Differential cross sections and polarization of in collisions

Abstract

The presence of an axially symmetric angular distribution of energetic protons in astrophysical plasmas can be detected by measuring the linear polarization of the lines emitted from collisionally excited levels of atoms. Another signature of proton beams can be found in the study of line profiles. For a quantitative interpretation of the observations of the line one needs to know all the relevant integrated and differential cross sections and the polarization fraction. Coupled-state calculations based on a two-centre expansion in atomic orbitals and pseudostates were carried out for protons colliding with H(1s) in the impact energy range 1-100 keV. Differential cross sections for excitation and capture from the level n = 1 to the level n = 3 as well as the total and differential polarization fraction of the line are presented. It is shown that the differential cross sections reach a maximum for an angle varying from 1 to degree when the collision energy varies from 1 to 40 keV. For 1 keV collisions, this leads to large recoil velocities and, as a consequence, to a large Doppler broadening. At the same energy, the resulting polarization fraction is found to have only small variations with the deflection angle.