The characteristics of DC laser photodetachment signals in a hot carbon cathode discharge

Negative ions in carbon containing plasma were detected by photodetachment method using a Langmuir probe coupled to a DC laser irradiation with the photon energy larger than the electron affinity of C-. A high temperature carbon filament served as a cathode for producing a carbon containing plasma with Ar gas sustaining the discharge. The amplitude of photodetachment signals was depended upon the gas pressure, the laser modulation frequency, and the geometrical arrangement between the laser injection path, and the detecting probe location. In an Ar gas operation, photodetachment signals increased as the Langmuir probe was moved toward the wall from the center of the ion source. Meanwhile, the signal for a hydrogen discharge exhibited descending characteristics from the center to the wall of the discharge chamber. Mechanisms causing these differences are discussed.