Laser Thomson scattering (LTS) is shown to provide a useful means for measurements of negative ion density in a glow discharge plasma. Because the saturation energy density is so low that complete photo-detachment occurs at a very early stage of the probing laser pulse for the LTS measurement, the main part of the laser pulse is Thomson scattered by the photo-detached electrons as well as those already generated in the discharge. The energies of photo-detached electrons are below the energy difference between the photon energy and the electron affinity of the negative ions, and Thomson scattering from these appears below the difference energy. The principle has been tested using the second harmonic of a YAG laser light (532 nm) against an inductively coupled plasma operated in a mixture of Ar(80%)/O 2 (20%) at a pressure of 20 mTorr, and yielded the negative ion density of 1.5 × 10 17 m −3 , which amounted to 37% of the electron density of 4.1 × 10 17 m −3 or 27% of the negative charges (electrons and negative ions).

The newly developed method of the negative ion density measurement in a plasma by laser Thomson scattering (LTS) was checked by comparing the obtained results against an independent technique, namely the Langmuir probe method. Both measurements were performed at the same position of the same inductively coupled plasma. The results agree quite well with each other and this has given confidence in the LTS method of negative ion density measurement. At the same time, both methods are complementary to each other, because the Langmuir probe measurement requires knowledge of the positive ion mass number.

A one-dimensional particle-in-cell simulation has been conducted in order to improve the physical understanding of the laser-pulse photo-detachment diagnostics of negative ions, and thus to assess the accuracy of the measurements. An emphasis is placed on a short time scale (20 ns after photo-detachment for parameters of a typical glow discharge plasma) behaviour, where the laser Thomson scattering technique combined with the photo-detachment has recently been developed. The plasma consisting of background electrons, positive argon ions (Ar + ), and negative oxygen ions (O -) is treated. It is assumed that all of the negative ions in the laser-irradiated region (photo-detachment region) are replaced by the photo-detached electrons in the time scale much faster than in the time characteristic of the electron plasma oscillation. The loss of photo-detached electrons from the photo-detachment region quickly forms a potential structure in 2 ns. The potential traps the remaining photo-detached electrons. Due to the thermal fluctuation of the electrostatic electric field, the slow decay of the photo-detached electrons follows the initial fast decay. Because of these motions of photo-detached electrons, the measured negative ion density was found to underestimate the actual value by 25 %.