Anomalous Contributions to the Signal in Laser-Enhanced Ionization Spectrometry from “Scattered” Laser Light

Abstract

Anomalous contributions to the measured signals in Laser-Enhanced Ionization (LEI) spectrometry in flames is investigated. It was found that a significant part of the one-step LEI signal (»90% in certain cases) can be attributed to ionization of atoms which are positioned outside of the “true” interaction region (the region out-traced by the laser beam in the flame) due to excitation of atoms by “scattered” light (the word “scattered” is here used in its widest meaning, i.e., it refers to scattered, diffracted as well as fluorescence light). This is especially significant when small beam diameters and high light intensities are being used. The anomalous contributions to the LEI signals from “scattered” light are investigated both in a study of the shape (the widths) of one-step LEI signals and in experiments with two nonoverlapping beams in the flame. Several origins for the “scattered” laser light are proposed and investigated: (1) diffraction of laser light from apertures, (2) emitted fluorescence from analyte atoms in the interaction region, and (3) scattering from flame related particles or molecules in the interaction region. The exact mechanism has not yet been determined, but the findings are indicative of some sort of saturable scattering process from a flame constituent which exists in higher concentrations in fuel-rich flames than in lean flames. The existence of “scattered” light outside of the interaction region implies, furthermore, that erroneous conclusions may be drawn when, for example, number densities, ionization yields, signal enhancements, and saturation effects are studied by LEI.