Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells

Transverse (2D) photoelectron velocity distributions are directly measured on 10 nm Au film and single Au nanoshells following multiphoton excitation/photoemission. This unique capability is achieved by combining scanning photoemission microscopy with velocity map imaging, yielding photoelectron spectra as a function of diffraction-limited position on a sample. Detailed 3D photoelectron velocity distributions are retrieved for Au film by fitting the 2D data with a ballistic (three-step) photoemission model, where contributions from two-photon, three-photon, and d-band processes are identified and further characterized as a function of photon energy using a broadly tunable, visible femtosecond optical parametric oscillator. These techniques are further applied to investigate the more complex behaviors of single plasmonic Au nanoshells with silica cores. The strong plasmonic near- and far-field signatures are first characterized via optical and photoemission measurements, along with theoretical methods (Mie theory and finite element analysis). This is followed by measurements of the transverse photoelectron velocity distributions for single nanoshells, which reveal at least one surprising result. Specifically, nearly perfect azimuthal symmetry is evident in the nanoshell electron momentum distributions, despite linearly polarized excitation and anisotropic near-electric-field distributions corresponding to the dipolar/quadrupolar plasmon modes. It is demonstrated that this isotropy is at least partially due to photoelectrons scattering with physisorbed or chemisorbed surface molecules during the photoemission process.