Sequential two-photon laser-induced fluorescence detection of mercury

We present initial experimental results on the two-photon laser-induced-fluorescence (TP-LIF) detection of mercury. This technique uses the sequential absorption of two photons, involving real atomic levels, to produce atomic fluorescence at a wavelength shorter than either pump wavelength. In this system, the pumping occurs at 254 and 408 nm, and optical sampling occurs at 185 nm. Based on preliminary findings using a first-generation system, together with modest evaluations of future improvements, we have estimated that the sensitivity of the TP-LIF mercury- detection system, under atmospheric sampling conditions, could approach ~10³ atoms/cm³. Under ideal sampling conditions, e.g., using He or Ar diluent gases, the detection limit could be as low as a few atoms per cubic centimeter. Attempts to realize both of these goals will soon be undertaken.