Determination of the negative hydrogen ion concentration in a cesium-hydrogen discharge

The concentration NH− of negative hydrogen ions in a low-voltage cesium-hydrogen discharge plasma NH− is determined from experiments with laser radiation absorption caused by the photodetachment of electrons from the H− ions. The resolution of a setup measuring the relative absorption is ≥10−5 for a signal-to-noise ratio of ∼10−2 or less. A heated-cathode diode is used to initiate the discharge at a voltage of U≤10 V and a current density of j≤5 A/cm2 (hydrogen pressure pH2 is equal to several torr, and the cesium concentration in the plasma NCs(0)∼1014 cm−3). The absorption due to the photoionization of excited Cs atoms is shown to be negligible. The measured concentration NH− of the H− ions is 1012–1013 cm−3. Experimental results are consistent with the theory.