New H2O–H2O collisional rate coefficients for cometary applications

Abstract

We re-introduce a semi-classical methodology based on theories developed for the determination of broadening coefficients. We show that this simple and extremely fast methodology provides results that are in good agreement with results obtained using the more sophisticated MQCT approach. This semi-classical methodology could be an alternative approach which allows to provide large sets of collisional data for very complex molecular systems. It saves time both on the determination of potential energy surfaces and on the collisional dynamical calculations. In addition this paper provides more complete sets of rotational de-excitation cross-sections and rate coefficients of H2O perturbed by a thermal average of water molecules. Those data can be used in the radiative transfer modelling of cometary atmospheres.

Adiabatic Trajectory Approximation within the Framework of Mixed Quantum/Classical Theory

“…55 We found, however, that the adaptive step-size control routine gave advantage only for the molecular systems with a long-range interaction potential, such as H 2 O + H 2 O. 31,36,41 For other cases, such as the H 2 O + H 2 system considered here, the "black-boxed" step-size control gave no noticeable increase of performance. However, we realized that the adiabatic trajectory approximation introduced above gives us an opportunity to implement a very efficient method for the variable-step integration of the quantum system of equations, eq 5, propagated during the second step of the AT-MQCT calculations (which is the only costly part of new method).…”

“…Inelastic collisions of molecules with atomic gasses − with other gas-phase molecules − or with solid surfaces − lead to the energy exchange between translational, rotational, and vibrational degrees of freedom, which is a fundamentally important phenomenon in the field of chemical physics. On the applied side, the inelastic molecular collisions play critical roles in many processes that span a huge range of sizes and time scales from the man-made microtraps − to the galaxy and extra-galactic sizes. − Therefore, theoretical prediction of inelastic cross sections has been and still is actively pursued, including practical applications of well-known codes to more and more complex molecular processes, ,,,− development of new computational tools, − and exploration of new theoretical methodologies. − …”

“…Within the framework of MQCT, the internal molecular degrees of freedom are treated quantum mechanically using the time-dependent Schrödinger equation. Therefore, this method takes into account many quantum features of molecular rotations and vibrations, such as state quantization and zero-point energy preservation, symmetry restrictions on allowed and forbidden transitions, propensities of state-to-state transition cross sections, quantum interference effects, and several other quantum features that we keep investigating. At the same time, the translational motion of colliding partners responsible for their scattering in space is treated classically within the Ehrenfest mean-field trajectory approach. , This simplification was found to work well even for the lightest collision partners such as the He atom and H 2 molecule in a broad range of collision energies. ,,, Since the quantum treatment of scattering is avoided, the mixed quantum/classical method enables an affordable computational treatment of heavier collision partners and/or at higher collision energies, the limits when the standard full-quantum methods are not affordable computationally anymore.…”

“…Within the framework of MQCT, the internal molecular degrees of freedom are treated quantum mechanically using the time-dependent Schrodinger equation. Therefore, this method takes into account many quantum features of molecular rotations and vibrations, such as state quantization and zeropoint energy preservation, 46 symmetry restrictions on allowed and forbidden transitions, 31 propensities of state-to-state transition cross sections, 36 quantum interference effects, 47 and several other quantum features that we keep investigating. At the same time, the translational motion of colliding partners responsible for their scattering in space is treated classically within the Ehrenfest mean-field trajectory approach.…”

Collisional energy transfer in the CO–CO system

“…In comets, the excitation of molecules is usually dominated by collisions with H 2 O. Several studies involving collisions with water molecules can be found in the literature, from which the collisional systems of H 2 O-H 2 O, [8][9][10] and H 2 O-CO, 7 have the most significant impact for modeling cometary spectra. An important exception is the case of comets at large heliocentric distances, for which the production of gaseous CO is larger than H 2 O.…”