In this thesis, I developed an approach to simulate and interpret the Sum-Frequency Generation (SFG) spectra of molecules adsorbed on different substrates. Vibrational SFG spectroscopy is a technique based on nonlinear optics to characterize surfaces. This approach encompasses two steps. First, the molecular properties (vibrational frequencies, IR and Raman quantities) are evaluated using methods implemented in standard quantum chemistry programs. Second, the macroscopic optical responses (the second-order nonlinear optical susceptibility tensors) of the adsorbate on its substrate are determined within the three-layer model of the interface. To carry out this latter step, a homemade Python program, named “SFG-from-QM”, has been elaborated from scratch and the necessary equations have been implemented in it to account for the pa- rameters of SFG measurements. This program can be installed on different operating systems (Mac OS X, Unix, Linux, Windows). The particularities of the approach consist in i) including a fragment of the substrate in the system during the quantum chemistry calculations. In par- ticular, the ONIOM embedding scheme has been employed; ii) performing the calculations of molecular properties using first principles methods. Moreover, though most of these calculations are carried out at DFT levels, for the first time SFG spectra have been simulated using molecular properties evaluated at the CCSD level. These latter constitute reference data to substantiate DFT calculations. This approach has been illustrated in the case of organic monolayers (alkyl and alkylsilane chains) covalently bonded to inorganic surfaces [H−Si(111) and SiO2] and for dif- ferent polarization combinations (ppp, ssp, and sps), showing a good agreement with experiment and therefore demonstrating its ability to unravel the surface structure. Moreover, equations for SERS enhancement factor have been derived and applied in the case of thiophenol molecules adsorbed on gold surfaces, highlighting its dependency on the nature of the adsorption site.
- Sum-Frequency Generation
- Functionalized surfaces