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The Journal of Chemical Physics

Velocity Dependence of the Total Cross Section for the Scattering of Metastable He(3S1) by Helium, Argon, and Krypton

Erhard W. Rothe, R. H. Neynaber, S. M. Trujillo

Published May 1, 1965• 134 citations

Abstract

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+).

DOI: 10.1063/1.1696416

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Physical Review A•Jul 1, 1974•12 citations

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+).

Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (2 1 S 0 ) and triplet (2 3 S 1 2) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*10 5 cm s -1 . Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.

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)

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)

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

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{1}S)]

target is higher than the corresponding value when the target is in the metastable triplet state

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{3}S)]

.

Previous semiempirical determinations by Fitzsimmons et al. of the asymptotic form of the potentials

{V}_{g}

and

{V}_{u}

for the helium molecular states $^{3}\ensuremath{\Sigma}_{g,u}^{+}$ are refined to include new experimental data. The resulting potentials yield cross sections for elastic scattering, diffusion, and metastability exchange in $\mathrm{He}(2^{3}S_{1})\ensuremath{-}\mathrm{H}\mathrm{e}(1^{1}S_{0})$ collisions in agreement with various experiments from 15 to 500 \ifmmode^\circ\else\textdegree\fi{}K.