Schematic view of the optical beamline dedicated to TR-PES. The FH is obtained through three sum-frequency generation processes: SHG, THG, and FHG are the three nonlinear crystals generating second, third, and fourth harmonics, respectively, DM are dichroic mirrors. WBS is the wedged beam splitter.
The UHV system is composed by three independent chambers: the fast-entry load-lock chamber with the transfer arm (right), the sample preparation chamber equipped with evaporators, sputter-gun, LEED, and Auger analyzers (center), and the photoemission chamber with the ToF spectrometer (left).
The ToF spectrometer: the flight length is about 500 mm. A bias can be applied to the sample and to the three separated elements of the drift tube creating two electrostatic lenses. The detector is a 64 anodes MCP (in chevron configuration).
(a) Simulated trajectories of electrons emitted at 0.8 and 1.6 eV. With a proper setting of the electrostatic lenses the acceptance angle can increase to ±10°. (b) Acceptance angle as a function of the electron kinetic energy for two different voltage settings. (c) Relationship between the electron kinetic energy and the inverse square of the ToF for the two voltage settings.
Fermi edges of the Cu(100) obtained with ; with no sample bias (right) the Fermi level is at 2.76 eV and the experimental resolution is . With a retarding bias of on the sample (left) the Fermi level shifts to 1.71 eV and the resolution improves to . The inset reports the photoemission spectrum with no bias. From its extension the work function of 4.54 eV is deduced.
Experimental 1PPE intensities from the Ag(111) surface with photons . The horizontal axis represents the detection angle (between the surface normal and the analyzer axis), the vertical axis in the binding energy referred to the Fermi level.
1PPE intensities of the surface state at various detection angles (from −10° to in steps of 2°). The inset shows the dispersion relation observed under different experimental conditions: with no bias on the sample (squares), after applying the angular correction (circles) and with a retarding bias on the sample. The corresponding effective masses are reported.
Ray-tracing simulation of the electron trajectories between the sample and the entrance of the ToF drift tube. The nominal detection angle is . The contact potential is 1.1 V. The table on the right shows the angular distribution at the photoemission point on the sample and at the entrance of the analyzer.
Upper panel: schematic energy diagram of the TR-2PPE process in Cu(111). 2PPE with photon energies (SHp) and (TH) can involve virtual intermediate states, starting at the surface state, or the IPS state as intermediate state. Lower panel: measured intensities of TR-2PPE from Cu(111). The right inset reports the energy spectra at 0 fs delay. The lower inset shows the (normalized) temporal evolutions of the SS and the IPS. The delay relates to the finite lifetime of the IPS.
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