Light Scattering versus Plasmon Effects: Optical Transitions in Molecular Oxygen near a Metal Nanoparticle

The localized surface plasmon of a metal nanoparticle can influence the optical properties of a molecule in the plasmon field. In a previous study of molecular oxygen adjacent to nanodisks on a flat substrate, we showed that a plasmon field can increase the probability of the O ₂(a ¹Δ _g) ← O ₂(X ³Σ _g ^-) radiative transition at 1275 nm. For the present study, we set out to ascertain if metal nanoparticles suspended in a liquid solvent could likewise induce measurable plasmonic effects on optical transitions in oxygen. Metal nanoparticles were prepared with the intent of selectively perturbing the 765 nm O ₂(X ³Σ _g ^-) → O ₂(b ¹Σ _g ⁺) absorption transition. Because O ₂(b ¹Σ _g ⁺) efficiently decays to O ₂(a ¹Δ _g), we used the spectrally distinct O ₂(a ¹Δ _g) ← O ₂(X ³Σ _g ^-) phosphorescent transition at 1275 nm to probe the potential plasmon effects at 765 nm. Although we indeed observed nanoparticle-mediated effects on the O ₂(X ³Σ _g ^-) → O ₂(b ¹Σ _g ⁺) transition, our present data are readily explained in terms of a nanoparticle-dependent change in the path length of light propagation through the sample. We modeled the latter using features of radiative transfer theory. As such, we cannot claim to observe a plasmonic effect on oxygen from these nanoparticles suspended in solution. Instead, our results point to the general importance of considering the effects of light scattering, certainly for experiments on suspended metal nanoparticles. Indeed, the extent to which light scattering can influence such optical experiments leads us to infer that many claims of a plasmonic effect could be misassigned.