Time-resolved Core-level Photoelectron Spectroscopy Research Articles

Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}Sr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}CaCu2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}O8+δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{\ extrm{8}+ \\delta }$$\\end{document} crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu–O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu–O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu–O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu–O plane in the normal and superconducting phases.

Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm.

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.

Dynamical observation of photo-Dember effect on semi-insulating GaAs using femtosecond core-level photoelectron spectroscopy

We investigated ultrafast photogenerated carrier dynamics on a semi-insulating GaAs surface using femtosecond time-resolved core-level photoelectron spectroscopy. We observed a transient change in the surface potential where the Ga-3d core-level peak instantaneously shifted by ∼350 meV toward a higher binding energy within 2 ps of a 100 fs laser irradiation. A comparison of the experimental results with numerical simulations based on a drift-diffusion model of a semi-insulating semiconductor revealed that the transient surface potential change was mainly due to the photo-Dember effect, whereby the large difference in the carrier diffusion between the electron and hole instantaneously induced a surface photovoltage.

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface.

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator–metal transition in VO2, by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.

Towards time resolved core level photoelectron spectroscopy with femtosecond x-ray free-electron lasers

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses.

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(0 0 1)

The surface phase transition on GaAs(001) was investigated in real time by core-level photoelectron spectroscopy and reflection high-energy electron diffraction (RHEED). The time dependence of the relative surface band bending was obtained from the deconvolution of core-level photoelectron spectra. We found that band bending on the 4 × 2 surface increases 0.06eV from that of the 2 × 4 surface and that the time dependence of relative surface band bending is in good agreement with that of core-level photoelectron intensities but not with the RHEED intensities. This indicates that the long-range ordering of surface superstructures and microscopic surface morphology does not influence surface band bending, whereas the termination atoms, As or Ga, does.

Real-time analysis of a surface phase transition of GaAs (0 0 1) by core-level photoelectron spectroscopy and photoelectron diffraction

The surface phase transition of GaAs (0 0 1) from the As-rich 2×4 surface to the Ga-rich 4×2 surface and the reverse transition were analyzed in real-time using time-resolved core-level photoelectron spectroscopy incorporating the photoelectron diffraction effect. From the analysis of the time dependence of core-level photoelectron intensities and spectra, we found a large anisotropy in the intensities and spectra caused by the photoelectron diffraction effect. This anisotropy in the intermediate state of the transition from the 2×4 to the 4×2 differs from that of the reverse transition, indicating that the surface phase transition pathways differ.

時間分解内殻準位光電子分光による半導体表面過程の解析

We describe the development of an equipment which can be employed for examining various surface phenomena in real-time by performing time-resolved core-level photoelectron spectroscopy. We demonstrate this unique capability by examining the desorption of Sb atoms occurring from GaSb(001) as well as Sb/GaAs(001) in the temperature range of 440oC∼580oC, which is the usual growth temperature. The time dependence of core-level photoelectron spectra revealed that the activation energy for Sb desorption varies from 0.7 eV to 2.5 eV due to the difference in local bonding structures. In addition, experiments on alternating growth of GaAs suggest that there are two Ga adsorption sites, the occupation of which is dependent on the Ga coverages. This finding is in agreement with theoretical studies. These results demonstrate the potential of the technique to elucidate the surface process by real-time analysis.

Real-time analysis of alternating growth on GaAs(001) by core-level photoelectron spectroscopy

The alternating growth of GaAs on a (001) surface was analyzed by time-resolved core-level photoelectron spectroscopy (CLPES) and measuring reflection high-energy electron diffraction (RHEED) specular beam intensity. When looking at a Ga supply period, a saturation of Ga coverage was found from the time-dependence of Ga CLPES intensity. The RHEED specular beam intensity decreased even after the saturation coverage of Ga. These results indicate that Ga droplets begin to grow after Ga has been grown laterally. The inflection points of photoelectron intensity change were found at about half a monolayer of Ga supply at a substrate temperature of 560°C. The spectrum analysis of the time-resolved CLPES clarifies that the Ga adsorption site changes at about half a monolayer of Ga supply. These results show the potential of using time-resolved CLPES for the real-time analysis of the growth process.

Sb desorption from Sb/GaAs(001) and GaSb(001) analyzed by core-level photoelectron spectroscopy

Abstract The process of Sb desorption from GaSb(001) and Sb-terminated GaAs(001) was investigated by using a real-time analysis system that uses time-resolved core-level photoelectron spectroscopy. From the photoelectron intensity analysis, two competing processes with fast and slow time constants were found on the Sb desorption from both substrates and the activation energies were determined to be 0.71 eV and 0.90 eV for GaSb(001) and 1.1 eV and 2.5 eV for Sb-terminated GaAs(001). We investigated the relationship between the activation energies and surface bonding structure using spectrum differences based on the assignment of conventional core-level photoelectron spectroscopy of the surfaces whose Sb coverage is well controlled. This indicated that the activation energy of the desorption related to the transition from 2×8 to 2×4 of an Sb-terminated GaAs(001) is larger than the other desorption processes and it can be explained that the large potential barrier for bonding structure rearrangement between 2×8 superstructure and 2×4 superstructure exist in addition to Sb atoms desorption.

Real-time analysis for MBE by time-resolved core-level photoelectron spectroscopy.

A system has been developed for the real-time analysis of surface reactions during molecular beam epitaxial growth which uses photoelectron spectroscopy with VUV light taken from synchrotron radiation. This system consists of a synchrotron radiation beamline and growth/analysis apparatus in which photoelectron spectroscopy is performed with sub-second time resolution. In this system, photoelectron spectra are measured in sequence by a 'non-scanning' measurement method that enables the acquisition of snapshot photoelectron spectra using a multi-channel detector. This non-scanning measurement method was enabled by equipping an electric field correction grid. This system was used to monitor the photoelectron spectra of a GaSb(001) surface.

Time-resolved core-level photoelectron spectroscopy on Sb-terminated GaAs(001) under Sb supply control at growth temperature

The Sb desorption process from Sb-terminated GaAs(001) substrates has been analysed by time-resolved core-level photoelectron spectroscopy to investigate the influence of the substrate on the desorption. From the time dependence of the Ga 3d intensity, and the spectrum changes of Ga 3d and Sb 4d under Sb flux and during Sb desorption, three series of time constants for Sb desorption were found and activation energies of 2.5 eV and 1.1 eV were obtained for two of the desorption series. Spectrum analysis clarified that Sb double layers lie on the surface under Sb flux exposure and that Sb atoms corresponding to activation energies of 2.5 and 1.1 eV desorb from the topmost site of the Sb double layers. This is the same in the case of GaSb (001) substrate without the activation energy and shows that the bonding strength between the Sb atoms of the topmost layer site and the second layer site depends on the substrate materials.