Laser-fluorescence experiments with a continuous argon-ion laser in an argon plasma

We have carried out laser-fluorescence experiments in the argon plasma of a hollow cathode arc (HCA) using a continuous argon-ion laser, operating at one of four different laser lines in the single-line mode. By measuring the fluorescence intensity of the lines originating from an upper level identical to that of the pumping lines, we observed negative fluorescence when the laser light is incident on the plasma, i.e., we found a decrease of the line intensity resulting from inversion. We have determined absolute density inversions between the 4p- and 4s-levels for electron density (ne)-values between 8 × 1018 and 1 × 1020m-3, as determined by Thomson scattering. All of the observed lines show quenching of the density inversions at an electron density of 1 × 1020m-3.