Ro-vibrational excitation of floppy triatomic molecules in collisions with He:
Rigid bender treatment of the bending-rotation interaction
Thierry Stoecklin, Otoniel Denis-Alpizar and Philippe Halvick
UMR5255-CNRS, Université de Bordeaux, 33405 Talence Cedex, France
Because of their importance to model the chemistry of interstellar clouds, quantum inelastic
scattering calculations involving small polyatomic molecules are the subject of many theoretical
studies. However most of them are limited to the use of the rigid rotor approximation as it is
expected to be a quite accurate approach to calculate rotational transitions which are the most
probable at the typical temperature of interstellar clouds. This is the case of most available studies
of energy transfer collisions involving an atom and a linear or bent triatomic molecule. The present
study is motivated by recent astrophysical measurements of vibrationaly excited HCN in the
interstellar medium [1] and by the detection of HCN l-type transitions in hot planetary nebula [2].
We present a new theoretical method [3,4] to treat atom-rigid bender inelastic collisions at the Close
Coupling level (RBCC) in the space fixed frame. The coupling between rotation and bending is
treated exactly within the rigid bender approximation and we obtain the cross section for the
rotational transition between levels belonging to different bending levels. The method is applied to
three molecules of astrochemical interest HCN, DCN and C3. We find that l-type transitions cross
sections have to be calculated at the RB-CC level for the He-HCN collision while pure rotational
transitions cross sections may be calculated accurately at the rigid rotor level. For C 3 [5,6,7] we
demonstrate that the rotational transition cross sections cannot be calculated using the usual rigid
linear approximation for collisions energies higher than the first excited bending energy threshold.
Fig. 1: Comparison of the elastic and de-excitation cross section of C3(=0, j=0) in collisions
with He as a function of collision energy calculated using the RB-CC and ALM-CC approaches. The final
level is indicated by two integers designating the bending and the rotational quantum number.
References
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