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Physical Review Letters

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

THOMAS W. MOSSBERG, Allen M. Flusberg, Ravinder Kachru, Sven R. Hartmann

Published Jun 18, 1979• 108 citations
Publisher

Abstract

A technique which employs heretofore unappreciated aspects of the optical stimulated echo was developed and used to measure the total scattering cross section for $\mathrm{Na}(3{S}_{\frac{1}{2}})$-He and $\mathrm{Na}(3{P}_{\frac{1}{2}})$-He collisions. The repulsive-core-dominated $\mathrm{Na}(3{S}_{\frac{1}{2}})$-He collision is found to be relatively soft. The cross section for degradation of the $3{S}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{1}{2}}$ 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.

DOI
10.1103/physrevlett.42.1665

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Photon-echo measurements in atomic Na perturbed by He provide the first demonstration of the contribution of velocity-changing-like effects to the transverse relaxation of atoms in superposition of two states even when the states follow different post-collision trajectories. The apparent anomaly that broadening cross sections derived from either absorption line widths or photon-echo relaxation measurements are smaller than the total cross section for scattering of atoms in either pure state is explained.
From work on the $2S\ensuremath{-}2{P}_{\frac{1}{2}}$ transition of atomic 7Li^{7}\mathrm{Li}7Li, 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.
With a three-level configuration in ytterbium vapor, we have performed the first quantitative investigation of single excited-level scattering using a stimulated photon echo (three-pulse echo). We have studied the collisional relaxation of the echo signal by rare-gas perturbers and have derived a theoretical expression for the relaxation rate. Total elastic cross sections for collisions between Yb(6s6p3P1\mathrm{Yb}(6s6p^{3}P_{1}Yb(6s6p3P1​) and Ne or Xe have been obtained.
The spectroscopy of metastable states is used to make the first experimental measurement of the total differential elastic-scattering cross sections of metastable helium atoms in 2 3 S and 2 1 S states by sodium atoms in the ground state at interaction energy 68 meV in the center-of-mass system. To analyze the experimental data, the partial scattering phases are calculated using the method of phase functions in the optical potential approximation. The analysis makes it possible to give a more detailed interpretation of the structure of the differential cross section. The computed integral cross sections, specifically, Penning ionization, diffusion, viscosity, and spin exchange are discussed.
It is shown that the decay in photon-echo amplitude with interval TTT between pulses provides a new method for the determination of the small-angle dependence of the forward-elastic-scattering cross section $\ensuremath{\sigma}(V,\ensuremath{\theta})$, and that the experiment of Schmidt, Berman, and Brewer yields an analysis of $\ensuremath{\sigma}(V,\ensuremath{\theta})$ for CH3{\mathrm{H}}_{3}H3​F collisions. The result is in agreement with the Cr3\frac{C}{{r}^{3}}r3C​ interaction between molecules with permanent electric dipole moments.