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Physical Review Letters

Observation of Quantum Effects in the Scattering of a Monoenergetic Li Beam by a Crossed Hg Beam

Hans U. Hostettler, Richard B. Bernstein

Published Oct 1, 1960• 25 citations
Publisher

Abstract

Data on total and differential cross sections are presented, which are believed to show quantum effects. A velocity-selected beam of lithium, chosen for its small mass and large lambda , was scattered by a crossed beam of mercury at 90 deg incidence. The dependence of the total cross section on the average relative velocity is shown graphically. Owing to the difficulty in determining the number of Hg atoms in the scattering zone, absolute total cross sections, Q, were not calculated. Typical angular distributions for Li scattering, observed in the plane of the crossed beams, are plotted as scattering intensities as a function of laboratory angle in the interior quadrant. It appears that the observed undulations are real interference effects originnating from the wave nature of the interacting beams. (B.O.G.)

DOI
10.1103/physrevlett.5.318

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•Sep 1, 1975•0 citations
Data on total and differential cross sections are presented, which are believed to show quantum effects. A velocity-selected beam of lithium, chosen for its small mass and large lambda , was scattered by a crossed beam of mercury at 90 deg incidence. The dependence of the total cross section on the average relative velocity is shown graphically. Owing to the difficulty in determining the number of Hg atoms in the scattering zone, absolute total cross sections, Q, were not calculated. Typical angular distributions for Li scattering, observed in the plane of the crossed beams, are plotted as scattering intensities as a function of laboratory angle in the interior quadrant. It appears that the observed undulations are real interference effects originnating from the wave nature of the interacting beams. (B.O.G.)
Measurements of the velocity dependence of the angular intensity distribution of potassium and cesium beams scattered by a crossed beam of mercury are presented. The alkali beam was velocity selected, with a triangular velocity distribution (half-intensity width 4.7% of peak velocity); the velocity was varied over the range 185–1000 m/sec. The Hg beam had a thermal distribution; the average Hg speed was ∼235 meters per second. The scattering data have been converted to the center-of-mass system. The angular distributions show the expected strong forward scattering and evidence the phenomenon of rainbow scattering. The energy dependence of the rainbow angle is used to evaluate the interatomic potential well depth, interpreted as the dissociation energy De of the 2Σ+ molecular ground state. Values (in erg×1014) thus obtained (±5%) are 7.46 for KHg and 7.72 for CsHg. Absolute values of differential cross sections could not be obtained; only relative cross sections D(θ) are reported. The observed low-angle behavior D(θ) ∝θ- 7/3 serves as direct experimental confirmation of the r—6 dependence of the long-range attractive potential for K–Hg and Cs–Hg systems.
Measurements of the velocity dependence of the angular intensity distribution of potassium and cesium atomic beams scattered by a crossed atomic beam of mercury are presented. The alkali atomic beam was velocity-selected, having a triangular velocity distribution with a halfintensity width of 0.047 of the peak velocity. The alkali velocity could be varied over the range 185 to 1100 m/s. The Hg beam had a thermal velocity distribution. The average speed of the Hg atoms in a typical experiment was 235 m/s (the calculated half-intensity width of the velocity distribution was 190 m/s). A typical angular distribution curve showed the expected very large low angle scattering, but evidenced a region in which the slope changed sharply, often resulting in a bump on the curve. This behavior is indicative of the phenomenon of rainbow scattering. The energy dependence of the rainbow angle was used to evaluate the depth of the interatomic potential wells; values (in ergs x 10/sup 14/) obtained are 7.7/sub 2/ for Cs-Hg and 7.4/sub 6/ for K-Hg. It is difficult to estimate the magnitude of the systematic errors in the absolute differential cross sections. However, reliable values for relative differential cross sections, which are velocity independent, can readilymore » be obtained by the methods described. From the low angle behavior of the differential cross sections, direct experimental evidence for the inverse sixth power dependence of the long-range attractive part of the potential was obtained. (auth)« less
Collisions studies using optically state selected beams is discussed. Measurements of spin resolved differential cross sections for superelastic scattering of electrons from excited sodium atoms (3P) are reported. The excited states are prepared by laser optical pumping with circularly polarized light thus giving a pure atomic state before the collision. A crossed beam arrangement of sodium atoms and electrons is used with a scattered electron detector. Scattered electron intensity as a function of scattering angle, incident electron energy, and relative orientation of the incident electronic and atomic spins is measured. 8 refs., 10 figs.
A velocity selected Tl beam has been crossed by an electron-bombarded Hg beam which contains a small percentage of the Hg(6 3P) metastable levels (an approximate 5:1 ratio of 3P2:3P0). By means of an interference filter–photomultiplier combination beneath the collision zone, the cross section for excitation of the 5350 Å (7 2S1/2 → 6 2P3/2)Tl fluorescence has been determined to increase monotonically with decreasing relative collision speed, g, approximately as g−s for s=2.0±0.5. Implications of these results for the sensitized fluorescence phenomenon in vapor mixtures of metallic elements are briefly discussed.
The scattering of a monoenergetic beam of atoms by a hard hemisphere on a hard flat surface is formulated quantum mechanically. Differential and integral cross sections are calculated as a function of incident beam energy and incident angle and compared with the familiar results of scattering from a free hard sphere. Two types of interference processes are identified. In addition to the previously discussed off-specular reflection symmetry interferences, Rayleigh oscillations are found in the integral cross sections. They result from the interference of specular scattering from the hemisphere and from the flat surface. Their phases are found to be independent of the angle of incidence when plotted versus the normal component of the incident wave vector.
Sensitized fluorescence in crossed atomic beams: Hg(6 sup3sup 3sup3Psub0sub 0sub0,sub2sub 2sub2)+Tl(6 sup2sup 2sup2Psub1sub 1sub1/sub2sub 2sub2)
A velocity selected Tl beam has been crossed by an electron-bombarded Hg beam which contains a small percentage of the Hg(6 sup3sup 3sup3P) metastable levels (an approximate 5:1 ratio of sup3sup 3sup3Psub2sub 2sub2:sup3sup 3sup3Psub0sub 0sub0). By means of an interference filter--photomultiplier combination beneath the collision zone, the cross section for excitation of the 5350 A (7 sup2sup 2sup2Ssub1sub 1sub1/sub2sub 2sub2 YieldsYieldsYields 6 sup2sup 2sup2Psub3sub 3sub3/sub2sub 2sub2)Tl fluorescence has been determined to increase monotonically with decreasing relative collision speed, g, approximately as gsup−sup -sup−/subs/ for s=2.0plus-or-minus0.5. Implications of these results for the sensitized fluorescence phenomenon in vapor mixtures of metallic elements are briefly discussed.