Effect of electric fields on the decay branching ratio ofPe1doubly excited states in helium measured by time-resolved fluorescence

Abstract

We have measured the lifetimes of $^{1}P^{e}$ $(n=9--12)$ doubly excited states in static electric fields $(1--6\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm})$ by observing the decay of the fluorescence signal as a function of time. The effects of the field on these helium states below the second ionization threshold are twofold: their excitation becomes possible due to the Stark mixing with the optically allowed $^{1}P^{o}$ series, and their lifetime is strongly modified by the opening of the autoionization channel, not accessible in zero field. Although the electric field represents only a tiny perturbation of the atomic potential, a substantial shortening of the lifetimes below $100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ is observed. This is the simplest quantum system where the ratio of autoionization to fluorescence decay probability can be effectively controlled by an electric field of moderate strength.