Reduction of Intrinsic Electron Emittance from Photocathodes Using Ordered Crystalline Surfaces

The generation of intense electron beams with low emittance is key to both the production of coherent x rays from free electron lasers, and electron pulses with large transverse coherence length used in ultrafast electron diffraction. These beams are generated today by photoemission from disordered polycrystalline surfaces. We show that the use of single crystal surfaces with appropriate electronic structures allows us to effectively utilize the physics of photoemission to generate highly directed electron emission, thus reducing the emittance of the electron beam being generated.

Figure 1

(a) QE vs excess energy for 3 angles of incidence ${\theta }_i=60°$ (blue), ${\theta }_i=35°$ (red), and ${\theta }_i=0°$ (green). The dotted lines are experimentally measured values and the solid lines are the results from our photoemission model. The experimental measurements have an error bar of 10%. The purple dotted-dashed line is the QE measured for a polycrystalline Cu surface [12]. (b) rms transverse momentum vs excess energy for the Ag(111) surface. Red curve is the measured values for ${\theta }_i=35°$ in unpolarized light. The blue curve is calculated from our photoemission model. The purple curve is the rms transverse momentum expected from a polycrystalline Cu cathode [11].

Figure 2

(a) Band structure of Ag (within the nearly free electron model) projected onto the [111] direction. (b) [111] projected band structure zoomed in near the Fermi level. The 4 green curves are free electron parabolas corresponding to various incident photon energies.