Infrared reflection spectroscopy is a tool that can be used to study coated plastics, but the spectra can show unexpected
features. In this report, we calculate the specular reflectance for a flat surface of two different polymers as well as how
their spectra change when the other polymer is added as a film with a thickness of up to 2.5 µm. One of the unusual phenomena
we observe is that "derivative"-shaped substrate bands invert in sign as the other polymer is added as a coating. We also
show how the reflection of the surface changes and becomes polarized as the angle of incidence increases.
With support from the National Institutes of Justice, our laboratories recently worked on the spectroscopy of fabrics for
nondestructive, standoff (~1 m) detection of biological fluid stains (for example, bloodstains) as a crime-scene survey tool.
This application largely demands reflection, scattering, or emission measurements. The nature of the measurement we hope to
make constrains the methods and approaches we can take. We felt that a mid-infrared (MIR) reflection imaging approach was
best suited to our particular application.
Diffuse reflection (DR) is a method more commonly applied in cases where the absorption of incident radiation is weak.
One of the main reasons DR is not often applied to MIR analysis of strongly absorbing materials is that the spectra we obtain
are frequently not recognizable even to a specialist trained in infrared spectral identification. Instead, the reflection
spectra we observe are often low in intensity because of strong absorption in the sample. The spectra may have peaks in unexpected
places, and none where one might expect an absorption peak. The spectra may change with angle of incidence and detection,
and may be very sensitive to how the sample is prepared.
The Specular Component of Reflection
The source for all these problems is front-surface specular reflection. When the surface is rough or irregular, as with a
fabric, such a front-surface reflection is called Fresnel diffuse reflection (FDR). It is always present, resists intuition,
and analytical spectroscopists (like the authors) tend to hate even thinking about it. In DR measurements we do everything
in our power to ignore it, and we would make it go away if we could — but we can't.
FDR usually does not exhibit features expected in an absorption spectrum. But that doesn't mean it is featureless: It has
plenty of features, in fact too many features that are all in the "wrong" places. Underlying the complex nature of FDR is
a phenomenon most of us didn't learn about in school; the dispersion of optical constants in real materials (1).
We study the infrared absorption spectrum of matter because it contains characteristic features related to the chemical composition
of the matter. From an optical standpoint, matter can be characterized by its refractive index, n. The refractive index is frequency and wavelength dependent, and is also a complex quantity, meaning that it can be written
as the sum of a real and an imaginary part:
In equation 1, the roman n is the real component of index, lower case i is the square root of negative 1, and k is the imaginary
refractive index. The imaginary index k is related to the linear absorption coefficient, α, by equation 2:
where λ is the wavelength of light. Thus, the imaginary refractive index k varies with wavelength along with the absorption
coefficient. In other words, if you know the absorption coefficient for matter, you also know its imaginary index.
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