Above-threshold ionization in a strong dc electric field

High-lying Rydberg states of Xe have been ionized using intense 108 mu m radiation from a free-electron laser. Measured two-dimensional photoelectron images reveal significant above-threshold ionization and contain an indirect contribution resulting from the combined action of the atomic Coulomb field, laser field, and dc electric field of the spectrometer on the electron. The observation of indirect ionization contains information about the electron localization directly after the laser excitation and indicates that the experiments are performed in the multiphoton regime of strong-field ionization. The experiments are compared to and interpreted by means of both classical and quantum-mechanical simulations.