Low temperature pressure broadening of OCS by He

We report experimentally measured cross sections for pressure broadening of OCS by He from 4.2 to 23 K. These measurements were made in a quasiequilibrium cell using the collisional cooling technique. Cross sections were obtained for the broadening of the J=2<--1, J=3<--2, and J=4<--3 rotational transitions of OCS. Theoretical cross sections were also calculated using a recent He-OCS potential surface. While at the higher temperatures, approximately 20 K, there is only modest disagreement between experiment and theory, this disagreement increases steadily with decreasing temperature, with predicted cross sections rising steeply while the experimental data remains constant or decreases slightly. Comparisons to similar recent experimental studies are made and reasons for the observed discrepancy between experiment and theory are considered.