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Physical Review Letters

Velocity Dependence of the Total Scattering Cross Section for a Superposition of Dissimilar States

John E. Thomas, Richard A. Forber, L. A. Spinelli, Michael S. Feld

Published Dec 12, 1983• 7 citations
Publisher

Abstract

A measurement is reported of the velocity dependence of the total cross section for Yb optical radiator-Xe perturber collisions in which the active atom is in a superposition of dissimilar ($S$ and $P$) states. The total cross section is found to vary with relative velocity in a simple manner, consistent with van der Waals scattering as the dominant interaction for this nonclassical system.

DOI
10.1103/physrevlett.51.2194

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A measurement is reported of the velocity dependence of the total cross section for Yb optical radiator-Xe perturber collisions in which the active atom is in a superposition of dissimilar (SSS and PPP) states. The total cross section is found to vary with relative velocity in a simple manner, consistent with van der Waals scattering as the dominant interaction for this nonclassical system.
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Collisions of atoms in superpositions of dissimilar states have special significance in that, classically, each state follows a different trajectory. Hence the scattering of such a system as a unit has no classical analog and is of fundamental interest. We report studies of velocity-changing collision kernels in 174Yb for two types of superposition states: optical radiators, consisting of a ground S state and an excited P state, and P-state multipole moments, involving magnetic substate superpositions.
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.
A thermal K beam was velocity selected, and scattered by Ne, Ar, and Xe gases. The dependence of the total cross section on relative velocity is approximately v/sup -0.4/ for K--Ar and K--Xe, and v/sup 0.8/ for K--Ne. The average relative velocity ranges were 400 to 1430 m/sec for Ne, 480 to 1350 m/sec for Ar, and 380 to 1320 m/sec for Xe. The dependence of the cross section on relative velocity was obtained by removing the cross section from the collision frequency integral. For the v/sup -0.4/ dependences, this gives results correct to 3%. For the v/sup -0.8/ dependences, the error is most likely greater. Three gas temperatures were used: liquid air, room, and 200 deg C. The results for a given gas at different temperatures are consistent with kinetic theory predictions. (auth)
We calculate the total and differential cross sections for radiative-collisional excitation of the metastable 2 1 S state of He atoms at collisions with Ne atoms in external radiation fields of various frequencies and polarizations. The calculations are performed for a thermal collision energy of E = 10 −3 atomic units and light intensity of I = 1 MW cm −2 , which corresponds to a single photon absorption by a quasi-molecule during the collision. Both the differential and total cross sections are shown to depend strongly on the relative orientation of the radiation polarization vector and the initial relative velocity vector of the colliding atoms. We analyze the azimuthal scattering asymmetry related to the orientation of the angular momentum of the absorbed photon.
We present a time-dependent calculation of the velocity-dependent forces which act on an excited atomic dipole in relative motion with respect to ground state atoms of a different kind. Both its interaction with a single atom and with a dilute atomic plate are evaluated. In either case, the total force consists of a conservative van der Waals component and a nonconservative R\"ontgen component. On physical grounds, the former corresponds to the velocity-dependent recoil experienced by the excited atom in the processes of absorption and emission of the photons that it exchanges with the ground-state atoms on a periodic basis. The latter corresponds to the time-variation of the R\"ontgen momentum, which is also mediated by the periodic exchange of quasiresonant photons. We find that, at leading order, all these interactions are linear in velocity. In the nonretarded regime the van der Waals force dominates, being antiparallel to the velocity. On the contrary, in the retarded regime the velocity-dependent forces oscillate in space, van der Waals and R\"ontgen forces are of the same order in the atom-atom interaction, and the R\"ontgen component dominates in the atom-surface interaction.