Rotational excitation of HC₃N by H₂and He at low temperatures

Abstract

Aims. Rates for rotational excitation of HC 3 N by collisions with He atoms and H 2 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 HC 3 N-He PES is in excellent agreement with the recent literature. The HC 3 N-H 2 angular dependence is approximated using 5 independent H 2 orientations. An accurate angular expansion of both PES suitable for low energy scattering is achieved despite the severe steric hindrance effects by the HC 3 N rod. Results. The rod-like symmetry of the PES strongly favours even ∆J transfers and efficiently drives large ∆J transfers. Despite the large dipole moment of HC 3 N, rates involving ortho-H 2 are very similar to those involving para-H 2 , because of the predominance of the geometry effects. Except for the even ∆J 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 HC 3 N levels at H 2 densities in the range 10 4 −10 6 cm −3. Conclusions. The HC 3 N 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.

Combined IRAM andHerschel/HIFI study of cyano(di)acetylene in Orion KL: tentative detection of DC3N

“…The column density of this molecule was derived using HC 3 N-pH2 collisonal rates from Wernli (2007), and the modeled line profiles are shown in Figs. 1 (IRAM) and 2 (Herschel).…”

“…A.13-A.15 show the results for the 13 C isotopologue lines from HIFI. The collisional rates were taken from HC 3 NpH 2 (Wernli et al 2007). All lines from HC 15 3 N except one are hidden by other stronger species.…”

“…To model the lines of DC 3 N we assumed LTE conditions for the hot core and LVG calculations for the rest of the components, as we did for HC 3 N, using collisional rates from Wernli et al (2007) (HC 3 N-p-H 2 ). Given that this molecule has previously only been detected in dark clouds, we first tried to use only components with rather low temperatures (T K ∼ 60 K).…”

Recent Advances in Development and Applications of the Mixed Quantum/Classical Theory for Inelastic Scattering

“…But in larger molecules the number of accessible internal states may reach hundreds and even thousands. [9][10][11][12] Second, the molecule-molecule interaction is described by a complicated multidimensional potential, represented by the potential energy surface (PES), or hypersurface to be exact, that should be constructed using the tools of electronic structure theory and the state-of-the-art methods of surface fitting. 13,14 Third, the range of scattering energies we have to cover is often broad, with different mechanisms dominating in different collision regimes (e.g., scattering resonances at low energies 2,12,15 in contrast to vibrational excitation at high energies).…”

“…It appears that, when the PES is complicated, too many expansion terms (hundreds) are required for accurate surface representation, and it is hard to truncate the expansion without introducing artifacts into the PES. 9,12 In our MQCT calculations for HCOOCH3 + He 62 we used the same PES as in ref 12, but we did not expand it over the basis set of spherical harmonics. Instead, we computed matrix elements directly by integration of eq 4 over Euler angles.…”