  Thin molecular films under model conditions are often exploited as benchmarks
and case studies to investigate the electronic and structural changes occurring
on the surface of metallic electrodes. Here we show that the modification of a
metallic surface induced by oxygen adsorption allows the preservation of the
geometry of a molecular adlayer, giving access to the determination of
molecular orbital symmetries by means of near-edge x-ray absorption fine
structure spectroscopy, NEXAFS. As a prototypical example, we exploited Nickel
Tetraphenyl Porphyrin molecules deposited on a bare and on an oxygen
pre-covered Cu(100) surface. We find that adsorbed atomic oxygen quenches the
charge transfer at the metal-organic interface but, in contrast to a thin film
sample, maintains the ordered adsorption geometry of the organic molecules. In
this way, it is possible to disentangle {\pi}* and {\sigma}* symmetry orbitals,
hence estimating the relative oscillator strength of core level transitions
directly from the experimental data, as well as to evaluate and localize the
degree of charge transfer in a coupled system. In particular, we neatly single
out the {\sigma}* contribution associated with the N 1s transition to the mixed
N 2px,y-Ni 3dx2-y2 orbital, which falls close to the leading {\pi}*-symmetry
LUMO resonance.
