Knowledge Bank

The Ar-HBr cluster has a large amplitude intermolecular bending motion. The excited states of the van der Waals vibrations were observed by $infrareda$ and far $infraredb$ laser spectroscopy. The cluster has a nearly linear'' structure of Ar-HBr in the ground state ($\Sigma_{0}$ state), while the cluster is estimated to have a nearly anti-linear'' structure of Ar-BrH in the first excited state ($\Sigma 1$ state) of the intermolecular bending vibration. In the present study, we have observed submillimeter-wave transitions of the $\Sigma 1-\Sigma 0$ band of Ar-HBr generated in a pulsed supersonic jet expansion. The observed band origins of the $\Sigma 1-\Sigma 0$ bands of $Ar-H79Br$ and $Ar-H81Br$ are 6 and 27 MHz lower than those reported by combination differences of infrared $dataa$. The cQq constants in the $\Sigma 1$ states were determined for the first time to be $260.90(12)$ and $217.854(98)MHz$ for $Ar-H79Br$ and $Ar-H81Br$, respectively, which agree well with the estimated values (238 and 199 MHz for $Ar-H79Br$ and $Ar-H81Br$) from a potential $calculationc$. The $cos-1⟨\cos 2\theta ⟩$ value is $144.3c$ in the $\Sigma 1$ state, which is different from the value, $42.1\circ$, in the ground state, where the $\theta$ is an angle between a cluster axis and a HBr monomer axis.