Correlation of surface production of negative ions and the work function measured by laser‐induced photoelectron current

Dynamic change of a negative‐ion production surface in an ion source due to cesium introduction and plasma ion bombardment has been studied by simultaneous measurements of photoelectron current by laser irradiation and negative‐ion current. Strong dependences of production rates for negative ions by desorption (H−) and by sputtering (Au−) on the work function monitored by photoelectron current were observed. Peaks on mass spectra corresponding to He− were observed only when the monitoring signal indicated that the work function of a molybdenum‐cesium surface was near the minimum.