Gas phase elemental mercury: a comparison of LIF detection techniques and study of the kinetics of reaction with the hydroxyl radical

We have examined the sensitivity of single and sequential two-photon laser-induced fluorescence (LIF) techniques for the detection of elemental mercury, Hg(0), in the gas phase. Single photon LIF involves excitation of the 63P1–61S0 transition at 253.7 nm, followed by observation of resonance fluorescence. Sequential two-photon techniques follow the initial 63P1–61S0 excitation with a second excitation step to either the 71S0 or 73S1 levels followed by observation of blue or red shifted fluorescence. We have examined four variants of these approaches which all exceed the sensitivity of single photon LIF. The most sensitive detection approach involves the initial 253.7 nm excitation followed by excitation of the 71S0–63P1 transition at 407.8 nm. Fluorescence is observed on the 61P1–61S0 transition at 184.9 nm. Using this approach, our limits of detection are 0.1 ng m−3 with a 10 s integration time in air. We have also examined the effects of saturation, quenching and line-width on detection sensitivity. We have used the pulsed laser photolysis–pulsed laser-induced fluorescence (PLP-PLIF) technique to study the kinetics of the reaction of elemental mercury with the hydroxyl radical under atmospheric conditions at 298 K. We see no evidence for reaction and obtain an upper limit of 1.2×10−13 cm3 molecule−1 s−1 for the rate coefficient.