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ACS Publications

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ACS Earth and Space Chemistry

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doi: 10.1021/acsearthspacechem.1c00418


A computationally affordable methodology is developed to predict cross sections and rate coefficients for collisional quenching and excitation of large molecules in space, such as PAHs. Mixed quantum/classical theory of inelastic scattering (MQCT) is applied, in which quantum state-to-state transitions between the internal states of the molecule are described using a time-dependent Schrodinger equation, while the scattering of collision partners is described classically using mean-field trajectories. To boost the numerical performance even further, a decoupling scheme for the equations of motion and a Monte Carlo sampling of the initial conditions are implemented. The method is applied to compute cross sections for rotational excitation and quenching of a benzene molecule (C6H6) by collisions with He atoms in a broad range of energies, using a very large basis set of rotational eigenstates up to j = 60, and close to one million nonzero matrix elements for state-to-state transitions. The properties of collision cross sections for C6H6 + He are reported and discussed. The accuracy of the approximations is rigorously tested and is found to be suitable for astrophysical/astrochemical simulations. The method and code developed here can be employed to generate a database of collisional quenching rate coefficients for PAHs and other large molecules, such as iCOMs, or for molecule–molecule collisions in cometary comas.


Accepted version. ACS Earth and Space Chemistry, Vol. 6, No. 3 (October 2022): 521-529. DOI. © 2022 American Chemical Society. Used with permission.

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