  We present the experimentally obtained single-molecule (SM) surface-enhanced
Raman scattering (SERS) spectrum of 4-aminibenzenethiol (4-ABT), also known as
para-aminothiophenol (PATP). Measured at a 4-ABT concentration of 8 * 10^-10 M,
the spectra show Raman forbidden modes. The SM-SERS spectrum of 4-ABT obtained
using a non-resonant visible laser is different from the previously reported
SERS spectra of 4-ABT, and could not be reconstructed using quantum mechanical
calculations. Careful classical assignments (not based on quantum-mechanical
calculations) are reported, and indicate that differences in the reported
spectra of 4-ABT are mainly due to the appearance of Raman forbidden bands. The
presence of Raman forbidden bands can be explained by the charge-transfer (CT)
effect of 4-ABT adsorbed on the silver nanostructures, indicating a breakdown
of Raman selection rules at the SERS hotspot.
