Lifting of Accidental Degeneracy in Molecular Vibrations Probed with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy*

Ultrahigh Vacuum Tip-enhanced Raman spectroscopy (UHV-TERS) combines the imaging capability of scanning probe microscopy (SPM) with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). In combination with time-dependent density functional theory (TDDFT), insights into the influence of intermolecular interactions on the vibrational spectra of a N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer is revealed. In particular, we have interrogated the lifting of an accidental vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale.