A STUDY OF THE PHOTO-ELASTIC PROPERTIES OF THE MOLECULAR CRYSTAL ANTHRACENE (PIEZOMODULATION, ELASTO-OPTIC, SPECTROSCOPY, REFLECTION, STRESS)

The photo-elastic properties of crystalline anthracene have been studied in the energy region of the first singlet exciton. Polarized single crystal uniaxial stress modulated reflection spectra have been obtained at room temperature (298K) using a microspectroreflectometer in the energy region of 25kK to 31kK. Modulated reflection spectra were taken along chosen stress and polarization directions within the crystal lattice that would facilitate the determination of all non-zero members of the stress photo-elastic tensor in this energy region. Utilizing the experimental elastic constants of anthracene the stress photo-elastic tensor was transformed into the strain photo-elastic tensor yielding the corresponding uniaxial strain modulated reflection spectra. The modulated reflection spectra were compared to both room temperature (298K) and low temperature (10K) unmodulated reflection spectra. The comparison revealed an enhancement of spectral structure approaching the resolution of the 10K spectral results. Comparison of the uniaxial strain modulated data to the numerically calculated logarithmic derivatives of the room temperature (298K) reflection spectra allowed the determination of exciton strain shifts for all corresponding strain and polarization directions. These strain shifts were compared to theoretically calculated strain shifts which resulted in a microscopic understanding of the intermolecular interactions (Van der Waals) for defined directions in the crystal lattice