Abstract. The present paper extends the method of Anstee, Barklem and O'Mara (Anstee 1992;Anstee & O'Mara 1991, 1995Anstee et al. 1997;Barklem & O'Mara 1997;, developed during the 1990s for collisional line broadening by atomic hydrogen, to the depolarization of spectral lines of neutral atoms by collisions with atomic hydrogen. In the present paper, we will limit the calculations to p (l = 1) atomic levels. The depolarization cross sections and depolarization rates are computed. In Table 2 cross sections as functions of the relative velocity and effective quantum number are given, allowing for the computation for any p atomic level. Our results are compared to quantum chemistry calculations where possible. The sensitivity of depolarization cross sections to regions of the potential is examined. We conclude that the accuracy obtained with our method (<20% for the depolarization rates) is promising for its extension to higher l-values for the interpretation of the "second solar spectrum". This will be the object of further papers.

Abstract. The semi-classical theory of collisional depolarization of spectral lines that we have applied to neutral atoms in previous papers is extended to spectral lines of singly ionised atoms. In order to validate our general theory, we compare our results to quantum chemistry calculations obtained for the particular cases of the 3d 2 D and 4p 2 P states of the CaII ion. As a demonstration of the universality of our theory and the easiness of its application, we calculate depolarization and polarization transfer rates for the 5p 2 P state of the SrII ion. Analytical expressions of all rates as a function of local temperature are given. Our results for the CaII ion are compared to recent quantum chemistry calculations. A discussion of our results is presented.