Porous silicon samples were prepared by anodizing p-doped Si(100) substrates in both aqueous (HF/H20, HF/CHOH, HF/CH3OH/H20, HF/C2H5OH/H20) and nonaqueous (MeCN/HF) media. The time-dependent porous silicon photoluminescence (PL) was monitored during the etch (in situ) and after removal from the etch solution (ex situ). Correlation of the ex situ and in situ PL indicates that the composition of the etchant solution plays an extremely important role in the onset, time-dependent intensity, and lifetime of the emission, both in and out of solution. The effect of etchant solution additives (ethylene glycol, CH3OH, C2H5OH, NaF, HC1, and NaC1) on the porous silicon PL both during arid following the etching cycle, was also determined. The distinct and different correlations found between aqueous and nonaqueous etchants provide insights into the mechanism of PL. These results, when considered in the context of quantum chemical modeling, strongly suggest surface-bound silicon oxyhydride moieties as the source of the porous silicon PL. * Electrochemical Society Active Member. select environments, we also consider the net effect of these in situ treatments on ex situ behavior. We have carried out an extensive series of experi-ments5'79'12 in both aqueous (HF/H20, HF/CHOH, HF! CH3OH/HZO, HF/C2H5OH/H20) and nonaqueous (MeCNf HF, anhydrous) etching media. We monitor the time-dependent PL, in situ, during the etching cycle and before the PS sample is removed from the etching solution, including a study of the effect of the additives ethylene glycol, CH3OH, C2H5OH, NaF, HC1, and NaCI on the in situ luminescence process. We also consider the effect of these treatments on the ex situ behavior of the PL after removal of the PS from the etching solution. By correlating the ex situ with the in situ behavior of the PS, we find that the aqueous electrolyte composition plays an extremely important role in the onset, time-dependent intensity, and lifetime (solution) of the PL emitters.7-912 Comparable studies on nonaqueous electrolytes are found to demonstrate distinctly different correlations between the in situ and ex situ behavior and we use these resuilts to provide possible insights into the mechanism of the PL process. Further, in complement to the experimental studies, we introduce a quantum chemical modeling which suggests that silicon oxyhydride-like fluorophors9 strongly bound to the PS surface are the source of the PS PL. The silicon oxyhydride moiety can be viewed as a highly reactive intermediate in the oxidative pathway between the hydrophobic Si-H surface generated in the HF etching process and the hydrophilic Si02 surface.