Two-channel competition of autoionizing Rydberg states in an electric field

Abstract

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 ~increasing! 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. Decay of a highly excited electron most often occurs through interaction with the ionic core. The rate of, e.g., rotational and vibrational autoionization, fluorescence and predissociation is proportional to the frequency with which an electron returns to the vicinity of the ionic core. Through- out the Rydberg series the ratio between these decay chan- nels remains constant. In this respect field ionization is a special decay channel. A static electric field distorts the Cou- lomb potential by raising the potential in the direction of the field and lowering it on the other side. The maximum on the down-potential side is called the saddle point. An electron that is excited above the saddle point is energetically allowed to escape from the ionic core. The rate for electron emission depends strongly on the opening angle at which the electron can escape. This opening angle rapidly increases above the saddle point making the field ionization change from zero to the dominant pathway of decay in a small energy range. How rapidly the field ionization dominates depends on the com- petition with the core-dependent decay. This competition is reflected in the overall behavior of the ionization spectra, allowing one to retrieve the rate of one channel with the spectrum of the other. In a recent paper we investigated the field ionization yield above the saddle point energy of nitric oxide in a strong electric field @1#. A competition between the field ionization and predissociation of the NO molecule is observed. The field ionization yield just above the saddle point was strongly reduced by the other decay channel and the ionization thresh- old appeared to have shifted to higher energy. Computing the field ionization spectrum yielded the rate of predissociation. In that experiment only the field ionization channel was monitored. To investigate a two-channel competition prop- erly, however, one wants to monitor the two channels simul- taneously. An ideal system of investigation is the autoioniz- ation from the Rydberg states converging to the upper spin state of a noble gas in a strong electric field. The field ion- ized electrons have nearly zero kinetic energy whereas the electrons that autoionize by changing the core spin-orbit state, carry a kinetic energy of 1-2 eV, which makes the two ionization channels separable. The two channels are distin- guished by velocity map imaging. The detected spectra, as presented in Sec. III, are simulated by full quantum calcula- tions based on multichannel Stark theory @1-3#. Moreover, we show that the overall behavior of the spectra can already be simulated by relative simple rate equations. In Sec. IV we derive these equations and compare with the experimental observations.