A N Grum-Grzhimailo

Coherent control with a short-wavelength free-electron laser

Researchers demonstrate correlation of two colours (63.0 and 31.5 nm wavelengths) in a free-electron laser and control photoelectron angular distribution by adjusting phase with 3 attosecond resolution.

Angle-resolved electron spectroscopy of the resonant Auger decay in xenon with meV energy resolution

The angle-resolved resonant Auger spectrum of Xe is investigated with a record high meV energy resolution in the kinetic energy region of 34.45-39.20 eV at hv = 65.110 eV, corresponding to the resonant excitation of the Auger Xe* 4d(5/2)(-1)6p state. New lines have been observed and assigned in the spectra. The results of previous measurements concerning energies, intensities and angular distribution asymmetry parameters have been refined, complemented and, for some of the lines, corrected.

Angular distribution of photoelectrons in resonant photoionization of polarized atoms

The angular distribution of photoelectrons produced by excitation of polarized atoms to autoionizing states is analysed. In particular, core-excited autoionizing states produced in absorption of synchrotron radiation by laser-pumped excited atoms are considered. A general expression for the angular distribution of arbitrarily oriented laser and synchrotron beams is obtained and further reduced to a simpler form for a particular geometry used in experiments. A case of an isolated autoionizing state (resonance) interacting with several continua is considered and parametrization of the observable parameters in the region of the resonance is discussed. Peculiarities of the circular dichroism in the region of autoionizing resonances are analysed. Formulae suitable for particular resonances with J<or=5/2 are presented and all parameters necessary for analysis of experimental angular distributions are tabulated. As examples, excitations of autoionizing 2p53s3p and 2s3p2 states in Na and 3p53d4s4p states in Ca are considered. Theoretical angular distributions are compared with recent experimental data.

Angular distributions and angular correlations in sequential two-photon double ionization of atoms

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.

Photoelectron spectroscopy of sequential three-photon double ionization of Ar irradiated by EUV free-electron laser pulses

We have investigated the ionization of the Ar atom by 51 nm extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan. The angle-resolved photoelectron spectra contain lines due to sequential three-photon double ionization with the second ionization step proceeding via the resonantly enhanced two-photon absorption. The relative intensities of the corresponding photoelectron peaks and their angular dependence are explained in the framework of a three-step model of the process.

Sequential two-photon double ionization of Kr atoms

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.

Sequential two-photon double ionization of the 4d shell in xenon

The sequential two-photon double ionization of 4d electrons of xenon in intense free-electron laser (FEL) radiation is theoretically investigated for photon energies of 70–200 eV. At these energies, which are available at the free-electron laser FLASH, the 4d photoionization dominates over the ionization of the valence shells, showing a giant resonance. The 4d vacancy produced by the first photon strongly couples to the (Auger) autoionization continuum with a lifetime of only ~6 fs and thus the Auger decay competes with the second photoionization in a typical FEL pulse. The photoelectron angular distributions and angular correlation between emitted electrons are investigated as associated with the peculiar 4d character of the photoionization and Auger decay.

3D visualization of XFEL beam focusing properties using LiF crystal X-ray detector.

Here, we report, that by means of direct irradiation of lithium fluoride a (LiF) crystal, in situ 3D visualization of the SACLA XFEL focused beam profile along the propagation direction is realized, including propagation inside photoluminescence solid matter. High sensitivity and large dynamic range of the LiF crystal detector allowed measurements of the intensity distribution of the beam at distances far from the best focus as well as near the best focus and evaluation of XFEL source size and beam quality factor M2. Our measurements also support the theoretical prediction that for X-ray photons with energies ~10 keV the radius of the generated photoelectron cloud within the LiF crystal reaches about 600 nm before thermalization. The proposed method has a spatial resolution ~ 0.4–2.0 μm for photons with energies 6–14 keV and potentially could be used in a single shot mode for optimization of different focusing systems developed at XFEL and synchrotron facilities.

On the angular distributions in molecular photoionization beyond the dipole approximation

The theory of angular distributions and angular correlations of photoelectrons and recoil ions in molecular photoionization is reformulated in terms of the density matrix and statistical tensor formalism, which incorporates a full multipole expansion of the radiation field. The dynamical parameters of the angular distributions are expressed in terms of the multipole photoionization amplitudes. Photoionization of linear molecules is analysed in more detail. New possible nondipole effects in the molecular photoionization are outlined.

Angular distribution of Auger electrons from fixed-in-space and rotating C 1s→2π photoexcited CO: Theory

The one-center approach for molecular Auger decay is applied to predict the angular distribution of Auger electrons from rotating and fixed-in-space molecules. For that purpose, phase shifts between the Auger decay amplitudes have been incorporated in the atomic model. The approach is applied to the resonant Auger decay of the photoexcited C 1s-->2pi resonance in carbon monoxide. It is discussed how the symmetry of the final ionic state is related to features in the angular distributions and a parametrization for the molecular frame Auger electron angular distribution is suggested. The angular distribution of Auger electrons after partial orientation of the molecule by the sigma-->pi-excitation process is also calculated and compared to available experimental and theoretical data. The results of the one-center approach are at least of the same quality as the available theoretical data even though the latter stem from a much more sophisticated method. As the one-center approximation can be applied with low computational demand even to extended systems, the present approach opens a way to describe the angular distribution of Auger electrons in a wide variety of applications.