Vibration → Vibration Energy Transfer in CO2-Hydrogen Halide Mixtures

The relaxation of the asymmetric stretching vibration of CO₂ was observed in mixtures with HCl, DCl, HBr, DBr, HI, and DI. The observed cross sections were relatively large for vibration‐vibration exchange between CO₂(00⁰1) and HX (v=1). For HCl and DCl the cross sections were about 0.5 Ų and decreased slightly with increasing temperature in the range 298–510°K. The cross sections decrease more rapidly with increasing energy difference between the vibrations for the deuterium halides than for the hydrogen halides. Energy transfer to the HX or DX rotational degree of freedom may be responsible for this difference. Existing approximate theories of vibrational energy transfer due to long range multipolar forces do not quantitatively explain the data. However, for ${\mathrm{CO}}_2–\mathrm{HBr}$ , ${\mathrm{CO}}_2–\mathrm{DCl}$ , and ${\mathrm{CO}}_2–\mathrm{HI}$ , calculations show that it is possible that multipolar interactions could make significant contributions. The rate at which vibrational energy is removed from the CO₂(00⁰1) and HX (v=1) levels is usually much slower than the exchange between them. The deactivation rates due to CO₂‐hydrogen halide collisions could usually only be determined as a weighted sum of the deactivation of CO₂(00⁰1) by HX and HX (v=1) by CO₂. Rates for deactivation of CO₂(00⁰1) by HCl and of DBr (v=1) by CO₂ were established at room temperature within 25% ranges. The deactivation rates are much faster than for CO₂(00⁰1) with rare gases and molecules such as N₂. The data suggest the hypothesis that energy transfer among the vibrations of CO₂ is aided by attractive forces and transfer to rotation in CO₂(00⁰1)‐hydrogen halide collisions.