Spin–orbit branching in the photodissociation of HBr: Time-independent, time-dependent, and semiclassical calculations

Abstract

The dynamics of the photofragmentation of HBr is treated within time-independent, time-dependent, and semiclassical methods. The calculated relative cross sections for formation of the two accessible fine-structure channels [Br(2P1/2) and Br(2P3/2)] agree well with the experimental results, both in magnitude and in dependence on photon excitation wavelength. For relatively small photon wavelength (λ=193 nm), vertical excitation in the Franck–Condon region populates preferentially the A 1Π state, and only three states (A 1Π, the Ω=1 components of the a 3Π and 1 3Σ+), coupled by the spin–orbit interaction, are invoved in the dissociation process. For larger photon wavelength (λ=243 nm), the product branching is governed by initial excitation in both the A 1Π state and the a 3Π(Ω=0) component. Comparison of the redistribution of the time-independent photofragment fluxes as a function of the H–Br separation with the temporal evolution of the populations within a time-dependent framework shows that the two methods, although based on a different point of view, provide equivalent mechanistic information on the dissociation process.

Spin–orbit branching in the photofragmentation of HCl at long wavelength

“…As shown in Ref. 3, this time-independent approach can reproduce all the mechanistic information given by time-dependent methods, 14 while providing monoenergetic cross sections and branching ratios. All calculations were carried out with our HIBRIDON 4 code.…”

“…Four hydrogen halide potential energy curves ͑ 1,3 ⌺ ϩ and 1,3 ⌸͒ correlate with the H( 2 S)ϩX( 2 P) asymptote, as shown in Fig. 1 for HCl.…”

“…Figure 2 displays the total photodissociation cross sections for HCl and DCl for excitation energies below 70 000 cm Ϫ1 , determined by the calculations reported here. Branching of the nascent photofragmentation flux among the other electronic states occurs nearly exclusively at long range, [1][2][3] where the definite-⌳ electronic states coalesce, and become significantly mixed by the atomic spin-orbit Hamiltonian, which is the dominant term at long range.…”

Two-photon state selection and angular momentum polarization probed by velocity map imaging: Application to H atom photofragment angular distributions from the photodissociation of two-photon state selected HCl and HBr

“…In the single-photon A-band absorption spectrum of HBr centered around 243 nm one can hence find a considerable parallel component. 46,45 This contribution was reported to be between 8% and 20% in theoretical and experimental studies. 44 -46 Following the two-photon state selection, absorption of a third photon causes dissociation of HBr into H*(nϭ2) and Br( 2 P 1/2 ) as well as a second weaker channel producing spin-orbit ground state Br( 2 P 3/2 ).…”

Investigation on the photofragmentation of hydrogen chloride

“…In the case where the excitation amplitude stems from the initial low rotational level (m 00 = 0), the J-dependent Coriolis couplings are weak, and these interference terms become negligible. Similarly, the effect of rotation on the photofragmentation was shown in prior studies of analogous molecules [31] to be minor. The second category of transition is a homogenous transition (DX = 0) induced by the radial derivative coupling, which means a non-adiabatic transition from the A 1 P 1 state to the t 3 R þ 1 state may take place spontaneously due to the small spin-orbit splitting, and this seems to be the dominant factor, as the slope of the latter potential is steeper than the former.…”