LASER-INDUCED NONRESONANCE ATOMIC FLUORESCENCE IN AN ANALYTICAL LASER MICROPROBE PLUME

Laser-induced nonresonance atomic fluorescence is used in conjunction with absorption and emission measurements to study the spatial and temporal distribution of atomic species in a laser microprobe plume. Microplumes were formed in helium, neon, and argon atmospheres at pressures 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 than for resonance detection. The major source of noise is in the fluorescence signal for both modes of detection. Spatial scans at different pressures indicate that atmospheric density strongly affects the interaction of the microplume with the atmosphere. At constant atmospheric pressure, 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 and remains contiguous to the surface early in the lifetime of the microplume. Later in the lifetime, the microplume separates from the sample surface and eventually disperses into the atmosphere. The dispersal of the microplume occurs much faster in helium than in argon. (C) 1994 Academic Press, Inc.