SERAC simulation chamber
A simulation chamber 'SERAC' for adsorption measurements was designed and built in 2007 as a complement to the reflectance spectrometer.
Fig. 1 scheme of the complete experimental setup: spectrometer, chamber and adsorption device. The empty volume inside the chamber is 181 mL.
Picture: Scheme of the SERAC simulation chamber containing the sample and coupled to the spectro-Gonio radiometer, and the thermodynamic setup. Each of the five valves is numbered to be easily identified in the experimental protocol description
The dimensions of the sample are constrained by reflectance spectroscopy requirements (size of the incident and emergent beams and need for an infinite optical depth). Sample diameter is 30 mm and sample thickness can vary between 1 mm and 10 mm. According to theoretical arguments and experimental checking, sample thickness 100 times the grain size is sufficient to make negligible a potential contribution of the sample holder to the measured spectrum, even for transparent materials. Smaller thickness can be chosen when the material is not transparent over the whole spectral range measured.
The simulation chamber is closed on its upper surface by a sapphire window that is 150 mm in diameter and 3 mm thick. Sapphire has excellent transmission and is spectrally featureless in the visible and near-infrared. However, light reflection on the window as well as multiple reflections between the sample and the window can affect the absolute values of reflectance measured. More problematic, multiple reflections also result in a small reduction of contrast between continuum and absorption bands in the sample reflectance spectra.
Therefore, we developed a simple model of multiple reflections between the sample and the sapphire window that we use to correct the measured spectra. In this model, we assume that when light encounters the sapphire window, a constant fraction of light, T, is transmitted in the forward direction while the complementary fraction of light, 1 - T, is reflected in the backward direction. We apply this assumption for all multiple reflections between sample and window, which gives the following relationship between the sample actual reflectance, R, and the apparent reflectance, M (measured with the window), at each wavelength:
Application of this simple model to the data gives excellent results in correcting both the photometric bias and spectral effects of the sapphire window. Fig. 2 provides an example of the comparison between measurements made on the same sample without the window and with the window (raw and corrected). For this particular example, we choose an organic material that presents extremely contrasted values of reflectance in the visible and near-infrared spectral ranges to insure that the correction method is efficient from the lowest to the highest reflectance value.