A simple photoionization scheme for characterizing electron and ion spectrometers

A new endstation for extreme-ultraviolet spectroscopy of free clusters and nanodroplets

In this work, we present a new endstation for the AMOLine of the ASTRID2 synchrotron at Aarhus University, which combines a cluster and nanodroplet beam source with a velocity map imaging and time-of-flight spectrometer for coincidence imaging spectroscopy. Extreme-ultraviolet spectroscopy of free nanoparticles is a powerful tool for studying the photophysics and photochemistry of resonantly excited or ionized nanometer-sized condensed-phase systems. Here, we demonstrate this capability by performing photoelectron-photoion coincidence experiments with pure and doped superfluid helium nanodroplets. Different doping options and beam sources provide a versatile platform to generate various van der Waals clusters as well as He nanodroplets. We present a detailed characterization of the new setup and show examples of its use for measuring high-resolution yield spectra of charged particles, time-of-flight ion mass spectra, anion-cation coincidence spectra, multi-coincidence electron spectra, and angular distributions. A particular focus of the research with this new endstation is on intermolecular charge and energy-transfer processes in heterogeneous nanosystems induced by valence-shell excitation and ionization.

Desorption dynamics of RbHe exciplexes off He nanodroplets induced by spin-relaxation

Doped He nanodroplets are ideal model systems to study elementary photophysical processes in hetero-nanostructures. Here we study the formation of free RbHe exciplexes from laser-excited Rb-doped He nanodroplets.

A compact design for velocity-map imaging of energetic electrons and ions

We present a compact design for a velocity-map imaging spectrometer for energetic electrons and ions. The standard geometry by Eppink and Parker [Rev. Sci. Instrum. 68, 3477 (1997)] is augmented by just two extended electrodes so as to realize an additional einzel lens. In this way, for a maximum electrode voltage of 7 kV, we experimentally demonstrate imaging of electrons with energies up to 65 eV. Simulations show that energy acceptances ≲270 and ≲1200 eV with an energy resolution ΔE∕E≲5% are achievable for electrode voltages ≤20 kV when using diameters of the position-sensitive detector of 42 and 78 mm, respectively.

Electron Vortices in Femtosecond Multiphoton Ionization

Multiphoton ionization of potassium atoms with a sequence of two counter-rotating circularly polarized femtosecond laser pulses produces vortex-shaped photoelectron momentum distributions in the polarization plane describing Archimedean spirals. The pulse sequences are produced by polarization shaping and the three-dimensional photoelectron distributions are tomographically reconstructed from velocity map imaging measurements. We show that perturbative ionization leads to electron vortices with c_{6} rotational symmetry. A change from c_{6} to c_{4} rotational symmetry of the vortices is demonstrated for nonperturbative interaction.