Infrared Double Resonance in Sulfur Hexafluoride

The relaxation of sulfur hexafluoride has been monitored by observation of infrared absorption intensities following passage of an infrared laser pulse through the gas. The laser pulse saturates a small number of rotational lines in the $ν3 ← 0$ transition of SF₆ at 944 cm⁻¹, producing a transient “hole” in the absorption spectrum at that frequency. This hole is filled in very rapidly by rotational relaxation processes. The specific vibrational excitation is, at essentially the same time, transformed into a vibrational temperature in excess of the gas's translational temperature, by means of very efficient vibration ↔ vibration energy transfer collisions. The energy released in this step also heats the gas translationally by the order of 25°K. The vibrational temperature then relaxes to the translational temperature by a binary collision process, with $pτ = (122 ± 8) μsec·torr$ in pure SF₆. The rate‐controlling step is concluded to be the vibration→translation relaxation of the $ν6$ level at 363 cm⁻¹. The mean efficiency per collision of this step has been measured for He, Ne, Ar, Kr, C₂H₆, (CH₃)₂O, CH₄, CH₃Br, CHF₂Cl, H₂O, H₂, N₂, and Cl₂, in addition to SF₆ itself. A transient absorption spectrum arising from the vibrationally hot SF₆ is also given. The slowest relaxation observed in the system is the bulk cooling of the translational temperature of the gas, which occurs in the order of milliseconds under typical experimental conditions. The dominant mechanism for this relaxation appears to be cooling by thermal conduction.