Knowledge Bank

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 $banda$ $(j=1\leftarrow 0)$ were analyzed to improve an empirical intermolecular potential energy surface. The intermolecular stretching frequency obtained is $9.1618cm-1$, while the dissociation energy $(D0)$ is $9.4411cm-1$. The average distance 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 $(4.306A~$ and $0.576A~$). 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}.