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Schmerge, Quantum efficiency and thermal emittance of metal photocathodes, Physical Review Special Topics - Accelerators and Beams, 2009","review_link":"https://doi.org/10.1103/PhysRevSTAB.12.074201","review_verified":"true","summary":"Electron beams for X-ray lasers and electron microscopes start when light knocks electrons out of a surface, and the sideways jitter of those electrons sets how sharp the beam can ever be. 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