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Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions
We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV, on the photoemission spectra of liquid water. We show that the nascent photoelectron spectrum and, hence, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible. Otherwise, quasi-elastic scattering leads to strong, down-to-few-meV kinetic energy scattering losses from the direct photoelectron features, which manifest in severely distorted intrinsic photoelectron peak shapes. The associated cross-over point from predominant (known) electronically inelastic to quasi-elastic scattering seems to arise at surprisingly large electron kinetic energies, of approximately 10–14 eV. Concomitantly, we present evidence for the onset of indirect, autoionization phenomena (occurring via superexcited states) within a few eV of the primary and secondary ionization thresholds. These processes are inferred to compete with the direct ionization channels and primarily produce low-energy photoelectrons at photon and electron impact excitation energies below ∼15 eV. Our results highlight that vibrational inelastic electron scattering processes and neutral photoexcitation and autoionization channels become increasingly important when photon and electron kinetic energies are decreased towards the ionization threshold. Correspondingly, we show that for neat water and aqueous solutions, great care must be taken when quantitatively analyzing photoelectron spectra measured too close to the ionization threshold. Such care is essential for the accurate determination of solvent and solute ionization energies as well as photoelectron branching ratios and peak magnitudes.
Terahertz emission from lithium doped silicon under continuous wave interband optical excitation
We report on experimental observation and study of terahertz emission from lithium doped silicon crystals under continuous wave band-to-band optical excitation. It is shown that radiative transitions of electrons from 2P excited states of lithium donor to the 1S(A1) donor ground state prevail in the emission spectrum. The terahertz emission occurs due to capture of nonequilibrium electrons to charged donors, which in turn are generated in the crystal as a result of impurity assisted electron-hole recombination. Besides the intracentre radiative transitions the terahertz emission spectrum exhibits also features at about 12.7 and 15.27 meV, which could be related to intraexciton transitions and transitions from the continuum to the free exciton ground state.
Ultrafast optical excitations of metallic nanostructures: From light confinement to a novel electron source
Combining ultrafast coherent spectroscopy with nano-optical microscopy techniques offers a wealth of new possibilities for exploring the structure and function of nanostructures. In this paper, we describe newly developed nano-optical methods based on short-pulse laser sources with durations in the 10 fs regime. These techniques are used to unravel some of the intricate dynamics of elementary excitations in metallic nanostructures. Specifically, we explore light localization and storage in plasmonic crystals, demonstrate field enhancement and second harmonic generation from metallic nanotips and describe a novel nanometre-sized source of electron pulses. The rapid progress in this area offers exciting new prospects for probing and controlling electron dynamics in metallic nanostructures with femtosecond temporal and nanometre spatial resolution. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.