CRYSTAL DICHROISM AND THE ORIENTATION OF MOLECULAR-TRANSITION DIPOLE-MOMENTS

The dichroic behavior of strongly absorbing orthorhombic (or more symmetric) crystals is considered. General expressions for the transmittance of crystal plates as a function of the material dielectric tensor, the direction of the incident beam and the state of polarization of the analyzed transmitted light are presented. These expressions are given in a form adequate to assist in the interpretation of dichroic measurements performed employing usual spectroscopic settings. In particular, the equations are used to obtain an accurate expression for the dichroic ratio as a function of spectroscopic data. The value of this ratio, in turn, provides information on (and in favorable cases, determine) the orientation of molecular transition dipole moments in the crystal lattice. Limiting forms of these expressions suited to study spectroscopically weakly absorbing crystals and molecules hosted as oriented impurities in strongly absorbing crystals are derived. An evaluation of the errors involved in these approximations is also given.