We investigate Topological Insulators with two-photon photoelectron spectroscopy. Our main focus lies on the compound Sb2Te2S, a so called Tetradymite.
The crystal structure of Sb2Te2S consists of quintuple layers with the stacking sequence (Te1-Sb1-S-Sb2-Te2). Chulkov et al.  and Lin et. al  predict this system to be a topological insulator with a single Dirac cone at the Γ point.
If the van der Waals gap between the two quintuple layers closest to the surface is increased by 40%, the surface band- structure changes. A parabolic surface state between the Dirac cone and the conduction band minimum appears, as well as a surface state with a negative dispersion at a binding energy of about -1.0 eV.
The investigated samples are p-doped. Therefore we expect the Fermi energy below the valence band maximum, a completely unoccupied Dirac cone and a work function of about 5 eV.
For two-photon photoemission we employ a laser system based on a regenerative amplifier (RegA, Coherent) seeded by a homebulid broadband oszillator and pumped by a Verdi G18. In the present experiment we used the compressed infrared laser beam of the RegA (790 nm/1.56eV) and the second harmonic of a Vis-OPA (250 nm/4.95eV). The cross correlation measured on Cu(111) had a FWHM of less than 80 fs.
To detect the photoemitted electrons we use the angular-resolving Time-of-Flight spectrometer THEMIS. This instrument allows us to measure energy E over parallel momentum kx|| and ky|| without rotating the sample.
The measurements were performed in the wide-angle mode (± 15°) with a kinetic energy of 1.45 eV and a pass energy of 10 eV. Since the work function difference between sample and spectrometer has not been balanced, it serves as an acceleration. This additional voltage was taken into account for the conversion of the raw data. In combination with the delay of the two laser pulses we get 4 dimensional data.