Experiments　have　proved　that　lowering　of　the　work　function　of　a　cathode　by　adsorbing　both　oxygen　and　electropositive　metallic　elements　on　its　surface　may　be　more　effective　than　by　adsorbing　simply　the　metallic　element.　However,　the　role　played　by　oxygen　is　not　yet　completely　clear.　In　this　paper　we　first　discuss　briefly　the　situation　of　the　thermionic　cathode.　Then　the　role　of　oxygen　or　fluorine　in　the　activation　of　the　negative　electron　affinity　GaAs　photocathode　is　discussed.　Emphasis　will　be　placed　on　the　atomic　core　potential　field　of　the　surface　atoms　of　the　cathode,　the　bonding　state　of　the　surface　adsorption　layer　(Cs　or　O),　and　the　transfer　of　electron　charges　in　the　surface　atoms.　The　conclusion　is　drawn　that　an　appropriate　quantity　of　oxygen　adsorbed　on　the　cathode　surface　would　be　essential　to　result　in　a　better　dispenser　cathode　and　also　high　quantum　yield　negative　electron　affinity　photocathodes,　while　fluorine　would　play　an　important　part　in　the　electron　emission　stability.　©　1988,　American　Vacuum　Society.　All　rights　reserved.