INFRARED AND MICROWAVE STUDY OF ANGULAR-RADIAL COUPLING IN Ar-HCN

$^{1}$ K.R. Leopold, G.T. Fraser, F.J. Lin, D.D. Nelson, Jr., and W. Klemperer, J. Chem. Phys. 81. 4922 (1984). $^{2}$ T.D. Klots, C.E. Dykstra. and H.S. Gutowsky, J. Chem. Phys. 90, 30 (1989).Author Institution: Molecular Spectroscopy Division, National Institute of Standards and TechnologyMicrowave and infrared spectra of Ar-HCN have been obtained using an electric-resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold $et al.^{1}$ and Klots $et al.^{2}$, allowing the determination of higher-order centrifugal distortion constants for this quasilinear, highly non-rigid complex. A Pad\'{e} approximate fit to the microwave data indicates a centrifugal alignment of the HCN subunit with the Ar-HCN axis due to angular-radial coupling. Infrared spectra are reported for the C-H stretching fundamental, $\nu_{1}$, and the combination band $\nu_{1}+\nu_{5}{^{1}}$, where $\nu_{5}$ is the van der Waals bending vibration. The band-origin difference between these two bands gives $\nu_{5} = -7.8 cm^{-1}$, in rough agreement with the $10 cm^{-1}$ harmonic value predicted from the microwave-determined nuclear quadrupole coupling constant. The complexation-induced shift of the C-H stretching vibration is $2.69 cm^{-1}$ and the vibrational predissociation linewidths. $\Gamma$. are $< 10 MHz$. The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime, $r < - 1 ms$