Collective Mid-Infrared Vibrations in Surface-Enhanced Raman Scattering

Area of Science:

• Chemical Physics
• Spectroscopy
• Surface Science

Background:

• Surface-enhanced Raman scattering (SERS) typically models molecular vibrations individually.
• Intermolecular vibrational coupling in SERS has been largely overlooked.
• Understanding collective effects is crucial for accurate spectral interpretation.

Purpose of the Study:

• To demonstrate collective vibrations in SERS from molecular monolayers.
• To investigate the role of infrared (IR) coupled dipoles in SERS.
• To establish SERS-based methods for measuring intermolecular distances and reaction dynamics.

Main Methods:

• Fabrication of molecular monolayers with controlled intermolecular separation using IR-active and IR-inactive spacer molecules.
• SERS measurements to observe vibrational frequency shifts.
• Microscopic modeling and density functional theory (DFT) calculations for validation.
• Tracking photochemical reactions of 4-nitrothiophenol to observe changes in collective vibrations.

Main Results:

• Observed vibrational frequency upshifts up to 8 cm⁻¹ due to intermolecular coupling.
• Frequency shifts tuned with mixing fraction and IR dipole strength, matching theoretical models.
• Demonstrated angstrom-resolution measurement of intermolecular distance and disorder.
• Successfully tracked photochemical reactions by monitoring the disruption of collective vibrations.

Conclusions:

• Collective vibrations of IR coupled dipoles are significant in SERS.
• SERS spectral analysis must account for intermolecular coupling.
• Cooperative frequency shifts offer a novel tool for nanoscale metrology and reaction monitoring in analytical chemistry and sensing.