Total Scattering Cross Section for Na on He Measured by Stimulated Photon Echoes

A technique which employs heretofore unappreciated aspects of the optical stimulated echo was developed and used to measure the total scattering cross section for $\mathrm{Na}\left(3\mathrm{}{S}_{\frac{1}{2}}\right)$-He and $\mathrm{Na}\left(3\mathrm{}{P}_{\frac{1}{2}}\right)$-He collisions. The repulsive-core-dominated $\mathrm{Na}\left(3\mathrm{}{S}_{\frac{1}{2}}\right)$-He collision is found to be relatively soft. The cross section for degradation of the $3S_{\frac{1}{2}}-3\mathrm{}{P}_{\frac{1}{2}}$ superposition, determined from photon-echo-decay measurements, is anomalous in that it is smaller than the total elastic-scattering cross section with Na in either state separately.