Molecule vibrational predissociation

Vibrational Predissociation, in this section we discuss the case of a transition from a predissociative state to the photofragment state that occurs on a single adiabatic pes. Such processes cannot occur for diatomic molecules, but they can be observed for polyatomic systems. The transition is caused by intramolecular energy transfer, that is, by internal redistribution of vibrational energy. 

Fig. 12.4. ln(r/n) plotted versus [m(en - en i)]x 2 for the vibrational predissociation of HeCl2. m is the reduced mass of the van der Waals molecule and en — en i is the energy spacing between adjacent levels of the Morse oscillator. Note that n increases from the right- to the left-hand side Adapted from Cline, Evard, Thommen, and Janda (1986). 

Beswick, J.A. and Jortner, J. (1977). Model for vibrational predissociation of van der Waals molecules, Chem. Phys. Lett. 49, 13-18. 

Delgado-Barrio, G., Mareca, P., Villarreal, P., Cortina, A.M., and Miret-Artes, S. (1986). A close-coupling infinite order sudden approximation (IOSA) to study vibrational predissociation of the Hel2 van der Waals molecule, J. Chem. Phys. 84, 4268-4271. 

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. 

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). 

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). 

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.). 

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). 

Hutson, J.M. "Vibrational Predissociation and Infrared Spectrum the Ar-HC Van der Waals Molecule", submitted to J. Chem. Phys. 

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. 

In cases where the yield of molecular ions is higher than 10% and where the fragmentation pattern depends upon the atomic site of the core hole, the dissociation processes clearly depend upon the electronic structure of the molecule and the details of the electronic relaxation, i.e. not all pathways produce essentially the same result. The mechanism then may involve vibrational dissociation or electronic or vibrational predissociation as well as direct dissociation. Even in these cases, some of the electronic relaxation channels may rupture all the bonds in a molecule and high-kinetic-energy fragments can be produced. Such channels sometimes are labeled a Coulomb explosion, but this terminology should not be confused with the more specific use of the term that is proposed above. 

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