  We study the dipole formation at the surface formed by -CH3 and -CF3
terminated shortchain alkyl-thiolate monolayers on Au(111). In particular, we
monitor the change in work function upon chemisorption using density functional
theory calculations. We separate the surface dipole into two contributions,
resulting from the gold-adsorbate interaction and the intrinsic dipole of the
adsorbate layer, respectively. The two contributions turn out to be
approximately additive. Adsorbate dipoles are defined by calculating dipole
densities of free-standing molecular monolayers. The gold-adsorbate interaction
is to a good degree determined by the Au-S bond only. This bond is nearly
apolar and its contribution to the surface dipole is relatively small. The
surface dipole of the self-assembled monolayer is then dominated by the
intrinsic dipole of the thiolate molecules. Alkyl-thiolates increase the work
function of Au(111), whereas fluorinated alkyl-thiolates decrease it.
