Issue |
A&A
Volume 464, Number 3, March IV 2007
|
|
---|---|---|
Page(s) | 1147 - 1154 | |
Section | Atomic, molecular, and nuclear data | |
DOI | https://doi.org/10.1051/0004-6361:20066112 | |
Published online | 02 January 2007 |
Rotational excitation of HC3N by H2 and He at low temperatures*
Laboratoire d'Astrophysique de l'Observatoire de Grenoble, UMR 5571 CNRS/UJF, Université Joseph-Fourier, Boîte postale 53, 38041 Grenoble Cedex 09, France e-mail: laurent.wiesenfeld@obs.ujf-grenoble.fr
Received:
26
July
2006
Accepted:
20
November
2006
Aims.Rates for rotational excitation of by collisions
with He atoms and H2 molecules are computed for kinetic
temperatures in the range 5-20 K and 5-100 K, respectively.
Methods.These rates are obtained from extensive quantum and
quasi-classical calculations using new accurate potential energy
surfaces (PES). The –He PES is in excellent agreement with
the recent literature. The
–H2 angular dependence is
approximated using 5 independent H2 orientations. An accurate
angular expansion of both PES suitable for low energy scattering
is achieved despite the severe steric hindrance effects by the
rod.
Results.The rod-like symmetry of the PES strongly favours even
transfers and efficiently drives large
transfers. Despite the large dipole moment of
, rates
involving ortho-H2 are very similar to those involving
para-H2, because of the predominance of the geometry effects.
Except for the even
propensity rule, quasi classical
calculations are in excellent agreement with close coupling
quantum calculations. As a first application, we present a simple
steady-state population model that shows population inversions for
the lowest HC3N levels at H2 densities in the range
104-106 cm-3.
Conclusions.The molecule is large enough to
present an original collisional behaviour where steric hindrance
effects hide the details of the interaction. This finding,
combined with the fair accuracy of quasi classical rate
calculations, is promising in view of collisional studies of
larger molecules.
Key words: molecular data / molecular processes
© ESO, 2007
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