2PPE Spectra Research Articles

Two-photon photoemission study ofCO/Pt(111)

Unoccupied electronic states induced by CO chemisorbed on a Pt(111) surface have been investigated by two-photon photoelectron (2PPE) spectroscopy. The 2PPE spectra obtained by excitation with4.3eV⩽hv ⩽ 5.4eV indicate that photoemission peaks prominent in the 2PPE spectra from the CO-covered Pt(111) surface originate from normally unoccupied CO-induced states above the Fermi level ( E f ), which are transiently occupied in the 2PPE process. The energy of the CO-induced unoccupied state depends on the adsorption site of CO. Below 0.17 ML where CO is adsorbed at the on-top site, the CO-induced state is located at 4.5 eV above E f . With an increase in the coverage to 0.5 ML, where CO is adsorbed at the bridge site in addition to the on-top site, a new photoemission peak at 4.2 eV grows. This peak is assigned to photoemission from the unoccupied state of bridge-bonded CO. From the polarization dependence of the photoemission peaks, these unoccupied states are assigned to the CO2π * derived states hybridized with the Pt 5d-band.

Electronic excited states of [formula omitted] studied by high-resolution two-photon photoelectron spectroscopy

A high-resolution two-photon photoelectron (2PPE) spectrometer was constructed and applied to NO adsorbed Cu(111). The apparatus was based on a high-repetitive tunable laser of picosecond pulse duration combined with a single-electron counting time-of-flight (TOF) system. Performance of the apparatus was examined with 2PPE spectra of Ag(111) and Cu(111) surfaces. Energy resolution better than 0.03 eV was achieved. A preliminary result for NO adsorbed Cu(111) surface showed that electronic states of adsorbed NO contain some fine structures, suggesting that the high-resolution 2PPE spectroscopy provides much information on electronic structure of adsorbed molecules.

A simple theory of two-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.

Electronic excitations of adsorbates analyzed with two-photon photoemission: [formula omitted

Photoinduced excitations of normally unoccupied electronic states of adsorbates are investigated with two-photon photoemission (2PPE). We report 2PPE spectra from an ordered (1.4 × 1.4)-CO layer on Cu(1 1 1) for excitation with 3.5 eV ≤ hv ≤ 4.5 eV photon energy. The energetic position, E kin, of a CO induced state is found to shift as ΔE kin = 1 × Δhv. This provides evidence for emission from a normally unoccupied state, located (3.3 ± 0.2) eV above the Fermi level. Based on the polarization dependence of the emission from this adsorbate-induced state it is identified as the 2π ∗ affinity level of chemisorbed CO on Cu(1 1 1) which is temporarily populated in the 2PPE process.