  The present paper extends the method of Anstee, Barklem and O'Mara (Anstee
1992; Anstee & O'Mara 1991, 1995; Anstee, O'Mara & Ross 1997; Barklem 1998;
Barklem & O'Mara 1997; Barklem, O'Mara & Ross 1998), developed during the
1990's 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.
