Mechanisms of energy transfer in hydrogen fluoride systems

Abstract

Rate coefficients are calculated for the energy-transfer processes that ocuur when HF(v1,J1) molecules collide with HF(v2, J2) molecules. Three-dimensional classical trajectories of the collision dynamics of these energy-transfer processes were calculated by means of a potential energy surface, which consists of a London–Eyring–Polanyi–Sato (LEPS) potential function for the short-range interactions and a partial-point-charge, dipole–dipole function for long-range interactions. This energy surface was used to predict an equilibrium geometry of the HF dimer. From the trajectory calculations it was predicted that the v→v energy-transfer processes occur by means of Δv=±1 transitions and that the rate coefficients for the processes HF(v)+HF(v=0) →HF(v−1)+HF(v=1) decrease with increasing vibrational quantum number v. A calculation of the v→v rate for the reaction HF(v=1)+HF(v=1) →HF(v=0)+HF(v=2) indicates a value of 1.2×1013 cm3 mol−1 s−1 at 300 K. This process corresponds to near-resonant vibration-to-vibration (v→v) intermolecular energy transfer. The major contribution toward the rate coefficients for the energy transfer mechanisms comes from the rotating HF molecules. The vibrationally excited HF rotor takes the energy mismatch ΔE, corresponding to rotationless HF molecules, away by means of a vibration-to rotation (v→R) energy-transfer process. This process corresponds to a nonresonant v→R intramolecular energy transfer. Multiquantum v→R processes are predicted. At low v it is predicted that one in three HF–HF collisions produces v→R energy transfer. For many of the important v→R energy-transfer processes the energy defect is less than 200 cm−1. The trajectory calculations indicate that the number of v→R open channels increases with increasing v. The multiquantum v→R transitions provide more ways to distribute the vibration energy of the vibrationally excited HF molecules into rotational energy, i.e., into very high rotational quantum states. The high rotational quantum states are quickly relaxed by R→v processes and by fast v→R processes in which even higher rotational quantum states are produced. The high rotational quantum states are relaxed slowly by R→R, T processes.