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Physical Review Letters

n Dependence of l -Changing Collisions between He+ Ions and Na

Keith B. MacAdam, D. A. Crosby, R G Rolfes

Published Apr 14, 1980• 30 citations
Publisher

Abstract

$l$-changing collisions were observed in a crossed ${\mathrm{He}}^{+}$-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 ${n}^{5}$ and have magnitudes of order ${10}^{8}$ ${\mathrm{\AA{}}}^{2}$, a few hundred times the geometric cross section of the Rydberg atoms.

DOI
10.1103/physrevlett.44.980

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lll-changing collisions were observed in a crossed He+{\mathrm{He}}^{+}He+-ion/Na-Rydberg-atom beam experiment. Transitions nd\ensuremath→n(l> 3)\mathrm{nd}\ensuremath{\rightarrow}n(l>~3)nd\ensuremath→n(l> 3) induced in Na by ion impact at 450 and 600 eV were studied for n=20\ensuremath−34n=20\ensuremath{-}34n=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.
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.
Ion formation in sodium vapor (\ensuremath{\sim}1011{10}^{11}1011 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) containing Rydberg atoms (18\ensuremath{\le}n\ensuremath{\le}35) was investigated. Both atomic ions and diatomic molecular ions resulting from collisions between highly excited sodium, Na∗∗{\mathrm{Na}}^{\mathrm{*}\mathrm{*}}Na∗∗, and Na(3s) were observed. However, the predominant production mechanism was found to be photoionization of Na∗∗{\mathrm{Na}}^{\mathrm{*}\mathrm{*}}Na∗∗ by blackbody radiation, the rate of which was measured at 500 K and found to be \ensuremath{\simeq}5.7\ifmmode\times\else\texttimes\fi{}103{10}^{3}103 ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=18, in good agreement with theory. The Na+{\mathrm{Na}}^{+}Na+ signal from blackbody photoionization is observed to be independent of n, which we interpret as implying rapid and nearly complete self-l-mixing, mixing by collisions with Na(3s) atoms, for both s and d states. It is probable, however, that the s states are mixed to the adjacent, n-1, manifold of l states. It was also found that the heavy-body collision cross sections for Na+{\mathrm{Na}}^{+}Na+ and Na2{\mathrm{Na}}_{2}Na2​+{\mathrm{}}^{+}+ formation, while roughly three orders of magnitude lower than our estimate of the self-l-mixing cross section, are appreciable, of order 100 A˚2{\r{}}^{2}˚2.
We have observed electron impact ionization of highly excited sodium Rydberg atoms in nsnsns and ndndnd states, n=35−−51n=35--51n=35−−51, below E=2  eVE=2\text{ }\text{ }\mathrm{e}\mathrm{V}E=2  eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in nnn. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.
We have made comprehensive measurements of the noble-gas-induced (He, Ne, Ar, Kr, and Xe) broadening cross sections of the Na $3S\ensuremath{-}nS$ ($3S\ensuremath{-}nD$) transitions, from n=5to38n=5 \mathrm{to} 38n=5to38 (n=4to40n=4 \mathrm{to} 40n=4to40). These measurements were made using the trilevel-echo technique and are an extension and refinement of our earlier work. We find that the cross sections increase as the geometric cross section at low values of nnn, and then, at a value of nnn which depends on the particular noble gas, begin to decrease and eventually reach an asymptotic value at high nnn. The broadening cross sections for the nearly degenerate DDD states are significantly larger than those for the SSS states and clearly show the effect of lll-changing collisions. We also compare our measured SSS-state broadening cross sections with the cross sections calculated using the theory of Omont and find good qualitative agreement.
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 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.