Abstract
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.
Highlights
Organic molecules with π -conjugated electron systems have been intensively studied in recent years
If there is a chemisorptive contribution to this bond, this often leads to charge transfer from the metal into the lowest unoccupied molecular orbital (LUMO) of the molecule [4]
We argue here for the example of MPc molecules that if a mode that indicates a particular symmetry reduction has a Fano line shape, this proves an electronic contribution to this symmetry reduction, because the Fano line shape indicates an interfacial dynamical charge transfer (IDCT), and for an IDCT to be observable in a MPc molecule, there must exist an imbalance in the occupation of the two Eg LUMOs
Summary
Organic molecules with π -conjugated electron systems have been intensively studied in recent years. The LUMO of isolated MPc is twofold degenerate and belongs to the irreducible representation Eg of the D4h point group This degeneracy is closely related to cross-conjugation in the central porphyrazine macrocycle of phthalocyanine molecules [4,15]. A strong geometric distortion may lead to a negligible symmetry breaking of electronic states For this reason, additional experimental methods have been employed to settle the issue of a possible lifting of the degeneracy. Photoemission tomography has the unique advantage that if a degeneracy lifting takes place, the actual orientation of the filled orbital can be determined [24] This orientational sensitivity can be a drawback, if many molecules with many different orientations are present at the surface.
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