Photoionization of Excited States of Atomic Hydrogen in a Strong Electric Field.

The photoionization of excited states of atomic hydrogen in a strong external electric field is examined theoretically and experimentally. A theoretical treatment which predicts novel effects in the excitation of the broad resonant states in the continuum from pure parabolic Stark states is presented. The study of these effects is not possible in complex atoms, since only in the hydrogen atom are the fine-structure splittings of the low-lying states small enough to allow the excitation of pure parabolic states in fields of laboratory strength. The first observation of broad resonances in the photoionization of atomic hydrogen in an electric field is reported. The various properties of the resonant structure are compared to theory. Under conditions in which pure parabolic n=2 intermediate states are excited, it is found that the resonant structure above the zero-field ionization limit can be selectively enhanced or suppressed through the choice of the intermediate state, in agreement with theory.