  In order to investigate the orientational ordering of molecular dipoles and
the associated electronic properties, we studied the adsorption of
chlorogallium phthalocyanine molecules (GaClPc, Pc=C_32N_8H_16) on Cu(111)
using the X-ray standing wave technique, photoelectron spectroscopy, and
quantum chemical calculations. We find that for sub-monolayer coverages on
Cu(111) the majority of GaClPc molecules adsorb in a 'Cl-down' configuration by
forming a covalent bond to the substrate. For bilayer coverages the XSW data
indicate a co-existence of the 'Cl-down' and 'Cl-up' configuration on the
substrate. The structural details established for both cases and supplementary
calculations of the adsorbate system allow us to analyze the observed change of
the work function.
