Exictation of Nanoparticle Plasmon Vortices with an Electron Beam

We observe the decay of an electron vortex beam upon interaction with a gold nanoparticle. Transmission electron micrographs and supporting electron energy loss spectra show scattering of an electrom beam with an incoming orbital angular momentum $L_z = 1\hbar$ and an outgoing $L_z = 0\hbar$ from the nanoparticle at surface plasmon energies. By conservation of angular momentum, we conclude that we have observed excitation of surface plasmon vortices by an electron beam. Electron vortex probes therefore present a new opportunity for orbital angular momentum-selected excitation of plasmon modes with high spatial resolution.