We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, $s$, $p$ and $d$ character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth $V$ as a function of the static field strength $F$ for otherwise identical experimental conditions. The $V\left(F\right)$ curve exhibits a maximum (typically $\sim 65\%–85\%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent $s$, $p$ and $d$ character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.

Photoionization microscopy (PM) is an experimental method allowing for high-resolution measurements of the electron current probability density in the case of photoionization of an atom in an external uniform static electric field. PM is based on high-resolution velocity-map imaging and offers the unique opportunity to observe the quantum oscillatory spatial structure of the outgoing electron flux. We present the basic elements of the quantum-mechanical theoretical framework of PM for hydrogenic systems near threshold. Our development is based on the computationally more convenient semiparabolic coordinate system. Theoretical results are first subjected to a quantitative comparison with hydrogenic images corresponding to quasibound states and a qualitative comparison with nonresonant images of multielectron atoms. Subsequently, particular attention is paid on the structure of the electron's momentum distribution transversely to the static field (i.e., of the angularly integrated differential cross-section as a function of electron energy and radius of impact on the detector). Such 2D maps provide at a glance a complete picture of the peculiarities of the differential cross-section over the entire near-threshold energy range. Hydrogenic transverse momentum distributions are computed for the cases of the ground and excited initial states and single- and two-photon ionization schemes. Their characteristics of general nature are identified by comparing the hydrogenic distributions among themselves, as well as with a presently recorded experimental distribution concerning the magnesium atom. Finally, specificities attributed to different target atoms, initial states, and excitation scenarios are also discussed, along with directions of further work.

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 ω/2ω 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.

Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (≤3 × 1011 W cm−2) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s2 1S0 → 4p2 1S0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca2+ yield is observable well before the saturation intensity of Ca+ and, moreover, it grows with intensity and saturates in parallel with the Ca+ yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as 'synonymous' with direct (or non-sequential) multiphoton double ionization.