Resonance and interference phenomena in the photoionisation of a hydrogen atom in a uniform electric field. I. Resonances below and above the potential barrier

The photoionization cross section (PCS) structure of an H atom in an external electric field is investigated analytically within the framework of semiclassical theory. The resonance and interference effects resulting from the superposition of Coulomb and uniform fields are related to the PCS structure, with a focus on the energy region with E less than zero. The semiclassical equations defining the resonance spectrum are discussed in detail, and the analytical results are compared to those obtained numerically in tables. The relative importance of resonance and interference at E greater than zero is found to depend strongly on the energy. The H model is used to interpret the experimental results of Luk et al. (1981), Freeman et al. (1978), and Freeman and Economou (1979).