Inverted Vibrational and Rotational Excitation of CN(BΣ) 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(BΣ) and M(sS) ( = 5, 4, and 3 for Rb, K, and Na, respectively) in collision with Kr metastable atoms, Kr(P). Both the vibrational and rotational distributions of CN(BΣ) produced in the reactions of RbCN and KCN were inverted as being peaked at = 1 and ' = 35, respectively, where ' and ' are the vibrational and the rotational quantum numbers of CN(BΣ), respectively. According to a state crossing model, it was derived that CN(BΣ) is produced by predissociation through a superexcited ion-pair state, CN(3Σ)·M(S), followed by an adiabatic transition to a repulsive state correlating to the dissociation limit of CN(BΣ) + M(sS). The inverted distributions are driven by structural changes during the excitation and the adiabatic transition. The maximum vibrational population at ' = 1 originates from a large Franck-Condon overlap between the vibrational wavefunctions of CN(3Σ) and CN(BΣ) at ' = 1. The rotational excitation of the CN(BΣ) 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.