Low energy H+CO scattering revisited

Abstract

Context. A recent modeling study of brightness ratios for CO rotational transitions in gas typical of the diffuse ISM by Liszt found the role of H collisions to be more important than previously assumed. This conclusion was based on recent quantum scattering calculations using the so-called WKS potential energy surface (PES) which reported a large cross section for the important 0 → 1 rotational transition. This result is in contradiction to one obtained using the earlier BBH PES for which the cross section is quite small and which is consistent with an expected homonuclear-like propensity for even ∆J transitions. Aims. We revisit this contradiction with new scattering calculations using two new ab initio PESs that focus on the important longrange behavior and explore the validity of the apparent departure from the expected even ∆J propensity in H-CO rotational excitation obtained with the WKS PES. Methods. Close-coupling (CC) rigid-rotor calculations for CO(v = 0, J = 0) excitation by H are performed on four different PESs. Two of the PESs are obtained in this work using state-of-the-art quantum chemistry techniques at the CCSD(T) and MRCI levels of theory. Results. Cross sections for the J = 0 → 1, as well as other odd ∆J, transitions are significantly suppressed compared to even ∆J transitions in thermal energy CC calculations using the CCSD(T) and MRCI surfaces. This is consistent with the expected even ∆J propensity and in contrast to CC calculations using the WKS PES which predict a dominating 0 → 1 transition. Conclusions. Inelastic collision cross section calculations are sensitive to fine details in the anisotropic components of the PES and its long-range behavior. The current results obtained with new surfaces for H-CO scattering suggest that the original astrophysical assumption that excitation of CO by H 2 dominates the kinetics of CO in diffuse ISM gas is likely to remain valid.

First detection of CO in a high-redshift damped Lyman-α system

“…Thus the collisional excitation of CO by H 2 is negligible. Collisions with H contribute little to the CO excitation compared to collisions with H 2 in astrophysical conditions (Green & Thaddeus 1976;Shepler et al 2007). This means that the CO excitation is dominated by CMBR, so we conclude that T ex = T CMBR = 9.15 ± 0.72 K.…”

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

“…The rotational excitation of CO is implemented as described in Liszt (2007a) including excitation by atomic helium and hydrogen, ortho and para-H 2 , although excitation by atomic hydrogen is actually negligible given the small rate constants (Shepler et al 2007;Walker et al 2015) and the absence of CO except when the molecular hydrogen fraction is large. The J=0 and 1 levels of H 2 are taken to be in thermal equilbrium at the local kinetic temperature (Savage et al 1977;Rachford et al 2002).…”

Quantum Calculation of Inelastic Co Collisions With H. I. Rotational Quenching of Low-Lying Rotational Levels

“…From Figure 1, one can see that the cross sections on the WKS PES are significantly larger than that obtained using the CCSD and MRCI potentials, especially at ultracold collision energies. In the energy region between 0.1 and ∼20 cm −1 , the cross sections computed on all three potentials exhibit resonances caused by the van der Waals interaction, but due to differences in the potential well depths (see Shepler et al 2007) very different structures are displayed, in particular the resonances on the WKS potential are largely suppressed. There exists generally good agreement between the cross sections on the CCSD and MRCI PESs above ∼100 cm −1 , while the cross sections on all three surfaces appear to converge above 10 4 cm −1 .…”

“…From Figure 1, it can also be seen that the CS approximation is in excellent agreement with the CC method. For initial rotational states j = 2, 3, 4, and 5, the state-tostate quenching cross sections were calculated using the MRCI PES of Shepler et al (2007) only. Figure 2 presents the cross sections from initial j = 2, 3, and 4 into the individual final states j .…”

“…Examples include a study of molecular excitation in interstellar shock waves by Flower & Pineau des Forêts (2010), an investigation of the molecular regions of the supernova remnants IC443C, W28, W44, and 3C391 and the Herbig-Haro objects HH7 and HH54 (Yuan & Neufeld 2011), and in radiation thermo-chemical models of protoplanetary disks (Thi et al 2013). Therefore, to provide accurate rate coefficients of CO rotational excitation due to impact by H atoms, we extend here the calculations of Shepler et al (2007), carrying out computations of stateto-state quenching cross sections and rate coefficients for CO from initial states j = 1-5 in the ground vibrational state to all lower j levels using the potentials presented in Shepler et al (2007).…”