15min:

KENSUKE HARADA, KEIICHI TANAKA AND TAKEHIKO TANAKA, Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashiku, Fukuoka, 812-8581 JAPAN; SHINKO NANBU, Institute for Molecular Science, Okazaki, 444-8585, Japan; MUTSUMI AOYAGI, Computing and Communications Center, Kyushu University, Hakozaki, Higashiku, Fukuoka, 812-8581 JAPAN.

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the observation of the intermolecular stretching excited state ( _s) and internal rotation second excited state (j=2) of the He-HCN complex. Two supersonic jet nozzles and 10 round trip multi-reflection optical path were used for the measurement. Transitions belonging to the _s, _s j=1, j=2 1, and j=2 0 bands were observed in the frequency region of 150--280 GHz, where _s refers to the intermolecular stretching excited state, and j=1 and 2 to internal rotation excited states. These transitions were definitely assigned using combination differences. The observed transition frequencies as well as the transition frequencies of the internal rotation fundamental band (j=1 0) were analyzed to improve an empirical intermolecular potential energy surface. The intermolecular stretching frequency obtained is 9.1618 cm^-1, while the dissociation energy (D₀) is 9.4411 cm^-1. The average distance from the He atom to the center of mass of HCN and the root mean square amplitude of the intermolecular stretching vibration is 5.572 Å and 2.182 Å for the _s, J=1 state, which is far longer and larger than those for the ground state (4.306 Å and 0.576 Å ). The wavefunction of the intermolecular stretching excited state has a node along the radial coordinate and the probability density extends up to more than 12 Å .