Knowledge Bank

In anisotropic crystals of orthorombic or higher symmetry the dichroic ratios can be correlated with the inclinations of the transition moment vectors if the radiation is incident normally on principal planes and it is plane polarised so that the electric vector is parallel to the principal axes.1 In crystals of lower symmetry this condition cannot be met. The case of tri-clinic crystals is discussed. It is shown by electromagnetic theory and the theory of anomolous dispersion that in those $cases1$. Strong absorption bands can give rise to very high dichroic $ratios.2$. Strong absorption bands will in general show a spurious splitting with different polarization characteristics for the components of the doublet. These can be used to obtain an expression for the orientation of the transition moment $vector.3$. Weak bands as overtone and combination bands will in general show small dichroic ratios. The theory is illustrated with reference to the spectra of $NN1$ diacethylhexamethylenediamine.2