Abstract
Abstract We report the results of the trajectory-based simulation of far-infrared collision-induced absorption (CIA) due to CH4–N2 pairs at temperatures between 70 and 400 K. Our analysis utilizes recently calculated high-level potential energy and induced dipole surfaces. Treating collision partners as rigid rotors, the time evolution of interaction-induced dipole is accumulated over a vast ensemble of classical trajectories and subsequently transformed into a CIA spectrum via Fourier transform. In our calculations, both bound and unbound states are properly accounted for, and the rigorous theory of lower-order spectral moments is addressed to check the accuracy of simulated profiles. Classically derived trajectory-based profiles are subject to two approximate desymmetrization procedures so that resulting profiles conform to the quantum principle of detailed balance. The simulated profiles are compared to laboratory measurements and employed for modeling Titan’s spectra in the 50–500 cm−1 range. Based on the desymmetrized simulated profiles, a new semiempirical model for CH4–N2 CIA is proposed for modeling Titan’s infrared spectra. Synthetic spectra derived using this model yield an excellent agreement with the data recorded by the Composite Infrared Spectrometer aboard the Cassini spacecraft at low and high emission angles.
Highlights
Nitrogen and methane were well established as the primary atmospheric constituents of Titan’s atmosphere after the Voyager 1 encounter (Hunten et al 1984)
Significant progress has been made with respect to previous consideration of CH4–N2 collisioninduced absorption (CIA) by Borysow & Tang (1993) in virtue of the use of high-level ab initio anisotropic PES and IDS (Finenko et al 2021), obviating the need for short-range empirical dipole terms
The synthetic spectra simulated with semiempirical CH4–N2 CIA spectra and a slightly modified temperature profile yielded an excellent agreement with Composite Infrared Spectrometer (CIRS) data
Summary
Nitrogen and methane were well established as the primary atmospheric constituents of Titan’s atmosphere after the Voyager 1 encounter (Hunten et al 1984). Later studies (Tomasko et al 2008; de Kok et al 2010) suggested that Borysow & Tang’s (1993) CIA data is possibly systematically in error at Titan’s temperatures Both studies derived that a multiplicative factor of approximately 1.5 needs to be applied to Borysow & Tang’s (1993) data to obtain satisfactory fits of the Cassini’s Composite Infrared Spectrometer (CIRS) observations in the 150–450 cm−1 region. Our consideration relies on the ab initio potential energy (PES) and induced
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