• Energy Research

The high flux of the gas-phase beamline at the Elettra storage ring, Trieste (Italy), and the efficiency of a multicoincidence end station have been combined to measure the triple-differential cross section (TDCS) for the double photoionization (DPI) of He at excess energies E = 20 and 80 eV. The TDCS has been measured in the equal-energy-sharing kinematics, in which the kinetic energies of the two photoelectrons are equal (E1 = E2 = E/2). The experiments are compared with the predictions of a many-body Coulomb wave method and a convergent close-coupling calculation. The symmetrized gerade amplitudes of the He (DPI) extracted by the experiments as well as by the two calculations have been fitted by a Gaussian function. The results support the validity of the parametrization of the gerade amplitude via only one parameter, the full width at half maximum 1/2, of the Gaussian function, up to 80 eV above threshold.

We present recent results obtained with a novel spectrometer developed within the EU projectVOLPE (VOLume PhotoEmission from solids with synchrotron radiation), now operational at ESRF (Beamline ID16), where we were able to achieve high energy resolution as well as good statistics photoemission spectra up to 6–8 keV of photoelectron kinetic energy. The spectrometer is a hemispherical deflector analyzer with electrostatic input lens and 2D position sensitive detector. Ag 3d, Au 4f and valence band spectra have been measured from polycrystalline Ag and Au, displaying an overall energy resolution (photons + analyzer) of about 71 meV at 6 keV, as measured at 18K on the Fermi edge. The corresponding resolving power of 8×104 is one of the highest values achieved in photoemission measurements from solid samples.

The double-differential spectrum of coincidences between backscattered electrons and secondary electrons (SEs) emitted from a polycrystalline Al surface bombarded with 100-eV electrons was measured. For energy losses of the scattered electron in between the work function of Al and the bulk plasmon energy, a sharp peak is observed in the SE spectra, corresponding to ejection of a single electron near the Fermi edge receiving the full energy loss and momentum of the primary electron. This process predominantly takes place when the primary electron suffers a surface energy loss in vacuum, and leads to SE ejection from the very surface. At energy losses just above the bulk plasmon energy, a sharp transition is observed, corresponding to a sudden increase in the depth of ejection. The latter is a direct consequence of the complementarity of surface and bulk plasmons, the so-called Begrenzungs effect.