Thermionic and threshold photoemission energy distributions from LaB6(110)

Mogren, S.; Reifenberger, R.

doi:10.1016/s0039-6028(87)80045-6

Cited by 16 publications

(5 citation statements)

References 12 publications

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“…It has been shown that, if surface impurities are removed, sintered LaB6 can show a work function value of 2.36 eV, which is fully comparable to that of single crystals (2.28 ÷ 2.47 eV) 13 . In addition, photo-induced electron emission is documented in the literature both by single crystals and by polycristalline specimens 14, 15 , as well as simultaneous thermionic emission and photoemission under excitation of a polycrystalline film at 442 nm wavelength (2.8 eV) 16 .…”

Section: Introductionmentioning

confidence: 99%

Lanthanum hexaboride for solar energy applications

Sani

Mercatelli

Meucci

et al. 2017

Sci Rep

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We investigate the optical properties of LaB6 – based materials, as possible candidates for solid absorbers in Concentrating Solar Power (CSP) systems. Bulk LaB6 materials were thermally consolidated by hot pressing starting from commercial powders. To assess the solar absorbance and spectral selectivity properties, room-temperature hemispherical reflectance spectra were measured from the ultraviolet to the mid-infrared, considering different compositions, porosities and surface roughnesses. Thermal emittance at around 1100 K has been measured. Experimental results showed that LaB6 can have a solar absorbance comparable to that of the most advanced solar absorber material in actual plants such as Silicon Carbide, with a higher spectral selectivity. Moreover, LaB6 has also the appealing characteristics to be a thermionic material, so that it could act at the same time both as direct high-temperature solar absorber and as electron source, significantly reducing system complexity in future concentrating solar thermionic systems and bringing a real innovation in this field.

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Section: Introductionmentioning

confidence: 99%

Lanthanum hexaboride for solar energy applications

Sani

Mercatelli

Meucci

et al. 2017

Sci Rep

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“…However, in photoemission mode, the electron generation process is of a completely different nature. The initial energy distribution is related to the excess photon energy above the work function, 29,30 and the peak currents are much higher than in conventional TEM; thus space charge effects are more important. Photoelectron guns are typically operated in a linear photoemission regime governed by a constant quantum efficiency, so the bunch charge is proportional to the total energy of the UV pulse, at least until it reaches saturation due to space charge effects (e.g., the Child-Langmuir limit 31,32 ).…”

Section: Resultsmentioning

confidence: 99%

Influence of cathode geometry on electron dynamics in an ultrafast electron microscope

Piazza

Cao

et al. 2017

Structural Dynamics

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Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.

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“…The distributions were modeled as transmission coefficients for a free electron encountering a step potential, and photoemission was approximated by shifting the Fermi–Dirac distribution by the hv energy of the incident photon, following the approach taken by Mogren and Reifenberger for LaB 6 . 58 Note again that because we are presently focused on the gun region, the X-ray aperture is the final limiting element in the simulations. Electron packet populations were collected at a virtual screen positioned 35 cm from the photoemission plane.…”

Section: Methodsmentioning

confidence: 99%