Discussion and conclusion

Similar calculations have been performed on the HD-H and HD-H₂ systems by Flower & Roueff ([1999]). Table 6 gives a comparison between the quenching rate of the v = 1 state of HD due to collisions with He (present work), H and H₂ at different temperatures.

The magnitude of the quenching rate coefficients vary significantly with the perturber. At all temperatures considered, quenching by hydrogen atoms is more efficient by about one order of magnitude. However, collisions with helium predominate over those with molecular hydrogen.

Helium is sometimes taken as a prototype of the H₂ molecule in its ground state J = 0 in view of rotationally inelastic dynamics. The rate coefficients are then evaluated by scaling the values with the square root of the ratio of the reduced masses of the two systems:

$$\left(\frac{\mu_{\rm HD-He}}{\mu_{\rm HD-H_2}}\right)^{1/2}= 1.1934.$$ (2)

However, the results displayed in Table 6 show that this approximation is inadequate. In this work, we have extended the previous calculations of Flower & Roueff ([1999]) devoted to HD rovibrational excitation in H and H₂ collisions to the case of Helium. This should allow future infrared observations of HD in various environments to be interpreted without uncertainties related to basic molecular physics.

Acknowledgements

The present calculations were performed on the super-scalar computers at the IDRIS computer centre (Orsay, France) under contract No. 990939 (CP8).