Simulation of surface enhanced vibrational spectroscopy:
A journey to a unified method

T. Colleu-Banse1, A. Fekete1, M. Lobet1, V. Liégeois2, L. Henrard1

1 Department of Physics and NISM Institute, University of Namur
Department of Chemistry and NISM Institute, University of Namur

Surface Enhance Vibrational Spectroscopies (SEVS) are well known experimental methods to detect trace amounts of a compound. However, simulations of SEVS spectra are challenging although necessary to interpret experimental results. It requires to describe the vibrational modes of a molecule and the electro-magnetic response from a surface or from a nanoparticle within a single framework. This is the aim of the SURFASCOPE project. Here, we propose to combine a quantum approach to simulate the vibrational modes of ethanol and a classical one to simulate the plasmon excitation of gold nanodisks. 

First, the electro-magnetic response and the local field of two gold nanodisks are studied within the discrete dipole approximation (DDA) as a function of the distance between the nanoparticles. The figure illustrates the creation of a hot spot (region of high electric field) in the nano-gap.

Then, the influence of an inhomogeneous electric field on frequencies and IR intensities of the vibrational modes of ethanol is investigated with quantum chemistry method (Hartree-Fock self-consistent field (SCF) method). A field gradient leads to a redshift of the main modes and, in some case, to mode mixing. 

In another approach in the DDA frame, the IR response of a molecule lying nearby the gold nanodisks is modelled by a single dipole position in the hot spot region. The resonant absorption of the molecule is modelled with an eigen-frequency in the plasmonic excitation range.