FIG. 4: The isosurfaces of the charge-density difference for (a) the HSCH$_{2}$COOH adsorption on the Au(111) surface with blue (accumulation of electrons) and/or red (depletion of electrons) isosurface value, ± 0 . 02 e/ ˚ A $^{3}$, (b) SCH$_{2}$ COOH on the surface, (c) HSCH$_{2}$COO − on the surface, and (d) SCH$_{2}$COO − on the surface. Only three related gold atoms of the Au(111) surface are displayed. To support the above adsorption mechanism, we calculate the Raman vibrational spec- tra of the HSCH$_{2}$COOH, SCH$_{2}$COOH, HSCH$_{2}$COO − and SCH$_{2}$COO − adsorbed on the Au(111) substrate, respectively. The Raman vibrational peak frequencies (cm − $^{1}$) for exper- imental data and computational values for the HSCH$_{2}$COOH, SCH$_{2}$COOH, HSCH$_{2}$COO − and SCH$_{2}$COO − on the Au(111) surface are listed in Table V. The vibrational frequencies IV. THE RAMAN VIBRATIONAL SPECTRA 13