UV resonant Raman spectra of nitrobenzene adsorbed on Ni(111)

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. As a first test, we have measured the spectra of nitrobenzene adsorbed on Ni(111). We could detect the Raman signal from the NO₂ symmetric stretching mode (1346 cm^-1) of two monolayers of nitrobenzene when the average incident power was ∼0.6 mW at 266 nm. The average power density of incidence was ∼20 mW cm^-2, and Raman-scattered light was collected from an area of 0.03 cm² at the sample surface. The Raman scattering cross-section for an incident wavelength of 266 nm is enhanced by a factor of 5 × 10³ relative to that for an incident wavelength of 488 nm. This enhancement is due partly to the well-known ω⁴ factor, and partly to an electronic resonance in the UV range around 250 nm. The limitation of 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.