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

[摘要] 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.

[发布机构] University:University of Birmingham;Department:School of Physics and Astronomy

[关键词] Q Science;QC Physics