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Selfabsorption

When one atom emits a photon there is a chance some other atom will absorb it. To absorb the photon the energy of the photon must match a possible electron transition in the atom, normally this means the absorbing atom must be of the same type as the emitting atom, hence the term 'self-absorption'.

This means, for example, emissions from iron atoms can only be absorbed by other iron atoms, because only other iron atoms have matching electron energy levels. Also, to match energy levels, the emitting and absorbing atoms must normally be in the same excitation state, and since most atoms in analytical plasmas are in or near the ground state, self-absorption is normally only seen for electron transitions involving the ground or near-ground states. These transitions are called resonance or near-resonance transitions.

[Self-absorption] 

The effect of self-absorption is to rob signal from the emission line. As the number of emitting atoms increases, the likelihood of self-absorption increases and the there is no longer a linear relationship between the number of emitting atoms and the measured intensity.

In simple models of self-absorption, it possible to show that the severity of self-absorption depends on the product of the number of emitters and the effective absorption cross-section,(1) given by (2)

[sigma l] 

where M is the atomic weight of the emitting atom or ion, T is the absolute temperature of the plasma gas, l0 the wavelength emitted, and f the oscillator strength. Self-absorption then tends to be highest when there is a large number of emitters, of high atomic weight, at lower gas temperatures, longer wavelengths, and higher oscillator strengths. Clearly the problem of self-absorption will vary greatly from one emission line to another and one emission source to another.

Spark OES is known to have severe self-absorption on some lines, ICP-OES has moderate problems at high concentrations, and GD-OES has less severe problems, though still present for some important lines in some materials, e.g. Zn I 213.8 nm and Cu I 327.3 nm in brass.

For more details on self-absorption, especially related to glow discharge, the GDOES site.

References

  1. R Payling, M S Marychurch and A Dixon, in Glow Discharge Optical Emission Spectrometry, R Payling, D G Jones and A Bengtson (Eds), John Wiley & Sons (1997), pp 376-91.
  2. N P Ferreira and H G C Human, Spectrochim. Acta 36B, 215 (1981).
  3. Th. Nelis R. Payling, A practical Guide Glow Discharge Optical Emission Spectroscopy,RSC, 2004

First published on the web: 15 November 1999.

Author: Richard Payling