Multiphoton ionization of atomic hydrogen is investigated experimentally in the short-pulse regime (τ≤0.5 ps) at wavelengths around 600 nm and intensities up to ${10}^{14}$ W/${\mathrm{cm}}^2$. Sharp peaks in the energy spectrum of the photoelectrons can be attributed to resonant ionization processes via excited states which are ac-Stark shifted into resonance. Energies and angular distributions of the electrons allow one to identify some participating excited states with quantum numbers n≥4 and l≥3.

• Received 12 September 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.404