Astronomy & Astrophysics

Free access article

A&A 475, L15-L18 (2007)
DOI: 10.1051/0004-6361:20078693

Letter

Low energy H+CO scattering revisited

CO rotational excitation with new potential surfaces

B. C. Shepler¹, B. H. Yang², T. J. Dhilip Kumar³, P. C. Stancil², J. M. Bowman¹, N. Balakrishnan³, P. Zhang⁴, E. Bodo^{4, 5}, and A. Dalgarno⁴

¹ Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, GA 30322, USA
    e-mail: [bcshepl;jmbowman]@emory.edu
² Department of Physics and Astronomy and the Center for Simulational Physics, The University of Georgia, Athens, GA 30602-2451, USA
    e-mail: [yang;stancil]@physast.uga.edu
³ Department of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89154, USA
    e-mail: dhilip.thogluva@unlv.edu; naduvala@unlv.nevada.edu
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
    e-mail: [pezhang;adalgarno]@cfa.harvard.edu
⁵ Department of Chemistry and CNISM, University of Rome "Sapienza", Piazzale, A. Moro, 00185 Rome, Italy
    e-mail: e.bodo@caspur.it

(Received 17 September 2007 / Accepted 25 September 2007)

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 $\rightarrow$ 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 $\Delta J$ transitions.
Aims.We revisit this contradiction with new scattering calculations using two new ab initio PESs that focus on the important long-range behavior and explore the validity of the apparent departure from the expected even $\Delta J$ 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 $\rightarrow$ 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 the expected even $\Delta J$ propensity and in contrast to CC calculations using the WKS PES which predict a dominating 0 $\rightarrow$ 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 H₂ dominates the kinetics of CO in diffuse ISM gas is likely to remain valid.

Key words: molecular processes -- ISM: molecules

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