Development of a single-shot CCD-based data acquisition system for time-resolved X-ray photoelectron spectroscopy at an X-ray free-electron laser facility

In order to utilize high-brilliance photon sources, such as X-ray free-electron lasers (XFELs), for advanced time-resolved photoelectron spectroscopy (TR-PES), a single-shot CCD-based data acquisition system combined with a high-resolution hemispherical electron energy analyzer has been developed. The system's design enables it to be controlled by an external trigger signal for single-shot pump–probe-type TR-PES. The basic performance of the system is demonstrated with an offline test, followed by online core-level photoelectron and Auger electron spectroscopy in `single-shot image', `shot-to-shot image (image-to-image storage or block storage)' and `shot-to-shot sweep' modes at soft X-ray undulator beamline BL17SU of SPring-8. In the offline test the typical repetition rate for image-to-image storage mode has been confirmed to be about 15 Hz using a conventional pulse-generator. The function for correcting the shot-to-shot intensity fluctuations of the exciting photon beam, an important requirement for the TR-PES experiments at FEL sources, has been successfully tested at BL17SU by measuring Au 4fphotoelectrons with intentionally controlled photon flux. The system has also been applied to hard X-ray PES (HAXPES) in `ordinary sweep' mode as well as shot-to-shot image mode at the 27 m-long undulator beamline BL19LXU of SPring-8 and also at the SACLA XFEL facility. The XFEL-induced Ti 1score-level spectrum of La-doped SrTiO3is reported as a function of incident power density. The Ti 1score-level spectrum obtained at low power density is consistent with the spectrum obtained using the synchrotron source. At high power densities the Ti 1score-level spectra show space-charge effects which are analysed using a known mean-field model for ultrafast electron packet propagation. The results successfully confirm the capability of the present data acquisition system for carrying out the core-level HAXPES studies of condensed matter induced by the XFEL.

CHANGES INDUCED IN THE SURFACE ELECTRONIC STRUCTURE OF Be(0001) AFTER Si ADSORPTION

A study of effects induced in the Be 1s core level spectrum and in the surface band structure after Si adsorption on Be(0001) is reported. The changes in the Be 1s spectrum are quite dramatic. The number of resolvable surface components and the magnitude of the shifts do decrease and the relative intensities of the shifted components are drastically different compared to the clean surface. The surface band structure is also strongly affected after Si adsorption and annealing. At [Formula: see text] the surface state is found to move down from 2.8 to 4.1 eV. The band also splits at around 0.5 Å-1 along both the [Formula: see text] and [Formula: see text] directions. At [Formula: see text] and beyond [Formula: see text] only one surface state is observed in the band gap instead of the two for the clean surface. Our findings indicate that a fairly small amount of Si in the outer atomic layers strongly modifies the electronic properties of these layers.