The influence of wetting and drying cycles on mid-infrared attenuated total reflection spectra of quartz: Understanding spectroscopy of disturbed soil

Attenuated total reflection (ATR) spectroscopy is a well established optical technique investigating fundamental molecular vibrations in the mid-infrared (MIR) spectral regime for a wide variety of samples including liquids, thin films and powders. In the present study, first results simulating the influence of weathering processes on the spectral characteristics of soils are discussed. In particular, the effect of wetting and drying cycles on IR spectra of fine quartz NOD powders has been investigated with ATR techniques. Resulting from a wetting and drying cycle, the sample spectra of quartz powders revealed significantly increased absorption intensities throughout the spectral region of interest (1400-600 cm(-1)). We hypothesize that this effect results from a higher packing density of the particles following the wetting procedure with the fines packed into interstitial spaces closer to the ATR waveguide surface. Moreover, a strong red shift of approx. 40 cm(-1) of the absorption band assigned to asymmetric SiO4 stretching vibrations (1050 cm-1 to 1250 cm(-1)) could be observed. Both effects, increase in intensity and spectral shift, are reversed by mechanically disturbing the cemented powder after the wetting/drying cycle. Experiments with s- and p- polarized infrared radiation show similar (reversible) spectral shifts for this particular frequency range. It is expected that these findings will lead to better understanding of the spectral characteristics of soil in the mid-infrared spectral domain providing improved interpretation of data retrieved from disturbed soils e.g. potential landmine sites during hyperspectral imaging.