Photoelectron Spectroscopy of highly oriented pyrolytic graphite using intense ultrashort laser pulses

This thesis describes photoelectron emission measurements made at the surface of Highly Oriented Pyrolytic Graphite (HOPG) using ultrashort laser pulses. It concentrates on the observation and understanding of a new phenomenon whereby infrared laser pulses of just 1.5eV photon energy can be used to generate photoelectrons with kinetic energies of up to 80eV. Intensity dependence measurements depict a highly nonlinear excitation process and for p-polarised light observations can be explained by a high-order multiphoton excitation mechanism. Comparisons with photoelectron spectra taken using XUV pulse trains show a striking resemblance suggesting that the same final states excited by multiple IR pulses can also be reached by a single XUV photon. Interferometric autocorrelation measurements of the photoemission signal show increasingly high nonlinearity at greater photoelectron energies and a simulation of the interferometric data constructed using Optical Bloch Equations agreed with experiments showing that in the highly non-linear regime the autocorrelation shape depends almost exclusively on the nonlinearity of the excitation. XUV-IR pumpprobe measurements are also presented and the technical difficulties of such measurements discussed. Finally a novel technique of velocity map imaging of photoemission from a surface has been demonstrated for the first time at the HOPG surface