Vibrational Relaxation of Hydrogen Bromide in Gaseous Hydrogen Halide Mixtures

Abstract

A pulsed HBr chemical laser has been used to vibrationally excite HBr molecules in gaseous mixtures to their first vibrational excited state. The decay of infrared fluorescence following the laser pulse was studied. Cross sections for vibrational deactivation of HBr by HBr, HCl, n‐H2, p‐H2, and rare gases at room temperature have been found: σ HBr – HBr = (4.5 ± 0.4) × 10 −3 Å 2 , σ HBr – HCl = (8.1 ± 0.9) × 10 −3 Å 2 , σ HBr –n ‐H 2 = σ HBr –p ‐H 2 = (3.8 ± 0.2) × 10 −4 Å 2 , σ HBr – He ≤ (2.1 ± 0.2) × 10 −5 Å 2 , σ HBr – Ne ≤ (2.2 ± 0.2) × 10 −5 Å 2 , σ HBr – Ar ≤ (8.4 ± 0.7) × 10 −6 Å 2 . The cross sections for vibration‐to‐vibration energy transfer from HBr to HCl and HI are σ HBr – HCl = (4.56 ± 0.43) × 10 −2 Å 2 and σ HBr – HI = (1.86 ± 0.24) × 10 −1 Å 2 . The enhancement of probability for hydrogen halides at room temperature in Landau—Teller plots evident from extrapolating shock tube data available between 1800 and 800°K shows the importance of intermolecular forces. Direct comparison of vibrational relaxation rates among hydrogen halides (HCl, HBr, and HI) shows that the HCl – HCl interaction possesses a deeper attractive and steeper repulsive potential than HBr – HBr and HI – HI interactions. The small difference in cross section among rare gases indicates the unimportance of translational motion. Since equal cross sections are observed for n‐H2 and p‐H2, it is unlikely that energy is transferred into rotation of H2.