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Measurements of negative-ion densities by cavity ringdown spectroscopy

We report in this letter the first application of Cavity Ringdown Spectroscopy (CRDS) to the quantitative detection of negative ions—H−-ions in a magnetic multipole source in this specific case. The absorption connected with the photodetachment process is measured. A pulsed (τ ≈ 15 ns) green (λ = 532 nm) laser (Nd:YAG with SHG) beam is radiated into a high-Q cavity containing the discharge plasma. The temporal behaviour of the decaying pulse at the cavity exit yields the density of absorbing H−-ions 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 photodetachment process.