NEGATIVE-OXYGEN-ION DETECTION BY A CROSSED-BEAM PHOTODETACHMENT TECHNIQUE

A novel non-invasive photodetachment technique with sufficient spatial (10 mm) and temporal (15 ns) resolution for detection of negative ion s is described. Using the light of a frequency doubled Nd:YAG laser (l ambda = 532 nm), electrons are completely released from negative ions and simultaneously detected by a HCN laser interferometer beam (lambda = 337 mu m) crossing the irradiated volume perpendicularly. The diagn ostic method is examined using a microwave-excited oxygen surfatron di scharge in the pressure range 1-10 Pa. Typical densities at p = 2 Pa a re n(e) approximate to 2.7 x 10(17) m(-3) and n(o-) approximate to 8 x 10(16) m(-3). The temporal behaviour of the excess electron density a ppearing after the Nd:YAG laser pulse is evaluated in order to determi ne the initial negative ion density and to examine a kinetic model of electron motion developed by Stern at al. The evaluation procedure tak es account of the spatial profiles of the probing laser beams. A simpl e steady state modelling of the plasma delivers n(e) and O- concentrat ions nearly in agreement with the experimental finding.