Temporal behaviour 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 is observed in the post discharge. This increase is caused by the reduction of the collisional detachment reaction rate of hot electrons. The density of the negative hydrogen ions is determined by the photodetachment technique where the generated electrons are collected using a Langmuir probe. Information on the behaviour of the density of vibrationally excited hydrogen molecules in the electronic ground state is deduced from a comparison with the measured H-minus density on the basis of a semi-empirical model, which assumes that the dominant channel of H-minus formation is dissociative attachment of electrons to highly vibrationally excited hydrogen molecules. The model contains the population of the high vibrational states as an adjustable parameter. The concentration of vibrationally excited molecules is found to be independent of the discharge current in the limits 1-20 A (pressure range 0.1-2.5 Pa).