Direct measurement of thermal rate constants for state-to-state rotational energy transfer in collisions of CN(<i>X</i> 2Σ+, <i>v</i>=2, <i>N</i>) with He

Abstract

Stimulated emission pumping state preparation and laser induced fluorescence state detection in the CN(B 2Σ+–X 2Σ+) violet system have been employed to study relaxation of single rotational states of CN(X 2Σ+, v=2) in collisions with He at 295 K. Approximately 2/3 of the value of the total removal rate constant for CN(X 2Σ+, v=2, N=2, 11, or 14) corresponds to changes in the rotational quantum number of ‖ΔN‖≤3 and a strong propensity is evident which favors even changes in N for ‖ΔN‖≤4. The measured rate constants are consistent with detailed balance, can be described with statistical power-gap and exponential energy-gap fitting functions and, where compared, are in accord with an IOS-based scaling function. No significant difference is found between a sum of state-to-state rate constants and separately measured total removal rate constants for N=2, 11, and 14. The total removal rate constants decrease monotonically between N=0 and N=41 to about 1/3 of their value at N=0.