Spectrum and vibrational predissociation of the HF dimer. II. Photodissociation cross sections and product state distributions

Abstract

We study vibrational predissociation of the HF dimer both by a full coupled channels treatment as well as in the Fermi golden rule approximation. Photodissociation cross sections, linewidths, and rotational state distributions are computed for excitations from the ground state with rotational quantum numbers Jϭ1, Kϭ0 to monomer stretch excited states with JϭKϭ0, both for even and odd permutation symmetry. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the dimer stretch and dimer geared-bending modes. We find that dissociation is sufficiently slow for the Fermi golden rule approximation to be valid. The resonance positions and line strengths are compared with quasibound state calculations. The agreement with experimental data is fairly good for the photofragment angular distributions that were determined from the rotational state distributions, less good for some of the linewidths. Since we carefully checked that the results are converged with respect to the number of vibrational and rotational channels included, the remaining discrepancies are almost certainly due to small deficiencies in the SO-3 potential used in the calculations.

Vibrational predissociation in the HCl dimer

“…In a previous study on the vibrational predissociation of HF dimer it was shown that the Fermi golden rule approximation gives linewidths and product-state distributions that are in good agreement with full coupled channels calculations. 42 The same study also showed that the use of quasibound states to compute line strengths for the transitions to the vibrationally predissociating states is justified. We therefore applied the same approximations to the HCl dimer.…”

“…As in our work on the photodissociation of the HF dimer, 42 the scattering wave function JM (Ϫ) j A j B j AB l is expanded in a space-fixed basis ͉( j A j B ) j AB l;JM ͘ SF , which is related to the body-fixed basis via a unitary transformation:…”

“…1͒ has been described previously in our paper on the HF dimer. 42 It consists of the sum of the two monomer Hamiltonians, a dimer kinetic energy term, and the interaction potential:…”

“…In several previous publications 29,36,42 it was shown that the final HF-fragment rotational-state distribution resulting from vibrational predissociation of HF dimer can be understood in terms of the asymmetric equilibrium structure, which makes the proton donor and acceptor molecules in the parent complex quite different. In particular, the proton donor molecule receives a large torque as the molecules recoil, while the force on the proton acceptor acts nearly through its center of mass.…”

“…Although there are a large number of papers on the vibrational predissociation of the HF dimer, 21,[29][30][31][32][33][34][35][36][37][38][39][40][41][42] the corresponding data set for the HCl dimer is rather small. While there are a number of experimental studies that have been published, 28,[43][44][45][46][47][48] theoretical efforts have been restricted to bound-state calculations of the vibration-rotationtunneling levels of the HCl stretch excited dimer.…”