Quantitative absorbance spectroscopy with unpolarized light: Part I. Physical and mathematical development

Malcolm Sambridge, John Fitz Gerald, I. Kovács, Hugh St C O'Neill, Jörg Hermann

Research output: Contribution to journalArticle

61 Citations (Scopus)

Abstract

A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell's equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, Aunpol(φ, ψ), and the principal polarized absorbances, Aa, Ab, Ac in an anisotropic crystal. Aunpol(φ,ψ) = 1/2[Aa(cos2φcos2ψ + sin2ψ) + Ab(cos2φsin2ψ + cos2ψ) + Acsin2φ]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total (Aa + Ab + Ac) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.

Original languageEnglish
Pages (from-to)751-764
Number of pages14
JournalAmerican Mineralogist
Volume93
Issue number5-6
DOIs
Publication statusPublished - May 2008

Fingerprint

absorbance
spectroscopy
Spectroscopy
Crystals
crystals
crystal
transmittance
Light propagation
Maxwell equations
Light polarization
Crystal orientation
Infrared spectroscopy
ellipsoids
Maxwell equation
polarized light
Polarization
estimating
eigenvalues
infrared spectroscopy
propagation

Keywords

  • Absorption index theory
  • Anisotropic media
  • Spectroscopy
  • Unpolarized light

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Quantitative absorbance spectroscopy with unpolarized light : Part I. Physical and mathematical development. / Sambridge, Malcolm; Gerald, John Fitz; Kovács, I.; O'Neill, Hugh St C; Hermann, Jörg.

In: American Mineralogist, Vol. 93, No. 5-6, 05.2008, p. 751-764.

Research output: Contribution to journalArticle

Sambridge, Malcolm ; Gerald, John Fitz ; Kovács, I. ; O'Neill, Hugh St C ; Hermann, Jörg. / Quantitative absorbance spectroscopy with unpolarized light : Part I. Physical and mathematical development. In: American Mineralogist. 2008 ; Vol. 93, No. 5-6. pp. 751-764.
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