Measurement of Pressure-Broadening Parameters for the CO-He System at 4 K

A technique has been developed to observe collisions in a regime where the rotational energy-level separation is greater than the thermal energy. Pressure-broadening parameters of trace quantities of CO, thermalized by collisions with helium at 4 K, were measured and found to be 10.1 and 11.2 MHz/Torr (half width at half maximum) for the $J=0-1\mathrm{}$ and $J=1-1\mathrm{}$ transitions, respectively. At 4 K only a small number of collision channels are energetically available and comparisons between experiment and theory are facilitated.