## 3 Results and Discussion
### Pump-Probe Experiments  
The TA spectrum of the RITC solution in water (\(10^{-4}\) mol/L) excited at 560 nm exhibited a strong negative feature centered at 560 nm related to the fluorescence/stimulated emission of the photoexcited state, which leads to decreased apparent absorbance of the probe (negative differential optical density, \(\Delta\)OD) [57]. The lifetime of this transient fluorescence band can be characterized by a double exponential decay. This double exponential decay is assigned to the presence of both dimers and monomers of the dye. It is well-known that the lifetime of the fluorescence of the monomer is on the scale of a few nanoseconds (e.g., 1.6 ns for the case of rhodamine B [70]), whereas the interaction between molecules in the dimer (and larger aggregates) leads to strong quenching and a resulting reduction in the lifetime of the fluorescence (100 ps for the dimer [70]). Increasing concentrations of dimers, trimers, and larger aggregates result in a shortening of the fluorescence lifetime. The same experiment was performed on the AgNPs-RITC sample in which the synthesis used a silver/dye = 1/1 M ratio. The dynamics of the fluorescence band are  
Figure 5: (**a**) Ag3d SR-XPS spectrum and (**b**) S2p spectrum collected on AgNPs–FITC. Data for AgNPs–RITC are analogous (Table S2 in Supplementary Materials). almost identical for the two systems: RITC alone and AgNPs-RITC (Figure 6a). The data in Figure 6 are normalized to the areas under the curves for easy comparison, which is why the signals are positive. Furthermore, in this case, we took a temporal cut at 590 nm and not at 560 nm at the maximum of the fluorescence because the strongly scattered light of the pump pulse distorts the signal at this wavelength. The identical dynamics of the two samples suggest that they have similar equilibria between dimer/trimers and the monomers. No features due to the LSPR of the NPs were observed in these spectra because the excitation wavelength does not excite this resonance of the NPs. A comparison of the dynamics of the two samples when pumped at 400 nm revealed a significant difference in the dynamics of the transmitted probe light observed at 560 nm (Figure 6b). In particular, in the AgNPs-RITC case, we observed a strong reduction in the contribution of the short timescale constant (<100 ps) assigned to the typical dynamics of the dimers and higher aggregates to the signal. Exciting at 400 nm, which corresponds with the LSPR of the AgNPs, induces a strong concentration of the light on the surface of the NPs (this is the well-known effect responsible for surface-enhanced Raman spectroscopy [71]). In this way, molecules close to the surface of the AgNPs are preferentially excited (i.e., those chemically attached). It can be suggested that these dye molecules lack the short decay time as they are not quenched by the chromophore-chromophore interactions, which take place in the aggregates. Confirmation that we preferentially probed molecules on the surface of the AgNPs in this case can be found in the results of the polarization dependence of  
Figure 6: (**a**) Normalized dynamics of fluorescence of AgNPs–RITC (black) and RITC (red) probed by transient absorption following photoexcitation on the dye absorption band at 560 nm (the probe was at 590 nm to avoid the scattering of the pump). (**b**) The normalized dynamics of the same band (probed at 560 nm in this case) revealed by transient absorption following photoexcitation on the localized surface plasmon resonance (LSPR) band of the Ag nanoparticles at 400 nm. the latter signal for AgNPs-RITC. When probing freely rotating dye molecules in solution, relatively large differences of the dynamics at short times (t < 50 ps) with polarization of the probe light parallel to the pump were observed in comparison with the case in which the polarization of the probe light was at the magic angle [72].