Laser-Induced Ionization of Atoms in a Power-Modulated Inductively Coupled Plasma

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Abstract
Laser-induced ionization of atoms has been detected in a power-modulated inductively coupled plasma. The measurement is made 1.4 ms after complete interruption of the 40-MHz power to a 400-W plasma. Electrical conductivity measurements between probe electrodes in the plasma during the power-off cycle have been made, demonstrating the decay in plasma background ion/electron concentrations which makes detection of laser-induced ionization possible. Radio-frequency interference from the ICP on the ionization detection electronics is also avoided by this approach. The primary mode of laser-induced ionization was photoionization of the laser-excited atoms, i.e., resonance ionization spectroscopy (RIS). Detection limits of 80 µg Fe/L and 20 µg Ga/L were achieved.