Ion Collision and Spectroscopy Research Using Cold,
Confined Ions in Retrap
D. A. Church*, D. Schneider#, J. P. Holder#, J. McDonald#, and Yanbang Wang*
*
Physics Department, Texas A&M University, College Station, TX 77843-4242
#
Lawrence Livermore National Laboratory, Livermore, CA 94550
Abstract. The cryogenic Penning ion trap Retrap has been moved to Lawrence Berkeley National Laboratory,
where it is installed in collaboration with the weak interaction group. Present development of the facility for atomic
and plasma physics research will be described, directed toward extensions of low energy multiply charged ion
collision and spectroscopy studies previously completed, and to laser measurement of the fine structure of cold,
confined, multiply charged ions. Possibilities for extended development to highest-charged high-Z ions in the near
future will also be discussed. This facility will also be used in collaborations for nuclear research.
ground term fine structure of multiply-charged ions,
which are separately introduced but simultaneously
confined with the Be+ ions [5].
INTRODUCTION
Retrap [1] has been moved from LLNL to LBNL,
where it is installed on top of the shielding of the 88
inch cyclotron. A laser laboratory with an ion laser
which pumped two dye lasers having frequency
doubling capability has also been moved to LBNL,
and is presently being installed near Retrap, together
with appropriate safety shielding and laser beam
transport.
This apparatus has capabilities for the study of cold,
strongly coupled plasmas [6], in particular mixed
plasmas with different charge-to-mass ratio ions, and
for the study of low energy ion-atom collisions, and
precision spectroscopy. To augment these capabilities,
and to exceed original research specifications, a four
year Major Research Instrumentation Development
proposal was prepared and submitted to the National
Science Foundation by our collaboration.
This
proposal called for the development, test, and
installation at LBL of an EBIT source capable of
delivering pulses of ions of any charge up to fullystripped ions of all Z, as well as permitting in situ ion
spectroscopy in the visible, uv, and x-ray spectral
regions. This source was also expected to operate
without liquid helium, and to employ higher electron
currents and deliver more intense beams, then did
earlier sources of this type.
The original ion sources associated with Retrap
were a Metal Vapor Vacuum Arc (MeVVA) source for
ions with low charge, and an Electron Beam Ion Trap
(EBIT) used as a source of ions to neon-like electron
configurations for all Z [2]. The present ion sources
are MeVVA sources to be used primarily for Be+ ions,
and an Electron Cyclotron Resonance Ion Source
(ECRIS) called IRIS, for ions with charge q ≤ 15+.
The Be+ ions have a resonance transition near 313 nm,
which can be used for laser cooling [3] by a
frequency-doubled tunable beam set slightly to the
long wavelength side of the resonance.
Further, a second Penning trap in a warm bore
cryogenic magnet was to be installed. The magnet
coils of this Warm Retrap, or WRetrap, were to have a
twenty cm diameter bore, permitting the installation of
precision field shims, as well as provide large vacuum
pumping speeds at the trap site. The vacuum system
was expected to be bakeable, to reach local pressures
The second laser beam can be used as a probe to
monitor cooling effectiveness, or to measure ion
temperature through linewidth measurements on a
resonance transition [4]. Alternatively, this second
laser beam can be used to induce transitions in other
confined ions, in particular transitions within the
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below 10-11 Torr. These supplemental capabilities
would permit additional research in collisions and
spectroscopy on cold, confined ions, not feasible with
the original Retrap system.
would be implied, while a higher ratio would indicate
confirmation of the theory, and simultaneous capture.
With the proposed experimental system, this collision
could be studied with higher charge states, and with
more detail at intermediate charge states. A twoelectron He target could be used in addition to H2 in
Wretrap, to test for possible target structure effects,
and many-electron targets could be employed as well,
since the trap would operate at room temperature,
avoiding target gas density problems associated with
cryopumping.
This proposal received favorable reviews, but
insufficient to permit funding of the original
submission. In the present paper, some examples of
research are discussed, which were originally initiated
at Retrap, and which could be brought to completion
using the advanced system capabilities that were
proposed.
COLLISION RESEARCH
SPECTROSCOPY
Retrap was used to study low-energy electron
transfer collisions of confined Xe+n charge states (n =
35, 43 – 46) and Th+k charge states (k = 73 – 76, 79,
80). colliding with H2 molecules [7]. The choice of H2
as a target was based on several factors, including the
two target electrons, the vapor pressure of H2 over
partial surface coverages at liquid helium
temperatures, and the existence of theories for total
and for (true) two-electron capture. When two
electrons are captured to a highly charged ion, one
may autoionize before final stabilization of the product
ion, resulting in apparent one-electron capture if the
ion charge change is measured, or in a free electron if
charges are measured. This has been called capture to
the continuum. This effect strongly influences the
probability of true double capture, in which two
electrons are retained by the product ion. The theories
relevant to these collisions are an absorbing sphere
model calculation of the total electron capture cross
section [8], and a theory of the relative probability of
true double capture by highly charged ions, which
predicts a dependence on projectile (ion), rather than
target, structure [9].
A cloud of cold ions in the Penning trap is
equivalent to a one-component charged plasma. As
plasma temperatures are lowered, the Coulomb
coupling parameter Γ, which characterizes plasma
behavior, increases until it exceeds unity, when the
plasma becomes strongly coupled [6]. At still lower
temperatures a spatially ordered state can form. Mixed
plasmas contain ions with different charge-to-mass
ratios. Mixed strongly coupled plasmas are interesting
objects of study, due to astrophysical applications [10].
Strongly coupled plasmas have been produced in
Retrap by laser-cooling confined Be+ ions, using a
laser beam from a frequency-doubled dye laser tuned
to a wavelength slightly longer than that of a cycling
transition between the 2s and 2p states. Once the Be+
ions were cold, highly charged ions were captured into
the same trap without losing appreciable numbers of
the Be+ ions. The high- and low-charged ions were
coupled by Coulomb collisions, and were rapidly
cooled (in seconds) by the laser light [11]
In a mixed plasma, centrifugal separation of ions
with different charge-to-mass ratios occurs in a
Penning trap. A luminous annulus of laser-excited and
cooled Be+ ions surrounds highly-charged ions, which
condense in the center to an ordered state. This has
been verified using imaging, cyclotron resonance
excitation, ion temperature measurements from
resonance linewidths of laser excitation of Be+, and
molecular dynamics simulations [12].
In the Retrap measurements [7], rate coefficients
for electron capture by identified charge states were
measured for the confined ions, and converted to cross
sections using the measured mean collision speed in
the trap. It was found that the total capture cross
section agreed fairly well with the absorbing sphere
prediction, but fell slightly above the theory at high Z,
where the experimental uncertainties were largest.
True double capture probability was as high as 24 ± 7
% for the highest thorium charge states. However,
these charge states left the 2p shell intact.
The small, cold assembly of highly charged ions at
the center of the annulus presents a dense, localized
target with low Doppler widths for laser beam
spectroscopy. Many ions with intermediate charge
states have fine structure transitions within the ground
term, having wavelengths in the tunable cw laser range
and lifetimes the order of milliseconds [13]. Iron ions
important in astrophysics are particularly attractive for
Measurements with ion charge states which
opened the 2p shell were expected to increase the true
double capture ratio [9], provided that this ratio was
not already saturated. If the present ratio were near the
maximum, then sequential capture of the electrons
165
a first measurement, with some level lifetimes
measured in traps [14], and with certain wavelengths
already measured to relatively high precision by
conventional spectroscopy, which should aid the laser
spectroscopic search.
REFERENCES
1. D. Schneider, D. A. Church, G. Weinberg,
J. Steiger, B. Beck, J. McDonald, E. Magee, and
D. Knapp, Rev. Sci. Instrum. 65, 3472 (1994).
Other fine structure measurements of high interest
are found in Be-like and Mg-like ions excited levels of
intermediate charge. At much higher Z, the hyperfine
structure of hydrogen-like ions falls in the cw tunable
laser range. Laser measurements on ions in storage
rings (see e. g. [15]) are limited by Doppler effects,
which can be minimized in Retrap using ions from the
proposed advanced EBIT source. An example of an
interesting potential measurement related to atomic
structure is the g-factor of the bound electron in a
high-Z H-like ion [16], a long-term goal of our
collaboration. This g-factor has been measured by
another group on confined ions with Z < 8 [17], but
interesting QED terms which scale as powers of
α(Zα)2 can best be measured in laser-induced
hyperfine transitions, perhaps coupled with microwave
spectroscopy, at high Z on ions stored in a Penning
trap [18].
2. D. Schneider, D. DeWitt, M. W. Clark, R. Schuch,
C. L. Cocke, R. Schmieder, K. J. Reed, M. H. Chen,
R. E. Marrs, M. Levine, and R. Fortner, Phys. Rev.
A42, 3889 (1990).
SUMMARY
7. G. Weinberg, B. R. Beck, J. Steiger, D. A. Church,
J. McDonald, and D. Schneider, Phys. Rev. A57, 4452
(1998).
3. W. Itano and D. J. Wineland, Phys. Rev. A25, 35
(1982).
4. J. J. Bollinger, J. S. Wells, D. J. Wineland, and
W. M. Itano, Phys. Rev. A31, 2711 (1985).
5. J. P. Holder, D. A. Church, L. Gruber,
H. E. DeWitt, B. R. Beck, and D. Schneider, (AIP
Conf. Proc. 576, 2001) p. 118.
6. J. J. Bollinger, D. J. Heinzen, F. L. Moore,
W. M. Itano, D. J. Wineland, and D. H. E. Dubin,
Phys. Rev. A48, 525 (1993).
Particular collision, spectroscopy, and plasma
measurements are suggested, which could be
accomplished by a proposed enhancement of the
Retrap system currently under installation at Lawrence
Berkeley Laboratory.
Not discussed are other
potential measurements in ion spectroscopy in the
visible, uv, and x-ray regions on ions within the
proposed EBIT, lifetimes of ion levels, and collision
measurements using highest-charged ions in pulsed
beams.
Potential nuclear physics measurements
include measurements on rare highly charged ions.
Other proposals of measurements are encouraged.
8. R. E. Olson and A. Salop, Phys. Rev. A14, 579
(1976).
9. H. Cederquist, H. Andersson, E. Beebe,
C. Biedermann, L. Brostroem, A.Engstroem, H. Gao,
R. Hutton, J. C. Levin, L. Liljeby, M. Pajek,
T.Quintero, N. Selberg, and R. Sigray, Phys. Rev.
A46, 2592 (1992).
10. G. Chabrier, N. W. Ashcroft, and H. D. DeWitt,
Nature 360, 48 (1992).
11. J. Steiger, B. R. Beck, L. Gruber, D. A. Church,
J. P. Holder, and D. Schneider, Proc. Int. Conf.
Trapped Ions and Fund. Physics-1998, (AIP Conf.
Proc. 457, 1999) 284.
ACKNOWLEDGMENTS
D. A. C. is supported by the National Science
Foundation under grant PHY 9876899. Work by D. S.
J. P. H. and J. McD. was performed under the auspices
of the U. S. Department of Energy by University of
California Lawrence Livermore Laboratory under
contract no. W-7405-ENG-48. The authors particularly
wish to thank A. Kraemer for substantial work in
transfering the apparatus, S. Freedman, J. Burke, P.
Vetter, and others of the weak interactions group for
collaborating on the installation of Retrap at LBNL,
and C. Lyneis and the Nuclear Engineering Division of
LBL for support of the installation.
12. L. Gruber, J. P. Holder, J. Steiger, B. R. Beck,
H. DeWitt, J. Glassman, J. W. McDonald,
D. A. Church, and D. Schneider, Phys. Rev. Lett. 86,
636 (2001).
13. V. Kaufman and J. Sugar, J. Phys. Chem. Ref.
Data 15, 321 (1986).
14. D. P. Moehs, M. I. Bhatti, and D. A. Church, Phys.
Rev. A63, 032515 (2001).
166
15.
I. Klaft, S. Borneis, T. Engel, B. Fricke,
R. Greiser, G. Huber, T. Kuehl, D. Marx, R. Neumann,
S. Schroeder, P. Seelig, and L. Voelker, Phys. Rev.
Lett. 73, 2425 (1994).
17.
H. Haffner, T. Beier, K. Hermanspahn,
H.-J. Kluge, W. Quint, S. Stahl, J. Verdu, and
G. Werth, Phys. Rev. Lett. 85, 5308 (2000).
18. D. A. Church, J. Steiger, B. R. Beck, L. Gruber, J.
P. Holder, J. McDonald, and D .Schneider, (AIP Conf.
Proc. 457, 1999) 235.
16. S. A. Blundell, K. T. Cheng, and J. Sapirstein,
Phys. Rev. A55, 1857 (1997).
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