Probing the electronic absorption spectrum of single gold nanoparticles in the gas phase

Gold nanoparticles (Au NPs) exhibit a localised surface plasmon resonance (LSPR) which can be excited in the visible spectral range and is sensitive to the size, shape and environment of the NP. Here, we present electronic action spectra of single 38.5-52.5 nm diameter Au NPs in the gas phase measured by means of single nanoparticle action spectroscopy at cryogenic temperatures (cryo-SNAS). A single Au NP is trapped in a temperature-controllable Paul-type ion trap, thermalised by collisions with a buffer gas and coated with messenger molecules at sufficiently low temperature, while its absolute mass is non-destructively monitored. Absorption of monochromatic electromagnetic radiation (400-800 nm) is detected indirectly by monitoring the change in mass due to messenger desorption. We find that the LSPR is red-shifted by similar to 75 nm compared to the predicted value for a bare Au NP in vacuum. Laser heating changes the intensity, but not position or width of the LSPR and can be used to ultimately quench the LSPR. The measured spectra are modelled using Mie theory and assuming the formation and annealing of a refractive surface layer.