Momentum transfer in photo-ionization processes

Photo-ionization of an atom by a photon of energy hv involves absorption of momentum h nu /c, where c is the speed of light. That momentum is shared between the ion produced and the electron ejected. In order to calculate the fraction of the momentum given to the electron it is necessary to allow for interference between the dipole and quadrupole photoionization amplitudes. Results for H Is states were given by Sommerfeld in 1930. The present paper gives similar results for H 1s, 2s, 3s, 2p, 3p and 3d. These results are of interest in connection with calculations of selective radiation pressures which lead to diffusive separations of chemical elements in stars.