  A general theory of inhomogeneous broadening is rarely applied to nonlinear
spectroscopy in lieu of either a simple Lorentzian or Gaussian model. In this
work, we generalize all the important third-order nonlinear susceptibility
expressions obtained with sum-over state quantum calculations to include
Gaussian and stretched Gaussian distributions of Lorentzians. This theory gives
a better fit to subtle spectral features - such as the shoulder of the
electroabsorption peak, and is a more accurate tool for determining transition
moments from spectroscopy experiments.
