Transitions between the n = 2 fine-structure levels in H+p collisions

The transitions between the 2S and 2P states of a hydrogen atom in collisions with a proton are examined. Inclusion of electron spin, neglected in previous treatments, is shown to be crucial for the rigorous formulation of the problem. In a basis of the eight n = 2 eigenfunctions, a first-order system of coupled differential equations is obtained by assuming classical straight-line trajectories. The solution of these equations yields the cross sections as functions of velocity. The cross sections are used to compute the transition rates in a plasma over a wide range of temperatures