1 Introduction

Pure rotational transitions of HD have been detected recently. In the dense photon dominated region of the Orion bar the R(0) line at 112 m$\mu$ has been detected with the long wavelength spectrometer (LWS) of ISO (Wright et al. [1999]) and Bertoldi et al. ([1999]) report the detection of the R(5) line at 19.43 $\mu$m with the short wavelength spectrometer (SWS) of ISO in the Orion molecular outflow. On the other hand, infrared rovibrational transitions in HD have been predicted by Sternberg ([1990]) and Timmermann ([1996]) as possible occurrences in dense photon dominated regions (PDR). The interpretation of such observations require the knowledge of the different microscopic processes involved and in particular the rovibrational excitation rate coefficients due to the various perturbers, i.e. atomic and molecular hydrogen as well as helium. In a previous paper, Roueff & Zeippen ([1999]) have calculated the rotational excitation of HD due to collisions with helium. In the present work, these calculations are extended to the rovibrational excitation of HD by helium. Note that rovibrational excitation of HD by hydrogen atoms and molecules has already been calculated by Flower & Roueff ([1999]) in a complete close-coupling quantal approach. The same method is used in this study. Section 2 of the present paper summarizes the formalisms selected for the calculations. Results are given in Sect. 3 and a discussion plus a conclusion can be found in Sect. 4.