Plasmon Resonance, Thermal, And Optical Contributions To Anti-Stokes-To-Stokes Line Strength Ratios In Continuous Wave-Excited Surface-Enhanced Raman Scattering Spectra Of Molecules At Random Ag Surface

Surface-enhanced Raman scattering (SERS) by biochemically relevant organic reporter molecules, spread out over a promising localized surface plasmon resonance (LSPR) structure of randomly arranged silver nanoscale particles (AgPs) of various dimensions and shape, was induced using tightly focused continuous wave (CW) 785-nm laser radiation, and the spectra were registered simultaneously in the anti-Stokes and Stokes spectral ranges. The spectra were recorded as a function of the excitation laser power density in the range of their profile reproducibility. The power density dependences of the line strength ratio for three respective pairs of vibrational lines of a thiolate of 2-nitrobenzoic acid (TNB) in SERS spectra were derived. Using these data, we specify and quantify the contributions responsible for the discrepancy between this ratio and that defined by the thermal equilibrium populations of the upper and lower vibrational levels corresponding to the Raman-active transitions. These contributions are the following: (i) the spectral profile of an LSPR contour, (ii) local heating of the reporter molecule/AgP conjugates by the 785-nm radiation, and (iii) optical (Raman) pumping of the upper vibrational levels of the transitions involved. The extraction of the latter contribution enabled us to estimate the cross-sections of the TNB/AgP conjugates Raman vibrational pumping by the radiation for each of the three vibrational modes.