Collisional cooling between 5 and 20 K: Low-temperature helium pressure broadening of CH3F

Abstract

A recent adaptation of the collisional cooling technique which permits gas-phase spectroscopy from 5 K to ≥20 K is described. The new apparatus was used to measure pressure broadening parameters and cross sections for the the J=0–1, K=0 rotational transition of CH3F broadened by helium from 5 to 21 K. The cross sections show a general upward trend with decreasing temperature ranging from 55.9 Å2 at 20 K, to 114.0 Å2, at 5 K. This compares with a 295 K cross section of 48.8 Å2. While an accurate CH3F–He potential surface is not presently available for calculating theoretical cross sections, the rise in cross section at low temperature can be attributed to the dominance of resonant collisional processes at very low energy. Although resonant structure in many systems (e.g., CO–He, H(D)Cl–He) appears to be smoothed out by Boltzmann averaging, there is experimental evidence that at least one resonance may survive the thermal average in the CH3F–He system.