Emission spectra from CN(B(2)Sigma(+)) produced by dissociative excitation of MCN (M = Rb, K, Na) in collision with Ar-m(P-3(2,0)) were observed, and the mechanism of the energy partition between the two fragments 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 components, 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 proceeds on a repulsive potential energy surface correlating diabatically to M(ns(2)S) + CN(B(2)Sigma(+)) (n = 5, 4, 3 for M = Rb, K, Na, respectively). This mechanism was further supported by a molecular dynamics simulation. The predissociation, on the other hand, proceeds via a superexcited ion-pair state, M+.[CN-]**, having a much longer equilibrium C-N internuclear distance than that of CN(B(2)Sigma(+)), so that more than 90% of the excess energy is transmitted in the vibrational degree of freedom of the CN(B(2)Sigma(+)) product. In a framework of a state-crossing model, the extremely high vibrational excitation is explained by a large overlap between the vibrational wave functions of the superexcited [CN-]** and CN((BC+)-C-2).