Knowledge Bank

Recent low temperature pressure broadening results will be presented for low lying rotational transitions in $CH3F$ and OCS broadened by helium. These results were obtained using the collisional cooling technique at temperatures between 5 and 20 K. In the absence of reliable potential surfaces for OCS - He and $CH3F$ - He experimental results could not be compared to theoretical calculations. However, comparisons will be made to earlier low temperature experimental and theoretical results for CO- He and H(D)CI - He and experimental results for $CH3F$ - He in the 1.8 - 4.2 K region. Such comparisons can give insights into the effect of resonances on low temperature cross sections and their relation to rotational energy level spacings as well as the transition from the semi-classical ``hard sphere” scattering regime at intermediate temperatures to the resonance-dominated region at very low temperatures.