Experiments on ‘‘above-threshold ionization’’ of atomic hydrogen

The multiphoton ionization of atomic hydrogen is studied by means of photoelectron spectroscopy. The order of nonlinearity of the ionization process via the 3p resonance is measured for intensities in the ${10}^{11}$-W/${\mathrm{cm}}^2$ range, revealing that the three-photon excitation step is rate limiting. What may be the first measurement on ‘‘above-threshold ionization’’ (ATI) of atomic hydrogen is reported, performed as a function of the real momentary light intensity. Agreement between the measured relative ATI yield and the theoretically predicted one is within 10%. Possibilities to extend the measurements into the regime where perturbation theory breaks down are investigated by means of a two-color experiment.