Our general understanding of molecular collisions and energy transfer rests mainly upon classical mechanics. Except for particular quantum mechanical processes, such as transitions between different electronic states, and quantum mechanical features like resonances or interferences classical mechanics is a very useful tool for the study of molecular encounters (Porter and Raff 1976; Pattengill 1979; Truhlar and Muckerman 1979; Schatz 1983; Raff and Thompson 1985; Levine and Bernstein 1987:ch.4). This holds true for photodissociation as well (Goursaud, Sizun, and Fiquet-Fayard 1976; Heller 1978a; Brown and Heller 1981; Schinke 1986c; Goldfield, Houston, and Ezra 1986; Schinke 1988b; Guo and Murrell 1988a,b).

Classical mechanics is the limit of quantum mechanics as the de Broglie wavelength λB = 2πħ/(2mE)½ becomes small. The total energy released as translational and internal energy in UV photodissociation often exceeds 1 eV and therefore λB is of the order of 0.1 Å or shorter. On the other hand, the range of the potential is typically much larger so that the quantum mechanical wavefunction performs many oscillations over the entire interaction region (see Figures 2.3 and 3.2, for example). Furthermore, in many cases the fragments are produced with high internal excitation (Figure 3.3) which additionally favors a classical description.

The classical picture of photodissociation closely resembles the timedependent view. The electronic transition from the ground to the excited electronic state is assumed to take place instantaneously so that the internal coordinates and corresponding momenta of the parent molecule remain unchanged during the excitation step (vertical transition).