Vibrational relaxation of matrix-isolated HCl and DCl

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Abstract
Using infrared laser‐excited vibrational fluorescence, lifetimes have been measured for HCl (v=1,2) in Ar as a function of concentration and temperature, and for dilute samples of DCl (v=2) in Ar as a function of temperature. For dilute samples at 9 K, the nonradiative decay rates (s−1) are (8.1±0.7) ×102 and (3.8±0.4) ×103 for HCl (v=1,2), and (1.2±0.2) ×102 for DCl (v=2), respectively (the DCl decay may be partially radiative). These rates increase slightly (less than a factor of 2) between 9 and 21 K. The results are consistent with a relaxation process in which the rate‐determining step is intramolecular V→R transfer. No fluorescence signal is observed upon excitation of the overtone of the HCl dimer in Ar implying that relaxation of the dimer is faster than 0.3 μs. In concentrated samples the relaxation rate for HCl (v=1) (but not v=2) increases, due to energy‐migration aided V→V transfer to dimers, which are an energy sink. It is argued that this system belongs to the ’’fast migration’’ regime. In N2 and O2 matrices, there is no fluorescence following excitation of HCl (v=2), presumably due to rapid V→V transfer to the host. Spectroscopicmeasurements of overtone absorption transitions are reported. A measurement of the integrated absorption coefficient for HCl in Ar indicates that the transition dipole moment differs less than 9% from the gas phase value.