Laser induced double-resonance ionic fluorescence in an inductively coupled plasma

Abstract

Two pulsed dye lasers pumped by an excimer laser are simultaneously directed into the analytical zone of an inductively coupled argon plasma. When the two beams are tuned to the appropriate ionic transitions, highly excited ionic levels can be efficiently populated in saturated conditions, the resulting fluorescence being then spectrally isolated with a monochromator and measured. A theoretical outline of this technique, variously called double-resonance fluorescence or two-step fluorescence, is given. The experimental results obtained with the alkaline-earth metals Ca, Sr, Ba and Mg show that the technique does provide excellent sensitivity, freedom from scattering problems and unprecedented spectral selectivity. The laser characteristics, the time overlap between the pulses and the spectral characteristics of the transitions used are discussed. Finally, ionic fluorescence in the plasma is the most suitable analytical application of such double-resonance technique since its use in flame atomic fluorescence suffers from the strong depletion of the excited levels due to collisionally assisted ionization.