AI-3000K Infrared line list for hot CO2

Abstract

AI-3000K is a semi-empirical IR line list constructed for hot CO₂ spectra analysis and simulation up to 3000 – 4000 K. Compared to previously published Ames-1000K and UCL-4000, it represents a major upgrade, utilizing a new algorithm for optimization and including the latest improvements in potential energy surface (PES), dipole moment surface (DMS), and room temperature IR line list (Ames-2021 296K). To maximize the success of introducing experimental based accurate line positions, a new PES (X01d) was refined with respect to >800 selected CDSD2019 [Tashkun et al. JQSRT (2019) 228, 124] energy levels in the range of 0 – 24,000 cm⁻¹, with σ_rms = 0.5 – 0.7 cm⁻¹. Most differences between the X01d PES based levels and CDSD2019 energies are within ± 2 cm⁻¹. A new DMS is fitted from extrapolated CCSD(T)/aug-cc-pV(T,Q,5)Z dipole calculations, with σ_rms = 5.1 × 10⁻⁶ au for 11,155 geometries up to 40,000 cm⁻¹, denoted Ames-2021–40K. Compared to the best available Ames-2021 DMS and room temperature IR line list [Huang et al J. Phys. Chem. A (2022) 126, 5940], the relative intensity differences are expected to be ∼ 1 ‰. The line position accuracy of ''X01d + Ames-2021–40K'' IR line list is significantly improved by adopting CDSD2019 energy levels up to 24,000 cm⁻¹ (J ≤ 150). The Einstein A₂₁ coefficients for E' < 15,000 cm⁻¹ transitions are replaced by more accurate values from the Ames-2021 296K IR line list. In short, the AI-3000K is the X01d PES and Ames-2021–40K DMS based line list enhanced with the A₂₁ of Ames-2021 296K line list and CDSD energy levels. It provides continuous coverage from 0 to 20,000 cm⁻¹ for the four most abundant isotopologues: ¹²C¹⁶O₂ (6 2 6), ¹³C¹⁶O₂ (6 3 6), ¹⁶O¹²C¹⁸O (6 2 8), and ¹⁶O¹²C¹⁷O (6 2 7). The impacts of isotopologue and E'/E'' cutoffs have been examined. Intensity convergence (not accuracy) of AI-3000K line list is quantitatively estimated in 1 cm⁻¹ bins. It is better than 99% in the whole range of 0 – 20,000 (or 10,000) cm⁻¹ at 1000 K (or 2000 K), or better than 90% in the whole range of 0 – 15,000 (or 9000) cm⁻¹ at 2000 K (or 3000 K), respectively. Convergence beyond 3000 K will require a new PES and DMS for E' > 40,000 cm⁻¹. The AI-3000K and HITEMP based IR simulations are compared to high resolution shock tube experiments for CO₂-Ar mixture up to 2000 K. With line position accuracy comparable to that of HITEMP, AI-3000K IR line list yields better agreements at more frequencies. Potential sources of discrepancies with experiment are discussed.