XPS and bonding: when and why can relaxation effects be ignored

Select spectroscopic and chemical physics problems associated with atomic motion triggered by electronic transitions are the topics of this paper. The story starts with the initial stimulation provided by Dick Brundleʹs photoelectron spectroscopy studies of adsorbed molecules and continues to contemporary examples in photoelectron spectroscopy and Femtochemistry, all of which are theoretically modelled within a unified framework of time-dependent, driven oscillators and decaying states.