowledge of the exit potentials is essential both for any dynamical treatment of photodissociation or for any statistical phase space analysis which includes energy and angular momentum conservation. (vi) Much more attention needs to be focussed on the relationship between initial state preparation and the branching into alternative spin-orbit final states-indeed theoretical analysis of the spin-orbit problem or of curve crossings in the exit channels has not yet been satisfactorily addressed. Do the electron spins have time to relax during the dissociation event (cf. the CIDEP experiment) ? (vii) Finally, the photodissociation dynamics of large molecules and the transition from dynamical to statistical regimes remains poorly understood. For example, when is a measured vibrational energy distribution to be described as “statistical”? What are the criteria for statistical behavior? How are initial energy distributions in polyatomic nonfluorescent product fragments to be probed? What role will nonlinear photophysical techniques such as CARS (coherent anti-Stokes Raman spectroscopy) and REMPI play in the coming years? 111. An Experimental Rondo In 1982, Leone presented a thorough and enthusiastic review9 which surveyed the major impact that laser and molecular beam technologies were making on the experimental study of molecular photodissociation. At that time, however, very few experiments had addressed the problem of initial state selection-virtually the entire emphasis had been on final state resolution and the measurement of branching ratios. The emphasis is now shifting toward increasingly precise definition of the detailed state-to-state dynamics: the interface between “high-resolution” photochemistry and “high-resolution” molecular spectroscopy is becoming more and more evident. A second development is the increasing number of experiments involving the anisotropy of molecular photodissociation and the measurement of vector as well as scalar attributes of the dissociation dynamics. Most of the experiments described will fall into one or other of these categories. A . Quantum State Selection: The Ground Electronic State. When the molecular electronic photodissociation spectrum is a continuum, quantum state selection can be approached through supersonic nozzle beam expansion techniques20-22(a technique that is appropriate when the absorption “continuumn arises through rotational and vibrational state congestion) or infrared laser (15) Houston, P. L.; Moore, C. B. J . Chem. Phys. 1976, 65, 757. (16) Ho, P.; Smith, A. V. Chem. Phys. Lett. 1982, 90, 407. (17) Ashfold, M. N. R.; Bayley, J. M.; Dixon, R. N. Faraday Dissuss. Chem. Soc., in press. Chem. Phys., in press. (18) Heller, E. J. J . Chem. Phys. 1975, 62, 1544. (19) Imre, D. G . ;Kinsey, J. L.; Field, R. W.; Katayoma, D H. J . Phys. Chem. 1982,86, 1564. ( 2 0 ) Heaven, M. C.; Miller, T. A.; Bondebey, V . E. Chem. Phys. Left. 1981, 84, 1 . (21) DeKoven, B. M.; Baronavski, A. P. Chem. Phys. Lett. 1982,86,392. (22) Morrison, R. J. S.; Grant, E. R. J . Chem. Phys. 1982, 77, 5994.
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1289
Feature Article pumpingZ3(for vibronic states). Without independent experimental beam diagnostics, rotational state selection, or narrowing, must be a qualitative art though diagnosis may be possible if there happen to be rotationally structured transitions which are accessible in other regions of the parent molecular absorption spectrum. For example, Morrison and Grant22 were able to estimate the rotational temperature of their jet-cooled beam of NOz (used in a study of the far-UV dissociation dynamics) by comparing its laser-induced fluorescence (LIF) spectrum with that reported by Smalley and c o - ~ o r k e r s . Low ~ ~ rotational excitation in the N O fragments was attributed to dissociation via a linear upper state accessed through a two-photon transition. DeKoven and BaronavskiZ1found that cooling of H N 3 from 300 K to a calculated temperature -3 K reduced the average rotational energy of the primary NH(a'A) fragments generated by photolysis at 248 nm from 770 to 470 cm-I. Similar behavior was reported by Hawkins and HoustonZ5in their study of the energy disposal in SH(A2113/2,1/2) fragments, following excitation of H2Sinto the A'B, continuum at 193 nm. Unfortunately, using the same technique of laser-induced fluorescence Heaven et aL20 reported average rotational energies comparable to or greater than those measured by Hawkins and Houston under room temperature conditions (though their LIF spectra look very similar). Both groups agree on the spin-orbit state population ratio and an absence of significant vibrational excitation but this latter conclusion is in conflict with the photofragment spectroscopy experiments of van Veen et a1.;26 these display clearly resolved time-of-flight structures corresponding to >lo% of the fragments in vibrational levels u = 1-5. The disagreement probably reflects the insensitivity of the LIF technique when it operates on the predissociated A X transitions in SH but the differences in the reported rotational energy distributions are more difficult to explain. They may reflect uncertainties in the "collision-free timing" of the probe laser pulse in the photodissociated molecular beam. Similar uncertainties might explain differences in the energy disposal reported in CN(X28+)following photodissociation of BrCN at 193 nm3.27The present moral is that caution should be exercized before rotational cooling via nozzle beam expansion can be used confidently for definitive rather than qualitative rotationally state-selected or state-narrowed photodissociation experiments. B . Quantum State Selection: The Excited Electronic State. A much better approach to rotational state selection is available when the photoexcited molecule is sufficiently long lived to sustain resolvable rotational structure in its absorption spectrum. Rotationally resolved features present in Rydberg states of H20,17,28 H2S,29and NH3,3s32 for example, have been analyzed both by vacuum UV absorption and resonance-enhanced multiphoton ionization spectroscopy, REMPI. Competition between predissociation in the resonant intermediate state and further uppumping into the ionization continuum influences the relative intensities of rotational features, while their line widths provide a measure of intermediate state lifetimes.32 Ashfold, Dixon, and Bayley, for example, have used the REMPI technique to record the three-photon r e s o n p spectrum associated with the rotationally X'A, transition in H 2 0 (and D20)'' (see structured C'B, Figure 1). The frequencies of the rotational features can be
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(23) Kligler, K. J.; Pummer, H.; Bischel, W. K.; Rhodes, C. K. J . Chem. Phys. 1978, 69, 4652. (24) Smalley, R. E.; Wharton, C.; Levy, D. J. Chem. Phys. 1975,63,4977. (25) Hawkins, W. G.; Houston, P. L. J. Chem. Phys. 1980,73,293. 1982, 76 779
, -7
(26) van Veen, G. N. A.; Mohamed, K. A.; Baller, T.; deVries, A. E. Chem. Phys. 1983, 74, 261. (27) Halpern, J. B.; Jackson, W. M. J. Phys. Chem. 1982, 86, 3528. (28) Ashfold, M. N. R.; Bayley, . . J. M.; Dixon, R. N. Chem. Phys. Lett., in press. (29) Ashfold, M. N. R.; Dixon, R. N. Chem. Phys. Lett. 1982, 93, 5. (30) Nieman, G. C.;Colson, S. D . J. Chem. Phys. 1978,68, 5656. 1979, 71, 571. (31) Glownia, J. H.; Riley, S. J.; Colson, S. D.; Nieman, G. C. J . Chem. Phys. 1980, 72, 5998. 1980, 73, 4296. (32) Ashfold, M . N. R.; Dixon, R. N.; Stickland, R. J., personal communication.
374
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I
80500
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jbs
b s e r WavelengthI nm
81000
81000
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Three Photon Energylcm.'
Figure 1. (a) 3 + 1 MPI spectrum of the e l B l g l A l transition in H20.(b) Simulated spectrum which neglects any competitive predissociation channel. (c) Modified simulation which incorporates line width and intensity corrections dependent on Kan-the component of rotational angular moment about the a-inertial axis.17
reproduced through spectral simulation using the known rotational constants but the relative intensities and line widths can only be generated accurately when the K,-dependent predissociation channel is also accommodated within the simulation procedure. Rotation about the a axis would be consistent _with a predissociation pathway via radiationless transfer to the B'A, continuum. Rotationally resolved systems of this kind provide an ideal basis for experiments designed to isolate the influence of the total angular momentum in the system on the distributions of internal and orbital angular momentum among the products, the disposal of rotational energy, and the propensity for rotational alignment in the primary photofragments. Such experiments will benefit from current progress in the generation of tunable, narrow-line, vacuum ultraviolet radiation using excimer and/or dye laser nonlinear mixing schemes. Alternatively, two-photon excitation methods can be employed,33for example in H 2 0 , using tunable narrow-band injection locked excimer lasers; with this kind of approach there is the additional choice of accessing electronic states forbidden by the single-photon selection rules. A fascinating illustration of this possibility is displayed in the multiphoton ionization of methyl iodide promoted either by single UV photon or two visible photon resonance enhanced laser excitation pathways.34 When the resonant state(s) have predissociative lifetime(s) shorter than the inverse rate of further uppumping, then all that is seen in the MPI spectrum are the fragments of the "stepping stone", CH, and I. If the stepping stone remains whole then the parent molecular REMPI spectrum is recorded. The stepping stones in question are the A,, A2, and 3 X E spin-orbit valence states generated at wavelengths
