Inverted Vibrational and Rotational Excitation of CN(B²Σ⁺) Produced through Superexcited Ion-Pair States of MCN (M =...

This paper reports an energy-partition mechanism in dissociative excitation of alkali cyanide molecules, MCN (M = Rb, K, Na), to produce CN(B²Σ⁺) and M(ns²S) (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(B²Σ⁺) 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(B²Σ⁺), respectively. According to a state crossing model, it was derived that CN(B²Σ⁺) is produced by predissociation through a superexcited ion-pair state, CN^-(3¹Σ⁺)·M⁺(¹S), followed by an adiabatic transition to a repulsive state correlating to the dissociation limit of CN(B²Σ⁺) + M(ns²S). 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^-(3¹Σ⁺) and CN(B²Σ⁺) at v' = 1. The rotational excitation of the CN(B²Σ⁺) 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.