Definition of important factors in polarised infrared spectroscopy: vibrational circular and linear dichroism in normal transmission mode and in attenuated total reflectance

Infrared absorbance (IR) spectroscopy is seeing a resurgence in applications in both quality control and research laboratories. Vibrational linear dichroism (VLD) adds a dimension to the spectrum for oriented samples, which helps separate peaks and identify polarisations of transitions. Measuring VLD by rotating either the sample or a polariser is prone to error, so we use an adapted vibrational circular dichroism (VCD) instrument with a photoelastic modulator producing alternate polarisations to measure VLD. However, in contrast to VCD, transmission VLD spectra show a small contribution to the VLD that follows the absorbance even for isotropic samples. In this work, we show the origin of this contribution to VLD and why it does not affect VCD. This leads to a method for extracting the true VLD spectra from the measured data. In an attenuated total reflectance (ATR) mode of measurement, the situation is further complicated by the polarisation and refractive index-dependent changes of the light's electric field intensity. Equations are developed to account for these effects. It appears that there is a variable contribution of an absorbance spectrum to the ATR-VLD. Some speculation is given as to the origin of this effect.