Photoabsorption of potassium clusters isolated in helium droplets: From discrete electronic transitions to collective resonances

Photoabsorption spectra of neutral potassium clusters in helium nanodroplets are recorded by depletion spectroscopy using a supercontinuum laser source equipped with a variable bandpass filter. The results provide insight into the evolution of electronic spectra from molecular transitions to localized surface plasmon resonances in nanometer-sized clusters. Size selected spectra for potassium clusters Kn are acquired from n = 2-110. Indications for the transition from molecularlike absorption to a collective resonance are already observed below n = 20. For larger clusters, a splitting of the plasmon mode into two components is observed at around n = 600. Supported by simulations, this is explained by the presence of nonspherical potassium nanoparticles grown via multicenter aggregation inside the helium droplets. The presented supercontinuum laser-based experimental approach provides a new route for the mass-selective spectroscopic characterization of different materials and material combinations isolated in helium nanodroplets in a size regime ranging from single atoms and molecules to small sub-10-nm particles.