ABSTRACT
Since the observation of the anomalous intensification of the Raman
signal from pyridine molecules adsorbed on a rough metal surface
[1] and the successive explanation of the phenomenon as the
amplification of the Raman scattering cross section due to the
presence of a nanostructured metal surface [2, 3], the research
in this field grew rapidly and it is still growing as witnessed
by about 2000 of papers published in this field just considering
the year 2012a. Two enhancement mechanisms of different origin
were proposed to account for the observed giant amplification of
the Raman signal: an electromagnetic and a chemical one [4, 5].
In the electromagnetic mechanism, the enhancement results from
the coupling of the incident electromagnetic field with artificially
metallic corrugated surface, in the chemical one, electronic states
generated by the interaction between the metal and the molecules
adsorbed on it are involved. In fact the order of magnitude of the
resulting enhancements is very different: theoretical estimates for
the enhancement factors point out for values up to 1010 [6, 7] for
the electromagnetic and up to 103 for the chemical one [8]. Both
mechanisms are still at the focus of intense research activity to
clarify the physical basis underlying the phenomenon.
