DETERMINATION OF NEGATIVE-ION DENSITY WITH OPTICAL-EMISSION SPECTROSCOPY IN OXYGEN AFTERGLOW PLASMAS

A diagnostic scheme for the measurement of the O- density in afterglow plasmas is proposed and demonstrated. The present method is based on the fact that excited oxygen atoms (O) are produced by mutual neutral ization between positive and negative ions (O+ + O- --> O + O). The a bsolute value of the emission coefficient epsilon(n) due to O --> O h nu is determined by visible emission spectroscopy with a calibrated detection system. The O+ density n(o)+ is measured by a Langmuir prob e combined with mass spectrometry. With the rate coefficient for the a bove reaction k(n), the O- density can be evaluated as n(o)-= epsilon( n)/(k(n)n(o)+). In the present letter, the rate coefficient k(n) has b een determined experimentally from n(o)- measured by probe-assisted la ser photodetachment. (C) 1998 American Institute of Physics. [S0003-69 51(98)02419-X].