DIFFERENTIAL CROSS-SECTIONS AND POLARIZATION OF H-ALPHA IN H++H COLLISIONS

The presence of an axially symmetric angular distribution of energetic protons in astrophysical plasmas can be detected by measuring the lin ear polarization of the lines emitted from collisionally excited level s of atoms. Another signature of proton beams can be found in the stud y of line profiles. For a quantitative interpretation of the observati ons of the H alpha 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 ex citation and capture from the level n = 1 to the level n = 3 as well a s the total and differential polarization fraction of the H alpha line are presented. It is shown that the differential cross sections reach a maximum for an angle varying from 1 to 10(-2) degree when the colli sion 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 b roadening. At the same energy, the resulting polarization fraction is found to have only small variations with the deflection angle.