Infrared Intensities of Liquids XIX: A Simple and Effective Approximate Method for the Calculation of Infrared Optical Constant Spectra of Liquids from Transmission Measurements

A simple and effective approximate method is presented for the calculation of the optical constants of neat liquids from transmission measurements. The method calculates the apparent absorbance due to reflection losses by treating the liquid cell as a single slab of the window material. This approach makes the method far simpler than the exact iterative method that has been used to develop secondary infrared intensity standards and that applies Fresnel's equations to each interface in the cell. However, for all but the strongest absorption bands, the approximate method gives imaginary refractive indices that are within ∼1% of those from the exact method. The method is, thus, useful for nearly all common liquids in cells with alkali halide windows for all but the strongest bands. The effect of the size of the mismatch between the real refractive indices of sample and windows has been explored to some extent. It is recommended that results from the approximate method be regarded with caution if the refractive indices of the sample and windows differ by more than 0.15.