Collisional intramolecular energy transfer CN(X 2Σ→A 2Π) in a beam

The collision-induced intramolecular energy transfer from CN(X 2Σ+,v″) to CN(A 2Π) was observed in a thermal energy molecular beam through the CN(A→X) emission. In some preliminary experiments, CN(B→X) emission, resulting from collision-induced energy transfer from CN(X 2Σ+) to CN(B 2Σ+), was also observed. CN(X,v″) radicals were generated in a dc discharge burning in the expansion region of an Ar/CH3CN supersonic beam. Downstream from the source they interacted with a target gas in a collision cell. The relative formation rates of CN(A) with He, Ne, Ar, and Kr as collision gases were obtained. The relative vibrational population distribution of CN(A,v′=2–12) was also determined. It was found to be very similar for the four collision gases. In the wavelength region of the well-known perturbations between the A and X states, the rotational contours of the A→X emission bands show no pronounced enhancement. This demonstrates that in the spin-allowed collision-induced transition CN(A←X) the so-called gateway mechanism is much less important than in some spin-forbidden transitions [N2(B←A′) and NO(B←a)] studied previously by us.