15min:

KENSUKE HARADA AND KEIICHI TANAKA, Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan.

The He-HCN complex is a weakly bound complex with binding energy of about 9 cm^-1. We have measured the internal rotation bands (to the j = 1 and 2 states) and intermolecular stretching bands of the He-HCN complex by millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique, and reported the potential energy surface (PES) to reproduce the observed transition frequencies. The PES obtained has a global minimum in the linear configuration with a depth of 30.3 cm^-1 and has a saddle point at the anti-linear configuration with a depth of 20.3 cm^-1. The intermolecular stretching level and the second excited (j = 2) level of the internal rotation state are determined to be located 9.1407 and 9.0529 cm^-1, respectively, above the ground state, very close to the calculated dissociation limit (D₀) of 9.33 cm^-1 .

\par In the present study, we have extended the measurement to the transitions to the bound states above the "dissociation limit" (D₀) and observed several such transitions for the first time in the ground state of the molecular complex. The rovibrational levels of He-HCN with e label dissociate to the HCN molecule with J=0 and the He atom (D₀), while those with f label, due to the parity conservation, to the HCN molecule with J=1 and the He atom which is higher in energy by about 2B _HCN 2.96 cm^-1 than D₀, as discussed in the infrared study of He-HF. The PES obtained in the present analysis indicates that four f levels in the j = 1 and 2 excited states are bound above the "dissociation limit" (D₀) due to the parity conservation. In addition five levels (both of e and f labels) are predicted to be bound by centrifugal barrier with finite life times but not yet observed.