MILLIMETER WAVE SPECTROSCOPY OF THE INTERMOLECULAR STRETCHING BAND OF He-HCN.

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the observation of the intermolecular stretching excited state $(\\nu_{s})$ 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 $\\nu_{s}, \\nu_{s} \\leftarrow j = 1, j = 2 \\leftarrow 1$, and $j = 2 \\leftarrow 0$ bands were observed in the frequency region of $150-280$ GHz, where $\\nu_{s}$ 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 $band^{a}$ $(j = 1 \\leftarrow 0)$ were analyzed to improve an empirical intermolecular potential energy surface. The intermolecular stretching frequency obtained is $9.1618 cm^{-1}$, while the dissociation energy $(D_{0})$ is $9.4411 cm^{-1}$. The average distance $< R >$ 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 {\\AA} and 2.182 {\\AA} for the $\\nu_{s}, J = 1$ state, which is far longer and larger than those for the ground state $(4.306 \\tilde{A}$ and $0.576 \\tilde{A}$). 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 {\\AA}.