  Weakly collisional Ar-O2 electronegative plasmas are investigated in a dc
multidipole chamber. An electronegative core and an electropositive halo are
observed. The density ratio of negative ions to electrons ({\alpha}) in the
nondrifting bulk is found to be 0.43. The profile of {\alpha} is found using
both the phase velocity of ion acoustic waves and the drift velocity of
positive ions determined by laser induced fluorescence. The experiment shows
that negative ions are in Boltzmann equilibrium with a temperature of 0.06 \pm
0.02 eV. Double layers are not found separating the electronegative core and
the electropositive halo.

Plasma Physics