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

The concentration N _H ^? of negative hydrogen ions in a low-voltage cesium-hydrogen discharge plasma N _H ^? 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 p _{H 2} is equal to several torr, and the cesium concentration in the plasma N _Cs ⁽⁰⁾ ～10¹⁴ cm^?3). The absorption due to the photoionization of excited Cs atoms is shown to be negligible. The measured concentration N _H ^? of the H^? ions is 10¹²–10¹³ cm^?3. Experimental results are consistent with the theory.