Abstract
The spectral dependence of the mean transverse energy and quantum efficiency of photoemission from a single-crystal Rh(110) photocathode are determined at 300K using the solenoid scan technique and a sub-picosecond laser-based UV radiation source tunable from 3.0-5.3eV (235-410nm). The tunable UV radiation is generated by sum frequency mixing the second and third harmonics of a front-end, 2W, 28MHz repetition rate, femtosecond Yb:KGW laser with signal and idler radiation from nonlinear-fiber continuum-seeded optical parametric amplification. The measured properties of the Rh(110) photocathode are well explained by a one-step photoemission simulation employing the dispersion of the emitting Σ1 and Σ2 bulk band states evaluated by fully relativistic (including spin-orbit coupling) Ab initio density functional theory methods and an exact quantum solution for transmission through and over a triangular barrier that is extended into the transverse dimension. The inclusion of the joint density of states (bulk crystal and vacuum) in the simulation accounts for the observed spectral dependence of both the mean transverse energy and the quantum efficiency of the photoemission process. The consequent demonstrated base line for the evaluation of photocathode emission properties using Ab initio methods will allow for the development of screening tools to select promising (ultra)low emittance solid-state photocathodes.
Highlights
The performance of these cutting-edge research instruments is fundamentally linked to the quality of the electron source; in particular, to the mean transverse energy (MTE) of the photo-emitted electrons9 – even a modest factor of 2-3 reduction in the MTE likely providing for an order of magnitude increase in X-ray free electron lasers (XFELs) photon energy10 and a significant enhancement in dynamic transmission electron microscopes (DTEMs) and ultrafast electron diffraction (UED) spatial resolution due to the consequent increase in the transverse coherence length
The results obtained from the rhodium photocathode are directly compared to expectations from one-step photoemission model24,25 that is an extension of the exact analytical triangular scitation.org/journal/adv barrier solution of Forbes and Deane26 to include a parabolic approximation to the dispersion of the relevant photo-emitting states evaluated using Density Functional Theory (DFT), their local density of states, and a full description of the vacuum states
The excellent agreement of our one-step photoemission simulation with spectral dependence of both the MTE and quantum efficiency (QE) supports the veracity of the approach which employs the exact triangular barrier transmission solution of Forbes and Deane26 and a cylindrical parabolic approximation to the dispersion of the emission band(s) calculated from Ab initio methods
Summary
Planar pulsed-laser-driven photocathodes are routinely employed as the front-end electron source in high space-time resolution research instruments such as sub-picosecond X-ray free electron lasers (XFELs), and single-shot dynamic transmission electron microscopes (DTEMs) and ultrafast electron diffraction (UED) systems. The performance of these cutting-edge research instruments is fundamentally linked to the quality of the electron source; in particular, to the mean transverse energy (MTE) of the photo-emitted electrons9 – even a modest factor of 2-3 reduction in the MTE likely providing for an order of magnitude increase in XFEL photon energy and a significant enhancement in DTEM and UED spatial resolution due to the consequent increase in the transverse coherence length. The MTE (or intrinsic emittance12) is in turn dependent upon (i) the dispersion of the electronic states from which they are emitted into the vacuum (as transverse momentum is conserved in photoemission16), (ii) the temperature of the electron distribution in the photocathode material, (iii) surface effects, such as physical and chemical (surface work function variation) roughness that affect the transverse momentum of the electrons upon emission, and (iv) phonon scattering in materials with a sufficiently large electron-phonon scattering cross-section. Planar pulsed-laser-driven photocathodes are routinely employed as the front-end electron source in high space-time resolution research instruments such as sub-picosecond X-ray free electron lasers (XFELs), and single-shot dynamic transmission electron microscopes (DTEMs) and ultrafast electron diffraction (UED) systems.. Planar pulsed-laser-driven photocathodes are routinely employed as the front-end electron source in high space-time resolution research instruments such as sub-picosecond X-ray free electron lasers (XFELs), and single-shot dynamic transmission electron microscopes (DTEMs) and ultrafast electron diffraction (UED) systems.5–8 The performance of these cutting-edge research instruments is fundamentally linked to the quality of the electron source; in particular, to the mean transverse energy (MTE) of the photo-emitted electrons9 – even a modest factor of 2-3 reduction in the MTE likely providing for an order of magnitude increase in XFEL photon energy and a significant enhancement in DTEM and UED spatial resolution due to the consequent increase in the transverse coherence length.. The good agreement of our photoemission simulation with the experimental measurements represents a singular benchmarking that provides a roadmap for the development (or discovery) of new (ultra)low emittance photocathodes; that is, the demonstrated base line for the evaluation of photocathode emission properties using Ab initio methods will allow for the development of screening tools to select promising solid-state photocathodes
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