Vibration→Rotation Energy Transfer in Hydrogen Chloride

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Abstract
A pulsed HCl chemical laser source has been used to carry out laser‐excited vibrational fluorescence experiments on HCl and mixtures of HCl with DCl, n‐H2, p‐H2, rare gases, and H2O. The following cross sections σA–B for vibrational deactivation of A by B at room temperature have been found: σHCl–HCl = (4.2 ± 0.4) × 10−19cm2, σDCl–DCl = (1.3 + 0.2, − 0.4) × 10−19cm2, σDCl–HCl = (2.9 ± 0.3) × 10−19cm2, σHCl–n‐H2 = σHCl–p‐H2 = (2.9 ± 0.5) × 10−20cm2, σHCl–rare gas < 10−21cm2 , and σHCl–H2O = (2 ± 1) × 10−16cm2 . The cross sections for HCl–HCl and DCl–DCl are about 16 times larger than predicted by linear extrapolation of the shock tube data between 2000° and 1000°K on a log σ vs T−1/3 plot. The isotopic changes in vibrational relaxation rates lead to the conclusions that: (1) Vibrational energy is transferred almost entirely into rotation in hydrogen chloride–hydrogen chloride collisions, and (2) for HCl–HCl and DCl–DCl collisions most of the vibrational energy is transferred into the rotation of the molecule which is initially vibrationally excited. A plausible explanation for these results is provided by the hydrogen‐bonding interaction between two HCl molecules.