Modeling the (HI)2 photodissociation dynamics through a nonadiabatic wave packet study of the I*-HI complex

The nonadiabatic photodissociation dynamics of (HI)2 is simulated by applying a wave packet approach which starts from the I*-HI complex (where I* denotes the I(2P1/2) excited electronic state) produced after the photodissociation of the first HI moiety within (HI)2. In the model, two excited electronic potential surfaces corresponding to I*-HI(A 1Pi1) and I-HI(A 1Pi1), which interact through spin-rotation coupling, are considered. The simulations show that upon photodissociation of HI within I*-HI, the dissociating H fragment undergoes intracluster collisions with the I* atom. Some of these collisional events induce an electronically nonadiabatic transition which causes the deactivation of I* to the I ground electronic state. The probability of such nonadiabatic process is found to be 0.37%. Most of the photodissociation process takes place in the upper excited electronic surface [that of the I*-HI(A 1Pi1) complex], where H dissociation is found to be mainly direct or involving weak H/I* intracluster collisions. These weak collisions with high collisional angular momentum, and therefore high collisional impact parameters associated, are responsible for most of the probability of nonadiabatic transitions found. The type of H/I* collisions leading to nonadiabatic transitions appears to be closely related to the nature of the spin-rotation coupling between the two excited electronic states involved.