Observation and Quasistatic Analysis of Structure in Microwave Ionization of Highly Excited Helium Atoms

Abstract

The ionization of $\mathrm{He}[1s(n\ensuremath{\simeq}30)s]^{3}S_{1}$ atoms by an oscillating electric field (9.92 GHz) is strongly dependent on the field amplitude. The ionization curves reveal structures which are absent in those for $\mathrm{H}(n\ensuremath{\simeq}30)$ atoms. The ionization rate does not increase monotonically near threshold and is approximately constant over one or more regions of higher field. A quasistatic model is considered which relates the structure in the ionization curves of He to the occurrence of anticrossings in He Stark states.