Single-state electronic structure measurements using time-resolved x-ray laser induced photoelectron spectroscopy
- PDF / 290,905 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 246 Views
Q4.6.1
Single-state electronic structure measurements using time-resolved x-ray laser induced photoelectron spectroscopy A.J. Nelson, J. Dunn, T. van Buuren and J. Hunter Lawrence Livermore National Laboratory, Livermore, CA 94551 ABSTRACT We demonstrate single-shot x-ray laser induced time-of-flight photoelectron spectroscopy on semiconductor and metal surfaces with picosecond time resolution. The LLNL COMET compact tabletop x-ray laser source provides the necessary high photon flux (>1012/pulse), monochromaticity, picosecond pulse duration, and coherence for probing ultrafast changes in the chemical and electronic structure of these materials. Static valence band and shallow core-level photoemission spectra are presented for ambient temperature Ge(100) and polycrystalline Cu foils. Surface contamination was removed by UV ozone cleaning prior to analysis. In addition, the ultrafast nature of this technique lends itself to true single-state measurements of shocked and heated materials. Time-resolved electron time-of-flight photoemission results for ultra-thin Cu will be presented. INTRODUCTION Photoelectron spectroscopy is one of the preferred techniques for probing the static electronic structure of materials. This technique routinely uses synchrotron radiation or laboratory x-ray sources for photoionization, and various electron energy analyzers operating in a continuous mode. Time-of-flight (ToF) photoelectron spectroscopy (PES) with a pulsed synchrotron radiation source was first investigated as a means to determine energy and angular distributions in low photon energy regimes. [1] Researchers fairly recently started to use compact laser plasma x-ray sources (LPX) with 255 eV, 2.5 ns duration x-ray pulses at a repetition rate of 10 Hz in conjunction with the ToF photolectron spectroscopy technique. They demonstrated sufficient energy resolution to observe static chemical shifts in the Si 2p photoemission for SiO2 and Si3N4 using the time-of-flight technique after integrating for ~100 shots. [2, 3] The LPX ToF studies while successful have significant limitations which rule out a pump-probe experiment to study dynamic processes in a material induced by shocks, e.g. melting, phase changes. In particular, the low photon fluxes ~2.5 x 108 photons/pulse on the sample required multiple shots to record a photoelectron spectrum. Pushing towards observations of dynamical effects, a high harmonic generation (HHG) extreme ultraviolet (EUV) source coupled with a ToF spectrometer has been used in a number of pump-probe experiments. These have included semiconductor electron population dynamics as well as monitoring the surface chemical reaction of molecular oxygen and CO on Pt(111) with femtosecond time resolution. [4 – 6] These higher order harmonic sources have the advantage of shorter duration since the process is driven by ultrafast laser pulses, e.g. less than 60 fs, at a high repetition rate which makes them very attractive for dynamic pump-probe experiments. However, the presence of multiple harmonics requires furthe
Data Loading...
