Showing papers by "Yasumasa Hikosaka published in 2000"

Photoelectron-fragment ion correlations and fixed-molecule photoelectron angular distributions from velocity imaging coincidence experiments

Abstract: A new technique, based on velocity imaging, has been developed to examine correlations in energy and angle between the several particles formed in dissociative single and double photoionisation at VUV wavelengths. Electrons and positive fragment ions are imaged in coincidence at position-sensitive detectors; the resulting multidimensional data sets contain separable energy distributions, angular distributions and correlations in energy and angle between the particles. In some cases internal or external evidence indicates that pure axial recoil occurs, without molecular rotation. In such cases fixed-molecule photoelectron angular distributions can be extracted and parametrised. Results for H2, N2, NO, CO and O2 are presented. Effects of shape resonances in the ionisation channels are apparent in several cases.

Laser photoionization of polarized Ar atoms produced by excitation with synchrotron radiation

Abstract: The laser-synchrotron radiation combination technique has recently been incorporated into an apparatus for two-dimensional photoelectron spectroscopy of atoms and molecules in order to investigate photoionization dynamics of polarized atoms. Ground state Ar atoms are excited with linearly polarized synchrotron radiation to Rydberg states lying below the first ionization potential. The aligned atoms thus formed are ionized by irradiation of a laser which is also linearly polarized. Photoelectrons emitted in the direction of the electric vector of the synchrotron radiation are sampled and energy analysed. The photoelectron angular distribution is measured with respect to the electric vector of the laser. Expressions which correlate the asymmetric coefficients for the angular distribution with theoretical dynamic parameters involving transition dipole matrix elements are derived. The anisotropy of the present angular distribution can be reasonably explained, assuming that the matrix elements and phase shift differences are essentially independent of the total angular momentum quantum number of the final state and that the spin-orbit interaction in the continuous spectrum is small.

An Auger electron-threshold photoelectron coincidence spectrometer for studies of atomic and molecular dications

Abstract: An Auger electron-threshold photoelectron coincidence spectrometer has been developed to obtain spectroscopic information on atomic and molecular dicationic states. The spectrometer consists of two electrostatic hemispherical analysers. Auger electrons are detected by one of the analysers with a position sensitive detector. The other analyser captures threshold photoelectrons over a 4π sr solid angle with a penetrating electric field. The signal-accumulation circuit is based on a fast-slow coincidence technique. In order to demonstrate both the performance and advantages of this type of Auger electron spectrometer, the Kr MNN Auger transitions are investigated. Auger electron spectra associated with each 3d subshell are obtained by coincidence detection with the corresponding threshold photoelectrons, thus providing unambiguously assignments for the initial core-hole states of the Auger lines. The total energy resolution in a coincident Auger electron spectrum reaches 80 meV (FWHM), a value which corresponds to a sub-natural linewidth as the natural width of the Kr 3d core-hole states is 101 meV.

New results on photoion pair formation from application of the velocity imaging photoionisation coincidence technique.

Abstract: The velocity imaging photoionisation coincidence (VIPCO) technique is shown to be a powerful tool for studies of ion pair formation. Sequential mechanisms are demonstrated for some three-body ion pair formation reactions. Observation of a new type of reaction producing one negatively and two positively charged ions plus an electron is reported. Copyright © 2000 John Wiley & Sons, Ltd.

Molecular frame photoelectron angular distributions in inner valence photoionisation of CO

Abstract: The velocity imaging photoionisation coincidence (VIPCO) technique is applied to the photoionisation of CO into the inner-valence 4 2Σ+ state of CO+, and the molecular frame photoelectron angular distributions are measured on the high energy side of the known σ* shape resonance. The angular distributions at 40.8 and 48.4 eV show that partial waves with the angular momentum l⩽3 provide the dominant contribution to both parallel and perpendicular transitions, and the contributions from the l4 partial waves are relatively negligible. By restricting the partial wave expansion to l⩽3, eight and four sets of dipole matrix elements and phase shift difference are extracted from the measured angular distributions for parallel and perpendicular transitions, respectively. Although it is not possible to choose unambiguously among these sets, combined enhancement among the l⩽3 partial waves for the parallel transition is obvious at the shape resonance.