Picosecond Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy at a Synchrotron Using a Fast Time-Tagging Approach

Time-resolved X-ray photoemission spectroscopy (TRXPS) is a powerful tool that provides quantitative information about transient electron and hole populations at surfaces and interfaces with unique site- and element-specificity. The technique uses a “pump” pulse to initiate electronic and/or nuclear dynamics, the evolution of which is monitored by X-ray “probe” pulses. Using synchrotrons for picosecond TRXPS experiments is commonly associated with the major challenge of a significant disparity between the repetition rates of laser pump and synchrotron probe pulses. One option to address this challenge is to suppress XPS signals from all X-ray pulses save for one at the same repetition rate as the pump pulse. However, this approach often leads to impractically long data acquisition times and significant sample damage issues as all X-ray pulses damage a sample, but only a small fraction contributes to the XPS signal. We present a different approach, implemented at the Advanced Light Source (ALS), that utilizes the combination of a hemispherical analyzer with a fast delay line detector to record the kinetic energy and emission time of every photoelectron in the 500 MHz X-ray multi-bunch pattern. The time-tagging approach allows the use of all X-ray pulses simultaneously, dramatically reducing data acquisition times and X-ray induced sample damage. The surface photovoltage effect in n-Si and electron transfer dynamics in Au nanoparticle sensitized TiO₂ are used as model systems to demonstrate the capabilities and limitations of the technique.