Near-saddle-point-energy photoionization microscopy images of Stark states of the magnesium atom

Abstract

Photoionization microscopy (PM) is a photoelectron imaging method based on the measurement of the electron current probability density in the case of photoionization of an atom in an external uniform static electric field. The resolution of PM is high enough for observing the quantum oscillatory spatial structure of the outgoing electron flux. We examine the Stark states of the magnesium atom and focus on PM features which are specific to the energy range just above the saddle-point energy. Particularly, the existence of m-dependent saddle-point thresholds is clearly demonstrated by following the evolution of the recorded angular distributions with energy. Furthermore, the outer inflection point radii of the radial distributions of the images are generally found to increase with energy monotonically but occasionally abruptly, signaling parabolic channel openings (above which these channels are converted to continua). This observation is in accord with recently reported quantum mechanical PM calculations for hydrogenic systems. More importantly, there are a number of nonmonotonic variations (local maxima) of the outer inflection point. The latter are attributed to the local emergence of an intensity halo at the outer part of the images, a feature relevant to the excitation of quasibound Stark states (resonances). The emergence of the halo for a given resonance is found to depend on the static electric field strength. These observations along with the difficulties in observing resonant effects on the recorded images of nonhydrogenic atoms, such as the medium-size magnesium atom, are discussed in detail.

Glory oscillations in photoionization microscopy: Connection with electron dynamics and Stark spectral structures in the continuum

“…On the other hand, for m≠0 and/or weak fields the experimental and theoretical [ 2,29,31,32,33,34,35,36,37 ] results showed quite weak or no modulation contrasts at all. Furthermore, even for m=0, the contrast was found to critically depend on the particular initial state and excitation scheme as well [ 16,38 ]. An example employing the same field as in Fig.…”

“…according to recent suggestions [ 13,16,38 ], ρΙΙ may be associated to the outermost inflection point ρip of the radial distribution R(ρ). In turn, the radius ρip follows in general closely the simple analytic expression of [ 3,4,5 ], but additionally exhibits some localized abrupt magnitude variations.…”

“…In turn, the radius ρip follows in general closely the simple analytic expression of [ 3,4,5 ], but additionally exhibits some localized abrupt magnitude variations. For ε<0 these variations are due to either the transformation of n1-channels to continua or the presence of resonances [ 38 ], while for ε>0 they are due to the presence of SFIS [ 16 ]. For the purpose of obtaining the bow signal over a smoothly energy-varying and atom-independent radius we adopt throughout in the present work the approximation ρΙΙ≈ , which is very good for ε<0 and excellent for ε≥0 [ 4,5 ].…”

“…Details about the experiment were previously provided in [ 16,38 ] and we will only give a brief description here. Magnesium vapor is produced in an oven mounted at the top of the laseratom interaction chamber held at a background pressure of ≈7´10 −7 mbar.…”

“…[ 13,16,38 ]. Briefly, a rough estimate of and consequently of F was obtained from the lowest energy where an image of quantifiable signal could be recorded.…”