Achieving a low mean transverse energy or temperature of electrons emitted from the photocathode-based electron sources is critical to the development of next-generation and compact x-ray free electron lasers and ultrafast electron diffraction, spectroscopy, and microscopy experiments. In this Letter, we demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled (100) surface of copper using nearthreshold photoemission. Further, we also show that the electron energy spread obtained from such a surface is less than 11.5 meV, making it the smallest energy spread electron source known to date: more than an order of magnitude smaller than any existing photoemission, field emission, or thermionic emission based electron source. Our measurements also shed light on the physics of electron emission and show how the energy spread at few meV scale energies is limited by both the temperature and the vacuum density of states.

Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) could be an enabling material platform for photocathode applications due to its high emissivity. While the quantum efficiency (QE) of UNCD was reported by many groups, no experimental measurements of the intrinsic emittance/mean transverse energy (MTE) have been reported. Here, MTE measurement results for an (N)UNCD photocathode in the photon energy range of 4.41 to 5.26 eV are described. The MTE demonstrates no noticeable dependence on the photon energy, with an average value of 266 meV. This spectral behavior is shown to not to be dependent upon physical or chemical surface roughness and inconsistent with low electron effective mass emission from graphitic grain boundaries, but may be associated with emission from spatially-confined states in the graphite regions between the diamond grains. The combined effect of fast-growing QE and constant MTE with respect to the excess laser energy may pave the way to bright UNCD photocathodes.Photocathode-based RF and pulsed DC guns are bright electron injectors for free electron lasers and advanced time resolved microscopes 1 . Further progress of electron laser and microscopy facilities (improved sensitivity, spatiotemporal resolution, high throughput) largely depends on development and understanding of materials with the potential to be utilized as photocathodes. Photocathode development challenges include achieving simultaneously (i) high QE, (ii) high transverse coherence (meaning low intrinsic emittance/low MTE), (iii) rapid response time.The ratio of the charge to the MTE determines the photocathode brightness, which in many applications is the most critical figure of merit. For a classical metal photocathode such as copper, the Fowler-Dubridge law 2 predicts that the emitted charge is a fast-growing function of excess energy (a power law), where excess energy ∆E is the difference between the laser primary incident photon energy ω and the work function φ defined as ∆E = ω−φ. Dowell and Schmerge 3 have found that the transverse momentum for metals also grows with excess energy as ∼ √ ω − φ. For the latter reason, to attain the highest quality (low divergence) electron beam metal photocathodes are often operated in the near threshold region (having the smallest ∆E, with the primary photon energy nearly matching the work function), although brightness increases with excess energy.A great number of metal and thin film alkali antimonide photocathodes obey the Dowell-Schmerge (DS) model 3-5 . However, some semiconductor photocathodes, e.g. GaAs and PbTe, show various MTE versus excess energy trends that are different from those specific to metals. Negative electron affinity (NEA) GaAs photocathodes 6 , for instance, demonstrate ∼1,000-fold QE increase as the excess energy increases from 0 to about 1 eV while the MTE remains low and nearly constant with the same ∆E range (within measurement precision).(N)UNCD is another example of a NEA photocathode that has high electron conductivity through the bulk of a semi-me...

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.

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A one-step photoemission analysis is developed, using the exact one-dimensional quantum solution for transmission over and through a triangular barrier presented by Forbes and Deane (2011 Proc. R. Soc. A 467 2927), to evaluate the emission properties of a photocathode in an electron gun. The analysis, which employs transverse momentum conservation in electron emission, includes the physical attributes (density of states and energy-momentum dispersion) of both the bulk band emission states and the recipient vacuum states in its evaluation of the mean transverse energy and relative quantum efficiency of the emitted electrons.