Absolute density measurement of SD radicals in a supersonic jet at the quantum-noise limit

The absolute density of SD radicals in a supersonic jet has been measured down to $(1.1\pm 0.1)\times {10}^5$ cm${}^{-3}$ in a modestly specified apparatus that uses a cross-correlated combination of cavity ring-down and laser-induced fluorescence detection. Such a density corresponds to $215\pm 21$ molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantum-noise-limited and measured to be $(7.9\pm 0.6)\times {10}^{-11}$ cm${}^{-1}$, in 200 s of acquisition time, corresponding to a noise-equivalent absorption sensitivity for the apparatus of $(1.6\pm 0.1)\times {10}^{-9}$ cm${}^{-1}$ Hz${}^{-1\mathrm{/}2}$.