ИСТИНА

Accurate knowledge of absorption induced by intermolecular interaction is indispensable for the planetary climate simulations (see e.g. [1,2]). For many decades the theory of collision-induced absorption (CIA) was based on assumption of spherically symmetrical interaction potentials and simplified induced dipole [3]. Now it is becoming feasible to rely on complete ab initio calculated intermolecular potential energy (PES) and induced dipole (IDS) surfaces. Because of this there exists a need to refine the CIA theory in order to achieve both better accuracy and credibility in upcoming spectral simulations. Remarkable breakthrough in CIA modeling was made in recent years through the use of sophisticated quantum theory [4], classical trajectory approach [5], and molecular dynamics simulation [6]. In this paper we suggest a new classical approach which is apt to make accurate computation of spectral moments affordable even in case of interacting polyatomics. CO2-Ar pair was chosen in terms of a prototypic system possessing notable anisotropy. First, ab initio calculations of both PES and IDS were carried out using CCSD(T) method in assumption of rigid CO2 molecule. Second, the PES and IDS were represented in terms of analytical expansions. The quality of the PES was checked by calculation of the second virial coefficient. Then a classically rigorous expression for the CO2-Ar Hamiltonian was derived in the body-fixed coordinate frame. This Hamiltonian was used further whilst doing Boltzmann averaging in the calculation of both zeroth order and second order spectral moments. The derivatives of the Hamiltonian and the dipole were also used to find the Poisson bracket that appears in the expression for the second spectral moment. Our calculated temperature variations of the zeroth and second spectral moments are in good agreement with available experimental data [7,8]. It has to be noted that explicit relationship for the second moment which is derived here for the first time for molecular system with anisotropic interaction, can be extended to any other molecular system. References: [1] Robin Wordsworth et al. Geophys. Res. Lett., 44, 665 (2017). [2] Andrey A. Vigasin, Igor I. Mokhov, Izv. Atmospheric and Oceanic Phys. 53, 188 (2017). [3] Lothar Frommhold. Collision-induced absorption in gases. Cambridge University Press, 2006. [4] Tijs Karman et al. J. Chem. Phys., 142, 084306 (2015). [5] Daniil V. Oparin et al., JQSRT, 196, 89 (2017). [6] Jean-Michel Hartmann et al., J. Chem. Phys., 134, 094316 (2011). [7] Ian R. Dagg et al., Can. J. Phys. 64, 1485 (1986). [8] Mikhail. V. Tonkov In: Collision- and Interaction-Induced Spectroscopy. Kluwer AP, 1995.