Abstract
Integral cross sections and rate constants for the prototypical chemical reactions of the fluorine atom with molecular hydrogen and deuterium have been calculated over a wide interval of collision energy and temperature ranging from the sub-thermal (50 K) down to the ultra-cold regimes (0.5 mK). Rigorous close coupling time-independent quantum reactive scattering calculations have been carried out on two potential energy surfaces, differing only at long-range in the reactants' channel. The results show that tunnel, resonance and virtual state effects enhance under-barrier reactivity giving rise to pronounced deviations from the Arrhenius law as temperature is lowered. Within the ultra-cold domain (below 1 mK), the reactivity is governed by virtual state effects and by tunneling through the reaction barrier; in the cold regime (1 mK–1 K), the shape resonances in the entrance channel of the potential energy surface make the quantum tunneling contribution larger so enhancing cross sections and rate constants by about one order of magnitude; at higher temperatures (above 10 K), the tunneling pathway enhanced by the constructive interference between two Feshbach resonances trapped in the reaction exit channel competes with the thermally activated mechanism, as the energy gets closer to the reaction barrier height. The results show that at low temperatures cross sections and rate constants are extremely sensitive to small changes in the long-range intermolecular interaction in the entrance channel of the potential energy surface, as well as to isotopic substitution.
Highlights
The F + H2 reaction has been extensively studied for many years from a variety of perspectives, as reported in numerous papers and reviews appeared in the literature
In the quantum mechanical low-temperature regime, chemical reactivity is most sensitive to the details of the potential energy surface and small changes in the entrance channel interaction can enhance cross sections and rate constants by orders of magnitude
The effects of quantum mechanical tunneling and resonances on reaction cross sections and rate constants of the title reactions are discussed in the following
Summary
The F + H2 reaction has been extensively studied for many years from a variety of perspectives, as reported in numerous papers and reviews appeared in the literature (see e.g., Manolopoulos, 1997; Liu, 2001; Der Chao and Skodje, 2002; Althorpe and Clary, 2003; Qiu et al, 2006; Wang et al, 2018 and references therein). In the quantum mechanical low-temperature regime, chemical reactivity is most sensitive to the details of the potential energy surface and small changes in the entrance channel interaction can enhance cross sections and rate constants by orders of magnitude. To this purpose, numerically exact quantum scattering calculations of integral cross sections and rate constants have been carried out on two potential energy surfaces, the Stark and Werner potential energy surface (SW PES hereafter) (Stark and Werner, 1996) and PES-II (Aquilanti et al, 2001, 2005b), differing only in the long-range interaction of the F atom with the H2 molecule. Quantum reactive scattering calculations serve to generate differential and integral cross sections as well as rate constants, the observable quantities in reaction dynamics and kinetics experiments. In the range of temperature investigated, it is essentially negligible (
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