Atomic photoionization processes under magnification

Abstract

Recently, classical simulations of threshold photoionization in the presence of an electric field have shown that a clear distinction between direct and indirect trajectories followed by the outgoing electron can be observed in the patterns of electron impacts on a two-dimensional detector. Subsequently, slow photoelectron imaging experiments have been reported where this distinction could be observed in atomic xenon. Furthermore, using a magnifying electrostatic lens to improve the velocity-map imaging technique, oscillatory patterns were observed modulating the classical envelope that was measured in the experiments of Nicole et al. [Phys. Rev. Lett. 88, 133001 (2002)]. This extension of slow photoelectron imaging, called photoionization microscopy, relies on the existence of interferences between various trajectories by which the electron moves from the atom to the plane of observation. In this article we present the main experimental results obtained both in slow photoelectron imaging and in photoionization microscopy. The formation of the interference pattern is discussed in the framework of a semiclassical model that is described in detail elsewhere. The qualitative information that can be drawn from the experiments is discussed, and the potential applications of photoionization microscopy are considered. Particular attention is paid to the role of continuum Stark resonances that appear between the saddle point in the Coulombϩdc field potential and the field-free ionization limit.

Maximum-information photoelectron metrology

“…The central spot was not observed in the previous 2D images, although such a feature is quite typical of VMI measurements and is usually assumed to indicate photoelectrons generated via many possible pathways to low-k continuum states (e.g., field ionization of high-lying excited states). Such pathways may be laser-intensity and VMI-parameter dependent and have been exploited in "photoionization microscopy" studies [40,41], and more recently investigated in the context of strong-field atomic and molecular ionization [42,43]. In the former case, near-threshold ionization is analyzed in a joint atom-electric-field potential, leading to the formation of complex quasibound states; in the latter case, low-and zeroenergy photoelectrons are associated with electron tunneling, followed by scattering and Coulomb focusing or recapture into high-lying Rydberg states, with subsequent field ionization of these states.…”

Photoionization of Rydberg hydrogen atom in a magnetic field

“…Blondel et al firstly studied the photodetachment microscopy of Br − in an electric field and then to O − [9,10]. The observation of the atomic system's wave function has recently been developed into an experimental technique, called photoionization microscopy [11][12][13]. In the photoionization microscopy, the image formed by the electron on the detector is the square modulus of the transverse component of the electronic wave function.…”

Photoelectron imaging of autoionizing states of xenon: Effect of external electric fields

“…Some experiments have already been performed that make use of the applied electric field in imaging experiments [39][40][41][42][43][44][45][46]. As indicated schematically in Fig.…”