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Physical Review A

Collisional depopulation of He(n1P)(4≤n≤13) in thermal collisions with He(1S1)

W. R. Pendleton, Mats Larsson, B. Mannfors

Published Dec 1, 1983• 9 citations
Publisher

Abstract

Total collisional depopulation rates for $\mathrm{He}(n^{1}P)(4\ensuremath{\le}n\ensuremath{\le}13)$ in thermal collisions with $\mathrm{He}(1^{1}S)$ have been measured using the transient-decay method. Related loss cross sections increase in proportion to ${n}^{4}$ in the limited range $4\ensuremath{\le}n\ensuremath{\le}6$, reach a maximum of 2600\ifmmode\pm\else\textpm\fi{}600 ${\mathrm{\AA{}}}^{2}$ at $n=10$, and decrease approximately in proportion to ${n}^{\ensuremath{-}2.5}$ for $11\ensuremath{\le}n\ensuremath{\le}13$. The measurements were found to be inconsistent with a strong "selection rule," $\ensuremath{\Delta}L=2$, for the $\mathrm{He}({n}^{1}P)\ensuremath{-}\mathrm{H}\mathrm{e}$ collisions. A model in which $\ensuremath{\Delta}L$ for the collision is largely unrestricted provides a satisfactory interpretation of the observations, in agreement with recent $l$-mixing studies of atomic Rydberg levels. The experimental cross sections compare favorably with values calculated using an approximate scaling formula for collisional $l$ mixing and, for $n>10$, with predictions based on a simple perturbation treatment in the weak-collision approximation.

DOI
10.1103/physreva.28.3223

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Total collisional depopulation rates for He(n1P)(4\ensuremath≤n\ensuremath≤13)\mathrm{He}(n^{1}P)(4\ensuremath{\le}n\ensuremath{\le}13)He(n1P)(4\ensuremath≤n\ensuremath≤13) in thermal collisions with He(11S)\mathrm{He}(1^{1}S)He(11S) have been measured using the transient-decay method. Related loss cross sections increase in proportion to n4{n}^{4}n4 in the limited range 4\ensuremath≤n\ensuremath≤64\ensuremath{\le}n\ensuremath{\le}64\ensuremath≤n\ensuremath≤6, reach a maximum of 2600\ifmmode\pm\else\textpm\fi{}600 A˚2{\mathrm{\AA{}}}^{2}A˚2 at n=10n=10n=10, and decrease approximately in proportion to n\ensuremath−2.5{n}^{\ensuremath{-}2.5}n\ensuremath−2.5 for 11\ensuremath≤n\ensuremath≤1311\ensuremath{\le}n\ensuremath{\le}1311\ensuremath≤n\ensuremath≤13. The measurements were found to be inconsistent with a strong "selection rule," \ensuremathΔL=2\ensuremath{\Delta}L=2\ensuremathΔL=2, for the He(n1P)\ensuremath−He\mathrm{He}({n}^{1}P)\ensuremath{-}\mathrm{H}\mathrm{e}He(n1P)\ensuremath−He collisions. A model in which \ensuremathΔL\ensuremath{\Delta}L\ensuremathΔL for the collision is largely unrestricted provides a satisfactory interpretation of the observations, in agreement with recent lll-mixing studies of atomic Rydberg levels. The experimental cross sections compare favorably with values calculated using an approximate scaling formula for collisional lll mixing and, for n>10n>10n>10, with predictions based on a simple perturbation treatment in the weak-collision approximation.
We have measured total cross sections for the collisional depopulation of high-lying nP\mathrm{nP}nP states ($12\ensuremath{\le}n\ensuremath{\le}22$) of rubidium induced by thermal collisions with ground-state rubidium or rare-gas atoms. It is observed that for $n\ensuremath{\ge}14$ the rubidium-rare-gas cross sections do not vary with the principal quantum number, contrary to previous hypotheses. Our results are discussed using the simple argument of quasifree behavior of the valence electron involving either an elastic-scattering or a binary-encounter approach. However, the disagreement observed between the experimental and the computed cross sections suggests that a three-body approach is generally needed for a good understanding of collisional processes involving highly excited states.
The total depopulation cross sections by N2{\mathrm{N}}_{2}N2​ of high rubidium Rydberg states, $\mathrm{Rb}(\mathrm{ns}) (n=25\ensuremath{-}46)$ and $\mathrm{Rb}(\mathrm{nd}) (n=23\ensuremath{-}44)$, have been measured using time-resolved field ionization technique and compared to previous results obtained on the quenching of Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) by CO. It is shown that the Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) depopulation by N2{\mathrm{N}}_{2}N2​ is much less efficient than the Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) depopulation by CO and leads mainly to the quasielastic process of angular momentum mixing via the short-range interaction between the quasifree Rydberg-state electron and the molecule. It is clearly demonstrated that the presence of a permanent dipole in CO changes the collisional processes drastically. The agreement between experimental and theoretical predictions based on the impulse approximation is quite satisfactory.
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In thermal-energy collisions between sodium atoms in ns\mathrm{ns}ns or np\mathrm{np}np states (32< n< 5232<~n<~5232< n< 52) and xenon atoms, depopulation of the Rydberg states is observed to occur predominantly through state-changing reactions of the type $\mathrm{Na}(ns or np)+\mathrm{Xe}\ensuremath{\rightarrow}\mathrm{Na}(n\ensuremath{-}1, l>~2)+\mathrm{Xe}$. For the present range of nnn, the measured cross sections increase with nnn and lie between 1500 and 2600 A˚2{\mathrm{\AA{}}}^{2}A˚2 for ns\mathrm{ns}ns states and between 2000 and 4200 A˚2{\mathrm{\AA{}}}^{2}A˚2 for np\mathrm{np}np states. Implications of the data are discussed.
An experimental study of the depopulation of Na(30s) Rydberg atoms under He + ion bombardment has been undertaken to identify the range of collision velocities at which strongly coupled behaviour of the 30s and nearby levels at low velocity changes to predominantly dipole-allowed coupled at high velocity. Ion energies ranged from 700 to 3900 eV corresponding to collision velocities of 2.4 to 5.6 times the mean orbital speed of the excited 30s electron. The ratio of the cross section for the dipole-allowed transitions (30s to 30p and 30s to 29p) to the total depopulation cross section was determined for each collision velocity. (Other dipole-allowed transitions were observed to be negligible.) This ratio was found to increase from only 20% at a reduced velocity of 2.4 to 65% at a reduced velocity of 5.6. The ratio of the two dipole cross sections, sigma (30s-30p)/ sigma (30s-29p), was also determined and compared to calculations based on the dipole approximation with first-order perturbation theory.