{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"6a087fce13f08a776ecb1fc4ccb966e47611ce060e9e9ed7346c92edf83521b6","created":"2026-10-03T07:17:56Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011","c5ef3fbe2a8115d2489d6a660f71a5ce785dd095e6a538d7e3ec9c19a2a95faf"],"salt":"2a7297835fb121ea060a170af82c73f6158cbc2ed1da78646d65aad56606f973","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"beams.beam-brightness.minimum-photocathode-mte","kind":"well-posed","literature_status":"open","n":"1","parents":[{"note":"Minimum MTE at QE above 1e-5 is one point of the MTE-QE bound","parent_revision":"c5ef3fbe2a8115d2489d6a660f71a5ce785dd095e6a538d7e3ec9c19a2a95faf","relation":"special_case"}],"plain":"Electrons leave a cathode with some random sideways energy that blurs the beam; how low it can go, and what sets the floor, is open.","posed_since":"","precise":"The intrinsic normalized emittance of a photoemission source is eps_n = $\\sigma_x$ sqrt(MTE/(m c^2)), with sigma_x the rms source size and MTE the mean transverse energy of emitted electrons. Values of approximately $5\\,\\mathrm{meV}$ were reported around 2020 from near-threshold photoemission of cryogenically cooled single-crystal Cu(100). Determine the lowest achievable MTE at quantum efficiency $\\ge 1e-5$ and the physics that sets it (lattice temperature, surface roughness and work-function variation, electron-phonon and electron-electron scattering, multiphoton emission).","problem_ref":null,"references":"","settled_by":"MTE measurements versus temperature, photon energy and surface preparation on candidate cathodes, matched by a photoemission model including the surface.","status_note":"","title":"Lowest mean transverse energy obtainable from a photocathode","topic_ref":"3311684a3644100d5c4072cc7fb5698b312b296ec8b7139e230fbc39301d2361"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"cc1e984224de9035164a5150971df22d94799361735220ea88d7c9ff6331bd35","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"5645a3dde423280f64368fdd53fa856fdaacb7e5765d6ff2839248a62da26385","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"Uz_SJVQyqr4o7wckuoKSULZH1lWsfnEKMDa22MySY3k1Z-HO-dZuzqMmO9doArp0MYZkO6ewSTZoFyUnPJc5CA"},"schema":"pubphys.envelope/1"},"record_hash":"cc1e984224de9035164a5150971df22d94799361735220ea88d7c9ff6331bd35","leaf_index":683}