Spectroscopy of van der Waals molecules: Isomers and vibrational predissociation

The inert-gas-halogen complexes have been studied for several decades by jet spectroscopy. Much of the seemingly bizarre behavior has become understandable in terms of two virtually isoenergetic isomer forms. The recently recognized linear isomer of Ar-I2 has a virtually continuous B ¬ X excitation spectrum. It also undergoes a very rapid vibrational predissociation, and suffers no electronic quenching from the B state. The well-known Tshaped isomer shows slow vibrational predissociation, which is competitive with electronic quenching. The quenching distorts the vibrational distribution of the I2 B state photofragments, consequently leading to a false estimation of the T-shaped Ar-I2 (B) state dissociation energy. The binding energies for the T-shaped Ar-I2 (X) and Ar-I2 (B) are unambiguously determined from the recent dispersed fluorescence study, which are also in good accord with the ab initio calculation. We discuss aspects of pure vibrational laser-induced fluorescence of hydrogen fluoride complexes. We contrast the behavior of Ar-HF with Ne-HF and present new results for the vHF = 3 level of Ne-HF.