The SERS spectra of alizarin and its ionized species: The contribution of the molecular resonance to the spectral enhancement

Quantum chemical calculations are today extensively used to model the SERS spectra of molecular systems as the interpretation of the experimental data is facilitated by progress in the efficiency of programs for computing derivative properties, such as normal-mode frequencies and spectral intensities. After that quantum chemical calculations furnish an invaluable aid to spectroscopists in the analysis and assignment of experimental vibrational spectra for complex poliatomics.The analysis of the vibrational properties of alizarin and its anions is presented. The experimental results obtained by Raman and SERS spectroscopy are discussed in comparison with the results of DFT calculations carried out at the B3LYP/6-31g(d) level of theory. The surface selection rules for adsorbed molecule to metal nanoparticles are also applied for the explanation of the SERS experimental results. From the calculated spectra a hint to the contribution of molecular resonances to the SERS spectra can be obtained. The general spectral features of the SERS experimental data obtained with 514, 632, 785 and 1064 nm laser excitation wavelengths are satisfactorily reproduced.