Pressure Shifts of Highly Excited States of Atoms

A reinvestigation is given of Fermi's work on the shift of highly excited states of atoms embedded in a gaseous medium due to the interaction of the active electron with the medium particles. For the case treated by Fermi, spherically symmetric medium particles, it is found that effects neglected by him cancel and that his results are essentially correct. The method is extended to the case where the medium is composed of diatomic molecules, and it is shown that when rotational excitation is not important, only low-energy elastic scattering contributes in a manner similar to the spherically symmetric scatterer. A simple two-rotational-state model is also analyzed in the case where rotational excitation may be important. It is concluded from the model that when the method used is applicable, real rotational coupling is never important.