A time-dependent approach to flux calculation in molecular photofragmentation: Vibrational predissociation of HF-DF

We present in this paper a time-dependent approach to the calculation of photofragmentation dynamics using the flux formulation. The method is essentially a time-dependent version of the flux formulation for photodissociation calculation recently pursued by Manolopoulos and Alexander. In the present approach, the partial decay width of photofragmentation is obtained by calculating the flux at a given surface using a time-dependent method. This particular time-dependent approach for photofragmentation has two principal advantages. First, it is superior in computational scaling: CPU time ∝N^α(α<2) vs N³ in standard time-independent propagation method. Second, it is quite straightforward to handle the photofragmentation process involving rearrangement with the application of optical potentials. In addition, no bound state projection is necessary using the time-dependent flux method, which is required using the time-dependent golden rule method. This time-dependent method is applied to the calculation of decay width for vibrational predissociation of hydrogen-bonded HFDF, and the computed lifetime are compared with the recent experimental measurement of Farrell and Nesbitt. We also present the results of the full dimensional (6D) calculation of bound state energies for the HFDF complex. The exact dissociation energies are calculated to be 1057.33 cm^-1 for (HF)₂, 1166.6 cm^-1 for (DF)₂, 1142.7 cm ^-1 for HF-DF, and 1078.4 cm^-1 for DF-HF. All theoretical calculations have used the SQSBDE potential energy surface due to Quack and Suhm.