Theory for the measurement of the forward-scattering cross section with photon echoes

It is shown that the decay in photon-echo amplitude with interval $T$ between pulses provides a new method for the determination of the small-angle dependence of the forward-elastic-scattering cross section $\sigma (V,\theta )$, and that the experiment of Schmidt, Berman, and Brewer yields an analysis of $\sigma (V,\theta )$ for C${\mathrm{H}}_3$F collisions. The result is in agreement with the $\frac{C}{r^3}$ interaction between molecules with permanent electric dipole moments.