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

A coherent-transient laser technique which employs heretofore unappreciated aspects of the optical stimulated echo was developed and used to measure the total scattering cross section for Na(3S 1 /2)-He and Na(3P 1/2)-He collisions. The repulsive-core-dominated Na(3S 1/2)-He collision is found to be relatively soft. The cross section for degradation of the 3S 1/2-3P 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.