J . Phys. Chem. 1991,95,4627-4635 1.46 for the analytic potential function). So, in fact, the net contribution of the librational degrees of freedom of the water molecules to the solvent KIEs is still normal, which would indicate water structure breaking at the transition state, although not of such importance as for the Vs structure. A net inverse solvent KIE seems not to be such a straightforward indication of water structure making as has previously been assumed, at least on the basis of solvent molecule librations.
4. Summary and Concluding Remarks We have carried out semiclassical rate calculations for the reaction C1-(H20), CH3Cl’ CH3Cl + Cl”(H2O), with n = 0, 1, and 2. Instead of using an analytical potential energy function, we calculated the energy and gradient whenever we needed them by NDDO molecular orbital theory with parameters adjusted specifically for these individual reactions. The parameters we readjusted from the general AM1 parameter set are the atomic core matrix elements for p orbitals on C1 and C. The interface of the molecular orbital calculations with the dynamics calculations is accom lished by the use of a new direct dynamics program MORATE! The results are compared in detail to previous calc u l a t i o n ~based ~ ~ on a multidimensional semiglobal analytic potential f u n ~ t i o n ~ fit ~ . ~in’ part to ab initio calculations with a polarized basis set and electron correlation.36.’6 The comparison with the analytical surface has revealed some important differences that could be tested by experimental measurements, but, perhaps even more significantly, in many respects the two quite different approaches agree remarkably well. The correspondences between Arrhenius parameters and the kinetic isotope effects (the adeuterium secondary kinetic isotope effect as well as the heavywater microsolvent kinetic isotope effect) and their interpretation in terms of specific modes are very encouraging. For example, temperature-dependent energies of activation calculated from the surfaces agree within 0.2 kcal/mol over the 800 K temperature
+
-
4627
range studied. The contributions to the secondary C H 3 / C D ~ kinetic isotope effect (or, for short, the CD3 KIE) on the NDDO-SRP potential energy function are 1.23,0.75, and 1.04 from rotation, vibration, and the other factors, respectively, whereas on the analytic potential function these factors are4s 1.22, 0.76, and 1-04, respectively. Furthermore, both approaches predict that the effect on the vibrational contribution to the CD3 KIE of adding a single water molecule is traceable almost entirely to a single transition-state mode, the CH3 or CD3 internal rotation around Cl-Cl axis. In both calculations there is an important normal KIE due to water librations for the microhydrated cluster with n = 2. For the microsolvent H20/D20 kinetic isotope effects at 300 K, the contributions from low-, medium-, and high-frequency vibrations to the KIE are 0.46,1.61, and 0.85, respectively, for n = 1 and 0.64, 1.74, and 0.79 for n = 2. These compare well, for the most part, with the results calculated from the analytic potential energy function, which are450.43, 1.65, and 1.00 for n = 1 and 0.45, 3.25, and 0.81 for n = 2. Thus we have shown that molecular orbital theory with parameters adjusted specifically for an individual reaction, or-as in the present paper-for a set of reactions, leads to similar dynamical predictions, in many respects, as much more arduous methods, and so it can be a very useful technique for obtaining potential surfaces for reactions with many degrees of freedom. This kind of approach is especially well suited for direct dynamics calculations where the potential energy surface is defined implicitly by the electronic structure calculation without fitting an explicit analytic potential energy function.
Acknowledgment. We are grateful to Da-hong Lu and Xin Gui Zhao of our research group and James Lisy of the University of Illinois for helpful discussions. A.G.-L. gratefully acknowledges a Fulbright Scholarship. This work was supported in part by the U.S.Department of Energy, Office of Basic Energy Sciences.
Vlbronlc Hole-Burning Spectroscopy of Small Clusters Involving Perylene Stacey A. Wittmeyer and Michael R. Topp* Department of Chemistry, University of Pennsylvania. Philadelphia, Pennsylvania 19104-6323 (Received: December 19, 1990)
Spectral hole-burning measurements have been applied to small clusters of argon, methane, cyclopropane, and n-pentane nucleated by single perylene molecules under jet-cooled conditions. Distributional isomers are resolved, involving up to six clustering species around perylene. These species were chosen to explore the different kinds of low-frequency mode structure generated when an aromatic molecule is complexed by atoms and small molecules of different sizes and symmetries. Perturbed internal modes of perylene are also observed. Notably, the butterfly motion of perylene is damped in small argon clusters and rather less perturbed in methane clusters. On the other hand, methane clusters affect a Fermi resonance doublet near 545 cm-’ more strongly than argon. Low-frequency modes are also observed for cyclopropane 2:1 complexes of perylene, revealing different symmetries for the two isomers of the 1,1 species. Pentane 1:l and 2:1 complexes show strong perturbation of the Fermi doublet and also provide evidence for a butterfly mode fundamental resulting from lowering of the effective point-group symmetry.
1. Introduction When a large aromatic molecule is clustered with one or more atoms or small molecules, changes must occur in the vibronic spectrum that are specific to the structure and constitution of a given aggregate. The presence of clustering atoms or molecules represents an initial step in the formation of extended media, and the interactions and dynamics that arise in small clusters can be used to understand more extended systems: intermolecular modes of van der Waals complexes form the basis for the phonon mode structure of fluids and solids. Small clusters present valuable opportunities to demonstrate anisotropic interactions, such as the hindered rotation of small adsorbed molecules. The vibronic
spectrum of an isolated symmetric molecule is governed largely by group theoretical rules which are modified by the symmetry and strength of intermolecular forces. Detailed examination of the low-frequency mode structure generated by intermolecular interactions, particularly those features sensitive to complexation symmetry, can play an important role in structural assignments of molecular aggregates. One of the more interesting aspects of experiments on small molecular clusters is that more than one stable structure may exist for a given level of aggregation. Detailed studies of such phenomena provide avenues for quantitative comparison with theoretical predictions and are of key importance in the extension to
0022-365419112095-4627%02.50/0 0 1991 American Chemical Society
Wittmeyer and Topp
4628 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
larger aggregates. Some general aspects of isomerism in clusters have already been studied, including distributional isomers such as the (1,l (trans)) and (2,O (cis)) cases for 2:l aggregation of planar aromatic The interchange of such isomers is not readily achieved, since it requires the movement of atoms or molecules between different sides of the aromatic plane. For example, for perylene/Ar, the energy barrier to interchange is estimated at 200 cm-l (i.e., nearly 40% of the binding energy). Consequently, such isomers are commonly observed together in molecular beams, although many cases are also known where differences in interaction energy tend to stabilize one form over the other. For example, while two octane molecules adsorb separately on opposite faces of perylene, the complex anthracene/(octanc), contains a contacted octane dimer. Semiempirical potential energy calculations also frequently predict distinguishable arrangements of molecules as uibrutional isomers on the same side of the aromatic plane, which can be observed at the 1:l level of aggregation. These are calculated to have small barriers to conformational interchange (i.e., 80% depletion in some cases. Finally, most of the spectra we show here were saturated in excitation, in order to reveal low-intensity features: they do not accurately record band strengths, although qualitative comparisons are still valid. The principal resonances of the hole-burnt spectrum in Figure 1, shown in upper case type, are seen to be red-shifted by 142 cm-I from those of the perylene host (lower case). The electronic origin region shows evidence of low-frequency mode structure, including a doublet having components at -96 cm-' (+46 cm-' relative) and -89 cm-' (+53 cm-I) and a broad feature near 4 7 cm-' (+95 cm-I). These features are repeated in combination with the major vibronic bands, subject to lower cross sections and congestion effects. Significantly, these low-frequency components of the depletion spectrum lie close to excitation resonances (-102, -93, -52 cm-l) for lower aggregates, as seen in the bottom trace. Hole-burning spectroscopy allows weak vibronic bands of the selected species to be confidently distinguished from overlapping stronger resonances due to other clusters. Most of the bands are
-
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4629
Small Clusters Involving Perylene
TABLE I: Measured Red Shifts for Different Clusters involving
TABLE II: hmgb (in c d ) of Principal SIVibrrtioarl Lcr&
PerYkse
Perykae in Different Argon md Methane C h m t d
aggregate
argon
1,O
52 93 102 121 142 181 21 1 237
2,o 191
38 2,1 22
32 3,3
red shift, cm-I methane cyclopropane 82' 138 16d (150) 216b 271 289
156 282 300, 304
aggregate
4
G$
ul
vu
536
550
705
541 540 542 541 543 543 543 544
552 550 551 550 551 552 555 554
706 706 705 706 707 707 705 706
550 552 552 556 n/o
705 705 706 706 707
543 542 544 544 546 546
553 553 555 555 (558)b (558)b
708 706 706 706 707 708
543 545
552 (558)*
705 705
Perylene
pentane 244
95
477
95 96 93 n/o 95 (br)
427
353
Perylene/ Argon
422,431 542,-
n/o n/o n/o
@Principalresonance of doublet. *Peak of broadened resonance. 800,
Bb
Of
I
428 428 428 427 425 426 426 427
354 353 352 352 352 353 353 353
PerylenelMethane 94 97 95 95 98
429 427 427 427 427
352 352 352 354 352
541 543 543 546 547
Perylene/Cyclopropane 353 352 353 353 353 353
429 425 427 427 427 427
Perylene/Pentane 426 (4241
353 353 01 300
400
500
600
700
I
Relalive Wavenumber
'Data have an uncertainty of il cm-I. bEstimate, based on weak signal.
Figure 2. Perylene/argon hole-burning spectra for the various levels of
aggregation and corresponding distributional isomers. Changes in the Fermi resonance doublet ((FR))and the butterfly combination band at 448 cm-l (A + G2) are observed with increasing complexation. The spectra have been overlapped vertically for comparison.
I
I
800
700
labeled according to ref 9, except for the feature ((FRI) assigned to a Fermi resonance doublet, which appears in Figure 1 both as a fundamental and in combination with the A mode. The two traces in this introductory figure are shown as they were recorded, whereas the other figures in this paper focus on comparison of hole-burning spectra for different aggregates and display all traces aligned. The relevant spectral displacements are listed in Table
I. Figures 2 and 3 present a sequence of spectra for nine different clusters involving up to six argon atoms in different arrangements around perylene: the 2,2 trace is repeated for comparison. Detail in each spectrum is specific to the isomer selected: even though the fluorescence excitation spectrum of the jet-cooled ensemble is highly complex and mass resolution was not used, the spectra are distinct. The in-plane agmodes A (353 an-')and B (427 an-') of perylene, and their overtones and intercombinations, are not substantially affected by complexation, even with large molecules such as benzene and naphthalene.I0 The higher energy modes D (1292 cm-I), E (1 396 cm-I), and F (1603 an-'),not shown here, behave similarly.'lJ2 In all cases we have studied, these bands have similar relative intensities and their frequencies are about the same. The C mode (-550 cm-I) also has formal ag symmetry and would be expected to behave like the A and B modes. However, the observed resonance in jet-cooled bare perylene results from mixing with mode N in SIto form a Fermi doublet, as was recently ( 9 ) Foumnn, B.; Jouvet. C.; Tramer, A.; LeBars, J. M.; Millid, Ph. Chem. Phys. 1985, 95. 25. (10) Motyka, A. L.; Wittmeyer, S.A.; Babbitt, R. J.; Topp, M. R. J. Chem. Phys. 1988,89, 4586. (1 1) Doxtader, M. M.; Mangle, E. A.; Bhattacharya, A. K.; Cohen, S.M.; Topp. M. R . Chem. Phys. 1986,101,413. (12) Wittmeycr, S. A.; Topp, M. R. Chcm. Phys. Leu. 1990, 171, 29.
200
100
I 300
400
500
700
600
Relative Wavenumber
Figure 3. Perylene/argon hole-burning spectra for higher levels of aggregation. Each (3,n) cluster contains a low-frequency progression of 14-16 cm-I, which is not present in other clusters.
shown by hot-band experiments in this laboratory." In those experiments, it was shown that fluorescence following excitation of "N" populates mode C in the ground state, as evidenced by a prominent hot-band component at 550 cm-' due to Cy.14 Also, levels of the ground state identifiable with N (u(So) 585 ~ m - ' ) ~ are generated by fluorescence following excitation of 'Cy. On complexation with a single argon atom, the frequency of N in SI is upshifted toward C, forming a Fermi resonance doublet ((FRJ) spaced by 9-10 cm-I. (For convenience, we denote the lower energy component as vI and the upper component as uu.) In uncomplexed perylene, the vu component (550 cm-') is dominant, while, in larger perylene/argon aggregates, the intensity ratio
-
(13)Witfmeyer, S.A.; Topp, M. R. Submitted for publication. (14) Kaziska, A. J.; Wittmeyer, S.A,; Topp, M. R. J . Phys. Chcm., in
press.
4630 The Journal of Physical Chemistry, Vol. 95, No. 12, I991
begins to favor vI as it shifts upward. In the argon complexes studied, this shift is incomplete, and we note that the doublet persists at the highest levels of aggregation for which we have good data. For example, the 3,2 isomer of perylene/Ar5 exhibits vl = 544 cm-I and vu = 554 an-';see Figure 3. More extreme behavior will be seen below for molecular clusters. The fluorescence excitation spectra of many jet-cooled aromatic molecules show evidence for Franck-Condon activity in out-ofplane modes. Perylene has an active 'butterfly" model5 of b3" symmetry (seelater), overtones of which (e.&, Gi, 95 cm-'; G:,195 cm-l) are readily Seen in a fluorescence excitation spectrum. This activity is due to a change in the shape of the potential, in which the mean squared deviation from planarity is greater in the ground state than SI.Since resonances due to this out-of-plane motion are not evident in the spectra of perylene complexes with large molecules, it was of interest here to consider the effect on this mode of forming clusters of different size and symmetry. Geometrically, the D2h symmetry of perylene is reduced in an aggregate such as perylene/Ar to C,, which should cause some out-of-plane modes to become Franck-Condon active as fundamentals. Yet, the intensities of such transitions depend on the strength of the perturbation and the consequent functional symmetry of the system. In the argon 1,O and 1,l aggregates, the perylene butterfly mode appears at about the same frequency and strength as in the bare molecule. However, for the 2,O and 2,1 clusters this band is weakened and broadened to - 5 cm-I. The combination band with the A mode (A G2;448 cm-l) behaves similarly. The spectrum of the 2,1 cluster shows that, while the neighboring 427 cm-I (a,) band and both components of the Fermi doublet remain narrow, the 448-cm-I butterfly combination band is obviously weak and broadened. (Figure 1 shows similar behavior for the 95-cm-l parent band.) In higher argon clusters, including 3,0, 3,1, (2,2), 3,2 and 3,3, there is little evidence for the 95- or 448-cm-' bands. The low-frequency mode structure observed for argon complexes is relatively sparse, but quite characteristic of different aggregates, and may be used to support assignments of cluster types. Thus, a single argon atom generates a resonance at 46 cm-I, an argon dimer 53 cm-I, and a trimer -16 cm-I. The 2,1 cluster, which involves an argon dimer as well as a single argon atom, shows modes characteristic of both, at 46 and 53 cm-l. These frequencies are in the general range of intermolecular 'stretch" modes, but possible coupling to out-of-plane motions of the aromatic substrate should also be considered (see later). Figure 3 shows that all of the clusters assigned as (3,n) contain a low-frequency mode progression in 14-16 cm-I, which is not possessed by clusters having only 2 or 1 argon atoms per side. This low-frequency vibration of the trimer is probably due to argon-argon motion. 3.2. PerylenelMethane. Semiempirical potential energy c a l ~ u l a t i o n s ~predict ~ J ~ that the equilibrium van der Waals bond distances in perylene/Ar and perylene/CH, are both about 3.4 A. The calculated binding energies (600 and 900 cm-I, adjusted for SI)are quite close to the most recent experimental values (555 cm-l in a strong complexation field. At the same time, the zero-order C mode (a,) remains at about 548 cm-', which is consistent with other modes of the same symmetry. This interaction generates an avoided crossing as the energy of N is tuned, the minimum separation being 9 cm-I. There is also some dependence of the shift on complexation symmetry, which is seen most clearly from the band intensity ratios of different argon and methane clusters. Continuing work will use a vibrational mode analysis after the work of Cyvin et a1.22 to derive a pictorial representation of the zero-order N mode, for qualitative comparison of the different perturbations in the argon and methane complexes, and to examine in greater detail the different responses of the N and butterfly modes to complexation. 4.3. Low-Frequency Modes in MethanelPerylene Aggregates. ( a ) 2-cm-' Splitting. Figure 5 showed that the resonances of the perylene/methane 1,O complex have two components, indicating the presence of two distinct ground-state levels, either in distinct species or involving separate levels of the same species. One possible explanation for this doubling is that there are two nonequivalent sites for a single methane on perylene, which give rise to slightly different spectral shifts. Figure 14 shows the results of a potential energy calculation for a long-axis displacement of argon and methane on perylene: the parameter set used" gave a reasonable approximation for the binding energy of perylene/methane, which was recently measured by hot-band spect r o s ~ o p yto ' ~ be C890 cm-' in the SIstate, equivalent to C808 cm-' in So.The plot shows no strong indication of an off-center location for either methane or argon. Given the centrosymmetric nature of perylene, the low asymmetry of an adsorbed methane and the absence of a similar effect for argon complexes, distinct vibrational isomers involving different equilibrium structures are probably not responsible for the observed splitting. Also, we recall that only a single isomer was observed for the cyclopropane complex (C, symmetry). Leutwyler and Jortner' reported experimental and calculation data for rare-gas complexes of perylene and several linear polyacenes. The Occurrence of double-minimum potentials in several cases raised the possibility to observe splitting due to vibrational tunneling. This kind of behavior is probably ruled out for the argon complex of perylene (it certainly was not indicated in ref l ) , but there is a possibility that the extended potential well for perylene-methane, with a small central barrier (Figure 14), could result
Wittmeyer and Topp in splitting due to vibrational tunneling for the methane species. A further possibility is that the ground state is split by rotational tunneling, involving exchange of hydrogen atoms. White and c o - w ~ r k e r sreported ~~ just such an effect for submonolayer methane adsorbed onto graphite. Neutron diffraction experiments indicate that methane has C, adsorption symmetry, with three hydrogen atoms closest to the surface. This orientation is also predicted for perylene/CH4, with an energy barrier on the order of 100-150 cm-I. At this stage, however, calculation procedures are not sufficiently refined to provide a satisfactory computation of either kind of tunneling splitting, especially since the observed band separations depend on differences in the intermolecular surface between SIand So. ( 6 ) 5-IO-cm-' Splitting. The 1,0, 1,1, and 2,l clusters of methane on perylene (Figure 4) show in addition prominent low-frequency mode structure in the range 5-10 cm-I, which is also not present for argon. Although this study is still in progress, two possible explanations for this periodicity deserve mention. In principle, the 10-cm-' feature of the 1,0 complex could be attributed to motion of methane parallel to the plane of the perylene substrate-a "bending" mode-for example constrained by a potential such as in Figure 14. A semiempirical pair potential calculation does indeed predict that the zero-point motion of methane has a large amplitude along the long (y)axis of perylene, and a Morse potential fit of this profile gives a fundamental frequency of 10-12 cm-I. The observation of such a FranckCondon-active mode implies either a change in the position of the potential minimum or a substantial change in the shape of the potential on excitation. There is also the question of the relevant point group. In Da and C, long-axis motion would be non totally symmetric and would not be observable in the spectrum as a fundamental. On the other hand, in C,, the vibronic transition is allowed. Comparison with the cyclopropane and pentane complexes appears to rule out this assignment since, while those species further lower the symmetry of the system, the lowest energy modes observed are in the range 23-29 cm-I, which are almost certainly overtone bands. The vibronic selection rules in such cases are evidently not predictable from simple geometrical models. Another possibility for the low-frequency mode assignment is rotational motion of the methane about an axis (z) perpendicular to the perylene plane, for which potential energy calculations predict a negligible barrier. The rotational constant for free tetrahedral methane is 158 GHz (1, = 5.31 X l P Skg-m2),which should be virtually unaltered for a quasi-free methane rotor on perylene. The level spacing should be 2 X 158 X 109/c = 10.5 cm-I. Similar internal rotations has been documented for methyl a ~ e t y l e n e in , ~ infrared ~ measurements of the v6 antisymmetric stretching mode, where the rotational spacing is reduced about 10% by Coriolis coupling to 9.214 cm-I. This result is consistent with the 10-cm-' spacing we observe for perylene/(CH,)!. The 1,l isomer of ~erylene/(CH,)~ offers the opportunity to examine possible coupling between two adsorbed molecules in contact with the same aromatic molecule. The splitting observed in the 1,l case is about 5 cm-I, which is substantially different from that of the 1,0 complex and indicates that there may indeed be significant coupling between the two methane species. However, at this stage the experimental data in hand do not permit a more detailed analysis. Continuing work in this area will use holeburning techniques to investigate the effects of deuterium substitution on the low-frequency mode structure of different perylene/methane aggregates. For example, CD, will test the feasibility of assigning the 2- and 5-10-cm-' modes to internal rotation. 5. Conclusion
Spectral hole-burning measurements have been applied to small clusters of atoms and molecules nucleated by single perylene molecules under jet-cooled conditions. Changes in both low(23) Newbery, M. W.; Rayment, T.; Smalley, M. V.; Thomas, R. K.; White, J. W. Chem. Phys. Lerr. 1978, 59, 461. (24) Thomas, R. K.; Thompson, H. W. Specrrochim. Acra 1968, 24A, 1337.
4635
J . Phys. Chem. 1991,95,4635-4647 frequency and other modes have been identified, which are characteristic of particular two-dimensional aggregates adsorbed onto perylene. Different cases have been presented that show coupling of internal modes of the aromatic molecule to intermolecular modes. Cases studied include "damping" of the butterfly mode of perylene in argon clusters and the perturbation of a pair of levels in Fermi resonance, in molecular clusters. Hole-burning
studies of simple methane complexes reveal low-frequency mode structure which may be due to hindered rotational motion. Continuing work will explore details of the methane cluster spectra with deuterium substitution.
Acknowledgment. This project was supported by the Research Foundation of the University of Pennsylvania.
Femtosecond ReaCTlme Probing of Reactions. 6. A Joint Experimental and Theoretical Study of Blp Dissociation
R. M. Bowman, J. J. Cerdy,' G. Roberts, and A. H. Zewail* Arthur Amos Noyes Loboratory of Chemical Physics,$ California Institute of Technology, Pasadena, California 91I25 (Received: December 19, 1990)
The dynamics of the ultraviolet photofragmentation of bismuth dimer are studied experimentally and theoretically in the time domain. Employing the technique of femtosecond transition-state spectroscopy,the evolution of the dissociative process along two reaction channels leading to the 6p3(%03/2)+ 6p3(2D03/2)and 6p3(4Soy2)+ 6p3(2D05/2) levels of the atomic products is investigated following initial excitation of Bi2 at X = 308 nm. The broad spectral width of the ultrashort probe laser pulse coupled with the closely spaced excited energy levels of Bi enables fluorescence via some 14 atomic transitions to be monitored in real time, rendering possible the detection of dissociating molecules at different internuclear separations on the controlling potential surfaces. Long-time detection of the D ' O312 spin-orbit level permits clocking of the reaction along the lower energy exit channel, for which we report a dissociation time rl12 of approximately 1 ps, at which time the product Bi atoms are separated by some 10.7 A. Analogous measurements for the reaction giving rise to the higher-lying J = level of the 2D, term yield a value of r112= 1.5 ps, corresponding to an interfragment distance of 7.5 A. From the dissociation times so obtained, values for the length parameters that characterize noninteracting model potential curves Vl(r)and V&) for dissociation via both exit channels may also be determined. Early time detection of [Bi--Bi]** reflects dynamical behavior Over transition-state regions of the potential surfaces and allows various aspects of the nature of the force field governing fragmentation to be deduced. Finally, model quantum and classical calculations of the dissociation process are presented, which reproduce many of the salient features of the observed reaction dynamics.
-
I. Introduction of femtosecond transition-state ~pectroscopy.'J~,*~' On-resonance (longtime) detection of CN(X2Z+) via the B2Z+ X2Z+band Experimental efforts to study molecular reaction dynamics in real time' have been directed toward the following hierarchy of problems: ( I ) the internal motions executed by molecules on (1) Khundkar, L. R.; Zewail, A. H. Annu. Rev. Phys. Chem. 1990,41,15. bound potential energy surfaces (PESs); (2) direct dissociation (2) Williams, S.0.;Imre, D. G. J . Phys. Chem. 1988, 92, 6648. (3) Heather, R.; Metiu, H. Chem. Phys. Lett. 1989, 157, 505. Over a repulsive PES;(3) predissociation involving surface hopping (4) Lee, SPY.;Pollard, W. T.; Mathia, R. A. Chem. Phys. Len. 1989,160, between two intersecting diabatic PESs; (4) dissociation over 531. multidimensional PESs involving several exit channels leading to (5) Krause, J. L.; Shapiro, M.; Bersohn, R. J . Chem. Phys., submitted for product formation: ( 5 ) multiphoton excitation, ionization, and publication. (6) Engel, V.; Metiu, H.; Almeida, R.; Marcus, R. A.; Zewail, A. H. fragmentation processes. Such studies have served to initiate a Chem. Phys. Lett. 1988, 152, 1. number of theoretical investigations of these processes that aim (7) Engel. V.; Metiu, H. J . Chem. Phys. 1989, 90, 6116. to calculate the temporal dynamics as revealed by experiment. (8) Choi, S.E.; Light, J. C. J . Chem. Phys. 1989, 90,2593. Most quantum dynamical calculations2-12 based upon such an (9) Engel, V.; Metiu, H. J . Chem. Phys. 1989, 91, 1596. (IO) Lin, S. H.; Fain, B. Chem. Phys. Letr. 1989, 155, 216. approach have involved propagation of wave packets on approLetokhov, V. S.; Tyakht, V. V. Isr. J . Chem. 1990,30, 189. priately chosen PESs, while c l a ~ s i c a l ' ~and - ~ ~s e m i ~ l a s s i c a l ~ ~ (11) (12) Gruebele, M.; Roberts, G.; Zewail, A. H. Philos. Tram. R. Soc. A treatments have also been successful in describing the time evo(London) 1990, 332, 223. lution of the dissociating system. One reaction that has been (13) Dantus, M.; Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1988.89, examined in detail in the time domain, both t h e ~ r e t i c a l l y ~ - ~ J ~ J6128. ~~~ (14) Rose, T. S.;Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1989, 91, and e ~ p e r i m e n t a l l y , ' ~is~the ~ ~ dissociation ~' of ICN: 7415. ICN(X'Z+)
+ hu
+
[I-..CN]'*
+
CN(X22+) + I(2P3/2,1/2) (1)
which evolves over interacting PESs that are essentially repulsive at all interfragment separationsa (category 2 above). Following excitation of ICN in the A continuum at X = 306 and 285 nm, corresponding to excess energies for fragment separation of 6550 and 8960 cm-I, electronically excited transition-state configurations ([I.*.CN] **) of the parent molecule undergoing spatial separation to form products have been monitored in real time by the technique N S F Predoctoral Fellow. *Contribution Number 8373.
(15) Dantus, M.; Bowman, R. M.; Gruebele, M.; Zewail. A. H. J . Chem. Phys. 1989, 91, 7437. (16) Benohn, R.; Zewail, A. H. Ber. Bunsen-Ges. Phys. Chem. 1988,92, 373. (17) Bernstein, R. B.; Zewail, A. H. J . Chem. Phys. 1989, 90, 829. (18) Benjamin, I.; Wilson, K. R. J . Chem. Phys. 1989, 90, 4176. (19) Lee,S.-Y.; Pollard, W. T.; Mathies, R. A. J . Chem. Phys. 1989, 90,
6146. Yan, Y. J.; Fried, L. E.; Mukamel, S.J . Phys. Chem. 1989,93,8149. Fried, L. E.; Mukamel, S. J . Chem. Phys. 1990, 93, 3063. Mukamel. S.Annu. Rev. Phys. Chem. 1990,41, 647. Marcus, R. A. Chem. Phys. Leu. 1988, 152, 8. Scherer, N. F.; Knee, J. L.; Smith, D. D.; Zewail, A. H. J . Phys. Chem. 1985,89, 5141. (25) Dantus, M.; Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1987,87, (20) (21) (22) (23) (24)
2395.
0022-3654/91/2095-4635$02.50/0
0 199 1 American Chemical Society
