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

Area of Science:

• Chemical Physics
• Spectroscopy
• Nanotechnology

Background:

• Tip-enhanced Raman scattering (TERS) offers high-resolution vibrational analysis.
• Existing theories struggle to describe resonant vibronic Raman pathways involving dark intermediate states.

Purpose of the Study:

• To develop a validated, fully electrodynamic description for TERS resonant vibronic Raman pathways.
• To investigate the role of electronically forbidden (dark) intermediate states in TERS.

Main Methods:

• Developed a nonlocal quantum-electromagnetic framework using transition-dipole densities.
• Unified Franck-Condon and Herzberg-Teller Raman amplitudes, including interference.
• Self-consistently treated the Stokes frequency near field via dyadic Green functions and discrete-dipole approximation for lossy plasmonic geometries.

Main Results:

• Demonstrated that dark intermediate resonances enable the emergence of Herzberg-Teller-derived parity information in TERS images.
• Showcased the ability to discriminate vibrational identity and parity even under strong resonance conditions.
• Predicted that neglecting Stokes frequency backaction can alter apparent image symmetry.

Conclusions:

• The developed framework provides a comprehensive description of TERS, including dark states.
• This theory enhances the interpretation of single-molecule TERS data, enabling detailed vibrational and parity analysis.
• Highlights the importance of self-consistent near-field treatment for accurate TERS image interpretation.