Ultrafast charge transfer at surfaces accessed by core electron spectroscopies

Charge transfer at surfaces, which is very important for surface photochemistry and other processes, can be extremely fast. This tutorial review shows how high resolution correlated excitation/decay spectroscopies of core excitations can be used to obtain charge transfer times at surfaces around or below 1 fs. Some results are described in more detail, and their meaning and theoretical modelling are discussed. A brief comparison to laser methods shows that there are differences in the processes they look at.