15min:

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

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the measurement of the internal-rotation band of the He-HCN complex in the frequency region of 95--125 GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internal-rotation band, j = 1 - 0. The observed transition frequencies and the previous MBER data were analyzed to yield an empirical intermolecular potential energy surface, which was obtained by improving the recent ab initio potential surface. The surface obtained has a global minimum in the linear configuration (He H--C--N) with a well depth of 30.2 cm^-1 , and the saddle point located in the anti-linear configuration (H--C--N He) is higher in energy than the global minimum by 8.92 cm^-1 . The distance R _m from the He atom to the center of mass of HCN along the minimum energy path shows a large angular dependence; R _m is 4.169 Å and 4.039 Å in the linear and anti-linear forms, respectively, and has a minimum value of 3.528 Å in a T-shaped configuration. The He-DCN was also measured and analyzed. The empirical potential energy surfaces are compaired with the ab initio potential energy surface^b for both isotopic species.