Inverted Vibrational And Rotational Excitation Of Cn(B-2 Sigma(+)) Produced Through Superexcited Ion-Pair States Of Mcn (M = Na, K, Rb)

This paper reports an energy-partition mechanism in dissociative excitation of alkali cyanide molecules, MCN (M = Rb, K, Na), to produce CN(B-2 Sigma(+)) and M(ns(2)S) (n = 5, 4, and 3 for Rb, K, and Na, respectively) in collision with Kr metastable atoms, Kr-m(3P(2,0)). Both the vibrational and rotational distributions of CN(B2 Sigma+) produced in the reactions of RbCN and KCN were inverted as being peaked at v' = 1 and N' = 35, respectively, where v' and N' are the vibrational and the rotational quantum numbers of CN(B-2 Sigma(+)), respectively. According to a state crossing model, it was derived that CN(B(2)v(+)) is produced by predissociation through a superexcited ion-pair state, CN-(3(1)Sigma(+))center dot M+(1S), followed by an adiabatic transition to a repulsive state correlating to the dissociation limit of CN(B-2 Sigma(+)) + M(ns(2)S). The inverted distributions are driven by structural changes during the excitation and the adiabatic transition. The maximum vibrational population at v' = 1 originates from a large Franck-Condon overlap between the vibrational wavefunctions of CN-(3(1 Sigma+)) and CN(B2 Sigma+) at v' = 1. The rotational excitation of the CN(B-2 Sigma(+)) product is explained with changing from a T-shape geometry of MCN in the ground state to a linear one in the superexcited ion-pair state.