Kaori Niki

Structure determination of molecules in an alignment laser field by femtosecond photoelectron diffraction using an X-ray free-electron laser

We have successfully determined the internuclear distance of I2 molecules in an alignment laser field by applying our molecular structure determination methodology to an I 2p X-ray photoelectron diffraction profile observed with femtosecond X-ray free electron laser pulses. Using this methodology, we have found that the internuclear distance of the sample I2 molecules in an alignment Nd:YAG laser field of 6 × 1011 W/cm2 is elongated by from 0.18 to 0.30 Å “in average” relatively to the equilibrium internuclear distance of 2.666 Å. Thus, the present experiment constitutes a critical step towards the goal of femtosecond imaging of chemical reactions and opens a new direction for the study of ultrafast chemical reaction in the gas phase.

Theory of Pump-Probe Ultrafast Photoemission Spectra

Keldysh Green’s function approach is used in order to derive a practical formula to study pump-probe ultrafast photoemission spectra. The pump pulse is strong whereas the probe pulse can be treated by use of a perturbation theory: We expand the full Green’s function in terms of electron-probe pulse interaction and renormalized Green’s function including pump pulse excitation. Random Phase Approximation (RPA)-boson approach is introduced to estimate some important factors. Well above the edge, ultrafast photoelectron diffraction provides us with information on the transient structural change after the pump pulse excitation. In addition to these slow processes, the rapid oscillation plays an important role related to electronic excitation by pump pulse.

Plasmon Losses in Core Photoemission Spectra

A short review is presented on theoretical approach to plasmon losses in core-level photoemission from simple metals. Full multiple elastic scatterings of photoelectrons are taken into account before and after the plasmon losses within the quantum cumulant (Landau) formula that can describe both intrinsic and extrinsic losses, and overall features of core level photoemission. Far from normal emission and in low energy excitation, we cannot apply these simple exponential formulas.

Real-space multiple-scattering theory of XMCD including spin-orbit interaction in scattering process

The effects of the spin-orbit interaction on surrounding atoms for XMCD spectra are studied by a real-space multiple-scattering theory. The present numerical calculation for Fe K-edge XMCD spectra from BCC iron demonstrates the importance of the spin-orbit interaction on scattering atoms, which has been disregarded in previous works. These effects will be inevitable for K-edge XMCD analyses of light elements surrounded by heavy magnetic atoms.

Theory of Photoelectron Spectroscopy

Some features in photoemission theory from organic solids are reviewed starting from many-body scattering theory. This theoretical approach is direct and transparent, however, the extension to discuss temperature effects is rather difficult. In order to discuss phonon and many-body effects we build a theoretical frame based on Keldysh Green’s functions. Phonon effects such as Debye–Waller factor, electron–phonon interaction and recoil effects are extensively discussed. For the practical calculations multiple scattering formulas work so well. Different features obtained from UPS and XPS analyses are discussed for the excitations from extended levels.