Experiments are reported on the detection of slow photoelectrons resulting from the photoionization of Xe atoms in a dc electric field by electron imaging. In the far-field photoelectron velocity distributions we can distinguish between direct and indirect ionization processes (involving long range Coulomb interactions with the Xe+ ion). Also, a new modulation of the velocity distribution is observed which cannot be explained by previously discussed mechanisms. Classical and quantum mechanical calculations are presented to support the interpretations.

We present experimental data on the decay of xenon Stark states converging to the upper spin limit. In an electric field the Rydberg electron has two qualitatively different decay paths. If the electron changes the core state from the upper spin state into the lower spin state, it gains sufficient energy to escape the ionic core and autoionizes. Moreover, if the electronic state is above the saddle point, created by the electric field, it can field ionize. The probability to autoionize is nearly constant around the saddle point whereas the probability to field ionize rapidly increases above the saddle point. With the velocity map imaging technique we monitor both ionization channels as a function of ͑increasing͒ photoexcitation energy. We observe that the field ionization channel dominates the competition and gains yield at the expense of the autoionization channel. The spectra are explained both with full quantum calculations and with a relatively simple description for the overall behavior. These experiments show that the field ionization can be used in general as a clock for total core-dependent decay.