Piezomodulation spectroscopy of molecular crystals. I. Methods and principles

The effects of piezomodulation on the spectra (particularly reflectivity spectra) of molecular crystals for the strong, weak, and no coupling regimes are considered. The stress or strain modulation of the crystal is shown to affect the dispersion energy and exciton resonance interaction. The experimental approach is outlined. Methods of data analysis are described for obtaining differential changes in the real and imaginary parts of the dielectric function, for determining deformation potentials for vibronic transitions, and for applying the Seraphin coefficients. Previous microscopic theory for interaction of excitons with static lattice deformations is adapted for the modulated case. Application of classical dipole theory to model one and two vibronic level systems yields expressions for calculation of the piezomodulated reflection spectrum. Relative contributions to the spectrum are determined for a model polariton stop band by consideration of piezomodulation of the exciton energies, plasma frequency, oscillator strengths, and background dielectric function. Applications of piezomodulation spectroscopy to investigations of molecular and ionic crystals are discussed