Infrared characterization of surfaces and coatings by internal-reflection spectroscopy

Area of Science:

• Materials Science
• Spectroscopy
• Surface Chemistry

Background:

• Internal reflection spectroscopy (IRS) is crucial for characterizing surfaces and coatings.
• Absorption bands from water (H2O) and hydrocarbon impurities can complicate spectral analysis.
• Calcium fluoride (CaF2), Thorium fluoride (ThF4), and Zinc selenide (ZnSe) are common optical coating materials.

Purpose of the Study:

• To investigate and describe problems in surface and coating characterization using IRS.
• To analyze the spectral characteristics of bare and coated CaF2, ThF4, and ZnSe.
• To understand the adsorption and desorption behavior of water and hydrocarbons on these materials.

Main Methods:

• Infrared (IR) spectroscopy was used to analyze bare CaF2 trapezoids and ThF4- and ZnSe-coated trapezoids.
• Experimental absorptances were analyzed using a three-layer system reflectance model.
• Spectra were examined for absorption bands indicative of H2O and hydrocarbon impurities.

Main Results:

• Absorption bands for H2O and hydrocarbons were observed in the IR spectra of all samples.
• Absorptance due to water was significantly higher in ThF4 films compared to ZnSe films and CaF2 surfaces.
• Water appears to be distributed throughout ThF4 films, while primarily residing on the surface of ZnSe films.
• Observations on water and hydrocarbon desorption/adsorption were documented.

Conclusions:

• The distribution of water differs significantly between ThF4 and ZnSe thin films.
• IRS analysis, coupled with a three-layer model, allows for quantitative determination of absorption coefficients in thin-film coatings.
• Understanding impurity adsorption is vital for accurate surface and coating characterization.