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The Journal of Chemical Physics

Velocity Dependence of the Differential Cross Sections for the Scattering of Atomic Beams of K and Cs by Hg

Fred A. Morse, Richard B. Bernstein

Published Nov 1, 1962• 90 citations
Publisher

Abstract

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.

DOI
10.1063/1.1733421

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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

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Journal of Physics B Atomic and Molecular Physics•Jun 28, 1978•30 citations
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
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.)
The velocity (v) dependence of the total cross section (Q) has been measured for the scattering of helium in the metastable 2 3S1 state by ground-state helium, argon, and krypton at relative velocities in the range 1000–3300 m/sec. The results are compared with those previously obtained for the scattering of lithium. For each scattering gas, the shape of Q(v) is similar to that of lithium, but the absolute values for Q are approximately 20% higher. The scattering is largely elastic. It is estimated that the inelastic contribution to Q is about 10% for the He*–He case, and that it is less than 3% for the He*–Ar and He*–Kr cases. The Q(v) for scattering by argon and krypton show undulatory behavior. From the extrema velocities, the product εσ is shown to be close to that for Li scattering, where ε is the depth of the interatomic potential well and σ is the interatomic separation at the zero of the potential. The separate parameters ε and σ are also deduced. A minimum to the number of diatom bound states is established for HeAr(3Σ+) and HeKr(3Σ+).
We present measurements of differential elastic cross sections for various alkali-alkali pairs, as well as the angular dependence of the probability of electron spin exchange in these collisions. The resolution is sufficient to reveal interference structure in the angular dependence of both quantities. We describe our apparatus, which polarizes one incident beam and analyzes the polarization of the scattered atoms, and discuss the measurement and data-reduction procedures. Results are presented in the range 0.1-0.2 eV for the systems Na-Cs, Na-Rb, Na-K, K-Cs, and K-Rb.
The differential cross section has been measured for the interaction of Na 2 P 3/2 with Hg. The excitation of Na has been achieved with linearly polarised light of a CW dye laser. The measurements have been evaluated by fitting the parameters of a pseudopotential from which the interatomic potentials have been obtained. The scattering calculation for these interatomic potentials is performed by integrating the coupled Schrodinger equations for the six coupled channels resulting from the P states with J= 3 / 2 and 1 / 2 and their respective values of m j . This experiment and the experiment for the ground-state interaction are now both described by a common calculation using the pseudopotential method.