Nonadiabatic photodissociation dynamics in (HI)2 induced by intracluster collisions

Abstract

The sequential photodissociation dynamics of (HI)2 is studied by means of a nonadiabatic wave packet treatment starting from the I*-HI complex. The model reproduces the main experimental findings for photolysis with 266 nm radiation. The results confirm that some of the H atoms dissociated from the I*-HI complex deactivate the I* atom through a HI* intracluster collision which induces an I*-->I electronically nonadiabatic transition. As a consequence, these H fragments become very fast by acquiring nearly all the I* excitation energy, equivalent to the I*I spin-orbit splitting. A most interesting result is the high production of bound I2 fragments in highly excited rovibrational states in the photolysis, indicating that the H dissociation is mainly direct.

Effect of the Excitation Energy on the (HI)2 Nonadiabatic Photodissociation Dynamics

“…Wave packet simulations of the (HI) 2 nonadiabatic photodissociation were reported recently, , and they confirmed the mechanism proposed in ref to explain the appearance of the β peak in the H fragment spectrum. The model applied in the simulations took advantage of the sequential character of the (HI) 2 photodissociation dynamics.…”

“…In ref , the fragment state distributions produced with the 266 nm radiation wavelength used experimentally were reported . The I*−HI excitation energy reached with that wavelength was E = 13 244 cm -1 ( E = 0 corresponds to H + I* + I separated atoms).…”

“…The potential surfaces for the two excited electronic states are modeled as a sum of pairwise interactions . The H−I and H−I* interactions (associated with the A 1 Π 1 and a 3 Π 0 + electronic states, respectively) are described by ab initio potential curves, and the I−I and I−I* interactions (corresponding to the X 1 and B 3 Π u electronic states, respectively) are represented by empirical potentials. , The wave packet initially prepared in the I*−HI( A 1 Π 1 ) electronic state is propagated on the two coupled electronic surfaces, assuming zero total angular momentum for the system.…”

“…For each excitation energy, such degenerate states correspond to the product fragments H + I + I* and H + I*−I [i.e., I 2 ( B 3 Π u )] in the upper excited electronic surface, associated with I*−HI, and to the product fragments H + I + I and H + I−I [i.e., I 2 ( X 1 )] in the electronic surface populated after a nonadiabatic transition takes place (the one associated with I−HI). Details on the wave packet projection procedure have been given elsewhere. ,− In this way, the state distributions of the product fragments on the two electronic surfaces are obtained for a specific excitation energy.…”

“…A most interesting result of the distribution calculated for excitation with 266 nm radiation was the finding of a high probability of bound I 2 product fragments in highly excited rovibrational states. Indeed, about 89% of the photolysis on the I*−HI( A 1 Π 1 ) electronic surface produced bound I 2 ( B ) species, while more than half of the nonadiabatic transitions to the I−HI( A 1 Π 1 ) surface led to I 2 ( X ) products . It is stressed that highly excited bound Cl 2 product fragments were also found in the photodissociation of the Cl−HCl complex …”