Resonant above-threshold ionization of atomic hydrogen

An all-order multiphoton resonant theory for above-threshold ionization is presented within the resolvent formalism. An analytical expression displaying all the contributions to the electron yield is discussed. The theory is applied to the calculation of photoelectron energy spectra for ionization of H by picosecond and subpicosecond pulses of linearly polarized light at 608 nm, with laser intensities in the range ${10}^{13}$–${10}^{14}$ W/${\mathrm{cm}}^2$. The role of intensity, interaction time, and transition couplings, in the resonantly enhanced electron yield coming from each (nl) intermediate state, is discussed. The modifications which affect the shape of the spectrum as the laser peak intensity increases are described in great detail. An inversion of the relative amplitudes of the resonant structures is observed. This effect is attributed to the relative dimensions of the effective interaction volumes in which the field reaches sufficient intensities to give rise to the resonances. Our results reproduce the main features of recently measured spectra [H. Rottke et al., Phys. Rev. Lett. 64, 404 (1990)].