Simulation of collision-induced absorption spectra based on classical trajectories and ab initio potential and induced dipole surfaces. I. Case study of N2-N2 rototranslational band.

This paper presents theoretical formalism and some results of the collision-induced absorption (CIA) spectral simulation based on the classical trajectory analysis. Our consideration relies on the use of ab initio potential energy and dipole moment surfaces for two interacting rigid monomers. Rigorous intermolecular Hamiltonian is represented and used in the body-fixed reference frame. The complete set of dynamical equations with Boltzmann-weighted initial conditions is solved to render a large number of classical trajectories. The spectral shape is calculated as an ensemble-averaged Fourier spectrum issued from the time-dependent induced dipole along individual scattering trajectories. Considering a pair of N2 molecules as an example, we have calculated the rototranslational CIA band profiles at T = 78, 89, 109, 129, 149, 179, 228, 300, and 343 K. The classical trajectory-based spectral shape was corrected to satisfy the quantum principle of detailed balance. Good accuracy of our semiclassical approach was demonstrated by comparison with available experimental data as well as with results of the previously published purely quantum simulation by Karman et al. [J. Chem. Phys. 142, 084306 (2015)] in which the same ab initio calculated N2-N2 potential energy and induced dipole moment surfaces were used.