Absolute density measurement of SD radicals in a molecular beam at the quantum-noise-limit

The absolute density of SD radicals in a molecular beam 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 quantum-noise-limited noise-equivalent absorption sensitivity of the apparatus was found to be $(3.5\pm0.3)\times10^{-11}$ cm$^{-1}$ Hz$^{-1/2}$ based on a 200 s acquisition time at 10 Hz, therefore competing favourably with more sophisticated techniques.