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Infrared Complex Refractive Index Measurements and Simulated Reflection Mode Infrared Absorption Spectroscopy of Shock-Compressed Polymer Thin Films

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Abstract:

Thin film interference effects complicate the interpretation of reflection-mode infrared absorption spectra obtained in shock-compressed thin film materials and must be carefully accounted for in any analysis attempting to unravel shock-induced energy transfer or reactivity. We have calculated such effects for spectrally simple model systems and also, to the extent possible, for real systems such as polymethylmethacrylate (PMMA) and nitrocellulose (NC). We have utilized angle-dependent infrared (IR) reflectometry to obtain the ambient spectral complex index for PMMA and NC for use in the calculations and to interpret experiments. A number of counterintuitive spectral effects are observed versus film thickness and during uniaxial shock compression: absorption band shifts, changes of shape, and changes in both absolute and relative peak intensities. The film thickness effects can be predicted by thin film interference alone, while additional assumptions are required to predict the effects due to shock compression. Since it is very difficult to obtain the complex index in the shock state, we made very simple assumptions regarding the change in vibrational spectra upon shock loading. We illustrate general thin film interference effects that could be expected and compare them to experimental results for the antisymmetric NO2 stretch mode of NC.

Keywords: INFRARED ABSORPTION SPECTROSCOPY; INFRARED COMPLEX REFRACTIVE INDEX; INTERFERENCE EFFECTS; POLYMER THIN FILMS; REFLECTION MODE; SHOCK COMPRESSION

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370204774103291

Affiliations: Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Publication date: May 1, 2004

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