TEMPORAL BEHAVIOR OF THE H-MINUS DENSITY IN A PULSED MULTIPOLE DISCHARGE INVESTIGATED BY THE PHOTODETACHMENT TECHNIQUE

In the plasma of a pulsed hydrogen multipole discharge with densities below 10(17) m(-3) an increase in the concentration of negative ions i s observed in the post discharge. This increase is caused by the reduc tion of the collisional detachment reaction rate of hot electrons. The density of the negative hydrogen ions is determined by the photodetac hment technique where the generated electrons are collected using a La ngmuir probe. Information on the behaviour of the density of vibration ally excited hydrogen molecules in the electronic ground state is dedu ced from a comparison with the measured H-minus density on the basis o f a semi-empirical model, which assumes that the dominant channel of H -minus formation is dissociative attachment of electrons to highly vib rationally excited hydrogen molecules. The model contains the populati on of the high vibrational states as an adjustable parameter. The conc entration of vibrationally excited molecules is found to be independen t of the discharge current in the limits 1-20 A (pressure range 0.1-2. 5 Pa).