Dynamics vibrational predissociation

Zewail and co-workers performed a series of time-resolved experiments characterizing the vibrational predissociation dynamics of He I2(B, v ). 

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). 

Cline, J.I., Reid, B.P., Evard, D.D., Sivakumar, N., Halberstadt, N., and Janda, K.C. (1988). State-to-state vibrational predissociation dynamics and spectroscopy of HeCl2 Experiment and theory, J. Chem. Phys. 89, 3535-3552. 

Ewing, G.E. (1981). Vibrational predissociation of van der Waals molecules and inter-molecular potential energy surfaces, in Potential Energy Surfaces and Dynamics Calculations, ed. D.G. Truhlar (Plenum Press, New York). 

Hennig, S., Untch, A., Schinke, R., Nonella, M., and Huber, J.R. (1989). Theoretical investigation of the photodissociation dynamics of HONO Vibrational predissociation in the electronically excited state Si, Chem. Phys. 129, 93-107. 

Le Roy, R.J. (1984). Vibrational predissociation of small van der Waals molecules, in Resonances in Electron-Molecule Scattering, van der Waals Molecules, and Reactive Chemical Dynamics, ed. D.G. Truhlar (American Chemical Society, Washington, D.C.). 

Waterland, R.L., Lester, M.I., and Halberstadt, N. (1990). Quantum dynamical calculations for the vibrational predissociation of the He-ICl complex Product rotational distribution, J. Chem. Phys. 92, 4261-4271. 

E.E.Nikitin, Vibrational relaxation and vibrational predissociation as dynamical tunneling processes, Uspekhi Khimii 62,3 (1993)... 

Y.Kami and E.E.Nikitin. Vibrational predissociation rate from dynamics of the full collision a test of the Eandau method against the exact results, J. Chem. Phys. 100, 8065 (1994)... 

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). 

The first photodlssoclatlon experiments with vdW molecules were actually done In the visible and UV spectral regions by Levy and coworkers (] ). These beautiful experiments reveal the vibrational predissociation dynamics within an electronically excited state, and are therefore subject to the possible complication that the dynamics following vertical excitation of the system may be Influenced by structural differences between the ground and excited electronic states. Another complication Is that Internal conversion may compete with fluorescence. However, Important advantages of vlbronlc excitation are (1) that many Initial vibrational states are easily accessible, (2) that product states may be Identified from the dispersed fluorescence and (3) that the fluorescence lifetime provides an Internal "clock" against which the rates of energy redistribution and dissociation may be compared ( ). Because of the significant differences between the vibrational and vlbronlc excitation experiments the present discussion will be limited to the former. 

There is a growing body of evidence that energy gap laws are useful in rationalizing the relative rates of vibrational predissociation of various van der Waals molecules. As molecular complexity increases, so do the possible number of product channels. It remains to be seen if the dynamics of a molecule like ethylene dimer can be quantitatively understood. 

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