Rotationally and torsionally inelastic scattering of methanol on helium

Abstract

Cross-sections and rate coefficients have been computed for rotationally and torsionally inelastic transitions between levels of the torsional manifolds ν = 0, 1 and 2 of methanol; the collision partner was the helium atom. We find the torsional inelasticity to be most pronounced for transitions between the excited states, ν = 1 and 2. The torsionally inelastic cross-sections are smaller, on average, than the cross-sections for pure rotationally inelastic transitions, by typically an order of magnitude. We compared cross-sections calculated using the full CH 3 OH-He interaction potential, which is a function of the torsion angle, with the cross-sections computed for the staggered conformation, i.e. for a fixed value of the torsion angle. The agreement between these two sets of results improves with increasing magnitude of the cross-section. Accordingly, the use of previous results, for the rotational excitation of methanol by He and by H 2 , obtained for the staggered conformation, should not compromise the analysis of observations of methanol in the interstellar medium.

Weak Maser Emission of Methyl Formate Toward Sagittarius B2(n) in the Green Bank Telescope Primos Survey

“…Larger differences are expected between He and ortho-H 2 (J = 1), as observed e.g. in H 2 CO (Troscompt et al 2009) and CH 3 OH (Rabli & Flower 2011). However, since the ortho state (J = 1) lies 170 K above the para state (J = 0), it should not be significantly populated at the temperatures investigated here (see e.g.…”

“…Gottlieb et al 1973). For all complex molecules except methanol (see Rabli & Flower 2011, and references therein), however, such treatments have been hampered so far by the lack of accurate collisional cross sections. expansion adapted to close-coupling scattering calculations 1 .…”

Modelling CH3OH masers: Sobolev approximation and accelerated lambda iteration method

“…This assumption will tend to underestimate the contribution of ortho-H2 to the collisional transfer amongst these excited levels, as the ortho-H2 rate coefficients tend to be larger than the para-H2 (Rabli & Flower 2010a). Rabli & Flower (2011) computed rate coefficients for a restricted set of torsionally inelastic transitions in collisions of methanol with He atoms. For torsionally inelastic transitions induced by para-H2, Rabli & Flower (2011) suggested using the available rate coefficients for He atoms, and three times these values for collisions with ortho-H2.…”

“…Rabli & Flower (2011) computed rate coefficients for a restricted set of torsionally inelastic transitions in collisions of methanol with He atoms. For torsionally inelastic transitions induced by para-H2, Rabli & Flower (2011) suggested using the available rate coefficients for He atoms, and three times these values for collisions with ortho-H2.…”

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“…The position of He atom relative to the center-of-mass of H2CS is expressed in terms of the spherical polar coordinates (r, θ, φ). The interaction potential between the H2CS 16 and He is calculated for various positions (r, θ, φ). We have considered 17,18 532 positions specified by (r, θ, φ), with r=2 (0.25) 6.5 Å, θ=0• (30•) 180•, φ=0• (30•) 90•.…”