Photoemission band mapping with a tunable femtosecond source using nonequilibrium absorption resonances

Abstract

The authors show that the tunability of a femtosecond optical parametric amplifier combined with its high-repetition rate and short pulses provide a powerful tool for an alternate approach to conventional nonresonant band mapping by two-photon photoemission (2PPE). The authors demonstrate this 2PPE mapping via use of two model systems, i.e., the pair of sp surface and image states on flat Cu(111) and vicinal Cu(775) surfaces, over a photon energy range of 3.9–4.6 eV by making use of direct resonant band-to-band electronic transitions. Since the experimental excitation of the Cu image state from the surface state is comparable in time to the electron-electron equilibration time, the authors measure sharp resonant features in the electron energy distributions. In this approach, the authors track these resonant electronic transitions using 2PPE by varying the photon energy so as to achieve resonant excitation at each value of photoelectron emission angle over a large wavelength range on both types of surfaces. In addition, the authors explore the range of photon energies and optical intensities which may be used for this approach. The repetition rate of this laser was sufficient to yield a good signal-to-noise ratio while maintaining pump pulse intensities at levels that were low enough to prevent significant photon-induced space-charge broadening and electron-kinetic-energy shifting, even for photon energies close to the work function of the sample.