The footprint of linear dichroism in infrared 2D-Correlation spectra

On the level of the Bouguer-Beer-Lambert approximation, the effects introduced by linear dichroism into absorbance spectra can be simulated by classic linear dichroism theory. If wave optics and dispersion theory are employed, linear dichroism can be modelled with a 4x4 matrix formalism. For linear dichroism theory, the angle between polarization direction and transition moment can be seen as a perturbation which allows to calculate corresponding infrared 2D correlation spectra. Similarly, with help of an orientation representation based on Euler’s angles, varying the latter allows the same if electromagnetic theory is employed. Correspondingly, we compare the substantially different footprints of linear dichroism according to both theories in infrared 2D correlation spectra and show that only those based on wave optics and dispersion theory are in accordance with experimental results. Accordingly, in particular asynchronous 2D correlation spectra allow to detect orientation with a sensitivity that is unparalleled in case of conventional spectra, even if they are recorded with help of a polarizer and an analyzer.