Measurements of negative ion density in high-density oxygen plasmas by probe-assisted laser photodetachment

Probe-assisted laser photodetachment has been developed and applied to measure the negative ion density (n_) in high-density plasmas. Temporal variation of n_ is obtained in high-density and low-pressure oxygen plasmas generated by helicon wave excitation. Negative ions are not observed in the active discharge phase and rapid increase of n_ is seen only in the afterglow phase. This efficient production of negative ions is considered to be due to dissociative electron attachment to metastable molecular states, O-2(M)(A(3) Sigma(u)(+), A'(3) Delta(u), and c(1) Sigma(u)(-)), located at 4-5 eV above the ground state. (C) 1998 American Institute of Physics. [S0021-8979(98)03602-0].