10min:

KENSUKE HARADA, ASAO MIZOGUCHI, STEPHANE BAILLEUX AND KEIICHI TANAKA, Institute for Molecular Science, Myodaiji, Okazaki, 444-8585 Japan.

The Ar-HBr cluster has a large amplitude intermolecular bending motion. The excited states of the van der Waals vibrations were observed by infrared and far infrared laser spectroscopy. The cluster has a nearly "linear" structure of Ar-HBr in the ground state ( ₀ state), while the cluster is estimated to have a nearly "anti-linear" structure of Ar-BrH in the first excited state ( ₁ state) of the intermolecular bending vibration.

In the present study, we have observed submillimeter-wave transitions of the ₁ - ₀ band of Ar-HBr generated in a pulsed supersonic jet expansion. The observed band origins of the ₁ - ₀ bands of Ar-H⁷⁹Br and Ar-H⁸¹Br are 6 and 27 MHz lower than those reported by combination differences of infrared data^a. The eQq constants in the 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 calculation. The cos^-1 \sqrt\langle cos² \rangle value is 144.3^o in the ₁ state, which is different from the value, 42.1^o, in the ground state, where the is an angle between a cluster axis and a HBr monomer axis.