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Physical Review A

l-changing depopulation of NasandpRydberg states by ion impact

R G Rolfes, D. B. Smith, Keith B. MacAdam

Published Apr 1, 1988• 15 citations
Publisher

Abstract

${\mathrm{Ar}}^{+}$ and ${\mathrm{Na}}^{+}$ ion beams bombarding Na ns and np Rydberg-state targets at impact velocities near the Bohr-orbital velocity of the target atoms (i.e., v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\approxeq}1) induce transitions to (n-1)l states (l\ensuremath{\ge}2) with larger-than-geometric cross sections. Depopulation of ns states proceeds directly into the full n-1, l\ensuremath{\ge}2 manifold rather than populating the np or (n-1)p states as the first of a sequence of dipole-allowed steps. Depopulation of np states leads to a distribution of final states that is dominated by the nearer or high-l part of the n-1 manifold. n dependences of the cross section for ns depopulation are given at several energies for n=32--41 and at a single energy for np, n=26--32. The absolute cross section for Na(36s) depopulation falls gradually but steadily for reduced velocities v\ifmmode \tilde{}\else \~{}\fi{} increasing from 0.3 to 1.35. Bombardment of Na 39s and 39p states in applied fields 0--18.5 V/cm reveals that the final-state distributions become narrower as manifold states are Stark shifted close to the initial states. No theoretical results are available for direct comparison with these experiments, but the method of coupled channels seems to be the most suitable candidate.

DOI
10.1103/physreva.37.2378

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Physical Review Letters•Apr 14, 1980•30 citations
Ar+{\mathrm{Ar}}^{+}Ar+ and Na+{\mathrm{Na}}^{+}Na+ ion beams bombarding Na ns and np Rydberg-state targets at impact velocities near the Bohr-orbital velocity of the target atoms (i.e., v\ifmmode \tilde{}\else ~{}\fi{}\ensuremath{\approxeq}1) induce transitions to (n-1)l states (l\ensuremath{\ge}2) with larger-than-geometric cross sections. Depopulation of ns states proceeds directly into the full n-1, l\ensuremath{\ge}2 manifold rather than populating the np or (n-1)p states as the first of a sequence of dipole-allowed steps. Depopulation of np states leads to a distribution of final states that is dominated by the nearer or high-l part of the n-1 manifold. n dependences of the cross section for ns depopulation are given at several energies for n=32--41 and at a single energy for np, n=26--32. The absolute cross section for Na(36s) depopulation falls gradually but steadily for reduced velocities v\ifmmode \tilde{}\else ~{}\fi{} increasing from 0.3 to 1.35. Bombardment of Na 39s and 39p states in applied fields 0--18.5 V/cm reveals that the final-state distributions become narrower as manifold states are Stark shifted close to the initial states. No theoretical results are available for direct comparison with these experiments, but the method of coupled channels seems to be the most suitable candidate.
An experimental study of the depopulation and final-state distribution of Na 36s and 30p Rydberg states under Na + bombardment in the presence of electric fields up to 50 V cm -1 has been carried out. Incident energies E from 171 to 967 eV span the range from 0.6 to. 1.4 times the mean orbital speed of the excited electron in the initial state. Absolute depopulation cross sections of order 10 -8 cm 2 , believed accurate within 25%, are reported as functions of field and energy. sigma 36s is proportional to E -1 , but sigma 30p is nearly independent of energy. The distribution of final states was measured by selective field ionization. At weak fields for which the initial state is well separated from the nearest Stark manifold, final states are broadly distributed across the nearest manifolds. But for stronger fields that shift the initial state close to a Stark manifold the final states are strongly concentrated among the energetically closest manifold sublevels.
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.
The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow Na+{\mathrm{Na}}^{+}Na+ ion impact at reduced velocity v\ifmmode \tilde{}\else \~{}\fi{}=0.1--0.9 has been measured. Na+{\mathrm{Na}}^{+}Na+ beam energies 7.3--684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities v\ifmmode \tilde{}\else \~{}\fi{} and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion--Rydberg-atom collisions at v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.7. At very low energy (v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.
New experimental results have been obtained in collisions of singly charged ions with Na Rydberg atoms. (a) The distribution of final n states populated by electron capture shows non-monotonic variations of width with incident energy over the range 60-2100 eV. (b) The total capture cross section for alkali ions on Rydberg targets shows peaks and dips over the same energy range, correlated with particular reduced velocities v and the polarization of the exciting laser. (c) {ell}-Changing collision of Na{sup +} on Na(nd) targets, n=26 and 28, at low v preferentially transfer multiple units {h_bar} of orbital angular momentum under single-collision conditions, despite very large impact parameters. (d) Inelastic impacts of Na{sup f+} on Na 36s and 30p states, perturbed by electric fields, populate a strongly field-dependent mixture of nearby Stark levels. A synopsis of recent progress will be given.
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.
lll-changing collisions were observed in a crossed He+{\mathrm{He}}^{+}He+-ion/Na-Rydberg-atom beam experiment. Transitions $\mathrm{nd}\ensuremath{\rightarrow}n(l>~3)$ induced in Na by ion impact at 450 and 600 eV were studied for $n=20\ensuremath{-}34$. Cross sections vary approximately as n5{n}^{5}n5 and have magnitudes of order 108{10}^{8}108 A˚2{\mathrm{\AA{}}}^{2}A˚2, a few hundred times the geometric cross section of the Rydberg atoms.