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

Abstract

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.

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.…”

“…The calculated β peak accounted for 0.15% of the H-fragment kinetic energy distribution, in good qualitative agreement with the experimental estimate of 0.1−1% for the signal of this peak . The simulations indeed showed that the β peak is produced by nonadiabatic transitions induced by relatively weak intracluster collisions within the I*−HI complex, between the nascent H fragment and I*, which lead to deactivation of I* . 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.…”

“…Then, the second step of the sequential photodissociation dynamics, namely, the photodissociation of the remaining HI subunit within I*−HI, was simulated by means of a three-dimensional nonadiabatic wave packet treatment. Such a treatment involved photodissociation of the system on two coupled excited electronic surfaces corresponding to the I*−HI( A 1 Π 1 ) and I−HI( A 1 Π 1 ) complexes …”

New insights into the semiclassical Wigner treatment of photodissociation dynamics

“…(2) d.e by the modulous d of d, which amounts to suppose that d is parallel to e at the instant of the photon absorption. This approximation appears to be very satisfying provided than one is not interested in vector properties [6,26,30,[41][42][43]. d is either parallel or perpendicular to the triatomic plane and d does only depend on the configuration of ABC, i.e., d ≡ d(R, r, θ).…”

Semiclassical Wigner theory of photodissociation in three dimensions: Shedding light on its basis

“…9 Assuming the electronic problem of a process under scrutiny has been solved by the usual methods of quantum chemistry [10][11][12] and the potential energy surfaces (PESs) involved in the process are available as well as their possible nonadiabatic couplings, state-of-the-art descriptions of its dynamics are performed within the framework of rigorous quantum treatments of nuclear motions. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] More often than not, however, these treatments can hardly be applied to polyatomic processes involving more than three atoms, for the basis sizes necessary to converge the calculations may either be prohibitive, or require computation times of several months. a) Author to whom correspondence should be addressed.…”