The absolute density of SD radicals in a supersonic jet has been measured down to $(1.1\pm0.1)\times10^5$ 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 $215\pm21$ molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantum-noise-limited and measured to be $(7.9\pm0.6)\times10^{ -11}$ cm$^{ -1}$, in 200 s of acquisition time, corresponding to a noise-equivalent absorption sensitivity for the apparatus of $(1.6\pm0.1)\times10^{ -9}$ cm$^{ -1}$ Hz$^{ -1/2}$.
Categories: physics.chem-ph physics.atom-ph
