Binary ion-ion recombination coefficients determined in a flowing afterglow plasma

Abstract

The binary positive ion/negative ion recombination coefficients indicated (in cm 3s −1×10 −8) have been obtained for the following reactions at 300 K. ▪ The measurements were carried out in a flowing afterglow positive ion/negative ion plasma. The helium pressures and ionisation densities were chosen to avoid significant loss of ionisation by ambipolar diffusion, the coefficients for which we have measured in a separate experiment at the low helium pressures and which are also reported here. The reactant gas pressures were adjusted in order to establish the required ions as the dominant species in the plasmas. Mass spectrometric identification of the positive and negative ions (masses m +, m −) was carried out downstream of the flowing plasma and somewhat remote from the reaction zone, but confirmation of the reacting ion masses was obtained by determining the ion mass ratio m −/ m + in the reaction zone. This was achieved by inspection of the cylindrical Langmuir probe current-voltage characteristics which were primarily used to determine the ion density decay as a function of distance along the afterglow plasma column. Thus, using the measured plasma flow velocity, the recombination coefficients (and ambipolar diffusion coefficients) were obtained. α 1 and α 2 are very low compared to previous data, particularly those of the merged beam data, a result we ascribe to the absence of internal excitation of the negative ions in our collision dominated plasmas. Our values are consistent with those predicted by the Olson theory, which also predicts a reducing α with increasing electron affinity of the electron attaching species, from