The rate constant for radiative association of HF: comparing quantum and classical dynamics.

Radiative association for the formation of hydrogen fluoride through the A(1)Π → X(1)Σ(+) and X(1)Σ(+) → X(1)Σ(+) transitions is studied using quantum and classical dynamics. The total thermal rate constant is obtained for temperatures from 10 K to 20,000 K. Agreement between semiclassical and quantum approaches is observed for the A(1)Π → X(1)Σ(+) rate constant above 2000 K. The agreement is explained by the fact that the corresponding cross section is free of resonances for this system. At temperatures below 2000 K we improve the agreement by implementing a simplified semiclassical expression for the rate constant, which includes a quantum corrected pair distribution. The rate coefficient for the X(1)Σ(+) → X(1)Σ(+) transition is calculated using Breit-Wigner theory and a classical formula for the resonance and direct contributions, respectively. In comparison with quantum calculations the classical formula appears to overestimate the direct contribution to the rate constant by about 12% for this transition. Below about 450 K the resonance contribution is larger than the direct, and above that temperature the opposite holds. The biggest contribution from resonances is at the lowest temperature in the study, 10 K, where it is more than four times larger than the direct. Below 1800 K the radiative association rate constant due to X(1)Σ(+) → X(1)Σ(+) transitions dominates over A(1)Π → X(1)Σ(+), while above that temperature the situation is the opposite.