  The templated growth of the basic porphyrin unit, free-base porphine (2H-P),
is characterized by means of X-ray photoelectron spectroscopy (XPS) and
near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy measurements
and density functional theory (DFT). The DFT simulations allow the
deconvolution of the complex XPS and NEXAFS signatures into contributions
originating from five inequivalent carbon atoms, which can be grouped into C-N
and C-C bonded species. Polarization-dependent NEXAFS measurements reveal an
intriguing organizational behavior: On both Cu(111) and Ag(111), for coverages
up to one monolayer, the molecules adsorb undeformed and parallel to the
respective metal surface. Upon increasing the coverage, however, the
orientation of the molecules in the thin films depends on the growth
conditions. Multilayers deposited at low temperatures (LT) exhibit a similar
average tilting angle (30 degree relative to the surface plane) on both
substrates. Conversely, for multilayers grown at room temperature a markedly
different scenario exists. On Cu(111) the film thickness is self-limited to a
coverage of approximately two layers, while on Ag(111) multilayers can be grown
easily and, in contrast to the bulk 2H-P crystal, the molecules are oriented
perpendicular to the surface. This difference in molecular orientation results
in a modified line-shape of the C 1s XPS signatures that is dependent on the
incident photon energy, which is explained by comparison with depth-resolved
DFT calculations. Simulations of ionization energies for differently stacked
molecules show no indication for a packing-induced modification of the
multilayer XP spectra, thus indicating that the comparison of single molecule
calculations to multilayer data is justified.
