One- and two-photon phase-sensitive coherent-control scheme applied to photoionization microscopy of the hydrogen atom

Abstract

Photoionization microscopy (PM) is an electron imaging method designed for the measurement of the outgoing flux of slow electrons produced by photoionization of atoms in the presence of an external uniform static electric field. The high resolution of PM allows the observation of spatial quantum interference structures which are directly related to the squared modulus of the excited electron's wave function. The PM's range of interest lies above the saddle-point energy, where continuum Stark states are degenerate with quasibound ones (resonances). A principal aim of PM is to provide access to the wave functions of the latter, which in hydrogen atoms ionize exclusively via tunneling (in contrast to the continuum states where the electron escapes freely above the potential barrier). In nonhydrogenic atoms, however, quasibound states are coupled with the continuum ones. Among other consequences, this leads to comparable resonant and continuum excitation strengths and the weakening or disappearance of the resonant manifestations from the recorded PM images. Here we examine theoretically the possibility of bypassing these difficulties by applying two-excitation-pathway interference techniques. For this first case-study we employ the hydrogen atom but we, nevertheless, simulate a nonhydrogenic situation by selecting hydrogenic Stark resonances whose excitation strengths are comparable to or even smaller than the continuum ones. Specifically, we consider the interaction of ground-state atoms with an \ensuremath{\omega}/2\ensuremath{\omega} bichromatic laser field inducing one- and two-photon transitions to the final Stark states. It is shown that, under certain conditions and by appropriately adjusting the intensities of the two laser fields and their relative phase, the uncovering of resonant manifestations from PM images may indeed be achieved. Further work concerning theoretical extensions to complex atoms and possible experimental realizations are also discussed.