Infrared transmission equations in a five media system: gas and liquid

Equations are proposed for transmission cells in the presence of multiple reflections and absorption which generate unwanted fringes. These influence adversely band intensity measurements. The infrared (IR) spectra generated with these equations are compared successfully with experimental spectra obtained with BaF₂, ZnSe, and Si windows in parallel mount formation having μm range air gaps. Equations are extended for integration of a variable path length such as in wedge shaped cells that are used to mitigate fringe formations but generate other odd problems such as path length determination. These equations allow the evaluation of the transmission obtained from boxcar cells whose parallelism is a little offset. This phenomenon modifies the fringe intensities. The proposed equations were used to calculate the IR spectra of pure liquid D₂O between BaF₂ and ZnSe windows with path lengths of around 25 μm and compared with experimental spectra. Since the fit was very good it indicates that the proposed equations give better optical properties of pure liquids than that presently available. This is important for liquid used as standards and in particular water used here as reference.