Velocity Map Imaging VUV Angle-Resolved Photoemission on Isolated Nanosystems: Case of Gold Nanoparticles

We present an angle-resolved photoelectron spectroscopy study on isolated gold nanoparticles in the photon energy range between 6 and 12 eV performed at the DESIRS beamline of Synchrotron SOLEIL with a velocity map imaging spectrometer. The data are reported for polydisperse gold aerosols formed from hydrocolloids of bare and citrate-capped primary nanoparticles. The aerosols were brought to interact with the synchrotron radiation via a high-performance aerodynamic lens system developed for this study. The electronic structure of the aerosols was elucidated by threshold photoemission spectroscopy, which showed excellent agreement with the theoretical density of states. The photoionization cross sections of isolated gold particles were derived in the ∼10⁴ to 10⁶ Mb range between 8 and 12 eV. We introduce a new methodology to extract all of the photoemission observables from the electron images as a function of electron energy, including photoelectron spectrum, anisotropy parameter b₂, and asymmetry parameter, α­(E), which describes the shadowing asymmetry along the radiation propagation axis. The results show that electron energy distribution depends on the procedure used in the synthesis of the nanoparticles and on the ligand concentration. The dependence of the asymmetry parameter on electron energy along the valence band is rationalized through theoretical modeling in terms of the electric field distribution inside the particle and the losses due to inelastic scattering.