Optical dephasing and photon echoes from energetically and substitutionally disordered crystals

For a crystal with both diagonal disorder (inhomogeneous broadening) and substitutional disorder we have calculated the contribution to the photon echo decay due to purely electronic intersite interactions. We have considered the two important limiting cases of macroscopic and microscopic inhomogeneous broadening. In the case of macroscopic broadening we find that the optical dephasing rate constant 1/T2∼f J, where f is the fractional concentration of impurities and J is the nearest-neighbor intersite interaction in frequency units. This result is valid for f≪1. In the opposite limit of microscopic inhomogeneous broadening, we find that 1/T2∼f J2/δ̄, where δ̄ is the width in frequency units of the inhomogeneous distribution. This result is valid for arbitrary f, including f=1, and for J≪δ̄. In the case that f≪1 and J≪δ̄, 1/T2 is much smaller for microscopic broadening than for macroscopic broadening. We discuss our results in relation to several experiments on mixed molecular crystals and to an experiment on the stoichiometric material EuP5O14.