On the possibility of nonadiabatic transitions in the photodissociation of I2M clusters excited above the dissociation limit of the B state

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Abstract
The reaction I2M→hν I2(B,v’, j’)+M has been studied experimentally for excitation above the dissociation limit of the I2M B state. A surprisingly large amount of the available energy is found as relative translational energy of the I2 and M products. These results have been interpreted in terms of a one atom ‘‘cage effect,’’ where the iodine atoms are prevented from dissociating by the presence of the rare gas atom M. A purely kinematic cage effect could occur on a single electronically excited potential energy surface, namely the one correlating to the I2(B) state plus M in its electronic ground state. In this paper we discuss another possible mechanism for a pathway leading to bound I2, which involves an electronic nonadiabatic transition. Above the I2M(B) threshold the 1Π1u electronic state can also be excited. Since the 1Π1u and the B states can be coupled by the presence of the rare gas atom, there is a finite probability for an electronic transition from 1Π1u to B, with the energy difference being transformed into relative kinetic energy of the rare gas atom with respect to I2 after a fraction of the available energy has been used to break the van der Waals bond. The relationship between this mechanism and the electronic predissociation of I2M(B) van der Waals molecules at much lower energies, as well as the collision induced electronic predissociation of I2(B), are also mentioned. Finally the possibility of observing similar transitions in other halogen–rare gas clusters is considered.

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