A SIMPLE THEORY OF 2-PHOTON PHOTOEMISSION FROM CLEAN AND ADSORBATE COVERED METAL-SURFACES

A simple theory is presented to study the elementary processes of two-photon photoemission (2PPE) from clean metals and adsorbates on metal surfaces. The model can be applied to metals having the occupied surface state and unoccupied image-potential state and also to adsorbates whose unoccupied states are populated from the occupied valence levels or the substrate by photoexcitation. The 2PPE spectrum is calculated on the basis of a simple energy diagram. It is demonstrated that one-photon photoemission from an initially unoccupied state (a fixed intermediate state which is populated by photon-driven electron excitation from an occupied metal state), is accompanied by two-photon photoemission from the occupied initial state via the unoccupied state as intermediate. When an electron is nonresonantly excited from narrow metal states such as d-bands or surface states, the lineshape of the corresponding 2PPE spectrum exhibits a characteristic asymmetry due to the underlying direct 2PPE from the occupied states. At resonant excitation from the narrow initial state to the intermediate state the 2PPE spectra exhibit a narrowing of the linewidth. It is shown that the lifetime of the unoccupied state cannot be deduced from the linewidth of the 2PPE spectra if a width of the initial state is comparable or narrower than the unoccupied state. No resonance enhancement and narrowing effect in the 2PPE spectra appear when a broad bulk continuum serves as an initial state of the 2PPE process.