Birrell, Andrew Reid; (1996) Electron emission from selected metal surfaces by multiphoton processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Two possible mechanisms for the ejection of electrons from a metal surface are; (1) the photoelectric effect and (2) thermionic emission. The former process is observed when the energy of a single incident photon (ℏω)) is greater than the work function (Φ) of the surface; the excess photon energy being transferred to the released electron as kinetic energy. However, when very intense radiation fields are incident upon the surface, new multiquantum phenomena become possible, such that when nℏω>Φ, electrons are also emitted. The further phenomenon of above-threshold ionization (ATI) in which the ionised electron acquires additional quanta (sℏω) from the radiation field is well-known for gas-phase atoms, but is still the subject of intense investigation for solids. In this work, electron emission from three metal surfaces (copper, gold and silver) has been investigated using an 8ns Nd:YAG laser (λ = 1064 or 532nm, 30Hz repetition rate). The average laser power density at grazing incidence to the sample was ≈60MWcm-2 such that multiquantum processes should be observable. The emitted electrons were energy-analysed using a single hemispherical analyser. Ejected electron energy spectra have been investigated at different laser polarisations and intensities. When the laser polarisation was parallel to the surface, a single low energy ejected electron peak was observed. In contrast, when the polarisation was perpendicular to the surface a double-peaked structure was observed. However, the total electron yield was strongly dependent upon the incident laser flux and at high intensities the multiquantum process could be masked by thermionic emission. Nevertheless, this work clearly demonstrated that there is evidence for a multiquantum photoelectric process from pure metallic surfaces.

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