LASER-INDUCED NONRESONANCE ATOMIC FLUORESCENCE IN AN ANALYTICAL LASERMICROPROBE PLUME

Laser-induced nonresonance atomic fluorescence is used in conjunction with absorption and emission measurements to study the spatial and tem poral distribution of atomic species in a laser microprobe plume. Micr oplumes were formed in helium, neon, and argon atmospheres at pressure s from 50 to 500 Torr using stainless steel and niobium alloy samples. Studies of Fe, Ti, Zr, and Hf are reported. The signal-to-noise ratio for Fe is three times greater for Stokes direct-line fluorescence tha n for resonance detection. The major source of noise is in the fluores cence signal for both modes of detection. Spatial scans at different p ressures indicate that atmospheric density strongly affects the intera ction of the microplume with the atmosphere. At constant atmospheric p ressure, inner filter effects are more severe in higher density gases. Optical saturation helps to reduce some of the prefilter effect. The laser microplume undergoes a rapid upward expansion from the sample an d remains contiguous to the surface early in the lifetime of the micro plume. Later in the lifetime, the microplume separates from the sample surface and eventually disperses into the atmosphere. The dispersal o f the microplume occurs much faster in helium than in argon. (C) 1994 Academic Press, Inc.