Absolute Raman-scattering cross section of a surface-adsorbed layer: Amorphous nitrobenzene on Ni(111).

Abstract

We have determined the absolute Raman-scattering cross section of the ${\mathrm{NO}}_{2}$ symmetric stretching mode (1346 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) of an amorphous nitrobenzene layer adsorbed on Ni(111). The layer thickness ranged from several monolayers to several hundred \AA{}. The cross section was obtained by fitting the coverage dependence of the absolute Raman intensity to a theoretical curve where the electromagnetic Green's function for a multilayered geometry was used to account for the influence of the metallic substrate on the local incident electric field at the molecular site, and on the propagation of the scattered light. The molecules in the amorphous layer are assumed to be randomly oriented. The absolute differential Raman cross section and the depolarization ratio are 3.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}28}$ ${\mathrm{cm}}^{2}$/(molecule sr line plane of polarization) and 0.17, respectively. (This unit refers to the integrated intensity over the entire line width.) We verified that this cross section correctly predicts the Raman intensity for another arbitrary experimental geometry. We have also determined the absolute Raman-scattering cross sections for gaseous and liquid nitrobenzene. The ratios between these cross sections are gas:liquid:amorphous=1:7:29. Part of this change in the measured Raman cross section for different phases can be explained by the difference in the local microscopic field.