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Physical Review Letters

Observation of Collisional Velocity Changes Associated with Atoms in a Superposition of Dissimilar Electronic States

THOMAS W. MOSSBERG, Ravinder Kachru, Sven R. Hartmann

Published Jan 14, 1980• 35 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.44.73

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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.
We have measured the He-, Ne-, and Ar-induced relaxation properties of the $3S\ensuremath{-}nS$ ($3S\ensuremath{-}nD$) superposition states of atomic sodium, where nnn ranges from 5 to 20 (4 to 34). The measurements were made using the trilevel echo effect. We find that the effective collisional cross section first increases (as a function of nnn) at low nnn, and then decreases to a lower asymptotic limit at high nnn. This behavior is similar to that previously observed for the $3S\ensuremath{-}nP$ series. We also find that the difference between the collisional cross sections for SSS and DDD series superpositions closely approximates the inelastic collision cross sections measured elsewhere for the ddd states. A theoretical description of the trilevel echo effect used to make the measurements is presented.
A general theory of atomic dipole coherence under the influence of collisional phase changes, inelastic effects and optically active atom velocity changes, including those due to anisotropic interactions is presented. Velocity change effects are obtained in closed form. Line shapes appear as convolutions of standard pressure broadening contours with velocity-change contours. Width and shift parameters for the He-broadened Na D lines at 2 m bar pressure, 380 K are calculated, as are He-induced photon echo decay rates for these lines. Overall agreement with xperiment is reasonably good.