{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"c46cbe7f3cb85a5f1be04582d3d4999866e83dca7cb99d491097cabd3f1ce66c","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["cc1e984224de9035164a5150971df22d94799361735220ea88d7c9ff6331bd35","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"aa549c091ecc731e5e733f6855acda5a33ae514069a79183b3926bf3633f17ae","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":"2","parents":[],"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 $\\ge 10^{-5}$ and the physics that sets it (lattice temperature, surface roughness and work-function variation, electron-phonon and electron-electron scattering, multiphoton emission).","problem_ref":"cc1e984224de9035164a5150971df22d94799361735220ea88d7c9ff6331bd35","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":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"cf43139db3c384f48dc31485bb26198d3b204f96eca67be35877b0808161654a","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"d261395d49e6d0acedf7457caefcc004e6c15525fc34a72c8b4dcd0a8e5c7cad","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"6uMZL3ZkUKdG9DJ-ZuuEaJp-A9xr9eYUpbZ1etJeLpe5qfidc8IfBIrQTaDlCzWuo7YM9FXZd195UQIDyd0cBA"},"schema":"pubphys.envelope/1"},"record_hash":"cf43139db3c384f48dc31485bb26198d3b204f96eca67be35877b0808161654a","leaf_index":2456}