Density Measurement of Negative Hydrogen Ions by DC Laser Photodetachment Method

The change of the electron saturation current of a Langmuir probe in a hydrogen discharge due to irradiation of Ar+ laser has been measured as a function of the distance between the probe and the laser beam. The increase of the probe current caused by the photodetachment of negative ions of hydrogen ( H-) induced by the laser is inversely proportional to the distance. The H- density can be determined from this dependence of the increase of the probe current upon the laser-probe distance. However, a simplified treatment of the measured characteristics results in an overestimation of the H- density, and a correction to account for the effect of sheath formation around the probe is made. The H- density determined after the correction ranges from 15% to 50% of the electron density.