Detection of mercuric chloride by photofragment emission using a frequency-converted fiber amplifier

A real-time, noninvasive approach for detecting trace amounts of vapor-phase mercuric chloride (HgCl_2) in combustion flue gas is demonstrated using a near-infrared pulsed fiber amplifier that is frequency converted to the ultraviolet. Excitation of the HgCl_2(1∏1_u ? 1∑1g+) transition at 213 nm generates 253.7 nm emission from the Hg (6^3P_1) photoproduct that is proportional to the concentration of HgCl_2. A measured quadratic dependence of the HgCl_2 photofragment emission (PFE) signal on the laser irradiance indicates that the photodissociation process involves two-photon excitation. Additionally, low concentrations of HgCl_2 are detected with the PFE approach in an environment characteristic of coal-fired power-plant flue gas using this compact solid-state laser source. A detection limit of 0.7 ppb is extrapolated from these results.