A TUNABLE LASER-ATOMIC BEAM PHOTOELECTRON SOURCE WITH SUB-MILLI-ELECTRON-VOLT RESOLUTION - DESIGN, OPERATION AND APPLICATION TO THRESHOLD ELECTRON-ATTACHMENT STUDIES

A novel tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution, producing energy-variable free electrons (current 10(-12) A) by resonant two-step laser photo-ionization of metastable Ar*(4s P-3(2)) atoms in a collimated beam, is described. Operation and application of the source are demonstrated by a series of laser photoelectron attachment experiments, in which the process e(-)(E) + SF6 --> SF6- is investigated in the electron energy range 0 <E < 200 meV, both under nearly field-free conditions (electric stray fields 0.03-0.05 V m(-1)) and in defined static electric fields up to 650 V m(-1). In essential agreement with the results of model calculations, the effective electron attachment yield is characteristically reduced near threshold (E approximate to 0) with increasing electric field strength. The field also affects the contribution due to electron transfer from Rydberg atoms, either excited below the Ar+ (3p(5 2) P-3/2) ionization threshold (E < 0) or close to the Ar+ (3p(5) P-2(1/2)) threshold (E = 177.5 meV), where stabilization of auto-ionizing Ar**(nl') Rydberg states by I-mixing occurs. The possible influence of such electric field effects on previous threshold photoelectron spectroscopy for attachment data is examined. Prospects of future experiments involving the described or more intense laser photoelectron sources, based on resonant two-step photo-ionization of alkali atoms, are discussed.