The development of core electron spectroscopies of adsorbates

A short historical survey is given of the development of applications of core electron spectroscopies (X-ray induced core photoelectron spectroscopy, XPS; and Auger electron spectroscopy, AES) in surface physics, in particular in the investigation and characterization of well-defined adsorbate layers on single crystal surfaces. A personal perspective is used to show, with this subject and partly the author's group as example, the sometimes winding ways on which ideas and insights jump between communities, and the importance of personal relations in this process. Topics mentioned include qualitative and quantitative analysis of adsorbate species, interpretation of binding energy shifts induced by adsorption and connected topics such as reference levels, local potential, relaxation/screening and charge transfer, as well as final state splitting, satellites, peak shape changes and angular effects in both photo- and Auger electron spectra. The newly accessible possibilities from very high resolution in photon source and electron analysis, coupled with selective excitation with synchrotron radiation, are briefly discussed, and emphasis is laid on the access by these means to electronic dynamics and to their coupling to the dynamics of atomic motion, as visible in fragmentation of adsorbed molecules by these electronic excitations.