On the inversion of geometric parameters from absorption and circular dichroism spectroscopy of molecular dimers

Absorption spectra of molecular dimers usually exhibit complex line-shapes. In an approximate treatment the excited dimer states are described as a two-level system, leading to two bands separated by twice the electronic coupling element which is responsible for the splitting. The band intensities are related to the relative orientation of the monomer transition dipole-moments. We show, considering absorption and circular dichroism spectra, that extracting geometric parameters using the two-level description can lead to substantial errors when internal monomer vibrations become relevant. The application of sum-rules offers a more accurate approach to invert the data.