Measuring molecular interactions on surfaces requires subtle probes. Electron energy loss spectroscopy was used to determine that the C-O stretch mode energy of CO on Ir(111) gradually decreased due to the increasing coverage of coadsorbed phthalocyanine molecules. Density functional calculations rationalized the energy shift in terms of static electric dipole interactions, which affect the C-O stretch mode energy via the vibrational Stark effect. Starting from the lowest phthalocyanine coverage up to the nearly closed monolayer the spectroscopic signature of the C-O stretch mode adopted a rather broad and asymmetric line shape. This observation was traced to different phthalocyanine-CO distances and thus different dipole fields at the CO site, which gave rise to a finite distribution of vibrational energies. Additional calculations demonstrated that the influence of dynamic dipole coupling, intermolecular charge transfer and adsorbate-induced substrate lattice relaxations on the C-O stretch mode energy was negligible. The findings may be relevant to scanning probe methods striving for local force measurements.
