X-ray and photoelectron spectroscopy of surface chemistry; from bonding via femtosecond to operando

For the 60th anniversary of Surface Science, I present here a personal account of some of the most significant contributions I have made to the field over the past three decades. The utilisation of X-rays serves as the foundation for these studies, encompassing X-ray spectroscopy for the mapping of surface chemical bonds, probing of surface reactions on ultrafast timescales, and X-ray photoelectron spectroscopy under operando conditions. The direct projection of electronic states onto the adsorbed atom allowed the detection of bonding and anti-bonding states within the d-band model. The selective probing of orbitals of different symmetries on the two atoms in adsorbed N₂ provided a fundamental understanding of the nature of diatomic bonding to surfaces. Ultrafast optical pumping and X-ray laser techniques allowed the study of CO undergoing desorption leading to the observation of the precursor state. Pump-probed studies of co-adsorbed CO and O on Ru enabled the means to detect transition state species during catalytic CO oxidation. The use of operando X-ray photoelectron spectroscopy at near-atmospheric pressures opened the door to probe the surface chemistry and gain insight into the reaction mechanism during hydrogenation reactions to produce ammonia, hydrocarbons, methanol and ethanol. By inserting an electrochemical cell into the spectroscopic chamber, both fuel cell and water splitting electrocatalysis could be studied giving insight about the reaction mechanism.