Ab initio simulation of molecular beam experiments for the F+H-2->HF+H reaction

The celebrated 1985 molecular beam measurements for the F + H2 reaction of Lee and co-workers, consisting of time-of-flight (TOF) spectra and angular distributions (AD) at several collision energies, have been directly simulated using fully resolved differential cross-sections (DCS) obtained in accurate quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations on the most recent ab initio potential energy surface (PES) by Stark and Werner. The simulations performed using the QM calculations show an unprecedentedly good agreement with the experimental results for all final vibrational states of the HF product. In particular, the height of the peak in the experimental laboratory angular distribution corresponding to HF(v′=3) forward scattering is quite well reproduced by the simulation using the QM theoretical data at all three experimental collision energies for both para and normal hydrogen. The most important discrepancies between theory and experiment are found in the HF(v′=3) sideways and backward scattering. The simulations carried out with the QCT data, although accounting correctly for the backward scattering, fail to account for most of the v′ = 3 forward scattering. The analysis performed in this work indicates that an unbiased comparison between theoretical and experimental results in the laboratory frame (as opposed to the center-of-mass frame) is required to assess the quality of a theoretical calculation on a given ab initio PES.