1 Introduction

Recent observations of scattering polarization on the Sun have revealed the existence of "enigmatic'' linear polarization signals in several spectral lines, which have motivated new theoretical investigations on atomic polarization and the Hanle effect (see e.g., the recent reviews by Trujillo Bueno 1999; 2001). Probably the most popular observational example is the case of the D₁ and D₂lines of Na  I (see Stenflo & Keller 1997). Soon after the publication of such "enigmatic'' observations, Trujillo Bueno & Landi Degl'Innocenti (1997) showed, by means of selfconsistent radiative transfer calculations, that anisotropic radiation pumping processes in the solar atmosphere can lead to significant amounts of ground-level polarization, and to linear polarization signals in spectral lines with amplitudes in the observable range. In fact, Landi Degl'Innocenti (1998) could show, by adjusting of few free parameters, that a certain amount of ground-level atomic polarization in the hyperfine structure components of the ground level of sodium leads to a fairly good fit of the above-mentioned observations of linear polarization across the sodium doublet. More recently, Trujillo Bueno et al. (2002) have clarified the true physical origin of the ground-level polarization of Na  I by carefully investigating, within the framework of the quantum theory of polarization, how it is actually produced, and modified by the action of a magnetic field of given strength and inclination.

Because isotropic collisions with the H atoms present in the stellar atmosphere contribute to depolarization of the atomic energy levels, a rigorous interpretation of the polarization spectra is possible only if the effects of the depolarizing collisions predominantly with H atoms are well understood for the spectral lines under consideration. In this paper we give the elastic and inelastic collisional rates of interest for the interpretation of the Na  I, Mg  I, Ca  I, and Sr  I resonance lines. Section 2 gives a brief introductory overview to familiarize the reader with the basic theoretical concepts and techniques applied for the computation of depolarizing rates caused by collisions with H atoms. Section 3 discusses the potential energy curves, while Sect. 4 focuses on the details of the applied theory. Finally, we show the results in Sect. 5, and summarize our main conclusions in Sect. 6.