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

TL;DR

This paper demonstrates a highly sensitive method combining cavity ring-down and laser-induced fluorescence to measure the absolute density of SD radicals in a supersonic jet at the quantum-noise limit, achieving detection of very low densities.

Contribution

It introduces a novel measurement approach that reaches the quantum-noise limit for detecting SD radicals in a supersonic jet, with detailed sensitivity analysis.

Findings

Measured SD radical density as low as 1.1×10^5 cm^-3

Achieved quantum-noise-limited detection sensitivity of 7.9×10^-11 cm^-1

Demonstrated detection of molecules in a small probe volume

Abstract

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$^{-1/2}$.