MILLIMETER-WAVE SPECTROSCOPY OF He-HCN AND He-DCN: ENERGY LEVELS NEAR THE DISSOCIATION LIMIT.

_x000d_ The He-HCN complex is a weakly bound complex with binding energy of about _x000d_ 9 cm$^{-1}$. We have measured the the $j=1 \leftarrow 0$ internal rotation fundamental band of the He-HCN complex _x000d_ by millimeter-wave absorption spectroscopy and reported the potential energy _x000d_ surface (PES) to reproduce the observed transition frequencies.\footnote{K.~Harada, K.~Tanaka, T.~Tanaka, S.~Nanbu, and M.~Aoyagi, _x000d_ J.~Chem.~Phys. {\bf 117}, 7041 (2002).} _x000d_ \par_x000d_ In the present study, we have extended the measurement to the $j=2 \leftarrow 1$ internal rotation hot bands of the He-HCN and He-DCN complexes. _x000d_ In the analysis, the upper state of several observed transitions are found to be located above the "dissociation limit" ($D_0$). _x000d_ The rovibtrational levels with $e$ label dissociate to the HCN molecule with $j=0$ and the He atom ($D_0$), while those with $f$ label, _x000d_ due to the parity conservation, to the HCN molecule with $j=1$ and the He atom which is higher in energy by about 2.96 $cm^{-1}$ ($2B_{\rm HCN}$) than $D_0$._x000d_ The $f$ levels are bound up to $D_0 + 2B_{\rm HCN}$. _x000d_ \par_x000d_ The revised PES of He-HCN has a global minimum in the linear He--HCN configuration with a depth of 29.9 cm$^{-1}$ and _x000d_ has a saddle point at the anti-linear He--NCH configuration with a depth of 20.9 cm$^{-1}$. _x000d_ The $\nu_s$ intermolecular stretching first excited state and the $j=2$ internal rotation second excited state _x000d_ are determined to be located 9.1405 and 9.0530 cm$^{-1}$ above the _x000d_ ground state and very close to the calculated dissociation limit ($D_0$) of 9.32 cm$^{-1}$. _x000d_ Life times of several quasi-bound levels (both of $e$ and $f$ labels) and line widths of the related transitions are predicted for He-HCN and He-DCN _x000d_ from the revised PESs. _x000d_ \\_x000d