Collision Broadening by Neutral Hydrogen

The theory of collision broadening by neutral hydrogen is summarized and given in simplified form. The error introduced by averaging over azimuthal quantum numbers is exhibited and shown to be small. The multipole expansion as usually used is shown to give very slow convergence. The ezact expression is evaluated, avoiding the multipole expansion but using scaled hydrogenic functions, and is applied to the lines of Fe I. The predictions for the line width are considerably larger than the dipole-dipole estimate and are in reasonable agreement with the average broadening observed experimentally.