Surface-enhanced hyper-Raman and surface-enhanced Raman scattering from molecules adsorbed on nanoparticles-on-smooth-electrode (NOSE) substrate - I. Pyridine, pyrazine and benzene

Metal nanoparticles-on-smooth-electrode (NOSE) was used as the substrate in a potential-dependent surface-enhanced hyper-Raman(SEHRS), hyper-Rayleigh(SEHRyS) and Raman (SERS) study of adsorbed molecules of pyridine, pyrazine and benzene. The three molecules are chosen because they are known to adsorb in different orientations on an Ag surface, thus allowing us to examine the sensitivity of SEHRS and SERS signals to the orientation of adsorbed molecules on the NOSE substrate. A computational subroutine is developed and interfaced with Gaussian-98 to calculate the numeric infrared, Raman and hyper-Raman intensities, as well as their projections along any molecular-fixed orientation axis at the level of finite perturbation theory. In this study, we tried to make four types of systematic comparisons. First, we compared the performance of Ag(n)vertical bar Ag NOSE substrate with that of traditional substrates such as a randomly roughened Ag electrode or a randomly aggregated Ag sol in SEHRS and SERS. Secondly, we tried to compare between infrared, Raman and hyper-Raman spectra, which represent the parity-differentiated one-photon, two-photon and three-photon processes, respectively, with the aim of examining the complementarity between SEHRS and SERS as well as between SEHRS and infrared absorption. Thirdly, we compared isotropic normal spectra with surface-enhanced spectra (SEHRS and SERS), with the aim of examining the effect of surface adsorption on the selectivity and enhancement. Lastly, we made a systematic comparison between the observed and calculated spectra for both isotropic and surface spectra, which allowed us to assign and interpret the observed data and to infer the most likely orientation of the adsorbed molecules. The NOSE method can be generalized easily to other metal nanoparticles dispersed on a smooth inert metal electrode. The rapid development in the synthesis of metal nanoparticles of various sizes, shapes and crystalline states makes the NOSE method a much more flexible and versatile substrate in the study and applications of SERS and SEHRS techniques. Copyright (c) 2005 John Wiley & Sons, Ltd.