Stages of Se adsorption on Au(111): A combined XPS, LEED, TOF-DRS, and DFT study

We have studied the adsorption of Se on the surface Au(111) using XPS, TOF-DRS, LEED and DFT calculations. The use of a doser that operates in vacuum allowed us to investigate all the stages of the adsorption from the clean surface up to the formation of multilayers. In the monolayer regime we have found two ordered phases with distinctive LEED patterns. The LEED pattern of the first phase presents three fractional spots arranged symmetrically around the positions of the spots in a root 3x root 3 pattern. The analysis of this pattern suggests the formation of either a nxn superstructure of root 3x root 3 domains with n=19 or n=22, or that the adsorption occurs without removing the 22x root 3 herringbone reconstruction of the gold surface. This last possibility is in accordance with DFT calculations which show that the charge transfer to a Se adsorbate might not be enough to destabilize the surface reconstruction. Increasing the coverage, beyond 0.3 ML a new LEED pattern appears with broad spots which upon annealing at 150 degrees C become well defined indicating a 1x8 periodicity. At the highest doses we have observed the formation of multilayers with no discernible LEED pattern. The comparison with adsorption experiments carried out in liquid solutions show similarities and also some important differences.