A Theoretical and Experimental Investigation of the Technique of Laser-Intermodulated Fluorescence for Scattering Correction in Atomic Fluorescence Flame Spectrometry

A theoretical derivation is presented of the signal obtained with the technique of intermodulated atomic fluorescence spectroscopy. One laser beam, tuned at a selected atomic transition, is divided into two beams which are then amplitude-modulated at different frequencies and recombined in a flame containing the vapor of the element investigated. The fluorescence signal at the sum or difference frequency is measured. The derivation is given for both square-wave and sinusoidal modulation. It is shown that the intermodulated fluorescence amplitude depends upon the square of the laser spectral irradiance at low powers and reaches a plateau at high irradiances, but only in the case of square-wave modulation. For sinusoidal modulation, a maximum is reached followed by a roll-off at high irradiances. The theoretical predictions are verified experimentally with a square-wave-modulated cw dye laser for the case of sodium resonance fluorescence in an oxygen-argon-hydrogen flame. The scatter signal has no intermodulation component. Finally, it is shown that when the modulation waveform is not square wave, scattering correction can also be achieved with a single-beam excitation scheme.