Femtosecond ultraviolet photoelectron spectroscopy of ultra-fast surface processes

Conventional ultraviolet photoelectron spectroscopy (UPS) has been successfully used for decades to study static electronic properties of surfaces and their interaction with adsorbates. The recent progress in the creation of femtosecond and attosecond VUV pulses by means of high-order harmonic generation (HHG) opens the potential to operate UPS in a time-resolved mode, which is capable of monitoring ultra-fast dynamical changes of surface properties. It is now possible to evaluate the temporal evolution of chemical surface reactions on their fundamental time scales. This paper reviews technical aspects of time-resolved UPS and its application to the study of ultra-fast surface processes. Special emphasis is placed on the creation of ultra-short VUV pulses by means of HHG and also on the description of the stroboscopic experimental approach that enables temporal resolutions of the order of femtoseconds. This review will show that time-resolved UPS is indeed capable of probing with femtosecond resolution dynamic surface processes related, for example, to electronic excitations at surfaces or to the chemical state of adsorbates on a femtosecond time scale.