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Anal. Chem. 1982, 5 4 , 1057-1059
Atomic Absorption Spectrometry for Study of the Agglomeration of Atoms in a Glow Discharge D. C. McDonaHd CSIRO Divislon ofChemical Physics, P.O. Box 160, Clayton, Victorla, Australia 3 168
The agglomeration of sputtered atoms to form clusters whlch glve rlse to a “nonatomlc” absorption and scattering in a glow dlscharge has Important analytlcal consequences. Agglomeratlon has been studled by use of atomic absorption, and an estimate of the fraction of sputtered atoms contalned In clusters (30 % ) has been made.
the intensities of the light vibrating perpendicularly and parallel to the plane through the directions of propagation of the incident and scattered beams. The intensity of the beam decreases in a distance 1 by the fraction exp(-yl) for which the extinction coefficient y is found as
Y = NTU’QEXT In practical analysis using atomic absorption measurements on sputtered vapors it is usually necessary (1, 2) to make allowance for the attenuation of the light-beam by nonatomic (absorption/scattering) effects, which arise as a result of the agglomeration of some of the sputtered atoms. This study reports attemptw to estimate the numbers and sizes of the cluster formed under typical discharge conditions when a silver sample is used m the cathode in a a glow discharge sputtering cell. A better understanding of the phenomenon of agglomeration in sputtering discharges is needed before it will be possible to make full use of the sputtering technique in analytical atomic spectrometry. This is particularly so if it is desired to attempt “absolute” analysis, i.e., analysis without the use of reference materials (2).
THEORY Mie theory (311is a rigorous scattering theory for spheres of arbitrary size. It can be used to fit, models of cluster numbers and size distributions to an anomalous absorption curve measured experimentally for silver. Mie solved the vector wave equation for the case in which it was applied to the scattering of a plane wave by homogeneous spheres. He used boundary conditions to find expressions which described the field in terms of known functions at any point inside or outside the spheres. Efficiency factors for extinction (QEXT), scattering (QSCA), absorption (QMS),and radiation pressure (QPR)were subsequently introduced. In all cases QEXT
= QSCA + QABS
Using Mie coefficients 2 ” QEXT = - C (2n + 1)R(an + bn) x2n=1
(1)
(5)
The wave will also be retarded. Weakening and retardation are described together as the effect of a complex refractive index m. If the particles (spheres) have different radii and there are N(a)da with radii between “a” and “u + da” per unit volume, then
in which Q m T is also a function of “a” because of the x in the expression for QEXT. For absorbing spheres, the refractive index must be complex and the absorption is given by a = I0(1 - exp(-yNl)) (‘7) where 1 becomes the absorption path length. All of the above expressions are exact but general methods of computation may be introduced to simplify the solution in particular cases. For metal spheres and a small x (
