Berlin 2008 – scientific programme
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O: Fachverband Oberflächenphysik
O 11: Time-Resolved Spectroscopy I
O 11.1: Talk
Monday, February 25, 2008, 13:30–13:45, MA 043
Two-photon photoemission of image-potential resonances in front of the Si(100) surface — Jens Kopprasch1, •Christian Eickhoff1, Irina Ostapenko1, Cornelius Gahl1, and Martin Weinelt1,2 — 1Max-Born-Institut, Max-Born-Straße 2A, 12489 Berlin — 2Freie Universität Berlin, Arnimallee 14, 14195 Berlin
We have investigated the dangling-bond states and image-potential resonances on the Si(100) 2 × 1 surface by means of bichromatic two-photon photoemission. Optical parametric amplification generates 70 fs ultraviolet pulses with tunable photon energies between 4.5 and 5.5 eV. These allow us to populate unoccupied states up to the Si(100) vacuum level, probed by the IR fundamental at a fixed wavelength of 795 nm. Besides the occupied dangling-bond state Dup, we resolve the first two image-potential states with binding energies of E1 = 0.62 eV and E2 = 0.18 eV with respect to the vacuum level. Using these energies we obtain a surface dielectric-constant of є = 11.2 which is close to the silicon bulk-value of є = 11.9.
Tuning the photon energy of the pump pulse across the Dup to n=1 and Dup to n=2 transitions we find a significant variation of both the 2PPE peak positions and the intensities. Before resonance we observe the Dup initial state with the kinetic energy increasing with the pump-pulse photon-energy. Above resonance the Dup intensity is significantly reduced and shifted to the respective image-potential resonances at constant kinetic energy. These intensity variations indicate interference between the transition to the discrete image-potential resonance and transitions to the continuum of unoccupied bulk states.