On the possibility of nonadiabatic transitions in the photodissociation of molecular iodine-rare gas atom complex (I2M) clusters excited above the dissociation limit of the B state

The reaction I2M + hn -> I2(B,v',j') + M (M = rare gas atom) was studied exptl. for excitation above the dissocn. limit of the I2M B state. A surprisingly large amt. of the available energy is found as relative translational energy of the I2 and M products. These results were interpreted in terms of a one atom cage effect, where the I atoms are prevented from dissocg. by the presence of 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. Another possible mechanism for a pathway leading to bound I2, which involves an electronic nonadiabatic transition is discussed. Above the I2M(B) threshold the 1P1u electronic state can also be excited. Since the 1P1u 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 1P1u 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 was used to break the van der Waals bond. The relation between this mechanism and the electronic predissocn. of I2M(B) van der Waals mols. at much lower energies, as well as the collision induced electronic predissocn. of I2(B), are also mentioned. The possibility of observing similar transitions in other halogen-rare gas clusters is considered.