Intramolecular vibrational relaxation

Borkovec M, Straub J E and Berne B J The influence of intramolecular vibrational relaxation on the pressure dependence of unimolecular rate constants J. Chem. Phys. 85 146... 

Similar considerations have been exploited for the systematic analysis of room-temperature and molecular-beam IR spectra in temis of intramolecular vibrational relaxation rates [33, 34, 92, 94] (see also chapter A3.13 V... 

Evard, D.D., Bieler, C.R., Cline, J.I., Sivakumar, N., and Janda, K.C. (1988). The vibrational predissociation dynamics of ArCl2 Intramolecular vibrational relaxation in a triatomic van der Waals molecule , J. Chem. Phys. 89, 2829-2838. 

Janda, K.C. and Bieler, C.R. (1990). Rotational rainbows, quantum interference, intramolecular vibrational relaxation and chemical reactions All in rare gas-halogen molecules, in Atomic and Molecular Clusters, ed. E.R. Bernstein (Elsevier, Amsterdam). 

It should be noted that Wav and Whu,av are usually applicable only to the isolated (i.e., collision-free) molecule case. In this case, the intramolecular vibrational relaxation (IVR) is slow compared with the non-adiabatic... 

Alternatively, such prepared excited states may prove useful photochemically under particular circumstances. This is especially true for local-mode-type molecules [461, 462], that is, molecules for which vibrational eigenstates resemble localized excitation in individual bonds. As an example, in the case of HOD, the large frequency difference between the OH and OD oscillators is such that intramolecular vibrational relaxation does not destroy the localized excitation. (Similar effects arise if one excites a resonance state that displays local behavior see, for example, Ref. [463] for an ABA-type molecule.) As shown theoretically [464, 465], and confirmed experimentally [53-60], preparation of the OH stretch followed by an excitation laser leading to dissociation gives a marked enhancement of the H atom photodissociation in many molecules. 

Figure 2 Vibrational energy relaxation (VER) mechanisms in polyatomic molecules, (a) A polyatomic molecule loses energy to the bath (phonons). The bath has a characteristic maximum fundamental frequency D. (b) An excited vibration 2 < D decays by exciting a phonon of frequency ph = 2. (c) An excited vibration >d decays via simultaneous emission of several phonons (multiphonon emission), (d) An excited vibration 2 decays via a ladder process, exciting lower energy vibration a> and a small number of phonons, (e) Intramolecular vibrational relaxation (IVR) where 2 simultaneously excites many lower energy vibrations, (f) A vibrational cascade consisting of many steps down the vibrational ladder. The lowest energy doorway vibration decays directly by exciting phonons. (From Ref. 96.)...

We have presented experimental and theoretical results for vibrational relaxation of a solute, W(CO)6, in several different polyatomic supercritical solvents (ethane, carbon dioxide, and fluoroform), in argon, and in the collisionless gas phase. The gas phase dynamics reveal an intramolecular vibrational relaxation/redistribution lifetime of 1.28 0.1 ns, as well as the presence of faster (140 ps) and slower (>100 ns) components. The slower component is attributed to a heating-induced spectral shift of the CO stretch. The fast component results from the time evolution of the superposition state created by thermally populated low-frequency vibrational modes. The slow and fast components are strictly gas phase phenomena, and both disappear upon addition of sufficiently high pressures of argon. The vibrational... 

What evidence is there to support Neporent s n> arguments that intramolecular vibrational relaxations may generate diffuse and complex spectra in polyatomic molecules Neporent does stress the point that the differentiation between electronic and vibrational relaxation is based on the Bom-Oppenheimer approximation, and if the Bom-Oppenheimer approximation is not tenable, as may be the case for the higher-lying electronic states, no differentiation is possible. 

One can also assume that after the intramolecular vibrational relaxation process is completed the phase-space points are uniformly distributed inside the system... 

The competition between intramolecular vibrational relaxation and chemical reaction has been discussed in terms of the applicability of transition state theory to the kinetic analysis [6], If the environment functions mainly as a heat bath to ensure thermalization among the vibrational modes in the excited complex, then transition state theory is a good approximation. On the other hand, when the reaction is too fast for thermalization to occur the rate can depend upon the initial vibronic state. Prompt reaction and prompt intersystem crossing are, by definition, examples of the latter limit. 

Another relaxation process encountered in isolated molecules is the phenomenon of intramolecular vibrational relaxation. Following excitation of a high-lying vibrational level associated with a particular molecular mode, the excitation energy can rapidly spread to other nuclear modes. This is again a case of an initially prepared single state decaying into an effective continuum. 

There is a general view that, for most molecules of intermediate and large sizes, decay rates may be described by a statistical theory, such as the RRK.M model. However, modern experiments on decay rates of single vibronic levels (SVL) often show serious deviations from statistical behavior, even at energies where fast intramolecular vibrational relaxation (IVR) is presumed to exist. This suggests that apparent statisticality in previous experiments may have resulted from poor resolution. However, further work is necessary, since few SVL experiments have been performed thus far. 

Single vibrational levels of the 82 state of aniline, formed by excitation within a He-aniline molecular beam, have been shown to relax in low-energy collisions with the He diluent at rates which are markedly dependent upon the identity of the vibrational mode excited. Intramolecular vibrational energy transfer within the 82 state induced by collision with HjO and CH3F is also mode specific, and rates for these processes are of the same order for these two collision partners and considerably faster than for energy transfer caused by Ar. Within p-alkylanilines, collisionless intramolecular vibrational relaxation from the initially excited NHj inversion mode to the alkyl chain modes appears to be complete within 1 ns, ... 

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