A general expression for the angular correlation function of the two emitted photoelectrons in sequential two-photon double ionization of atoms is derived and discussed. The expression can be used in the analysis of angle-resolved coincidence experiments. The angular distributions of the emitted electrons as measured in non-coincidence experiments are also discussed. As an example, the cross sections, angular distributions of photoelectrons and angular correlation functions for the sequential two-photon double ionization of Ne and Ar atoms have been calculated within the MCHF and MCDF approaches. The results are compared with recent experiments performed at the free-electron laser (FLASH) facility.

We present kinematically complete data on two-photon double ionization of Ne induced by short (~25 fs) intense (~5x10 13 W/cm 2) free-electron laser pulses at 44 eV. The observed electron energy spectrum points to the dominance of "sequential" ionization. We analyze state-selective angular distributions as well as the two-electron angular correlation function, and suggest a method to determine the time delay between both ionization steps. The measured angular asymmetry (β-) parameters strongly deviate from the results of an earlier non-coincident experiment providing benchmark data for theory.

Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field. DOI: 10.1103/PhysRevLett.101.193002 PACS numbers: 32.80.Fb, 32.80.Rm Multiphoton single-color ionization in intense optical or infrared laser fields has been the subject of multiple experimental and theoretical studies for more than two decades and is by now a very well understood process (e.g., [1]). The extension of these studies to multiphoton absorption in the photoionization continuum was followed by the discovery that high order harmonics of the fundamental laser frequency are emitted in the extreme ultraviolet (XUV) when a strong femtosecond optical laser pulse interacts with a gas jet (e.g., [2,3]). The combination of different wavelengths, one in the XUV and the other in the visible or near infrared, opens new opportunities. It has recently permitted the investigation of above-threshold ionization (ATI) as the result of the combined interaction of both fields [4][5][6]. In this case the dominant contribution comes from processes in the course of which the emitted electron exchanges photons with the dressing laser field via stimulated emission (or absorption) resulting in a comb of sidebands disposed on both sides of the main photoelectron line.Theoretical studies have established that the sideband intensity depends on the electron kinetic energy as well as on the strength and polarization state of the optical laser field [7]. Fitting theoretical profiles to the measured sideband signals should yield the main parameters which govern the photon-atom interaction in this regime. For example, changing the polarization of either of the radiation beams gives rise to ''dichroic effects'' in the photoelectron spectrum. It therefore opens the possibility to control the relative contributions of photoionization channels with different angular momenta.This approach has been extensively used in studies of atomic ionization by weak monochromatic radiation from synchrotrons and continuous lasers, where at least one resonant intermediate state is involved, and the basic photon-electron interaction is completely dominated by this resonant excitation [8]. The use of high harmonic XUV sources to generate similar processes in the nonresonant continuum is complicated by very difficult analysis, since contributions from several harmonics and their mutual interferenc...

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Sequential two-photon double ionization of Kr atoms is theoretically considered. The angular distribution of emitted electrons is calculated with the dipole amplitudes evaluated within the multiconfigurational Hartree-Fock and Dirac-Fock approaches. The difference in angular distributions for two-photon and single-photon ionization is discussed.

Angular correlations between two emitted photoelectrons in the sequential two-photon double ionization (TPDI) of atoms are theoretically considered. The general expression for the angular correlation function is analysed and some particular geometries of possible experiments are discussed. The concept of true angular correlations is introduced in contrast to the angular dependence of the two-electron emission from two uncorrelated ionization events. The case of a pure LS coupling description of the TPDI is considered in detail and compared with the general case. As an example, calculations of the angular correlation functions are performed and discussed for sequential TPDI from the 4p valence shell of krypton.

Abstract. Motivated by the recent achievements of experiments using X-ray free electron lasers, we develop a theory for the angular distributions of photoelectrons in sequential two-photon double ionization (2PDI) of atoms beyond the dipole approximation. Expressions for the angular distributions are obtained taking into account the full multipole expansion of radiation in electric and magnetic moments. The formalism is further specified for the first-order corrections to the dipole approximation. As illustrative example, the sequential 2PDI of the 2p shell in atomic neon is studied. The numerical calculations predict distinct non-dipole effects observable in experiments at present X-ray Free Electron Lasers.

Multiple photoionization of neon atoms by a strong 13.7 nm (90.5 eV) laser pulse has been studied at the FLASH free electron laser in Hamburg. A velocity map imaging spectrometer was used to record angle-resolved photoelectron spectra on a single-shot basis. Analysis of the evolution of the spectra with the FEL pulse energy in combination with extensive theoretical calculations allows the ionization pathways that contribute to be assigned, revealing the occurrence of sequential three-photon triple ionization. The development of free electron lasers (FELs) emitting radiation in the extreme ultraviolet and the x-ray spectral range heralds a new era in the study of nonlinear processes at high photon frequency. Using these new sources (FLASH in Germany, SPring-8 in Japan, and LCLS in the USA), previously unexplored regimes of atomic and molecular strong field ionization become accessible. In the first experiments [1-3] strongly nonlinear multiple ionization of atoms has already been observed. Since mainly multiply charged ions were detected, it was impossible to unambiguously identify the ionization mechanisms. Accordingly, the interpretation of some of these experiments is still under debate [4,5], and the main question is whether the experimental results can be understood without introducing new concepts, e.g., collective effects, in the description of the multiphoton multiple ionization at high frequencies. In the simplest case of two-photon double ionization (2PDI) two basic mechanisms have been established: direct (nonsequential) ionization, where both photons are absorbed simultaneously, and sequential ionization, where, after absorption of a first photon and emission of the first electron, intermediate ionic states are formed that are further ionized by a second photon. In order to disentangle the ionization mechanisms, a number of experimental techniques have so far been used, which include measurements of ionic charge state distributions [6], energyand angle-resolved electron spectroscopy [7,8], measurements of recoil-ion-momentum distributions [9][10][11][12], and kinematically complete experiments using a reaction microscope [8]. Together with extensive accompanying theoretical work (e.g., Ref.[13] and references therein) these experiments have significantly advanced our understanding of 2PDI. In particular it was established, as first predicted theoretically [14], that sequential ionization is dominant if the photon energy is larger than the binding energy of the singly charged ion.As a logical next step in studies of multiphoton multiple ionization in the high-frequency regime, we report the first observation of sequential three-photon triple ionization (3PTI) of neon atoms, based on a measurement of angle-resolved photoelectron spectra using the velocity map imaging technique [15]. Strong field triple ionization is much more complicated than double ionization due to the larger number of processes that can contribute. Therefore, although triply charged ions have been reported [1,2,9], individual pathway...