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Faraday Discussions of the Chemical Society

Scattering of metastable mercury atoms

T. A. Davidson, Malcolm A. D. Fluendy, K. P. Lawley

Published Jan 1, 1973• 8 citations
Publisher

Abstract

Differential elastic cross sections for the scattering of the metastable 63P2 state of mercury from sodium, potassium and rubidium have been measured at thermal energies. Interference structure is resolved in all cases, suggesting that the atoms interact by a single effective potential in the attractive region probed at these energies. No attenuation of the interference structure is observed so that quenching of the metastable state is not an important process along trajectories sampling only the attractive part of the potential.

DOI
10.1039/dc9735500158

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New Journal of Physics•Jun 28, 2010•7 citations
Differential elastic cross sections for the scattering of the metastable 63P2 state of mercury from sodium, potassium and rubidium have been measured at thermal energies. Interference structure is resolved in all cases, suggesting that the atoms interact by a single effective potential in the attractive region probed at these energies. No attenuation of the interference structure is observed so that quenching of the metastable state is not an important process along trajectories sampling only the attractive part of the potential.
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.
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.
Quantal calculations for scattering of ground-state antihydrogen by metastable (n=2S)(n=2S)(n=2S) helium atoms have been performed using the nonadiabatic, atomic orbital expansion technique at thermal energies. The zero-energy elastic cross sections of the present systems are much greater than the corresponding value for the ground-state helium target. The low-energy elastic cross section for the singlet metastable helium [He(21S)][\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{1}S)][He(21S)] target is higher than the corresponding value when the target is in the metastable triplet state [He(23S)][\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{3}S)][He(23S)].
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
Two techniques have been used to study low energy electron scattering processes in the alkali metals. A transmission experiment was used to search for resonance structure in the total scattering cross section of sodium, potassium, rubidium, and cesium. A shape resonance was observed in sodium at 0.08 (+OR-) 0.03 eV. In rubidium, evidence was found for the existence of a shape resonance below 0.05 eV. No shape resonance structure was observed in potassium or cesium for energies above 0.1 eV. For energies near the first excited state to near the lowest ionization potential, a progression of Feshbach resonances was observed in each atom. The graphical analysis technique of Spence was used to group most of these resonances into seven families. An analysis is given of the mechanisms affecting the rejection of scattered electrons in the transmission spectrometer. A quantitative description of two modes of operation of the spectrometer is presented. The second portion of this dissertation concerns the measurement of the near-threshold excitation and ionization cross sections in sodium, potassium, and rubidium. A trapped-electron experiment was used to make a relative measurement of these cross sections. The shapes of the excitation cross sections are, in general, somewhat different from other experimental work but are in good agreement with the close-coupling calculations of Moores and Norcross. The shapes of the ionization cross sections are in reasonable agreement with previous work. The ratio of the excitation cross section at 2.5 eV in sodium, 2.0 eV in potassium, and 1.8 eV in rubidium to the respective ionization cross section at the first peak was measured and found to be 3.9 (+OR-) 13% in Na, 5.8 (+OR-) 10% in K, and 6.4 (+OR-) 13% in Rb. These ratios are generally larger than those derived from independent values of the individual cross sections. An analysis of several systematic effects associated with the measurement of the cross section ratio was made. Results from this analysis were used to correct the measured excitation cross section for effects of the elastically scattered electrons.