Dynamics of inelastic collisions of electronically excited rare gas atoms

Recent advances in the field of reactive and inelastic schattering of electronically excited rare gas atoms in metastable R*{(np)5(n+1)s} and shortlived R**{(np)5(n+1)p} states are discussed in case studies of the relevant processes: excimer formation, excitation transfer, Penning ionization and intramultiplet mixing. For reactive scattering the fine structure dependence of (KrBr) excimer formation is studied, showing that the propensity for core state conservation depends strongly on the halogen donor used. As a prototype of excitation transfer collisions the Ar*(3P0,2)–N2 system is analysed with respect to electronic, vibrational and rotational excitation. Vibrational excitation is well understood in terms of a Landau Zener type semiclassical model: the two observed rainbow singularities in the rotational distribution lead to a classical description of scattering on a hard shell with both a P2–and a P4–anisotropy. For Penning ionization new insight is given for the well-studied Ne*(3P0,2)–Ar system. Ab-initio calculations of the auto-ionization width confirm the validity of a simple two-state basis model for the metastable states. Polarisation effects in the ionization cross section for the Ne**(3p)–Ar system are used to establish a practical semiclassical criterion for ‘‘locking’’ of the total electronic angular momentum to the internuclear axis. Finally, intramultiplet mixing is discussed for the Ne**(3p)–He system. The observed polarised-atom cross sections are in good agreement with quantum mechanical coupled channel calculations, using the model potentials of Hennecart and Masnow-Seeuws as input. The results are equally well described with a semiclassical model that incorporates both rotational and radial coupling.