ABSOLUTE RAMAN-SCATTERING CROSS-SECTION OF A SURFACE-ADSORBED LAYER -AMORPHOUS NITROBENZENE ON NI(111)

We have determined the absolute Raman-scattering cross section of the NO2 symmetric stretching mode (1346 cm-1) of an amorphous nitrobenzene layer adsorbed on Ni(111). The layer thickness ranged from several mo nolayers to several hundred angstrom. The cross section was obtained b y fitting the coverage dependence of the absolute Raman intensity to a theoretical curve where the electromagnetic Green's function for a mu ltilayered geometry was used to account for the influence of the metal lic substrate on the local incident electric field at the molecular si te, and on the propagation of the scattered light. The molecules in th e amorphous layer are assumed to be randomly oriented. The absolute di fferential Raman cross section and the depolarization ratio are 3.7 X 10(-28) cm2/(molecule sr line plane of polarization) and 0.17, respect ively. (This unit refers to the integrated intensity over the entire l ine 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 gaseo us and liquid nitrobenzene. The ratios between these cross sections ar e gas:liquid:amorphous = 1:7:29. Part of this change in the measured R aman cross section for different phases can be explained by the differ ence in the local microscopic field.