MILLIMETER-WAVE SPECTROSCOPY OF THE vdW BANDS OF He-HCN NEAR THE DISSOCIATION LIMIT

Author Institution: Department of Chemistry, Faculty of Science, Kyushu University; Hakozaki, Higashiku, Fukuoka, 812-8581 JapanThe 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., 7041 (2002).} 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 ($D_0$) 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" ($D_0$) 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 ($D_0$), 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 $2B_{\rm HCN} \sim 2.96$ cm$^{-1}$ than $D_0$, as discussed in the infrared study of He-HF., 5387 (1990).} 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" ($D_0$) 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. \