Jun-ichi Adachi

Photoelectron?photoion?photoion momentum spectroscopy as a direct probe of the core-hole localization in C 1s photoionization of C2H2

Angular distributions of C 1s photoelectrons, relative to a dissociation axis for C2H2, have been measured with a photoelectron?photoion?photoion coincidence technique. The photoelectron angular distribution (PAD) for three two-body fragmentations (symmetric, non-symmetric and proton-migration fragmentations) is completely different. The PADs for non-symmetric fragmentation provide direct evidence of a localized core-hole and preferential bond breaking following Auger decays. Moreover, the PAD for symmetric fragmentation has been interpreted as the interference between photoemissions from the two carbon atoms.

Current status of the undulator beamline BL-2C at Photon Factory

Abstract The undulator beamline BL-2C was constructed for measurements of soft X-ray spectroscopy. It is equipped with a varied line-space plane grating monochromator. High energy-resolution, high photon-flux and tiny spot-size have contributed to research on electronic structure of gaseous samples, solid samples and surfaces. In order to carry out further sophisticated measurement, we have improved the beamline at the following three points. First, optical elements were re-aligned again with great care for higher energy-resolution. To demonstrate the energy-resolution, we measured absorption spectrum of oxygen 1xa0s σ → π * excitation and evaluated E/ Δ E to be more than 10,000. Secondly, mirror holders were altered to reduce the effect of heat load. The remodeling and the replacement of the optical-element holders resulted in energy-drift becoming negligible. Thirdly, the carbon-contaminated optical elements were cleaned by ozone-cleaning method. The photon flux at the carbon absorption-edge is recovered by the cleaning.

In situ removal of carbon contamination from optics in a vacuum ultraviolet and soft X-ray undulator beamline using oxygen activated by zeroth-order synchrotron radiation

Carbon contamination of optics is a serious issue in all soft X-ray beamlines because it decreases the quality of experimental data, such as near-edge X-ray absorption fine structure, resonant photoemission and resonant soft X-ray emission spectra in the carbon K-edge region. Here an in situ method involving the use of oxygen activated by zeroth-order synchrotron radiation was used to clean the optics in a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A at the Photon Factory in Tsukuba, Japan. The carbon contamination of the optics was removed by exposing them to oxygen at a pressure of 10(-1)-10(-4)u2005Pa for 17-20u2005h and simultaneously irradiating them with zeroth-order synchrotron radiation. After the cleaning, the decrease in the photon intensity in the carbon K-edge region reduced to 2-5%. The base pressure of the beamline recovered to 10(-7)-10(-8)u2005Pa in one day without baking. The beamline can be used without additional commissioning.

3D mapping of photoemission from a single oriented H2O molecule

3D O 1s photoelectron angular distributions from a fixed-in-space H2O molecule have been observed for the first time by quadruple coincidence measurements of photoelectron−O+−H+−H+. Obtained results for the non-axially symmetric molecules reveal the symmetry-imposed restrictions of the C2v symmetry group on the angular distributions and their strong orientation. Characteristic features of the angular distributions are explained with full quantum mechanical calculations and also in terms of Huygens approach on electron de Broglie waves.

Three-photon double ionization of Ar studied by photoelectron spectroscopy using an extreme ultraviolet free-electron laser: manifestation of resonance states of an intermediate Ar+ ion

Three-photon double ionization of an Ar atom has been investigated by photoelectron spectroscopy using an extreme ultraviolet free-electron laser. When the photon energy of the laser has been tuned to the resonances of the intermediate Ar+ ion, photoelectron peaks due to the resonant two-photon single ionization of the Ar+ ion have been observed. And it has been revealed that the appearance of the peaks is controlled by the mixing ratios of 3p4 core-configurations of the multi-configuration states. Furthermore, it has been shown that the laser intensity dependence of the Ar2+ ion production rate due to the resonant two-photon single ionization strongly deviates from that expected by perturbation theory.

Photoelectron angular distributions from dissociative photoionization channels of fixed-in-space molecular hydrogen

The angular distribution of photoelectrons from fixed-in-space molecular hydrogen has been measured in the 44-76 eV photon energy range for an ionic state, with a state energy of 38 eV measured from the H2 ground state, dissociating to H+ +H(n = 2). The photoelectrons are ejected nearly isotropically at 44 eV when their energy is relatively low, while ejection perpendicular to the molecular axis becomes more and more favoured as the photon energy increases. This behaviour is possibly explained in terms of direct photoionization to the 2p u and 2s g states of H2 + ; the former being favoured at 44 eV, and the latter at photon energies far from threshold.

Extensive study on the C 1s photoionization of CS2 molecules by multi-coincidence velocity-map imaging spectrometry

With the full use of the experimental data obtained by multi-coincidence velocity-map imaging spectrometry, we have determined the dipole amplitudes and their relative phases that describe the C 1s photoionization dynamics in CS2. It has been found that the σ shape resonance at about 15 eV above the C 1s ionization threshold appears only in the dipole amplitude dfσ. We have also carried out time-dependent density functional theory calculations so as to better understand the photoemission dynamics and shape resonances in the C 1s photoionization of CS2 molecules with heavy atoms (sulfur) having 3p valence orbitals, which distinguish the present work from well-studied works such as on CO and CO2 molecules.

New approach for a complete experiment: C1s photoionization in CO2 molecules

A new experimental approach for determining the photoionization matrix elements with the use of multi-coincidence velocity-map imaging spectrometry is presented for the case of C1s photoionization of CO2 molecule over the energy region of a shape resonance. Our experimental results provide the complete information on the shape resonance mechanism and clearly demonstrate the inadequacy of presently available theoretical calculations for this system. The analysis of a full set of experimental data from which absolute values of the dipole matrix elements and their relative phases can be extracted has been referred to as providing acomplete description of the photoionization process [1]. The complete experiment makes it possible to establish a close connection between the theory and experiment, and to check the quality of existing theories. During the last two decades, great efforts have been made for such experiments not only in atomic photoionization [2–4] but also in molecular photoionization. Although the latter started much later than the former, several experimental studies concerning the complete description of the molecular photoionization process have been published recently [5–8]. In these studies, one of the most promising approaches for the achievement of the complete experiment was developed, in which molecular frame photoelectron angular distributions (MFPAD) were measured using linearly and elliptically (or circularly) polarized light. However, this approach is tedious and not convenient because one must repeat the 7 Present address: Institute of Molecular Science, Myodaiji, Okazaki 444, Japan.

O1s photoionization dynamics in oriented NO2

We have performed extensive density functional theory (DFT) calculations, partial cross sections, dipole prepared continuum orbitals, dipole amplitudes and phase shifts, asymmetry parameters β, and molecular frame photoelectron angular distributions, to elucidate the O1s photoionization dynamics of NO(2) molecule with emphasis on the shape resonances in the O1s ionization continuum. In the shape resonance region, the β parameters and photoelectron angular distributions have been compared with our experimental results. Fairly good agreement between the theory and experiment has confirmed that the DFT level calculations can well describe the photoionization dynamics of the simple molecule such as NO(2). Interference due to equivalent atom photoionization is theoretically considered, and the possibility of detection of the effect in the two degenerate channels with different combinations of light polarization and photoemission direction is discussed.

Theoretical study of focusing and double slit effects in x-ray photoelectron diffraction

Double-slit and forward focusing effects in XPD spectra from H2O and GeCl2 molecules have been theoretically studied within the single scattering approximation. In low-and intermediate-energy region, the double-slit effect can be observed. In contrast, this effect wears off and only forward focusing peaks with simple structure are given in high-energy region. The double-slit effects can be observed under special conditions where the two scattering amplitudes have finite overlap in the bisecting direction.