Spectroscopy of van der Waals molecules: Isomers and vibrational predissociation

The inert-gas-halogen complexes have been studied for several decades by je t spectroscopy. Much of the seemingly bizarre behavior has become understan dable in terms of two virtually isoenergetic isomer forms. The recently rec ognized linear isomer of Ar-I-2 has a virtually continuous B <-- X excitati on spectrum. It also undergoes a very rapid vibrational predissociation, an d suffers no electronic quenching from the B state. The well-known T-shaped isomer shows slow vibrational predissociation, which is competitive with e lectronic quenching. The quenching distorts the vibrational distribution of the I-2 B state photofragments, consequently leading to a false estimation of the T-shaped Ar-I-2 (B) state dissociation energy. The binding energies for the T-shaped Ar-I-2 (X) and Ar-I-2 (B) are unambiguously determined fr om the recent dispersed fluorescence study, which are also in good accord w ith the ab initio calculation. We discuss aspects of pure vibrational laser -induced fluorescence of hydrogen fluoride complexes. We contrast the behav ior of Ar-HF with Ne-HF and present new results for the v(HF) = 3 level of Ne-HF.