Generation of a fast atomic-oxygen beam from O− ions by resonant cavity radiation

An apparatus has been developed for producing a beam of ground-electronic-state oxygen atoms with energies variable from 4 to 1000 eV with a 1.5 eV FWHM energy distribution. The technique involves extraction of negative ions from a low-voltage gas-discharge source, mass selection of the extracted O− with a Wien-type velocity filter, O− acceleration or deceleration and focusing by electrostatic ion optics, and electron detachment from O− by intracavity laser radiation. A 25 W argon-ion-laser cavity has been extended to include the ion-beam vacuum chamber so that the intracavity radiation intersects the O− ion trajectories normally. Depending on the laser configuration in use, ion-neutralization efficiencies between 5% and 25% have been achieved at 5 eV O− energy. Thus, 5 eV O-atom fluxes of ∼1011 atoms/s (∼1012 atoms/cm2 s) have been achieved for O− currents of ∼10−7 A. The advantages and limitations of the technique are discussed, and preliminary measurements of the secondary-negative-charge production from low-energy O-atom impact on copper and stainless-steel surfaces are presented.