Thermally averaged spectroscopic parameters of the weakly bound dimers

Spectroscopic parameters in an ensemble of the weakly bound dimers are deri ved making use of the quantum and classical statistical average over therma lly excited rovibrational states. Argon dimers and oxygen dimers are taken as examples. It is shown that thermally averaged characteristics of dimers rapidly change as temperature of a gas rises. In an ensemble of quasi-diato mic tightly bound dimers, the high-temperature limit of the effective disso ciation energy amounts to roughly D-0/3, where D-0 is the dissociation ener gy from the ground state. It is demonstrated that the effective rotational B-eff(T) constant for the oxygen dimers statistically averaged at near room temperature is merely one half of the ground state B-0 value. This conclus ion is in agreement with recent spectroscopic observations. Rotationally re solved spectroscopic probe of (O-2)(2) in the slit-nozzle expansion resulte d in B-0 = 0.095 cm(-1). while bandshape analysis of the room-temperature c ollision-induced absorption in the oxygen fundamental allowed an estimate o f B-eff approximate to 0.038 cm(-1). The role of anisotropy of intermolecul ar interaction and the formation of various types of metastable dimeric sta tes are discussed. (C) 2001 Academic Press.