Collision-induced dissociation dynamics of Ar-2(+) at high levels of vibrational excitation

The collision-induced dissociation (CID) dynamics of the Ar-2(+)+Ar collisi on system are investigated at different Ar-2(+) internal energy distributio ns in a guided-ion beam (GIB) apparatus. The internal energy of reactant io ns, assumed vibrational in a first approximation, is controlled by varying the position of ionization in a supersonic jet, electron impact ion source. Three conditions are investigated: cold, in which the ions are produced as vibrationally relaxed as possible; intermediate, in which a substantial sh ift in the CID onset is observed; hot, in which the apparent CID threshold is at near thermal collision energies. The vibrational distribution of the Ar-2(+) ions is probed at the same conditions by measuring the kinetic ener gy release of photofragment Ar+ following (2)Sigma(g)(+)<--(2)Sigma(u)(+) p hotodissociation. The derived internal energy distributions are then used t o model the observed CID cross sections with a modified line-of-centers app roach to assess vibrational effects in the single-collision cross sections. The intermediate CID cross sections are consistent with a negligible vibra tional enhancement beyond the statistical predictions. A substantial increa se in cross section is observed when going from intermediate to hot conditi ons, despite a weak increase in internal energy, as apparent from the photo dissociation measurements. Contributions from metastable states, not regist ered in the photodissociation experiment, can explain this disparity. (C) 2 000 American Institute of Physics. [S0021-9606(00)00924-7].