Measurements of negative-ion densities by cavity ringdown spectroscopy

We report in this letter the first application of Cavity Ringdown Spectrosc opy (CRDS) to the quantitative detection of negative ions -H- -ions in a ma gnetic multipole source in this specific case. The absorption connected wit h the photodetachment process is measured. A pulsed (tau approximate to 15 ns) green (lambda = 532 nm) laser (Nd:YAG with SHG) beam is radiated into a high-Q cavity containing the discharge plasma. The temporal behaviour of t he decaying pulse at the cavity exit yields the density of absorbing H- -io ns on the basis of the known (broadband) photodetachment cross-section. The densities determined by CRDS agree well with earlier measurements obtained with a multipass scheme and with the alternative method which consists in the detection with a Langmuir probe of the electrons released in the photod etachment process.