Surface-enhanced Raman scattering from p-nitrobenzoic acid

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Abstract
The surface-enhanced Raman scattering (SERS) from thin films formed by p-nitrobenzoic acid (PNBA) adsorbed onto silver island films has been investigated. SERS spectra obtained using low laser powers were very similar to the normal Raman spectra of the sodium salt of PNBA and were characterized by strong bands near 1600 and 1355 cm−1 and by weaker bands near 1395, 1115, and 875 cm−1. The band near 1395 cm−1 was assigned to the symmetric stretching mode of carboxylate groups, indicating that PNBA was adsorbed as a metal salt. When PNBA films were irradiated at high laser powers, a rapid reaction occurred. The bands near 1355 and 1115 cm−1 gradually decreased in intensity and a strong band near 1460 cm−1 and a strong doublet near 1150 cm−1 gradually appeared. The band near 1460 cm−1 and the doublet near 1150 cm−1 were attributed to azodibenzoate formed by the reductive coupling of PNBA molecules at the silver surface during laser irradiation. When adsorbed PNBA films were irradiated at low laser powers, the reaction still occurred but at a much lower rate. Reduction of PNBA was probably thermally induced but a photochemical mechanism may also be possible.