R. G. Pinnick, S. G. Jennings, Daniel C. Boice, and John P. Cruncleton, "Attenuated total reflectance measurements of the complex refractive index of kaolinite powder at CO2 laser wavelengths," Appl. Opt. 24, 3274-3285 (1985)
Attenuated total reflectance measurements of the complex refractive index of kaolinite powder–air mixtures are made for nine CO2 laser wavelengths. The Maxwell-Garnett effective medium theory and generalizations of it that account for either the shape distribution of kaolinite grains in the medium (in which the grains are approximated by a shape distribution of small arbitrarily oriented ellipsoids) or the size distribution of grains (in which finite grain sizes are accounted for by considering, in addition to the electric dipole interaction, magnetic dipole and electric quadrupole interactions) are used to deduce from these measurements the complex refractive index of kaolinite. Most success is achieved with a generalization which assumes a shape distribution of small ellipsoidal grains but which neglects all but electric dipole interactions. In spectral regions where kaolinite displays very strong absorption (in the 9.6–10-μm spectral region) all effective medium theory solutions for kaolinite refractive index either are plagued with ill-conditioning or are nonphysical. It appears that the attenuated total reflectance method, at least as we have applied it here to loosely packed powders comprised of nonspherical grains, is not suitable for measurement of powders in spectral regions of strong absorption.
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ATR system parameters: packing fraction 0.10; incident angles (degrees, minutes): 20.30, 22.15, 23.55, 26.20.
Since Al2O3 is weakly absorbing, the ATR technique is not sensitive enough to accurately determine the imaginary index.
Interpolated values using appropriate dispersion analysis parameters.
Table II
Attenuated Total Reflectance Measurements of the Complex Refractive Index of Kaolinite Powder–Air Mixtures
Wavelength (μm)
Packing fraction f
ATR system incident angles (deg, min)
Complex index of kaolinite powder–air mixture m = n − ik
These values are judged to be the best ones at the given wavelength and are used in the kaolinite refractive-index determinations.
Table III
Kaolinite Complex Refractive Indices Inferred from Attenuated Total Reflectance Measurements of Kaolinite Powder using Various Effective Medium Mixture Rules
Grain size distribution parameters are those of Fig. 3.
Determinations made using a generalized Bruggeman rule for uniformly sized grains with volume mean size parameter x = 0.3 [Eq. (3) of Ref. 11].
Determinations made using a further generalized Bruggeman rule for a size distribution of grains [Eq. (4) of Ref. 12] and for the gamma-type size distribution of Fig. 3.
Solutions led to unphysical roots for refractive index.
Tables (3)
Table I
Comparison of the Refractive Index of Al2O3 (at λ = 9.305 μm) as Measured by ATR and Reflectance Techniques
Refractive-index measurements
ATR measurementsa in conjunction with Maxwell-Garnett or Bruggeman effective medium theories
ATR system parameters: packing fraction 0.10; incident angles (degrees, minutes): 20.30, 22.15, 23.55, 26.20.
Since Al2O3 is weakly absorbing, the ATR technique is not sensitive enough to accurately determine the imaginary index.
Interpolated values using appropriate dispersion analysis parameters.
Table II
Attenuated Total Reflectance Measurements of the Complex Refractive Index of Kaolinite Powder–Air Mixtures
Wavelength (μm)
Packing fraction f
ATR system incident angles (deg, min)
Complex index of kaolinite powder–air mixture m = n − ik
These values are judged to be the best ones at the given wavelength and are used in the kaolinite refractive-index determinations.
Table III
Kaolinite Complex Refractive Indices Inferred from Attenuated Total Reflectance Measurements of Kaolinite Powder using Various Effective Medium Mixture Rules
Grain size distribution parameters are those of Fig. 3.
Determinations made using a generalized Bruggeman rule for uniformly sized grains with volume mean size parameter x = 0.3 [Eq. (3) of Ref. 11].
Determinations made using a further generalized Bruggeman rule for a size distribution of grains [Eq. (4) of Ref. 12] and for the gamma-type size distribution of Fig. 3.
Solutions led to unphysical roots for refractive index.
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