Direct photodissociation The reflection principle

Direct dissociation is the topic of this chapter while indirect photofragmentation will be discussed in the following chapter. Both categories are investigated with the same computational tools, namely the exact solution of the time-independent or the time-dependent Schrodinger equation. The underlying physics, however, differs drastically and requires different interpretation models. Direct dissociation is basically a classical process while indirect dissociation needs a fully quantum mechanical description. 

Because of the very short lifetime in a direct process the energy dependence of the absorption spectrum as well as the final state distri- 

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In contrast to indirect dissociation, which is the topic of Chapter 7, direct photodissociation is relatively simple to understand. The reflection principle describes qualitatively the fully state-resolved photofragmentation cross sections a E, n, j) as a multi-dimensional mapping of the initial coordinate distribution in the electronic ground state ... 

Here E,j, is the energy of the initial state and R is the nuclear geometry. The division by 3 in (14) comes from orientational averaging. In this form, calculation of the absorption cross section requires the initial vibrational wave function, the transition dipole moment surface and the excited state potential. The reflection principle can be employed for direct or near direct photodissociation. It is again an approximation where the ground state wave function is reflected off the upper potential curve or surface. Prakash et al and Blake et al. [84-86] have used this theory to calculate isotope effects in N2O photolysis. 

The HCl and DCl spectra calculated with the reflection principle model have too low intensities in comparison to the wavepacket results, which are for the same conditions. The differences in the results demonstrate that the dynamics of the system play an important role even for direct photodissociation reactions like HCl and DCl. 

Schinke, R. (1986c). The rotational reflection principle in the direct photodissociation of triatomic molecules. Close-coupling and classical calculations, J. Chem. Phys. 85, 5049-5060. 

Untch, A., Hennig, S., and Schinke, R. (1988). The vibrational reflection principle in direct photodissociation of triatomic molecules Test of classical models, Chem. Phys. 126, 181-190. 

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