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 O2(a1Δg) → O2(X3Σg–) 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 O2(X3Σg–) → O2(b1Σg+) absorption transition. Because O2(b1Σg+) efficiently decays to O2(a1Δg), we used the spectrally distinct O2(a1Δg) → O2(X3Σg–) phosphorescent transition at 1275 nm to probe the potential plasmon effects at 765 nm. Although we indeed observed nanoparticle-mediated effects on the O2(X3Σg–) → O2(b1Σg+) transition, our present data are readily explained in terms of a nanoparticle-dependent change ...