Abstract
Experiments investigating ionization, excitation, and quantal structure of highly-excited hydrogen and helium atoms in an intense microwave field are reviewed in terms of existing theory. Quantal theory is able to explain some of the results of the structure studies but is not yet at the level of producing quantitative agreement with ionization experiments, which involve very large numbers (hundreds) of microwave photons. Published Monte-Carlo calculations based on a classical theory of the interactions are in reasonable agreement with the results of published ionization experiments, but additional experiments are being carried out to see if quantal tunneling, which is ignored by the classical theories, plays an important role. New experiments show that the power dependence of microwave ionization of m=0 triplet helium Rydberg atoms is very different from that of hydrogen atoms. We are able to explain the threshold region of ionization in helium in terms of dynamic interactions at the first (and subsequent) avoided crossings(s) of adiabatic helium Stark potential curves.
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