The effects of optical pumping on laser enhanced electron impact ionization of atoms

Excitation of an atom by optical radiation reduces the atomic ionization energy thereby increasing its electron impact ionization cross section. Since the enhancement in the ionization rate depends on the population of atoms that can be maintained in the excited state, optical pumping of the ground state hyperfine levels can have pronounced effect on the magnitude of the observed signal. Here, experiments with Na atoms pumped by a narrowband dye laser are reported. Optical pumping is shown to have a significant effect on the signal-to-noise ratio in transitions that do not constitute a two level system. (orig.)