Spectroscopic probe of intramolecular predissociation dynamics in

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J. Phys. Chem. 1989, 93, 513-520

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FEATURE ARTICLE Spectroscopic Probe of Intramolecular Predissociation Dynamics in Clusters V. Vaida,* D. J. Donaldson? S. P. Sapers, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215

R. Naaman,t Department of Isotopes Research, Weizmann Institute of Science, Rehovot. Israel

and M. S . Child Department of Theoretical Chemistry, Oxford University, Oxford, U.K. OX1 3TG (Received: July 25, 1988)

This article presents a review of our recent work on intramolecular dissociation of molecules embedded in clusters. The experimental approach combines absorption spectroscopy and multiphoton ionization (MPI) studies of predissdating electronic states of molecules cooled in a supersonic expansion. The spectra of the isolated molecules are compared to those obtained when the same chromophore is bound in a cluster. The results show that predissociative states are especially sensitive to intermolecular interactions. In the specific systems discussed, methyl iodide, acetone, and acetaldehyde, the photodissociation dynamics are derived from changes in spectral line intensities, line widths, and MPI signals. A new model for cluster-induced potential energy shifts for the solvent effect on photochemical reactions has emerged from this work.

1. Introduction

Recent workl,2 in our laboratory has established a new effect that cluster formation has on molecular predissociation. In this effect, intermolecular forces stabilize the potential surfaces involved in predissociation, leading to changes in the dissociation dynamics. We have found that cluster-induced changes in the electronic spectra of predissociating molecules yield insight into which vibronic modes are coupled to the dissociation channel and also where the predissociative surface crossing occurs. In addition, a knowledge of the effects that cluster formation has on the excited-state photophysics suggests new chemistry, which takes place in the clustered species but not in the monomer. In this way, insight into condensed-phase chemistry may be gained. Spectroscopic studies of predissociating molecules carried out in the controlled environment of a molecular jet have recently proved very fruitful, providing information both on the structure and on the dynamics that take place on the relevant potential energy surface^.^,^ Molecules that predissociate are ideal probes for this type of spectroscopic study because, in spite of their rapid chemistry, they generally live for a sufficient number of vibrational periods to give rise to a structured electronic spectrum. In addition, the structured nature of the spectrum can provide a convenient diagnostic for the clustering conditions in the jet. Dimer formation will generally perturb the spectrum in a straightforward and quantifiable manner. This makes the spectral features that are due to dimer formation unambiguous. The information so gained can be interpreted to yield quantitative details of the dissociation dynamics, in both the monomer and the dimer. Studies of the dissociation of clustered molecules have become an area of intense activity in recent Much of this work has dealt with the dissociation of molecules that are “solvated” inside a cluster. This process is similar to dissociation in a solvent, where it is known that the solvent can “hold” molecular fragments together, even if the system contains energy in excess of that needed for the dissociation of the isolated molecule. Two mechanisms have been proposed to explain this effect. In the first Present address: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1Al. tJILA Visiting Fellow, 1986-1987.

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the solvent shell acts as a physical “cage”, preventing dissociation. Nascent fragments collide with the solvent shell before they can completely separate. Due to momentum and energy transfer, their relative direction of motion is reversed; their motion is slow enough to allow rec~mbination.~*’ In the other mechanism, the initially excited molecule transfers some of its internal energy to the solvent before it can fragment, leaving it with energy below that needed for dissociation. The former mechanism has been observed only in the case of large clusters, where the excited molecule is surrounded by a solvent hell.^-^ In this case there are enough “solvent” species to form a physical “cage”. In contrast, the second process has been observed even for the case of a single atom attached to the absorbing molecule.” Recently, a third mechanism has been proposed in which, for small clusters, an attractive potential is created between the nascent fragments by the “solvent” molecules. This mechanism is believed to be operative in clusters of Br2(CO):b and Br2ArnCat cluster sizes smaller than a complete solvent shell. In this article we summarize recent work, in which absorption spectroscopy in molecular jets is combined with multiphoton (1) (a) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Chem. Phys. 1987,87, 2522. (b) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Phys. Chem. 1988, 92, 1204. (2) (a) Gaines, G. A,; Donaldson, D. J.; Strickler, S. J.; Vaida, V. J. Phys. Chem. 1988, 92, 2762. (b) Donaldson, D. J.; Gaines, G. A,; Vaida, V. Ibid. 1988,92,2766. (c) Donaldson, D. J.; Richard, E. C.; Strickler, S. J.; Vaida, V. Ibid., in press. ( 3 ) Vaida, V. Acc. Chem. Res. 1986, 19, 1143. (4) Vaida, V. NATO ASI Ser., Ser. C 1987, No. 200, 253. (5) (a) Herschbach, D. R. NRC Report No. 16, 1988. (b) Naaman, R. Adu. Chem. Phys. 1988, 70, 181. (6) Saenger, K. L.; McClelland, G. M.; Herschbach, D. R. J . Phys. Chem. 1981.85, 3333. (7) Valentini, J. J.; Cross, J. B. J . Chem. Phys. 1982, 77, 572. (8) Amar, F. G.; Berne, B. J. J . Phys. Chem. 1984, 88, 6720. (9) (a) Alexander, M. L.; Johnson, M. A,; Lineberger, W. C. J . Chem. Phys. 1985, 82, 5288. (b) Alexander, M. L.; Levinger, N. E.; Johnson, M. A,; Ray, D.; Lineberger, W. C. Ibid., in press. (c) Amar, F. G. In The Chemistry and Physics of Small Clusters; Jena, P., Khanna, S., Rao, B., Eds.; , Plenum Press: New York, 1987. (10) (a) Celii, F. G.; Janda, K. C. Chem. Reu. 1986,86, 507. (b) Janda, K. C. Adu. Chem. Phys. 1985, 60, 201. (11) (a) Levy, D. H. Adu. Chem. Phys. 1981, 47, 323. (b) Levy, D. H. Annu. Rev. Phys. Chem. 1980, 31, 197.

0 1989 American Chemical Society

514 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

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Figure 1. Schematic representation of the crossing of a zero-order bound

potential energy surface and a repulsive zero-order dissociative surface. The solid lines represent unperturbed levels, while the broken lines represent the levels perturbed by the CIPS effect. In this case, the perturbed bound state is stabilized more than the unbound state; in principle, this effect could also be reversed. ionization (MPI) methods to provide detailed information on the predissociation of molecules embedded in van der Waals clusters. In particular, we will discuss the way in which cluster formation can yield information concerning the dissociation dynamics.

2. The Cluster-Induced Potential Shifts Model The new effect, named “cluster-induced potential shifts” (CIPS), arises from the differing degrees of stabilization experienced by different potential surfaces under the influence of external perturbation. Consider a molecule, AB, in which a (zero-order) Born-Oppenheimer bound electronic state is intersected by a (zero-order) dissociative state (see Figure 1). The absorption and emission spectra of the bound state will show evidence of the predissociative surface crossing in a well-documented fashion.12 Of specific interest to this discussion is the observation of line broadening of states lying at energies at or above the crossing region. If there is an attractive portion in the intermolecular potentials associated with AB, at some low enough temperature the dimer, (AB),, will be energetically more stable than two separated monomers, If this is also true for the molecules excited to the bound state (for instance, in the case of Rydberg states), then excitation to that state will not cause a large decrease in the attractive part of the intermolecular potential. However, excitation to the dissociative state often does involve a large change in electron density, due to the transfer of an electron from a nonbonding to an antibonding orbital. Therefore, the attractive part of the potential associated with the dimer in this excited state is expected to be substantially different from that in the bound states. Consequently, the stabilization of the dimer will be different in the bound vs dissociative states, and therefore, the position of the surface crossing between these states will shift, as demonstrated by Figure 1. Here, the shift in the curve crossing has the effect of stabilizing more bound levels a t energies below the crossing point. The spectroscopic consequences of this shift are the appearance (or disappearance) of rotational or vibrational states. This effect on the spectrum is the key to the following discussion. The model predicts that absorptions to vibrational levels will appear (or disappear) in the bound state as a consequence of cluster formation. It follows that the modes which display the most change between the clustered and unclustered spectrum will be those which lie closest to the surface crossing. These are the modes (12) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand Reinhold: New York, 1950; Vol. I.

Vaida et al. which are most strongly coupled to the dissociation channel in the unperturbed molecule. In the case where both the ground state and the bound excited state are more stabilized by cluster formation than the dissociative state, the energy of the predissociative surface crossing will increase. The increase in the surface crossing energy will, in turn, give rise to a decrease in the rate of dissociation from levels lying near or below the crossing region. This alteration of the dissociation rate by cluster formation represents a new mechanism for solvent-induced perturbations in molecular photodissociation. This mechanism will be important in molecules where predissociation involves states that have different gas-to-solvent shifts. Examples involve crossings of Rydberg and valence states, or the overlap of n r * and aa* transitions. These effects are known to be important in solutions, where the line width in many cases is affected by the solvent due to the different solvent-induced shifts of the excited states.I3 However, as will be discussed below, even a single molecule can serve as a “solvent” in this case. In the case of the CIPS effect discussed here, unlike the other solvent/cluster effects observed, it is not necessary for the dissociative molecule to be surrounded by a physical “cage” in order to prevent fast dissociation, and energy is not transferred to the surrounding atoms or molecules. The spectroscopic perturbation observed can occur in very small complexes, even dimers. It arises from the different interactions each electronic configuration of a probe molecule has with any attached molecule or atom, and it is especially pronounced in predissociation processes in which the energy at which levels cross plays an important role in the dissociation rate. One chemical consequence of the change in the dissociation rate in complexes may be the opening of new reaction channels. For example, in the complex (AB),, upon dissociation of the AB moiety, the products Az or B, may be formed. This is observed and CSZ.l7 This will be in clusters of methyl iodide,14 OCS,15316 particularly favorable if the energy of either the A-A or the B-B bond is larger than that of the A-B bond, as is the case in OCS clusters, where upon dissociation of the monomer species, the S2 molecule is observed.lsJ6 The opening of such new chemical channels may cause a significant change in the spectrum of the “solvated” molecule, which will be characterized by energy shifts and line width changes. The following discussion will deal mainly with methyl iodide as a model system. We have used this molecule as a probe because of the extensive literature existing on the dissociation of the m ~ n o m e r l * and - ~ ~ the relatively good theoretical understanding of the p r o c e ~ s . ~ & ~ ~ (1 3) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, 1978. (14) Sapers, S.P.; Naaman, R.; Vaida, V. J . Chem. Phys. 1988,88,3638. (15) Sivakumar, N.; Burak, I.; Cheung, W.-Y.; Houston, P. L.; Hepburn, J. W. J . Phys. Chem. 1985,89, 3609. (16) van Veen, N. J. A,; Brewer, P.;Das, P.; Bersohn, R. J . Chem. Phys. 1983, 79,4295. (17) Tzeng, W.-B.; Yin, H.-M.; Leung, W.-Y.; Luo, J. Y.; Nourbakhsh, S.: Flesh. G. D.: NK. C. Y. J . Chem. Phvs. 1988. 88. 1658. ‘(18) (a’)Riley, S.-J.; Wilson, K. R. D i s k s . Furuduy SOC.1972, 53, 132. (b) Dzvonik, M. J.; Yang, S. C . Rev. Sci. Instrum. 1974, 45, 750. (19) Baughcum, S. L.; Leone, S. R. J. Chem. Phys. 1980, 72,6531. (20) Sparks, R. K.; Schobatake, K.; Carlson, L. R.; Lee, Y. T.J . Chem. Phys. 1981, 7S, 3838. (21) Pence, W. H.: Baughcum, S. L.; Leone, S. R. J. Phys. Chem. 1981, 85, 3844. (22) Jiang, Y.; Giorgi-Amazzi, M. R.; Bernstein, R. B. Chem. Phys. 1986, 106. 171. (23) Hermann, H. W.; Leone, S . R. J. Chem. Phys. 1982,764759,4766, (24) van Veen, G. N. A.; Baller, T.; de Vries, A. Z.; van Veen, N. J. A. Chem. Phys. 1984, 87, 405. (25) Barry, M. D.; Gorry, P. A. Mol. Phys. 1984, 52, 461. (26) (a) Brewer, P.; Das, P.; Ondrey, G.; Bersohn, R. J. Chem. Phys. 1983, 79, 720. (b) Hess, W. P.; Kohler, S. J.; Haugen, H. K.; Leone, S . R. Ibid. 1986,84, 2143. (27) Khundkar, L. R.; Zewail, A. H. Chem. Phys. Lett. 1987, 142, 426. (28) Hess, W. P.; Naaman, R.; Leone, S . R. J . Phys. Chem. 1987, 91, 6085. (29) Lao, K.; Person, M. D.; Chou, T.; Butler, L. J. J . Chem. Phys. 1988, 89, 3463.

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3. Experimental Methods Our studies have relied on two experimental tahniques4irect absorption and multiphoton ionization (MPI) spectroscopies, both performed in a molecular beam. These methods have both been described at length in the original works.l.’J4 In the absorption experiments,’,’ both a I-mm commercial piezoelectric pulsed nozzle and a I-mm modified fuel injector valve were used, operated at 50 Hz. The output of a high-brightness deuterium lamp was dispersed by a I-m monochromator equipped with a holographic grating optimized for I500 A. The dispersed radiation intersected the molecular jet 0.5-1 cm downstream from the nozzle. Transmitted light was detected by a photomultiplier tube; the signal was amplified by a lock-in amplifier operating at the pulsed nozzle frequency and sent to a computer for processing. The spectra were normalized via division by the unattenuated signal; the result is an absorbance spectrum. The MPI experiments14were carried out using both the piezoelectric pulsed nozzle with a 0.5-mm orifice and a solenoid valve with a 0.8-mm orifice. The molecular beam was skimmed 1 cm from the nozzle; the skimmed beam was intersected by the probe laser 5.5 cm from the nozzle. Excitation was achieved with either excimer transitions at 248 or 308 nm or the tuned output of an excimer-pumped dye laser operating between 361 and 367 nm. The laser was focused with a 25-cm lens. Ions created in the laser field were deflected into the 40-cm flight tube of a time-of-flight mass spectrometer and detected by a channeltron electron multiplier. The transient signal was digitized and sent to a PC-type computer for signal averaging and processing. Both mass scans and wavelength scans are possible in this apparatus and have heen used in the work discussed here. 4. Effect of Dissociation on Spectral Line Shape Throughout this work, we use spectral line shapes as the major indicator of dissociation dynamics; therefore, a brief summary is presented of the theory that describes the dependence of spectral line widths on dissociation dynamics. In treatments of direct dissociation, one assumes that the wave function of the excited eigenstate spreads to infinity, resulting in a “short lifetime” which is observed as a broad absorption feature. However, in treatment of predissociative systems, the excited state is viewed as a bound state (in the zero-order Born-Oppenheimer approximation) that interacts with a dissociative continuum. In this case the lifetime of the excited state is longer, and so relatively sharp resonances can be observed in the spectrum. It is important to realize that there are but few cases, characterized as the “statistical limit”,’6 in which the predissociative state is coupled to a real continuum. In many systems the initial excited state ($,) interacts strongly with another vibronic manifold ($J which carries no oscillator strength but acts as an intermediate in the dissociation process; this manifold is mixed with the true continuum ([$& In this scheme, usually defined as the “intermediate case” in radiationless transition theory,lMOa set of eigenstates (qj)is obtained, each of which carries oscillator strength. These eigenstates can he represented as ‘kj = a,& + Z,bj,$,+ &?jk$k. Since we assume that only the $, state carries oscillator strength, the intensity in the absorption spectrum will he determined by the coefficients a,. The larger these coefficients are, the longer (30) Shapiro. M.; Bersohn. R. 3. Chem. Phys. 1980, 73,3810. (31) Lee. S.-Y.; Heller, E.J. 3. Chem. Phys. 1982. 76, 3035. (32) Gray,S. K.;Child, M. S . Mol. Phys 1984, 51, 189. (33) Shapiro, M. 3. Phyr. Chem. 1986, 90,3644. Kinsey, 1. L.;Coalson. R. (34) Sundbcrg, R. L.;Imre, D.; Hale, M. 0.; D. 3. Phys. Chem. 1986, 90, 5W1. (35) Donaldson, D. J.; Child, M.S.;Vaida, V. 3. Chem. Phyz. 1988.88, ,AI”

(36) Jortner. J.; Yukamcl. S . In The World o/ Quantum Chemtstry: Daudcl. R.. Pullman. R.. Edr.: Rcidcl: Baton. 1979 (37)Jonncr. J. I n Rodiorionlerr P m m r r : DiRartolo. R . Ed.:Plenum Res% New York. 1980. (38) Rice, S.A.; Gelbart. W.M.Pure Appl. Chcm. 1971,27,361 (39) Freed, K. F. Top. Cur,. Chcm. 1972, 31, 105. (40) Robinson. G. W.; Langhoff, C. A. Chem. Phys. 1974.5. 1.

The Journal of Physical Chemistry, Vol. 93, No. 2. 1989 515 /*,I

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Figure 2. Schematic representation of the intermediate state in a REMPI experiment. The solid line outlining the remnant state represents the line shape ~ b s e ~ in e da direct absorption experiment. The broken line is the

REMPI line shape.

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is the lifetime of the eigenstate and the narrower is its line width. If the excitation line width is broader than the energy gap between two ‘k, states, a nonexponential lifetime is measured for the excited state, and the line width in both absorption and emission is non-Lorentzian; in other words, the line width will not be homogeneous. On the other hand, the statistical limit gives rise to homogeneous line widths, which reflect the lifetime via the uncertainty principle. In an MPI experiment the issue of line width becomes more complicated. For simplicity, a resonance-enhanced two-photon process will be discussed, in which the first photon excites the molecule to a predissociative state, while the second one ionizes it. In this case the ionization step competes kinetically with the dissociation p r m s , and only molecules that do not dissociate can be ionized. In the statistical limit the signal intensity depends strongly on the dissociation time (assuming constant laser intensity), hut the line width always reflects the lifetime of the intermediate state due to the uncertainty principle, as it does in absorption experiments. However, the situation is quite different in the intermediate case where the line shape is non-Lorentzian. In this instance each state inside the excitation envelope has a different lifetime; therefore, long-lived states will show a strong MPI signal, whereas those with short lifetimes cannot be ionized efficiently. Consequently, it is possible that a line will appear narrower in the MPI spectrum than it does in the absorption spectrum. This concept is illustrated in Figure 2. Here the states closest to the true “line center” of the zero-order state, $8, are longer lived (have a larger value of a$*) than those in the wings of the absorption envelope. Therefore, the ionization step competes more favorably with dissociation in the states closest to the “line center”, and a signal is observed from these states only. Figure 2 implies that there are two variables which control the ion signal (and hence the line shape) in MPI: k., and q1. By varying either one of these parameters, the MPI line shape will change in a predictible manner. For instance, when a short-lived state is stabilized vi4 cluster formation, the magnitude and hence the relative importance of k , are decreased in the rate equations; ionization will become more efficient, and thus the line width will increase for the reasons given above. Alternatively, by increasing the laser intensity, I, the relative importance of the ionization step will increase, with the same consequence to the line shape. In Figure 3, the REMPI spectrum of methyl iodide is presented, in which two vibronic bands are excited in the second observed

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516 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

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ONE-PHOTON WAVELENGTH ( n m ) A

this excitation is that charge is moved from the iodine atom into the chemical bond, thereby reducing the dipole moment of the electronically excited molecule vs that of the molecule in its ground state. Hence, it is expected that if the intermolecular interactions between the members of a methyl iodide complex are governed by dispersive forces (dipole, polarizability, etc.), the transition between the ground and the valence state of methyl iodide will be shifted to higher energies. This is the outcome of the ground state being more stabilized than the excited state, as described above. This phenomenon is indeed observed for clusters of (CH3I),.I Under conditions in which the molecular beam contains mainly monomers of CH31, the maximum of the broad A-state absorption lies at 2610 A. When the methyl iodide stagnation pressure is increased, so that substantial clustering takes place, the maximum of the absorption is shifted to 2540 A, corresponding to a shift to higher energy of 1100 cm-'. The second type of excited electronic state in methyl iodide is of Rydberg character. The first two Rydberg states arise from the excitation of a nonbonding iodine electron to a 6s Rydberg . orbital centered on the I atom: ( u ) * ( ~ ) ~ ( n & ) ( U ) ~ ( T ) ~These two states, the B and C states, correlate to different spin-orbit states in the methyl iodide ion and are thus separated by approximately the spin-orbit splitting of that ion. In this case, the charge distribution remains basically the same as in the ground state, and the dipole moment of the molecule excited to these Rydberg states is expected to be similar to that in the ground state. As predicted by this simple description, it was found that the excitation energy to the Rydberg states in methyl iodide dimers is almost identical with that of the monomer,' except for a small (10 cm-l) red shift of the origin band. This phenomenon of differences in the "solvent shifts" of different potential energy surfaces as a result of cluster formation is of major importance when processes involving level crossings are studied. It finds a direct analogy in solution-phase spectroscopy, where it is known that, for instance, Rydberg and valence states behave differently upon solvation." In general, one expects that if two intersecting surfaces arise from two different electronic configurations, the position of their surface crossing will change upon complex formation as a result of the different solvent shifts experienced by the two states. In the present case, the 'solvent" is actually a single molecule!

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ONE-PHOTON WAVELENGTH ( n m ) Figure 3. REMPI spectra of the Rydberg C state of methyl iodide taken under different conditions: (a) high (1.33 mJ/pulse) and low (0.680 mJ/pulse) laser flux; (b) high (130 Torr) and low (1 Torr) stagnation pressure of neat methyl iodide. The band at 366 nm is the origin, and the band at 363 nm is the C-I stretch. The spectra are normalized to the origin.

Rydberg state (the C state). This electronic state is predissociated by an underlying manifold of dissociative The lowenergy band in the figure corresponds to the origin 0; transition; the other is the C-I stretch band, 3:, which dissociates faster than the origin.' The figure demonstrates that as the laser intensity is increased, the 3; line becomes stronger relative to the origin. The line width of this transition increases from 17 cm-' at 680 fiJ/pulse to 37 cm-' at 1330 J/pulse?' This is due to the higher probability that the ionization process can compete with the dissociation at higher laser fluxes. It is important to realize that although higher laser intensity will always enhance the ionization channel with respect to dissociation, if no saturation occurs, the line width will change only in the case of inhomogeneous broadening. Another observable effect of the dissociation lifetime on the line width in a MPI experiment may result from the saturation of the transition to the excited state. Vibrational states that are long-lived are easier to saturate than those that dissociate faster. Hence, the vibrational lines corresponding to dissociative states will become relatiuely stronger with increasing laser intensity. This will occur in addition to the line width increase outlined above. 5. Energy Level Shifting Due to Cluster Formation As two molecules or atoms approach one another, their eigenstates will be perturbed through their intermolecular interaction. This perturbation causes a shifting of the absorption bands in molecules embedded in a cluster. The amount by which the bands shift depends upon the nature of the interaction between the specific electronic configurations involved. For example, the two states involved in predissociation in methyl iodide are the repulsive valence A state and the higher energy Rydberg states. The dissociative valence state arises from the promotion of a nonbonding 5 p electron ~ centered on the iodine atom to a C-I . net result of antibonding orbital: ( U ) ~ ( T ) ~ ( U * ) ( U ) ~ ( T ) ~The

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(41) (1) The log-log plots of laser intensity vs signal intensity indicated that saturation was not occurring at the laser intensities used. (2) Power broadening is an issue relevant to our line shape interpretation. The fact that a line width change is not observed in the origin, a transition with larger oscillator strength than the C-I stretch, and not observed in the 130-Torr stagnation pressure runs for either bands indicates that it is not making a major contribution to the line widths in our data.

6. Effect of Dimer Formation on Spectral Line Intensities Dimer formation can perturb the dissociation by enhancing dissociation rates, for example, by opening low-energy channels not available to the monomer, or can hinder dissociation by increasing the energy of the intramolecular level crossing. Changing the energy of the surface crossing upon dimer formation may change the net coupling between the initially excited state and the dissociative surface and thereby cause a change in the dissociation rate. In the systems studied here, the common effect of dimer formation has been to increase the energy of the level crossing, thereby slowing the dissociation rate. Correlating these changes in the dissociation rate with changes in the line strength is simple only in the statistical limit. In this limit the homogeneous line width narrows upon dimer formation, due to the increase in lifetime. Since the integrated intensity of the band must remain constant, the intensity a t the band center will increase. A clear example of this effect is given by the case of acetone. In this molecule, the second excited singlet state, Sl, is the first member of 2. Rydberg series (no,3s). It is predissociated by an underlying level, probably the mixed singlet-triplet (h-r*)state, (SI,T1].2The absorption spectrum of this state shows cluster-induced differences in the line widths and in the relative peak intensities of several vibrational modes. The modes of interest to the present argument are the symmetry-allowed CCO in-plane bend, v8, and the out-of-plane skeletal deformation mode, vI6. (42) (a) Ito, M.; Inuzuka, K.; Imanishi, S . J. Am. Chem. Soc. 1960.82, 1317. (b) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1975; Vol. 11.

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Figure 4. Absorption spectra of the S2 (n0,3s)Rydberg state of acetone.

(a) Origin region of a jet containing mainly monomers. The main peak is the origin, with a torsional progression on the shoulder. (b) Same region in a jet that is heavily contaminated by clustering. The CCO

in-plane bend and the out-of-plane skeletal deformation are clearly present. In the limit at which the sample consists of isolated molecules with little cluster contamination (Figure 4a), these bands appear as a single broad feature. We believe that these modes act as promoting modes for the surface crossing to the dissociative state in the isolated molecule.2 Their homogeneous line widths are thus very broad, and the peaks cannot be resolved above the noise. Figure 4b displays the spectrum of the more heavily clustered species. The individual bands are clearly resolved in this case; the difference in the line widths between spectra a and b of Figure 4 implies an increase in lifetime of a factor of 5 in these modes. The relation between dissociation dynamics and line width is more elaborate for the “intermediate case”. Since in this case the line shape is non-Lorentzian, the part of it which relates to the short-lived component in the band can be diffuse enough to become part of the “background” in the absorption spectrum. As complex formation lengthens the lifetime of this component, the line width of the diffuse component narrows and contributes to the intensity of the peak. In this situation it may seem as though the integrated intensity in the particular band increases. For methyl iodide, the “intermediate case” model was found to be appropriate for describing the dependence of the MPI spectra on laser flux, as detailed above. In the absorption experiments this description must also be applied in order to explain the variation in line intensities upon dimer formation. Figure 5 shows the absorption spectra of the B Rydberg state of methyl iodide in the cases where there are predominantly monomers (upper trace) and where there are both monomers and dimers present (lower trace). Similar results are obtained for the C state and for both states in deuterated methyl iodide.lb In all instances, there are striking differences between the spectra due to monomers alone and those in which there is a dimer component. These differences may be summarized as (a) the appearance of vibronic structure in the dimer-containing spectra which is absent in the monomer spectra and (b) an increase in the relative peak intensities (compared to the origin) in the dimer spectra of the methyl umbrella vibrational mode; this mode also appears in the monomer. The “new” structure is assigned to the H3C-I stretching and “rocking” modes of the monomer species. These modes are the ones which are most strongly coupled to the dissociation channel.

+

Both of these dimer-induced effects are expected on the basis of our model, assuming “intermediate case” dynamics. The relative shifts of the absorption spectra of the valence and Rydberg states, presented in the previous section, indicate that the crossing between these states shifts to different (higher) energies in the clustered species. Since this crossing is responsible for predissociating the Rydberg states, the relative shift increases the effective lifetime of the vibrational modes most strongly coupled to the dissociation. The longer lifetime gives rise to an increase in peak intensity of the H3C-I stetching and bending modes, which dissociate very rapidly in the monomer. The change in relative peak intensities in the umbrella mode is affected both by a dynamical effect, also seen in v3, the C-I stretch, and v6, the methyl “rock”, and by structural perturbations of CH31 in the dimer, giving rise to different Franck-Condon factors. The structural contribution may be extracted in the manner described below; we thus obtain the contribution due to the change in surface crossing position. We use expressions for harmonic oscillator Franck-Condon overlap integrals appropriate for the case where the upper state frequency and equilibrium bond length differ from those of the ground state.43 The harmonic nature of the measured v2 frequencies suggests that this approach is appropriate in the present instance. Using these expressions and our observed intensity ratio for u’ = l/u’ = 0 in v2 of CH31, we calculate the difference in equilibrium bond length for this mode to be Ir’- r’l = 0.096 A, where r’and r’’are the equilibrium bond lengths in the excited and ground states, respectively. With this value we predict the intensity envelopes in CH31 and CD31; the results are shown in Figure 6a,b. The fit is seen to be very good for the transitions 0-0, 0-1, and 0-2 of both isotopes. However, in each isotope, there is intensity predicted for the transition to u‘ = 3 which is not observed experimentally. In addition, the CD31“monomern spectrum, whose intensities are shown in Figure 6b, contains an unknown but obvious contribution from the dimer, (CD31)2.7b Thus, the observed relative intensities are really upper limits to the “true” intensities in the monomer of CD31. The same procedure was attempted for the dimer spectra. The monomer contributions to the observed spectra were subtracted out, leaving a “dimer spectrum”. For CHJ, a reasonable fit to the spectrum is obtained using a bond length difference Ir’- r’l of 0.128 A. As for the monomer, this value reproduces the intensities in the transitions to lower vibrational levels correctly but predicts substantially more intensity in u’ = 3 than is observed, (43) (a) Hutchisson, E. Phys. Reu. 1930, 36, 410. (b) Brown, W. G.Z . Phys. 1933, 82, 7 6 8 .

518

The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

Vaida et al. ”

U M B R E LL A ” M 0 DE

I STRETCH” 0, ‘JMBQL-L:

‘V-!

EVE-

‘dW0REL.A”

(b) O2 dMBRELLA

V-LEVEL

UMBRELLA

MODE

‘+-LEVEL

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V-LEVEL

Figure 6. Predicted and calculated Franck-Condon intensity envelopes for the v2 mode (CHI umbrella) in methyl iodide for (a) CH31and (b)

CD31 monomers and (c) CH31 and (d) CD31 dimers. The solid bars represent experimental relative intensity data, while the hatch-marked and open bars are the calculated values. NotiCe the predicted intensity for u = 3, present in all calculations but not observed experimentally. as illustrated in Figure 6c. The CD31 spectrum predicted in this manner bears no resemblance to the observed spectrum (Figure 6d). Similar inadequacies exist in the spectra predicted by using lr’- r’l = 0.1 10 A, based upon the (u’ = 2 ) / ( u ’ = 0) intensity ratio in CD31. Therefore, we conclude that there is no FranckCondon envelope based on harmonic oscillator wave functions that will consistently predict the observed intensities in the umbrella mode of the dimer species. This would be disturbing, if the only contributions to the relative intensities in the spectra were due to structural differences. However, this result is not surprising, and is in fact expected, in the case where there are dynamical contributions to the relative spectral intensities. It is clear that the change in the line intensity distributions in the v2 progression cannot be explained by assuming a change in the Franck-Condon envelope alone. On the other hand, it is also clear from our observations that there is no significant change in the line width of these lines which can explain the changes in the peak intensities. The results can be understood by using the “intermediate case” model. We assume that each vibronic band exhibits at least two decay times-a long one corresponding to the line width of the observed peak and a short one which contributes a broad component to the band that appears as part of the absorption background. Upon dimer formation and the consequent reduction in the dissociation rate, this short-lived component becomes narrower and contributes to the peak intensity. In the MPI experiments, one expects that slowing the dissociation by dimer formation will enhance the MPI signal. This was discussed at length in the previous section. The effect is indeed observed, as displayed in Figure 3b. The figure shows REMPI spectra of CH31 in the region of the origin and the C-I stretch of the C state, taken under two different expansion conditions and low laser intensity (vide supra). As the stagnation pressure is increased, dimers are formed, and the intensity ratio between the 3; and the origin band increases substantially. Note that the intensity in the origin band also increases, but not by as much as that in 3;. As well as the total intensity increase, 3; displays an increased line width in the high stagnation pressure spectrum. This behavior is exactly that predicted in section 4 for an increase in lifetime, due to dimer formation. It serves as a perfect complement to the changes observed in the absorption spectra. 7. Calculation of the Surface Crossing Positions Using the structural information given by the Franck-Condon calculations and the observed vibrational frequencies, we can calculate empirical potential energy surfaces for two-dimensional

/ Figure 7. (a) Model empirical potential energy surface calculated for the Rydberg B state in methyl iodide using the linear triatomic approxima-

tion. The two coordinates Q1and Q2correspond to the H$-I stretch and the H3-C stretch, respectively. The “boxes” indicate the classical limits of motion associated with the vibrational levels of the two modes. The dashed line indicates the seam of intersection between this surface and a component of the A state. (b) Same as (a), but for CH31dimers. models of the B and C states of methyl iodide.34 In these models, methyl iodide is treated as a collinear triatomic, H3-C-I, with two degrees of freedom: the H3-C stretch and the H3C-I stretch. These are approximations to the methyl umbrella and C-I stretch normal modes of the real molecule. This model for methyl iodide was first applied to treat the direct dissociation dynamics,2e33but is also appropriate to the predissociation problem developed here. Figure 7a shows a contour plot of the model double-harmonic potential of the methyl iodide B state calculated in this way. The “boxes” superimposed on the contours indicate the classical limits of motion in the indicated u levels of the model vibrational modes. There have been several calculations of the dissociative valence potentials of methyl iodide. Most are based upon the same linear triatomic model as that described for the bound states above. A recent a b initio result44 shows the model potential of Gray and Child32 (GC) to have the correct form. The same a b initio calculation predicts another dissociative state of the same general shape but more repulsive in the C-I dimension. We have modeled this state by increasing the “steepness parameter” in the G C surface. The intersection seam between this modified GC potential and the empirical B state potential is displayed as the dashed line in Figure 7a. The dissociative surface intersects the bound state inside the classically bound regions of u = 1 of the C-I stretch mode and u = 2 of the umbrella mode. The model therefore predicts that these levels are predissociated more rapidly than lower lying levels. This is exactly what is inferred from the absorption data. The transition to one quantum in the C-I stretch mode appears in the (44) Tadjeddine, M.; Flament, J. P.; Teichteil, C . Chem. Phys. 1987, 118, 45.

Feature Article dimer-containing spectra of CH31and CD31; this is not observed in the monomer spectra. Similarly, u = 2 of the umbrella mode undergoes a major increase in peak intensity relative to the origin in the dimer spectra of the B and C states of CH31; this increase is not accounted for solely by a change in structure. The CIPS model infers from these observations that a predissociative surface crossing takes place in a region between u = 0 and u = 1 in the C-I stretch coordinate and in a region between u = 1 and u = 2 of the umbrella mode coordinate. Thus, the model potential surfaces correctly predict the experimental observations. If the modified G C surface is extended to higher energies, its crossing seam with the C state may be calculated. A much faster dissociation rate is predicted for the C state than for the B state, since the predicted crossing seam intersects the C-state surface at energies near the vibrationless level of that state.35 The dissociative surface now intersects the bound state in the origin region; no vibrational level is predicted to be classically bound in the model. This prediction is also consistent with experiment. The line widths observed for bands in the C state are uniformly larger, by a factor of 2 (40 vs 20 cm-I), than those of the corresponding bands in the B state, measured under identical experimental conditions. I Finally, Figure 7b shows the crossing seam predicted by this model for the methyl iodide B state in the dimer. Here, the seam lies at higher energies in both the Q,and Q, coordinates. This implies an increase in the lifetimes of the corresponding vibrational modes; consequently, absorption to higher levels in those modes may be observed, as outlined above. The combination bands involving one quantum in the C-I stretch and one and two quanta in the “umbrella” mode are both predicted to be predissociated, with the higher energy combination being the more strongly predissociated. These features of the calculation are also in agreement with experimental observations. In the dimer, absorptions are observed to the C-I stretch mode built upon zero, one, and two quanta in the umbrella mode. The pure C-I stretch displays a resolution-limited line width; the combination bands show line widths of 70 cm-l (one quantum in u2) and 100 cm-l (two quanta in u2). From experiment,I we know that the ug mode of methyl iodide is also affected by dimer formation; its role in the dynamics is not so clear. It might promote the actual surface crossing step, by destroying the C3, symmetry of the molecule and thereby removing any symmetry restrictions to the surface crossing. The rotational distributions of the methyl fragment of the dissociation might provide a clue as to the importance of Yg to the dynamics. It would be of great interest to extend the 2-D model surfaces to include Vg.

8. Rotational Effect in Dissociation A rotational effect in dissociation has been documented both for small and for large molecule^.^^*^ This effect may also be used as a measure of the changes in dissociation dynamics upon cluster formation. The coupling between different electronic surfaces can be strongly dependent on the angular momentum of the particular states involved. This effect is expected to be especially pronounced when a surface crossing mechanism is subject to symmetry restrictions that can be removed through, for example, Coriolis coupling. An interesting example of such a process is given in the case of methyl iodide. In Figure 8, MPI spectra of the origin and 3; region of the C state are shown, taken at different expansion conditions. As the expansion is made “colder” (more He is added to the expansion mixture), the integrated intensity of the 3; line increases. This indicates, as discussed above, that the dissociation rate from this state is reduced under these conditions. This effect was interpreted by us14 as a rotation-induced enhancement of the predissociation rate. As the methyl iodide molecule becomes colder (45) Novak, F. A.; Rice, S. A. J . Chem. Phys. 1979,71, 4680; 1980, 73, 858. (46) Henke, W. E.; Selzle, H. L.; Hays, T. R.; Schlag, E. W.; Lin, S . H. J . Chem. Phys. 1982, 76, 1335.

The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 519 t

k IJY

z W

L

362 94

364 14

365 34

366 54

ONE- P H O T O N WAVELENGTH ( n m )

Figure 8. REMPI spectra of the C state in methyl iodide for a mainly monomer neat jet expansion and a mainly monomer rotationally cooled (seeded with 650 Torr of He) jet expansion.

in the higher pressure expansion, it contains less rotational energy, and so the predissociation process becomes slower. That is, the population in higher rotational levels is compressed toward the “line center”, where the wave function contains more bound character. The effect may arise from the fact that methyl iodide behaves as a “pseudotriatomic” molecule (vide supra). Coriolis coupling will cause the molecule to bend when it is rotationally excited.47 Bending, in turn, causes a reduction in the symmetry which will increase the mixing of the initially excited Rydberg state with the repulsive valence states. The rotations may thus act in the same manner as u6, in coupling the bound and dissociative surfaces. The rotation-induced increase in the dissociation rate may be responsible for the fact that we do not observe an expected Jahn-Teller type splitting in the origin band of methyl iodide in the absorption experiment. This splitting, observed in roomtemperature spectra of CD31,48*49 is caused by the coupling of rotational motion with the electronic angular momentum.48 The splitting depends on J2,47 where J is the rotational quantum number. Since rotationally “hot” CH31molecules, containing high angular momentum, are expected to dissociate fast, their absorption spectrum will be broadened and will not rise above the background. Hence, only molecules in low-J states will be observed, and in this case the experimental resolution of 10 cm-’ will not be enough to resolve the splitting. 9. Covalent Bond Chemistry in Clusters One consequence of the perturbations to the dynamics induced by cluster formation is that new chemical channels may be open in clustered species that are not available to the monomer. For example, in the case of methyl iodide dimers, when the excitation occurs through the short-lived, purely dissociative valence state, the major signal carrier in the multiphoton ionization is I2+.I4We believe that this ion arises from the multiphoton ionization of 12. The formation of Iz is energetically allowed following absorption of one UV photon by the dimer: (CH31)2+ hu CH3 + CH3 I2

-

+

Another interesting possibility is that 1, is the product of a recombination of two I atoms, following the stepwise dissociation of each methyl iodide unit of the dimer. Another possible route to this product, the reaction in the dimer [I (or I*) CHJ] I2 + C H 3

+

-

has a heat of reaction of +20.4 kcal/mol (-2.2 kcal/mol for I*) and a high energy of activation,Mand so is not likely to occur prior to a second photon being absorbed. Yet, another possibility, I+ CH31 12+ CH3, though exoergic, requires a very efficient three-photon ionization of the I atom, in order that it compete with the one-photon dissociation of the remaining CH31. This

+

-

+

(47) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand Reinhold: New York, 1966; Vol. 111. (48) Walsh, A. D. J . Chem. Soc. 1953, 2321. (49) McGlynn, S. P.; Felps, W. S.; Scott, J. D.; Findley, G. L. J . Chem. Phys. 1980, 73,4925. (50) Kerr, J. A.; Moss, S . J., Eds. Handbook of Bimolecular and Termolecular Gas Reactions; CRC Press: Boca Raton, FL, 1981; Vol. 1.

J . Phys. Chem. 1989, 93, 520-525

520

is expected to be unlikely, and we therefore favor the iodine recombination process. This observation suggests that "new" chemistry may be initiated in a cluster. Further, recent examples of this type of process are the reactions of H atoms with C 0 2 5 1and OCS,52in which the reactant is the BrHSOCO, HSH-OCO, or BrH-OCS cluster. In addition, S2 is found to be a significant product of the photodissociation of CS2 and OCS c l u ~ t e r s . l ~This - ~ ~type of covalent chemistry in a cluster is qualitatively different from the simple cleavage of the (weak) van der Waals bond, normally associated with "cluster photochemistry". It is expected to be critically dependent on the relative time scales for covalent bond cleavage vs energy redistribution into the van der Waals bond. This exciting new direction promises to be of great significance to our understanding of condensed-phase processes in general.

occurs may be very different when the dissociating molecule is "solvated" in a cluster. The change in the surface crasing region will necessarily result in a different rate of predissociation for the clustered vs isolated molecule. These different dynamics can be observed spectroscopically, through changes in the vibronic structure of the predissociating state. We have observed this change in dynamics in the low-lying Rydberg states of methyl iodide,'J4 acetone,2 and acetaldehyde.2 In each of these cases, the lower Rydberg levels are predissociated by underlying valence states. The different dynamics in the clustered vs unclustered species are manifested in the absorption spectrum by the appearance of vibronic modes in the clustered spectrum, which dissociate too rapidly in the monomer to be observed. In the MPI spectra, we observe changes in the line shape of rapidly dissociating modes, as the dissociation rate is altered. We have quantified the spectroscopic behavior and developed a model, the cluster-induced potential shifts model, which can be used to interpret the differences in the clustered vs isolated spectra in terms of the dynamics of predissociation. The model has proved very successful when applied to methyl iodide, acetone, and acetaldehyde, and we believe it to be generally applicable to predissociating systems.

10. Conclusions

The effect of intermolecular interactions on the predissociation dynamics of molecules embedded in a cluster can be quite significant and with the methodology outlined here can be probed quantitatively. By virtue of the differing responses of different electronic states to external perturbations, the region of the bound potential surface in which the predissociative surface crossing

Acknowledgment. This work has been supported by the National Science Foundation. We thank Professors S. R. Leone, W. C. Lineberger, D. J. Nesbitt, S. J. Strickler, Dr. M. I. McCarthy, and L. M. Cousins for many stimulating discussions of the results presented herein.

(511 la) Buelow. S.: Noble. M.: Radhakrishnan. G . :Reisler. H.: Wittip. C. j . kh&. Chem. lk6, 90, 1015: (b) Rice, J.; Hoffman", G:;Wittig, J . Chem. Phys. 1988,88, 2841. ( 5 2 ) Hausler, D.; Rice, J.; Wittig, C. J . Phys. Chem. 1987, 91, 5413.

e:

ARTICLES Reaction of C(3P) Atoms with Azide Radicals D. J. May and R. D. Coombe* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: January 15, 1988; In Final Form: May 27, 1988)

Both continuous discharge-flow and pulsed methods have been used to investigate the reaction of carbon atoms with azide radicals. In the discharge-flow experiments, C(3P) atoms were produced by the reaction of CH4 with excess fluorine atoms. The C(3P) + N3 reaction was found to produce CN in the B2Z+and A2JI excited electronic states. The rate constant of this reaction was determined from experiments in which mixtures of C 3 0 2and HN3 were photolyzed at 193 nm. The results indicated the rate constant to have the value (1.1 & 0.2) X lo-'' cm3 s-'. This value is in accord with trends observed in other atom + N3 reactions.

Introduction

The high chemical potential of the free carbon atom and its ability to form strongly bound CN, CO, and C 0 2 molecules in reactions with oxygen- or nitrogen-containing molecules cause many such reactions to be both very exothermic and very rapid. In addition, spin and orbital angular momentum correlations may play an important role in carbon atom reactions with small molecules. For example, spin conservation is thought to constrain the reactions of C(3P) atoms with OCS('2+) and S02(IA') to produce electronically excited CS(a311) and SO(A311), respectively.l.* In the reaction of C(3P) atoms with N02(2A1),sufficient (1) Dorthe, G.; Caille, J.; Burdenski, S . J . Chem. Phys. 1983, 78, 594.

0022-365418912093-0520$01.50/0

energy is released for spin-allowed. production of any of several excited doublet or quartet states of NO. In this case, however, orbital angular momentum correlations constrain the reaction to produce the B(*II) states3 Angular momentum correlations have also been found to be important in reactions of the azide radical (N3) with various atoms.44 Apart from the role that spin conservation may play (2) Krause, H. F. Chem. Phys. Leu. 1981, 83, 165. Dorthe, G.; Costes, M.; Burdenski, S.; Caille, J.; Caubet, P. L. Chem. Phys. Lett. 1983, 94, 404. Wu, K. T. Chem. Phys. Lett. 1984, 107, 405. (3) Dorthe, G . ;Caille, J.; Burdenski, S.;Caubet, P.; Costes, M. J . Chem. Phys. 1985, 82, 2313. (4) Pritt, A. T.; Patel, D.; Coombe, R. D. Inr. J . Chem. Kinet. 1984, 16, 917.

0 1989 American Chemical Society