Electronically Forbidden Raman Pathways Create a New Contrast Mechanism in Single-Molecule TERS.

Tip-enhanced Raman scattering (TERS) provides vibrational fingerprints with subnanometer resolution, yet resonant vibronic Raman pathways involving electronically forbidden (dark) intermediate states lack a validated, fully electrodynamic description. We develop a nonlocal quantum-electromagnetic framework based on transition-dipole densities that unifies Franck-Condon and Herzberg-Teller Raman amplitudes, including their interference, and treats the Stokes frequency near field self-consistently via dyadic Green functions evaluated with a discrete-dipole approximation for arbitrary lossy plasmonic geometries. The theory shows that dark intermediate resonances can make Herzberg-Teller-derived parity information and mode-dependent nodal patterns emerge in single-molecule TERS images, enabling clear discrimination of vibrational identity and parity even under strong resonance. It also predicts that neglecting Stokes frequency backaction/renormalization (prescribed near field) can alter apparent image symmetry.