Raman Scattering from Surface Adsorbed Molecules Excited by Ultra-violet Laser Light

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For the purpose of extending the capability of Raman spectroscopy in investigating adsorbed molecules on solid surfaces, we have constructed a Raman spectroscopy system with a pulsed ultraviolet (UV) excitation source. By using the UV excitation we take advantage of the well-known omegaS^4 effect in light scattering and of resonant enhancement due to electronic transitions of many organic molecules in the UV range. As a first test we have measured the spectra of nitrobenzene adsorbed on Ni (111). We could detect the Raman signal from the NO_2 symmetric stretchig mode (1346 cm^<-1>) of two monolayrs of nitrobenzene, when the average incident power was -0.6mW at 266nm. (The average power density of incidence was -20mW/cm^2, and Raman scattered light was collected from an area of 0.03cm^2 at the sample surface.) The Raman scattering cross-section for the incident wavelength of 266 nm is enhanced by a factor of 5*10^3 relative to that for the incident wavelength at 488 nm. This enhancement is due partly to the well-known omega^4-factor and partly to an electronic resonance in the UV range around 250 nm. The limitation in the sensitivity arises from the fact that the power density of the incident laser cannot be increased, at least using a pulsed laser, beyond the level we have used without causing damage to the sample surface.