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

The ionization of $\mathrm{He}\left[1s\right(n\simeq 30\left)s\right]{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ 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 $\mathrm{H}\left(n\simeq 30\right)$ 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.