Relation to resonances in full collisions

Resonances in half and in full collisions have exactly the same origin, namely the temporary excitation of quasi-bound states at short or intermediate distances irrespective of how the complex was created. In full collisions one is essentially interested in the asymptotic behavior of the stationary wavefunction L(.E) in the limit R — 00, i.e., the scattering matrix S with elements Sif as defined in (2.59). The S-matrix contains all the information necessary to construct scattering cross sections for a transition from state i to state /. In the case of a narrow and isolated resonance with energy Er and width hT the Breit- Wigner expression 

The observation of resonances in a scattering experiment requires high resolution of the collision energy, which, however, is rather difficult to achieve in a conventional molecular beam machine. In a photodissociation experiment, on the other hand, the frequency of the light beam determines the collision energy in the upper state and hence the necessary energy resolution is much easier to accomplish. More crucial for the 

Despite the many theoretical predictions of inelastic resonances (mainly observed in calculations with reduced dimensionality Manz 1989, for example) we do not know of an unambiguous experimental observation of resonances in atom-molecule or molecule-molecule gas-phase collisions. In contrast to full atom-molecule collisions pronounced resonance-like structures are actually rather common features in bound-continuum absorption spectra (Robin 1974, 1975, 1985 Okabe 1978 Fano and Rao 1986). In fact, all sharp structures in UV absorption spectra can be considered as resonances and therefore photodissociation provides ideal opportunities to investigate resonance phenomena, such as the lifetime, the decay mechanism, and the final state distributions of the fragments, on a very detailed basis. 

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