Vibrationally-rotationally inelastic cross sections for H+SiO collisions

Abstract

Vibrationally-rotationally inelastic cross sections have been computed for H+SiO collisions over the energy range 200–9600 K. The calculations use a previously calculated ab initio potential energy surface which was fitted to an analytic form [P. Jimeno, M. D. Gray, and G. G. Balint-Kurti, J. Chem. Phys. 111, 4966 (1999)]. Potential optimized discrete variable representation techniques were used to compute vibrational matrix elements of this potential and the infinite order sudden method was then used to compute the required cross sections. The potential is very anisotropic and complicated. This is mirrored in the form of the vibrational matrix elements and in their behavior as a function of the scattering coordinate and of the Jacobi angle. The properties of the potential are markedly different from those of the He+SiO system which has previously been used to model the behavior of both H+SiO and H2+SiO in interstellar and stellar media. Selected vibrationally elastic and vibrationally inelastic cross sections are presented and discussed. Tables of cross sections covering a wide range of vibrational and rotational quantum numbers, as well as a large energy range, are made available through the journal’s EPAPS service. The inelastic cross sections computed here are needed in the modeling of circumstellar SiO maser radiation. Significant structure is observed in the computed inelastic cross sections. This structure is of a complicated nature and might well give rise to the vibrational-rotational quantum state population inversions which in turn leads to the observed circumstellar maser action.

Ro-vibrational excitation of CS by He

“…5 (upper panel) where we can see a distribution peak centered around higher j than the initial j. This kind of distribution is similar to the one found by Palov et al (2002) for the vibrational relaxation of SiO in collisions with H. However, some caution must be exercised regarding this distribution since the VCC-IOS method used in this work neglects the rotational structure of the molecule and by consequence the possible resonant vibrational relaxation when the initial and final rotational levels are close to each other.…”

Collisional depolarization of molecular lines. Application to the SiO+H isotropic collisions

“…We notice that our calculations are expected to be less accurate for high J (J > 50) because the rotational energy is comparable to the kinetic energy, so that the IOS approximation is not satisfied. Palov et al (2002) have performed calculations up to J > 100, accuracy of our calculations depends on the Palov et al results.…”

“…(7) and (8)), we have studied the variation of depolarization and polarization tarnsfer rates as a function of rotational levels J and temperature T . The calculations were possible in the case of the SiO molecules in collisions with neutral hydrogen, because generalized IOS cross sections σ(0 → L) are listed by Palov et al (2002) on the EPAPS home page for collision energy range 200−9600 K (92 K 64 cm −1 ). We notice that our calculations are expected to be less accurate for high J (J > 50) because the rotational energy is comparable to the kinetic energy, so that the IOS approximation is not satisfied.…”

“…The depolarizing and polarization transfer rates given in this section are obtained with the most recent and accurate available H-SiO interaction potentials (see Jimeno et al 1999;and Palov et al 2002). Variation laws such as those given by Eqs.…”

“…These calculations are based on the vibrational close-coupling rotational IOS (VCC-IOS) cross sections available in electronic format in the EPAPS home page (cited as EPAPS in the references). The VCC-IOS cross sections are computed by Palov et al (2002), who adopts accurate ab initio H-SiO interaction potential surfaces (see Jimeno et al 1999;and Palov et al 2002). We find that it is possible to make a good fit to the depolarizing rates as a function of J and T .…”