Vibrational relaxation of HCl (v=1, 2, 3) in Ar, Kr, and Xe matrices

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Abstract
The spectroscopy and vibrational relaxation of matrix isolated HC1 (v=1,2,3) have been studied as a function of host (M=Ar, Kr, and Xe) and temperature (9–42 K). FTIR and laser‐excited fluorescence excitation spectra were used to determine vibrational frequencies for the v=0−1, 0−2, a n d 0−3 transitions. The anharmonicity is found to increase relative to the gas phase value with increasing host size and polarizability. Relaxation occurs stepwise, nonradiatively from v=3 to 2, 2 to 1, and 1 to 0. For a given vibrational state, the relaxation rate increases in the series k(Ar)<k(Kr)<k(Xe), i.e., with increasing spectral shift. At 20 K, k 1 0(Ar)=0.9, k 1 0(Kr)=1.2, and k 1 0(Xe)=1.6 ms− 1. The deviation from the harmonic approximation, k v,v−1 =v k 1 0, increases in the same sequence with host change. At 20 K, k 3 2/k 2 1/k 1 0=11/4.3/1 for Ar, 53/11/1 for Kr, and 260/33/1 for Xe. These results are in remarkable contrast with those for most other molecules and suggest the importance of an attractive interaction, especially for HC1/Xe, which increases strongly with HC1 bond length. The temperature dependences of the rates are of the same modest magnitude as found in other systems. The smooth trends of rates with vibrational quantum number and host suggest that there are not strong resonances for specific final rotation‐local phonon states. Both rotational and translational degrees of freedom probably receive a significant fraction of the vibrational energy transferred.