Exploring the Role\nof Excited States’ Degeneracy\non Vibronic Coupling with Atomic-Scale Optics

Interactions between\nmolecular electronic and vibrational\nstates\nmanifest themselves in a variety of forms and have a strong impact\non molecular physics and chemistry. For example, the efficiency of\nenergy transfer between organic molecules, ubiquitous in biological\nsystems and in organic optoelectronics, is strongly influenced by\nvibronic coupling. Using an approach based on scanning tunneling microscope-induced\nluminescence (STML), we reveal vibronic interactions in optical spectra\nof a series of single phthalocyanine derivative molecules featuring\ndegenerate or near-degenerate excited states. Based on detailed theoretical\nsimulations, we disentangle spectroscopic signatures belonging to\nFranck–Condon and Herzberg–Teller vibronic progressions\nin tip-position-resolved STML spectra, and we directly map out the\nvibronic coupling between the close-lying excited states of the molecules.