DIRECT MEASUREMENT OF THERMAL RATE CONSTANTS FOR STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER IN COLLISIONS OF CN(X (2)SIGMA(+), V=2, N) WITH HE

Stimulated emission pumping state preparation and laser induced fluore scence state detection in the CN(B (2) Sigma(+)-X (2) Sigma(+)) violet system have been employed to study relaxation of single rotational st ates of CN(X (2) Sigma(+), v=2) in collisions with He at 295 K. Approx imately 2/3 of the value of the total removal rate constant for CN(X ( 2) Sigma(+), v=2, N=2, 11, or 14) corresponds to changes in the rotati onal quantum number of \Delta N\ less than or equal to 3 and a strong propensity is evident which favors even changes in N for \Delta N\ les s than or equal to 4. The measured rate constants are consistent with detailed balance, can be described with statistical power-gap and expo nential energy-gap fitting functions and, where compared, are in accor d with an IOS-based scaling function. No significant difference is fou nd between a sum of state-to-state rate constants and separately measu red total removal rate constants for N=2, 11, and 14. The total remova l rate constants decrease monotonically between N=0 and N=41 to about 1/3 of their value at N=0.