[Paper Review] Low energy H+CO scattering revisited: CO rotational excitation with new potential surfaces

Benjamin C. Shepler, Benhui Yang+7 more|ArXiv.org|Sep 18, 2007

Atmospheric Ozone and Climate29 references22 citations

TL;DR

This study revisits low-energy H+CO scattering using newly computed ab initio potential energy surfaces (CCSD(T) and MRCI) to resolve a contradiction in rotational excitation cross sections. Contrary to previous findings using the WKS potential, the new surfaces show strong even ΔJ propensity and a suppressed 0→1 transition, indicating H-CO collisions are less effective at exciting CO than previously thought, validating earlier astrophysical assumptions about H2 dominance in diffuse ISM.

ABSTRACT

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 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. New 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. Cross sections for the J=0->1, as well as other odd Delta J, transitions are significantly suppressed compared to even Delta J transitions in thermal energy CC calculations using the CCSD(T) and MRCI surfaces. This is consistent with a expected even Delta J propensity and in contrast to CC calculations using the earlier WKS PES which predict a dominating 0->1 transition. The current results 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.

Motivation & Objective

• To resolve the contradiction between previous quantum scattering calculations showing large 0→1 cross sections (using WKS PES) and the expected even ΔJ propensity in H-CO scattering.
• To compute new, high-level ab initio potential energy surfaces (CCSD(T) and MRCI) with improved long-range behavior for the H-CO system.
• To test whether the discrepancy in rotational excitation cross sections arises from inaccuracies in the long-range anisotropy of the WKS potential surface.
• To reassess the role of H collisions in CO rotational excitation in diffuse interstellar medium (ISM) environments.
• To validate or revise the astrophysical conclusion that H collisions significantly affect CO rotational populations, as previously claimed by Liszt (2006).

Proposed method

• Performs close-coupling (CC) rigid-rotor quantum scattering calculations for CO(v=0, J=0) excited by H at thermal energies.
• Uses four different potential energy surfaces (PESs): two new ab initio surfaces (CCSD(T) and MRCI) and two existing ones (BBH and WKS).
• Computes state-to-state rotational excitation cross sections for ΔJ = 1, 2, 3 transitions using the Legendre expansion of the PES, focusing on λ = |ΔJ| coupling terms.
• Analyzes the long-range behavior and anisotropy of the PES, particularly the λ = 1 component, to identify differences driving the cross-section discrepancies.
• Compares cross sections across PESs to isolate the origin of the anomalous 0→1 transition in the WKS surface.
• Performs additional calculations for J=1 and J=2 excitation to test consistency of the ΔJ=1 anomaly across rotational states.

Experimental results

Research questions

• RQ1 Does the H-CO system exhibit even ΔJ propensity in rotational excitation at low collision energies, as expected from homonuclear-like long-range interactions?
• RQ2 Why do previous calculations using the WKS PES predict a large 0→1 transition cross section, contradicting the even ΔJ propensity observed with the BBH PES?
• RQ3 How do fine details in the long-range anisotropy of the potential energy surface affect low-energy inelastic scattering cross sections?
• RQ4 To what extent do the new ab initio PESs (CCSD(T) and MRCI) resolve the discrepancy between the WKS and BBH results?
• RQ5 Does the 0→1 cross section from H collisions remain negligible compared to H2 collisions in diffuse ISM, as previously assumed?

Key findings

• The 0→1 rotational excitation cross section using the new CCSD(T) and MRCI PESs is suppressed and comparable in magnitude to that obtained with the BBH PES, in contrast to the WKS PES prediction.
• The new surfaces reproduce the expected even ΔJ propensity in rotational excitation, confirming the physical expectation for H-CO scattering at low energies.
• The anomalous large 0→1 cross section in the WKS PES arises from a distinct long-range behavior and enhanced λ=1 anisotropic component, peaking closer to the barrier and decaying more slowly than in other surfaces.
• The λ=1 component of the WKS PES is responsible for the exaggerated ΔJ=1 transitions, and its long-range anisotropy drives the discrepancy in scattering results.
• The 0→1 cross section for H-CO collisions is found to be much smaller than previously claimed, consistent with earlier studies by Chu & Dalgarno (1975) and Green & Thaddeus (1976).
• The results invalidate the astrophysical conclusion that H collisions significantly alter CO rotational populations in the diffuse ISM, reaffirming the dominance of H2 and He in CO excitation kinetics.