A charge-transfer surface enhanced Raman scattering model from time-dependent density functional theory calculations...

Area of Science:

• Computational Chemistry
• Spectroscopy
• Materials Science

Background:

• Surface-Enhanced Raman Scattering (SERS) is a powerful technique for analyzing molecules adsorbed on metal surfaces.
• Understanding the vibrational spectra of adsorbed molecules requires accurate theoretical models.
• Silver nanoclusters interacting with pyridine provide a model system for studying adsorption phenomena.

Purpose of the Study:

• To calculate and simulate vibrational spectra of a silver(10)-pyridine vertex complex using DFT and TD-DFT.
• To compare simulated spectra with experimental SERS data for adsorbed pyridine.
• To elucidate the contributions of different vibrational modes and scattering mechanisms (e.g., Herzberg-Teller) to the SERS spectrum.

Main Methods:

• Vibrational frequency calculations using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT).
• Employing B3LYP/LANL2DZ and BP86/TZP computational methodologies.
• Calculating excited states and molecular orbital isosurfaces to understand electronic transitions.
• Comparing simulated spectra with experimental SERS data obtained on roughened silver electrodes.

Main Results:

• Calculated vibrational frequencies for free pyridine showed good agreement with experimental values.
• Simulated SERS spectra for the Ag(10)-pyridine complex, particularly using BP86 TD-DFT with damping, closely matched experimental spectra in terms of relative intensities and frequencies.
• Excited states involved both Ag(10) intercluster and charge-transfer excitations, with a significant charge-transfer excitation around 500 nm.
• The calculations explained the observed spectral features, including the appearance of specific vibrational modes attributed to Herzberg-Teller scattering.

Conclusions:

• Small Ag(10)-pyridine vertex complexes can effectively simulate SERS spectra of adsorbed pyridine.
• DFT and TD-DFT methods provide accurate frequencies and relative intensities for SERS analysis.
• The study clarifies the underlying electronic and vibrational processes contributing to the SERS spectra of pyridine adsorbed on silver nanostructures.