Measurements of Collisional Energy Transfer between Rotational Energy Levels in CN

Abstract

A microwave-optical technique which selectively populates a single rotational level of CN and permits the observation of the redistribution of this population was utilized to measure the rates of collisional energy transfer between rotational energy levels of the B2Σ state of CN. CN was formed in the A 2Π state by the addition of CH2Cl2 to the afterglow of a nitrogen discharge. The near coincidence of the K=4, v=10 level of the A 2Π state with the K=4, v=0 level of the B 2Σ state permits microwave transitions near 10 GHz from the more populated Π level to the Σ level. The increased population in the rotational levels neighboring the K=4 level of the B 2Σ state was detected by measuring the increased optical emission due to the B 2Σ—A 2Σ transition near 3875 Å. Collisional energy transfer was measured over a pressure range from 0.1 to 5 Torr for changes in rotational quantum number ranging from one to ten. It is shown that rotational transitions having changes in rotational quantum number greater than unity take place with high probability, contrary to the optical selection rule ΔK=±1, and that approximately every gas kinetic collision produces a rotational transition. The relaxation time for the fourth rotational level was found to be 1.2×10−7 (±30%) sec at a pressure of 1 Torr.