Possible 2PPE-excitation schemes at the PE-I for the photon energy combinations UV/IR, UV/blue, and UV/UV for PTCDA/Ag(111). Depending on the photons used to photoemit electrons from the intermediate state, the signal is shifted in its final state energy. While the signal of the image-potential states (dashed lines) is shifted for the different excitation schemes due to different photon energies of the probe laser (blue, IR), the IS (thin solid line) is always probed by UV photons and its final state energy is hence independent from the applied photon combination (UV/IR, UV/blue, respectively). The combination UV/green used at PE-II is similar to the UV/blue case.
Work function change upon PTCDA adsorption. (a) Series of normalized 2PPE spectra for the clean Ag(111) substrate and for samples with different PTCDA coverages recorded with PE-I using UV/UV excitation with a photon energy of . The dashed spectrum represents a 15 ML film of inferior quality (see text). The workfunction of the clean Ag(111) sample is marked by the straight dashed line. (b) Relative shift of the work function plotted against the PTCDA coverage with reference to the clean Ag(111) substrate. The spectra were measured with both setups, PE-I as well as PE-II. The triangle represents the work function of the dashed spectrum in (a).
Attenuation of the IS signal with increasing PTCDA film thickness. The integrated thickness dependent signal of the respective IS peaks is plotted against coverage as points in the inset. An exponential fit (straight line) yields an electron attenuation length of , which is equivalent to a mean free path of 9.3 Å.
Shift of IS with photon energy for a 2 ML film recorded with different photon energies. For higher photon energies the IS clearly shifts to higher final state energies. The inset shows the dependence of the peak positions (diamonds) on the photon energy together with a linear fit with a slope of 0.84.
Image of the dispersion of the IS at the -point for different PTCDA coverages recorded in the PE-II chamber with the two-dimensional detector. The PTCDA coverages are from left to right: , , and .
(a) Emission angle dependent 2PPE spectra for 2 ML PTCDA/Ag(111). (b) IS dispersion for 1 and 2 ML samples of PTCDA/Ag(111). The lower -band and the SS data were extracted from Ref. 34. The PTCDA Brillouin zone boundaries are indicated by and . The inset shows a sketch of the real space arrangement of the PTCDA molecules in the case of 1 ML on Ag(111). and are the long and short unit cell vectors of the PTCDA superstructure, respectively (from Ref. 8).
Excitation of the IS with the photon energy combinations UV/IR (top) and UV/blue (bottom). Dashed lines show spectra at zero time delay and solid lines represent spectra at a delay of 200 fs.
Electron lifetime of the IS along the state’s dispersion for PTCDA/Ag(111). (a) Two-color (UV/blue) 2PPE pump-probe traces of a 1 ML film of PTCDA/Ag(111) recorded at the IS maximum for different . The lifetimes are determined from a fit of an exponential decay at positive time delays (shown as black lines). The dashed line represents the cross correlation of the two laser pulses. (c) Lifetime of electrons excited into the IS for 1 and 2 ML along the IS band compared to the corresponding dispersion shown in (b).
(a) Comparison of the line shape of the IS for a 1 ML sample recorded at temperatures between 110 (bottom) and 300 K (top). (b) Time-resolved IS signal at temperatures of 95 and 300 K.
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