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

摘要

A theoretical derivation is presented of the signal obtained with the technique of intermodulated atomic fluorescence spectroscopy. One laser beam, tuned at a elected 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 same 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 continuous wave 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. (Reprints)