Bare Molecules

This downhill descent of the wavepacket observed at 266/800 nm appears in figure 2 as monotonic, which should lead to the conclusion that the wavepacket is fully spread when landing on the Z surface. This is not the case, as for the bare molecule, and the coherence of the wave packet can be probed with a laser at 400 nm which allows its continuous observation down to the Z surface[9]. 

To discover smaller specific effects on the intramolecular dynamics after attachment of an Ar atom to the benzene molecule, we performed lifetime measurements of single rovibronic states in the 6q band of the benzene-Ar and the benzene-84 Kr complex. No dependence of the lifetime on the J K> quantum number within one vibronic band was found [38]. This is in line with the results in the bare molecule and points to a nonradiative process in the statistical limit produced by a coupling to a quasi-continuum, for example, the triplet manifold. 

The 2-naphthol/ammonia dusters studied by Droz et al. (1990) show a proton transfer also occurring for n > 4, but, in absorption, the broad S3 <- S0 bands are not red shifted with respect to the bare molecule as in l-naphthol(NH3) ... 

Sa1 excitation, which generates little bare molecule emission, fast IVR for the state pumped is also observed. While concrete proof for a serial mechanism is yet to come in the form of rise and fall times for intermediate states, the inference of this mechanism is quite strong in the results. The discussion below for anilinefNj) clusters, and the simple two parameter serial IVR/VP model based on Fermi s Golden Rule and RRKM theory, will provide the final demonstration for this mechanism. 

These decay measurements on the state excited can be repeated by a TRSEP technique (Hineman et al. 1994) to verify the IVR cluster kinetics. This has been done for the aniline(N2)1 P- vibronic excitation. The experiment involves excitation of the P- state, followed by stimulated emission with a time delayed pulse to deplete the P" population. The total emission from the excited Sj cluster and bare molecule as a function of time delay between the excitation and dump... 

The most remarkable and central observation of these experiments is the direct characterization of the emission kinetic curves for the T[, 0 and 0° transitions and the observation of the 10b transition following excitation at T7 of the aniline(N2)1 cluster (see Figure 5-7). The pumped state, an intermediate state populated by IVR, and two bare molecule product states populated by the IVR/VP process are observed. These data are consistent with, and therefore provide strong evidence in support of, the serial IVR/VP mechanism applied to clusters containing a polyatomic chromophore. A parallel mechanism simply cannot explain the observed results. 

The experimental results for these clusters are presented in Figures 5-10 and 5-11. The 4F.A(Ar)1 data show that the emission from the cluster/bare molecule systems changes dramatically as a function of vibrational energy in the cluster. At higher energies, only the 0° of the bare molecule is observed with a 12 cm"1 hot molecule sequence structure related to the ethyl group bend (toward and away... 

Figure 5-11. Dispersed emission spectra of 4EA(solvent)1 clusters expanded near the 4EA origin region (a) I2 excitation (b) I2 excitation. Positive numbers indicate a red shift from the 4EA bare molecule origin.

The data presented show features of the IVR/VP process which any theory of van der Waals molecule dissociation must be able to reproduce. First, only the bare molecule ethyl torsion mode and 0° are populated and emit following IVR and VP of 4EA/polyatomic solvent clusters. Second, specific product state distributions in the torsional mode manifold are observed which depend on the cluster and the excitation energy. Third, the rate of dissociation depends on the excitation energy. Fourth, the emission behavior of the three clusters studied is... 

Intermolecular vibrational energy redistribution may populate several different chromophore states with sufficient energy in the van der Waals modes that VP can occur. If VP, which is predicted to be very fast in the Ar cluster, competes with subsequent IVR, then these chromophore states will be populated in the bare molecule. This would give a more crowded spectrum for 4EA(Ar)x with greater intensity away from the 0 transition, as is observed. These predictions are in qualitative agreement with all cluster data however, quantitative comparison for the Ar cluster is rendered impossible by crowding of the spectra (see Figure 5-11). 

The potential surfaces can be qualitatively expressed for these clusters through a model quite similar to that employed by Levy and coworkers (Tubergen et al. 1990 Tubergen and Levy 1991) for substituted indole clusters. Polar solvation in this case lowers the polar La state below the local excited Lb state, which is the first excited singlet state of the bare molecule. 

As a result of the interaction between the molecule and the boson field, the bare molecule becomes dressed with a cloud of boson particles in the language of Sect. 2 the dressed molecule is an elementary excitation in the many-body system of molecules and boson particles. The number density of dressing boson particles is given by... 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

Our work on hydrated clusters manifests the value of gas phase experiments. Condensed phase studies reveal the properties of the bulk system. However, it is difficult to distinguish intrinsic vs. collective properties of a system. Gas phase studies, on the other hand, directly provide information on bare molecules. Moreover, the investigation of size selected water complexes can mimic the transition from an isolated molecule to the bulk. The comparison of gas phase experimental results with theoretical calculations can also provide a direct test of theoretical models. This test is in urgent need if theoretical modeling is to evolve into calculations of solvated systems with credibility. 

Fig. 4. An ortho-methyl group in diethylamino-pyrimidin induces some ground state twist and hence energetically destabilizes the B state but not yet sufficiently to make the population of the A state a najor process in supersonic jet spectroscopy. Upper panel dispersed fluorescence spectra of the jet-cooled bare molecule [36]. In clusters with methanol, the TICT state is preferentially lowered, and the majority of the ob rved red-shifted fluorescence can be assign l to arise from the TICT state (lower panel). This does not occur for the compound without an ortho-methyl group.

Fig. 6. The yield of dissociation vs. the squared veiocity of impact for a bare I2 moiecuie and for a molecule embedded in a cluster of 125 atoms of Ar and in a cluster of 125 atoms of Xe. Note how, at a given velocity, the cluster impact dissociation is significantly higher than for the bare molecule.

For smaller clusters the primary route for dissociation is the heterogeneous dissociation of the molecule. Just as for isolated collisions in the gas-phase, the probability of dissociation of a diatomic molecule upon impact at the surface is enhanced by its vibrational excitation. As the small cluster impacts the surface, it can be that the halogen molecule reaches the surface immediately, so that it still has the same velocity as that of the cluster center of mass. If that velocity is above the threshold, it will dissociate with about a 40% probability, just as a bare molecule would. The other process that can happen is that the halogen molecule collides first with a cluster atom, an atom that has already hit the surface, and therefore lost a fraction of its translational energy to the surface. Such a collision does two things. It slows the halogen molecule and so reduces its probability to dissociate at the surface. At the same time, at the supersonic velocities of... 

The effects of the mixed supersonic expansion of CDMA with various solvent molecules (such as cyclohexane, carbon tetrachloride, acetone, acetonitrile, methanol, dichloromethane and chloroform) on the emission spectra have been investigated by Phillips and co-workers [82d[. The cluster size distribution was varied by changing the nozzle temperature and the partial pressure of the solvent. Two emission components were observed in each case. The long-wave emission was attributed to dimers (which can be isolated or solvated) and to monomer complexed with chloroform or dichloromethane (of unknown stoichiometry). On the other hand, it has been reported by Bernstein and co-workers [84] that CDMA forms with acetonitrile two kinds of 1 1 complexes of different geometry. The first cluster has a structured excitation spectrum, similar to that of the bare molecule, but blue shifted by about 252 cm . The second exhibits a broad excitation spectrum with some resolvable features between 31400 and 31 600 cm (Table 2). The complexes show different fluorescence spectra excitation into the broad absorption leads to the red-shifted emission with respect to that of the monomer (Figure 8) and of the blue ... 

In the bulk, the low concentration of ground-state pairs excludes their observation by absorption. The formation of the excited-state complex, termed exciplex, is a collisional process electronic excitation of either the acceptor or the donor leads to the formation of a locally excited state (for instance, in hydrocarbon molecules, it is a nn state). During the lifetime of this state, a collision with the other partner (which is in the ground state) leads to the formation of the exciplex. This mechanism is compatible with the fact that the absorption and fluorescence excitation spectra of the system are identical with those obtained by superimposing the spectra of the individual components. At the same time, the fluorescence emission spectrum changes drastically—a broad band, red shifted with respect to the bare molecule s emission spectrum, appears. It is usually devoid of vibrational structure, and is shifted to longer wavelengths as the solvent polarity increases [1],... 

When two species collide, conservation laws may affect the outcome drastically. The requirement of momentum, as well as total energy conservation, determines both the angular scattering and the available channels for energy depositing in the bare -molecule reactions. Due to the existence of a third... 

Figure 15. LIF excitation spectra of S0 -> S, electronic origin (0-0) of several bare molecules M, and M-Ar complexes in supersonic expansions of Ar (Refs. 131, 132, 136). The coordination numbers, n, of the M - Ar complexes are marked.

Figure 19. Spectral shifts relative to the 0-0 transition of the bare molecule, and the widths (FWHM), of the 0-0 transition of tetracene in Ar clusters (Ref. 1 S3).

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