Vibrational predissociation complexes

Another example of current interest is the vibrational predissociation of hydrogen bonded complexes such as (HF) ... 

Ewing G E 1980 Vibrational predissociation in hydrogen-bonded complexes J. Cham. Phys. 72 2096-107... 

For complexes such as Ar-H2, Ar-HF and Ar-lTCl, vibrational predissociation is a very slow process and does not cause appreciable broadening of the lines in the infrared spectmm. Indeed, for Ar-ITF, ITuang et al [20] showed that... 

Figure C 1.3.5. Spectra of two different infrared bands of HF dimer, corresponding to excitation of the bound (lower panel) and free (upper panel) HF monomers in the complex. Note the additional line width for the bound HF, caused by vibrational predissociation with a lifetime of about 0.8 ns. (Taken from 1211.)...

Figure 12. Potential energy contour plots for He + I Cl(B,v = 3) and the corresponding probability densities for the n = 0, 2, and 4 intermolecular vibrational levels, (a), (c), and (e) plotted as a function of intermolecular angle, 0 and distance, R. Modified with permission from Ref. 40. The I Cl(B,v = 2/) rotational product state distributions measured following excitation to n = 0, 2, and 4 within the He + I Cl(B,v = 3) potential are plotted as black squares in (b), (d), and (f), respectively. The populations are normalized so that their sum is unity. The l Cl(B,v = 2/) rotational product state distributions calculated by Gray and Wozny [101] for the vibrational predissociation of He I Cl(B,v = 3,n = 0,/ = 0) complexes are shown as open circles in panel (b). Modified with permission from Ref. [51].

Gray and Wozny [101, 102] later disclosed the role of quantum interference in the vibrational predissociation of He Cl2(B, v, n = 0) and Ne Cl2(B, v, = 0) using three-dimensional wave packet calculations. Their results revealed that the high / tail for the VP product distribution of Ne Cl2(B, v ) was consistent with the final-state interactions during predissociation of the complex, while the node at in the He Cl2(B, v )Av = — 1 rotational distribution could only be accounted for through interference effects. They also implemented this model in calculations of the VP from the T-shaped He I C1(B, v = 3, n = 0) intermolecular level forming He+ I C1(B, v = 2) products [101]. The calculated I C1(B, v = 2,/) product state distribution remarkably resembles the distribution obtained by our group, open circles in Fig. 12(b). 

The Si PES, calculated by Nonella and Huber (1986), has a shallow minimum above the ground-state equilibrium, or expressed differently, a small potential barrier hinders the immediate dissociation of the excited S complex. Although the height of the barrier is less than a tenth of an eV, it drastically affects the dissociation dynamics, even at energies which significantly exceed the barrier. The excited complex lives for about 5-10 internal NO vibrational periods before it breaks apart. The photodissociation of CH3ONO through the Si state exemplifies indirect photodissociation or vibrational predissociation (Chapter 7). 

In this section we will explain the essential mechanism of vibrational predissociation by virtue of a linear atom-diatom complex such as Ar H2. Figure 12.1 illustrates the corresponding Jacobi coordinates, t In particular, we consider the excitation from the vibrational ground state of H2 to the first excited state as illustrated in Figure 12.2. The close-coupling approach in the diabatic representation, summarized in Section 3.1, provides a convenient basis for the description of this elementary process. For simplicity of presentation we assume that the coupling between the van der Waals coordinate R and the vibrational coordinate r is so weak that it suffices to include only the two lowest vibrational states, n = 0 and n = 1, in expansion (3.4) for the total wavefunction,... 

Drobits, J.C. and Lester, M.I. (1988b). Evidence for final state interactions in the vibrational predissociation of ICL-Ne complexes, J. Chem. Phys. 89, 4716-4725. 

Halberstadt, N., Beswick, J.A., and Schinke, R. (1991). Rotational distributions in the vibrational predissociation of weakly bound complexes Quasi-classical golden rule treatment, in Half Collision Resonance Phenomena in Molecules Experimental and Theoretical Approaches, ed. M. Garcia-Sucre, G. Raseev, and S.C. Ross (American Institute of Physics, New York). 

To focus the review, it is necessary to pass over some topics which are timely, relevant, and could be included were it not for space limitations. One of these is the area of electron photodetachment as applied to the study of the Franck-Condon region of the neutral PES accessed from the corresponding anion. This approach has been shown to provide information about the transition state region of several bimolecular reactions. This work was pioneered by Neumark and coworkers and an excellent review is already available (Manolopoulos et al. 1993 Neumark 1992). Another noteworthy area is state-to-state studies of vibrational predissociation in weakly bound complexes. Miller and coworkers have made impressive advances in which fully state and angle-resolved product distributions have been obtained (Bemish et al. 1994 Block et al. 1992 Bohac et al. 1986, 1992a,b Bohac and Miller 1993a,b Dayton et al. 1989), and these results have been used to bring theory and experiment into accord. The present review is limited to cases in which ultraviolet photodissociation of a complexed moiety initiates reaction. 

Audibert M-M, Palange E. Vibrational predissociation of the hydrogen-bonded (CH3)20-HF complex. Chem Phys Lett 1983 101 407-411. 

Vibrational predissociation of glyoxal.H2 complexes studied for several vibrational levels of the first excited singlet state of glyoxal... 

Y.Kami and E.E.Nikitin, Adiabatically corrected quasiclassical model for the vibrational predissociation of van der Waals complexes, Chem.Phys. 191, 235... 

E.I. Dashevskaya, I.Litvin, E.E.Nikitin, I.Orel and J.Troe, Classical diffusion model of vibrational predissociation of van der Waals complexes truncated mean first passage time approximation, Phys. Chem. Chem. Phys. 2, 2251 (2000)... 

Abstract Vibrational predissociation (VP) of van der Waals complexes occurs via an isolated resonance. An isolated resonance possesses no classical counterpart And yet, classical calculations of the decay yield the rates that are sometimes not too different from the quantum rates. We resolve this puzzle by addressing the following points ... 

Vibrational predissociation (VP) of a van der Waals triatomic complex A..BC is an example of a unimolecular reaction the rate of which is controlled by the intramolecular vibrational energy redistribution (TVR) [1]. Within a rigorous quantum mechanical approach, the VP dynamics is completely characterized by the complex-valued energies E = - /T / 2 that lie above the dissociation threshold of A..BC into an atom A and... 

Several recently published theoretical works predict an interesting effect of vibrational excitation on the reactivity. In a theoretical study Arnold et al. investigated the vibrationally assisted reaction of the Oj-NO vdW complex. They found that the dynamics resembled that of chemical reactions occurring under matrix isolation and differ significantly from the O3 + NO bimo-lecular collision dynamics. Mode sp>ecificity, vibrational predissociation, and intermodal energy transfer were found. The asymmetric stretching mode of O3 is found to be the most effective promoting mode. 

The present chapter mainly discusses the simplest class of atom-diatom Van der Waals molecules, the molecular hydrogen-inert gas complexes. While experimental information on the vibrational predissociation of these species is as yet relatively limited, our knowledge of the potential energy surfaces which govern their dynamics (9,10) is unequalled for any other systems. Moreover, the small reduced mass and large monomer level spacings make accurate calculations of their properties and propensities relatively inexpensive to perform. For these reasons, these species have come to be treated as prototype systems in theoretical studies of vibrational predissociation (17-25). 

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