sentences:  This paper reported the first-ever single electron spectroscopy work in which electronic energy levels of atoms at the surface of a metal were observed.In 1973, Plummer accepted a position in the Physics Department at the University of Pennsylvania where his work mainly focused on angle-resolved photoemission, momentum-resolved inelastic electron scattering and nonlinear optical response from surfaces.In 1988, he was appointed the William Smith Professor of Physics and in 1990 became the director of the NSF-funded Materials Research Laboratory (Laboratory for Research on Structure of Matter).