  We investigate the vibrational properties of Pt- and Pd-phthalocyanine (PtPc
and PdPc) molecules on Ag(111) with high resolution electron energy loss
spectroscopy (HREELS). In the monolayer regime, both molecules exhibit long
range order. The vibrational spectra prove a flat adsorption geometry. The red
shift of vibrational modes and the presence of asymmetric vibrational peaks
suggest a moderate interaction of the molecules with the substrate, accompanied
by a static charge transfer from the metal to the molecules. The appearance of
a particular vibrational mode, which (i) belongs to the $\mathrm{B_{1g}}$
representation of the original fourfold $\mathrm{D_{4h}}$ molecular symmetry
group and which (ii) exhibits interfacial dynamical charge transfer (IDCT),
proves that a preferential charge transfer from the Ag surface into one of the
originally doubly degenerate lowest unoccupied molecular orbitals (LUMOs) of
$\mathrm{E_g}$ symmetry takes place, i.e. the electronic degeneracy is lifted
and the molecule-surface complex acquires the twofold symmetry group
$\mathrm{C_{2v}}$. The vibration-based analysis of orbital degeneracies, as
carried out here for PtPc/Ag(111) and PdPc/Ag(111), is not restricted to these
cases. It is particularly useful whenever the presence of multiple molecular
in-plane orientations at the interface makes the analysis of orbital
degeneracies with angle-resolved photoemission spectroscopy difficult.

Materials Science