Resonance and interference phenomena in the photoionisation of a hydrogen atom in a uniform electric field. II. Overlapping resonances and interference

For pt.I see ibid., vol.17, no.10, p.1981 (1984). The photoionisation cross section for a hydrogen atom placed in a uniform electric field is investigated as a function of the light frequency, Watson transformation allows one to represent the partial cross sections sum (i.e. the total cross section) as a sum of contributions of two series of complex cross section poles and the background term. Analytical formulae are obtained describing the cross section structure in various regions of photon energy and fields strength. The contribution of one of the series of poles is related qualitatively with the population of resonance states in the superposition of Coulomb and uniform fields (these states exist even for photon energies exceeding the zero-field atomic ionisation potential). The Fano parametrisation of resonance peaks is generalised for the overlapping resonance case. The second series of poles generates cross section oscillations corresponding to interference of photoelectrons ejected from the atom along the field axis with electrons moving in the same direction after reflection from the potential barrier bounding the classically accessible region. The interference structure proves to be suppressed compared with the short-range potential case where this mechanism of structure formation is unique. If the photon energy is less than 4 pi 2* (ionisation potential) then the resonance structure is significantly larger than the interference one in amplitude and differs in phase. When the photon energy is close to the ionisation potential the resonance peaks are strongly asymmetrical. A comparison is made with experiments where the structure is observed in the photoionisation of sodium and rubidium atoms.