PRODUCTION OF VIBRATIONALLY EXCITED CN(B-2-SIGMA(-PAIR STATE OF TRIATOMIC ALKALI-METAL CYANIDES BY AR(P-3(2,0)) IMPACT()) VIA SUPEREXCITED ION)

Emission spectra from CN(B(2)Sigma(+)) produced by dissociative excita tion of MCN (M = Rb, K, Na) in collision with Ar-m(P-3(2,0)) were obse rved, and the mechanism of the energy partition between the two fragme nts was elucidated. The vibrational distribution of the CN(B(2)Sigma()) product is composed of two distinct components, P-L and P-H, ranged in the vibrational levels of v' = 0-3 and 11-19, respectively, where v' is the vibrational quantum number of CN(B(2)Sigma(+)). The componen ts, P-L and P-H, arise from direct dissociation and predissociation of MCN by Ar-m(P-3(2,0)) impact, respectively. The direct dissociation p roceeds on a repulsive potential energy surface correlating diabatical ly to M(ns(2)S) + CN(B(2)Sigma(+)) (n = 5, 4, 3 for M = Rb, K, Na, res pectively). This mechanism was further supported by a molecular dynami cs simulation. The predissociation, on the other hand, proceeds via a superexcited ion-pair state, M+.[CN-]*, having a much longer equilibr ium C-N internuclear distance than that of CN(B(2)Sigma(+)), so that m ore than 90% of the excess energy is transmitted in the vibrational de gree of freedom of the CN(B(2)Sigma(+)) product. In a framework of a s tate-crossing model, the extremely high vibrational excitation is expl ained by a large overlap between the vibrational wave functions of the superexcited [CN-]* and CN((BC+)-C-2).