Reconstructing high-order SERS transitions in an overlooked spectral region reveals surface complex formation

The complexity of information encoded in surface-enhanced Raman scattering (SERS) spectra provides both tremendous opportunities and challenges for molecular characterization. While conventional SERS studies focus on the spectral window below 2000 cm⁻¹, high-order transitions, including overtones and combination bands above this region, have received limited attention despite their potential to reveal detailed insights into molecular surface interactions. Here, we investigate these high-order transitions using 4-aminobenzenethiol (4-ABT) as a model system, which undergoes photon-induced chemical transformations and lacks electronic transitions in the visible range. SERS spectra were recorded at excitation wavelengths of 455, 532, 633, and 780 nm on highly enhancing Ag substrate, and theoretical spectra were constructed from fundamental bands to assign the high-order features. The results demonstrate that the observed high-order bands in these systems originate exclusively from 4,4'-dimercaptoazobenzene (4,4'-DMAB) formed on the surface, with excitation-wavelength dependence reflecting the number of dimers and surface complex formation. Control experiments on Au and Cu substrates and with other molecules confirm the generality of these observations and rule out instrumental artifacts. These findings establish high-order SERS transitions as reliable markers of molecule-metal surface complexes and provide a broader spectral perspective for analytical and fundamental SERS studies.