A surface enhanced hyper-Raman scattering study of pyridine adsorbed onto silver: Experiment and theory

This paper presents a combined experimental and theoretical study of the hyper-Raman spectrum of pyridine adsorbed onto roughened silver electrodes. The surface enhanced hyper-Raman spectra (SEHRS) were measured using a focused cw mode-locked Nd:YAG laser with a peak power density of approximately 10 ⁷ W/cm². Dominant bands in the pyridine spectra are the same (totally symmetric) bands as have been seen in the corresponding Raman (SERS) spectrum, although the relative intensities are different. To interpret these spectra, we present a semiempirical molecular orbital method for determining excitation energies, polarizability derivatives, and hyperpolarizability derivatives that is based on the π-electron Pariser-Parr-Pople (PPP) method. An empirical molecular force field is used to derive vibrational information, and the accuracy of the spectra is assessed by comparison with normal Raman spectra for liquid pyridine and with SERS spectra. The resulting SEHRS spectra are in good agreement with the measured spectra, particularly with respect to the intensity changes in the dominant lines in going from SERS to SEHRS. In addition, the theoretical/experimental comparisons indicate that SEHRS is more sensitive to adsorbate orientation than is SERS since the nontotally symmetric modes are predicted to be comparable in SEHRS (but not SERS) intensity to the totally symmetric modes for orientations other than perpendicular. Most important, a comparison of theoretical and experimental SEHRS/SERS ratios suggests that the enhancement factor associated with SEHRS is on the order of 10¹³ which is much larger than the 10⁶ enhancement seen for SERS.