On the validity of collidermass scaling for molecular rotational excitation
Abstract
Rate coefficients for collisional processes such as rotational and vibrational excitation are essential inputs in many astrophysical models. When rate coefficients are unknown, they are often estimated using known values from other systems. The most common example is to use Hecollider rate coefficients to estimate values for other colliders, typically H{sub 2}, using scaling arguments based on the reduced mass of the collision system. This procedure is often justified by the assumption that the inelastic cross section is independent of the collider. Here we explore the validity of this approach focusing on rotational inelastic transitions for collisions of H, paraH{sub 2}, {sup 3}He, and {sup 4}He with CO in its vibrational ground state. We compare rate coefficients obtained via explicit calculations to those deduced by standard reducedmass scaling. Not surprisingly, inelastic cross sections and rate coefficients are found to depend sensitively on both the reduced mass and the interaction potential energy surface. We demonstrate that standard reducedmass scaling is not valid on physical and mathematical grounds, and as a consequence, the common approach of multiplying a rate coefficient for a moleculeHe collision system by the constant factor of ∼1.4 to estimate the rate coefficient for paraH{sub 2} collisions is deemedmore »
 Authors:
 Department of Physics and Astronomy and Center for Simulational Physics, The University of Georgia, Athens, GA 30602 (United States)
 Department of Chemistry, University of Nevada, Las Vegas, NV 89154 (United States)
 Department of Physics, Pennsylvania State University, Berks Campus, Reading, PA 19610 (United States)
 Publication Date:
 OSTI Identifier:
 22365526
 Resource Type:
 Journal Article
 Resource Relation:
 Journal Name: Astrophysical Journal; Journal Volume: 790; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
 Country of Publication:
 United States
 Language:
 English
 Subject:
 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; CARBON MONOXIDE; COLLISIONS; COMPARATIVE EVALUATIONS; CROSS SECTIONS; ELECTRIC GROUNDS; EXCITATION; GROUND STATES; HELIUM 3; HELIUM 4; HYDROGEN; INTERACTIONS; MASS; MOLECULES; SCALING; SCATTERING
Citation Formats
Walker, Kyle M., Yang, B. H., Stancil, P. C., Balakrishnan, N., and Forrey, R. C. On the validity of collidermass scaling for molecular rotational excitation. United States: N. p., 2014.
Web. doi:10.1088/0004637X/790/2/96.
Walker, Kyle M., Yang, B. H., Stancil, P. C., Balakrishnan, N., & Forrey, R. C. On the validity of collidermass scaling for molecular rotational excitation. United States. doi:10.1088/0004637X/790/2/96.
Walker, Kyle M., Yang, B. H., Stancil, P. C., Balakrishnan, N., and Forrey, R. C. Fri .
"On the validity of collidermass scaling for molecular rotational excitation". United States.
doi:10.1088/0004637X/790/2/96.
@article{osti_22365526,
title = {On the validity of collidermass scaling for molecular rotational excitation},
author = {Walker, Kyle M. and Yang, B. H. and Stancil, P. C. and Balakrishnan, N. and Forrey, R. C.},
abstractNote = {Rate coefficients for collisional processes such as rotational and vibrational excitation are essential inputs in many astrophysical models. When rate coefficients are unknown, they are often estimated using known values from other systems. The most common example is to use Hecollider rate coefficients to estimate values for other colliders, typically H{sub 2}, using scaling arguments based on the reduced mass of the collision system. This procedure is often justified by the assumption that the inelastic cross section is independent of the collider. Here we explore the validity of this approach focusing on rotational inelastic transitions for collisions of H, paraH{sub 2}, {sup 3}He, and {sup 4}He with CO in its vibrational ground state. We compare rate coefficients obtained via explicit calculations to those deduced by standard reducedmass scaling. Not surprisingly, inelastic cross sections and rate coefficients are found to depend sensitively on both the reduced mass and the interaction potential energy surface. We demonstrate that standard reducedmass scaling is not valid on physical and mathematical grounds, and as a consequence, the common approach of multiplying a rate coefficient for a moleculeHe collision system by the constant factor of ∼1.4 to estimate the rate coefficient for paraH{sub 2} collisions is deemed unreliable. Furthermore, we test an alternative analytic scaling approach based on the strength of the interaction potential and the reduced mass of the collision systems. Any scaling approach, however, may be problematic when lowenergy resonances are present; explicit calculations or measurements of rate coefficients are to be preferred.},
doi = {10.1088/0004637X/790/2/96},
journal = {Astrophysical Journal},
number = 2,
volume = 790,
place = {United States},
year = {Fri Aug 01 00:00:00 EDT 2014},
month = {Fri Aug 01 00:00:00 EDT 2014}
}

The relative populations of rotational states in the v=0 and v=1 vibrational states of OH produced in the reaction of H+NO/sub 2/..>..OH+NO in crossed molecular beams were measured by laserinduced fluorescence. The excited vibrational state v=1 was found to be produced at 1.3 + 0.3 times the rate of v=0. High degrees of rotational excitation were also observed. The rotationalstate distributions were analyzed in information theoretic terms. A linear surprisal was found, which yielded a rotational surprisal parameter lambda/sub ROT/=2.69 + 0.57 for v=0. (AIP)

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