Measurement of a Total Atomic-Radiator-Perturber Scattering Cross Section

From work on the $2S-2P_{\frac{1}{2}}$ transition of atomic ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, perturbed by noble gases, and use of the photon-echo technique, the first measurement of a "total" atomic-radiator-perturber scattering cross section is reported. The phase-changing, inelastic, and velocity-changing aspects of collisions contribute to this cross section, which is significantly larger than the corresponding pressure-broadening cross section. Typical velocity changes are found to be roughly one percent of the mean thermal speed.