On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters

TL;DR

This paper investigates how chemical bonding influences Raman scattering in benzenethiol adsorbed on silver clusters using TDDFT, revealing significant off-resonant and resonant enhancements aligned with experimental SERS data.

Contribution

It introduces a computational scheme to analyze chemical enhancement effects in SERS, combining off-resonant and resonant regimes within a unified TDDFT framework.

Findings

Raman scattering is enhanced by an order of magnitude off-resonance.

Resonant transitions cause an additional enhancement of about 100 times.

Computed vibrational shifts agree with experimental SERS results.

Abstract

We study the effects of chemical bonding on Raman scattering from benzenethiol chemisorbed on silver clusters using time-dependent density functional theory (TDDFT). Raman scattering cross sections are computed using a formalism that employs analytical derivatives of frequency-dependent electronic polarizabilities, which treats both off-resonant and resonant enhancement within the same scheme. In the off-resonant regime, Raman scattering into molecular vibrational modes is enhanced by one order of magnitude and shows pronounced dependence on the orientation and the local symmetry of the molecule. Additional strong enhancement of the order of $10^2$ arises from resonant transitions to mixed metal--molecular electronic states. The Raman enhancement is analyzed using Raman excitation profiles (REPs) for the range of excitation energies $1.6-3.0$ eV, in which isolated benzenethiol does not have electronic transitions. The computed vibrational frequency shifts and relative Raman scattering cross sections of the metal--molecular complexes are in good agreement with experimental data on surface enhanced Raman scattering (SERS) for benzenethiol adsorbed on silver surfaces. Characterization and understanding of these effects, associated with chemical enhancement mechanism, may be used to improve the detection sensitivity in molecular Raman scattering.