Electron Correlation Leading to Double-K-Shell Vacancy Production in Li-Like
Ions Colliding with Helium
A.S. Alnaser 1,2, A.L. Landers1, D.J. Pole 1,3, S. Hossain1, S.M. Ferguson1, E.P. Benis2,
and J.A. Tanis1.
1) Department of Physics, Western Michigan University, Kalamazoo, MI 49008
2) James R. Macdonald Laboratory, Department of Physics, Kansas State
University, Manhattan, KS 66506-2604
3) Department of Physics, University of Virginia, Charlottesville, VA 22901
Abstract. Single and double K-shell vacancies in Be+, B2+, C3+, and O5+ Li-like ions
colliding with neutral helium at intermediate-to-high velocities have been investigated.
Specific excited states were observed using high-resolution Auger-electron spectroscopy.
The velocity dependence of the measured cross sections for the formation of doubly
vacant K-shell states was used to determine the mechanisms responsible for producing
these so-called “hollow states”. Electron correlation effects were inferred from the
spectral features and the velocity dependence of the observed hollow states. The
variation of the correlation strength was investigated as a function of the atomic number
of the Li-like ion.
produce a K-shell vacancy.
By
subsequent rearrangement of the residual
ion, in which electron correlation plays a
Introduction
role, the second electron may be excited
or ejected. This process is referred to as
The states of hollow lithium and
TS1 (two-step with one projectile
lithiumlike ions have recently been the
interaction) [4,5]. Additionally, at lower
subject of intense experimental and
projectile velocities, the projectile may
theoretical interest [1-3]. These hollow
interact more strongly with each of the
states consist of doubly or triply excited
target electrons independently to
configurations where the 1s orbital is
produce two-K-shell vacancies. This
vacant.
The Coulombic electronprocess is referred as TS2 (two-step with
electron interaction plays an important
two projectile interactions)[4,5]. For
role in the two-electron processes of
intermediate-to-high collision velocities,
double ionization, double excitation, and
where the projectile velocity is greater
ionization-excitation which lead to
than the velocity of the active bound
hollow state formation during the
electron, the Born approximation is
collisions of photons or charged particles
expected to be valid. In this case, the
with few-electron targets.
interaction with incident ions is expected
For
double-K-shell
vacancy
to resemble the interaction with incident
production
in
intermediate-to-high
photons where the corresponding
velocity atomic collisions where the
momentum transfer is small.
collision time is small, the projectile
In the present work, single and
interacts mainly with only one of the
double-K-shell vacancies produced in
target electrons transferring it to an
Li-like Be+, B2+, C3+ and O5+ ions
excited state or to the continuum to
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
77
separation of the lines of interest. The
spectrometer was scanned over the
energy range corresponding to Augerelectron emission from singly- and
doubly-vacant-K-shell states. Absolute
cross sections were determined by
normalizing to data obtained from
auxiliary measurements of binary
encounter electron emission [7].
interacting with neutral helium are
investigated at intermediate-to-high
velocity collisions, where the electronelectron (e-e) interaction is expected to
play an important role. Using different
Li-like ions allows an understanding of
the variation of electron correlation
when the Z of the parent ion changes.
Different collision energies are used in
order to study the dependence of
electron correlation on the velocity of
the collision. Singly- and doubly-Kshell excited ions are investigated by
detecting Auger electrons emitted at 0o
to the beam direction.
Results and Discussion
Typical high-resolution spectra for
Be+, B2+, C3+ and O5+ ions are shown in
Fig. 1, for the collision velocities
indicated. Similar spectra were recorded
for each of the Li-like ions with collision
velocities ranging from 0.5-2 MeV/u.
The data shown in Fig. 1 are mainly for
electrons emitted in the energy ranges
corresponding to the doubly-K-shell
excited configurations, as indicted by the
labeled peaks. Some of these peaks
represent singly-K-shell excited states
with configurations of the form 1s2snl,
with n ≥ 3 (these latter states are not
labeled in the spectra).
Additional
spectra for electrons emitted from the
decay of single-K-shell excited states
were also measured but are not shown
here. In Fig. 1, the hollow states in Be+
and B2+ ions consist of the
configurations 2lnl' and 2l2l'nl'' (the
latter only in B2+), with n ≥ 2, while for
C3+ and O5+ ions the hollow states are
composed of 2l2l' and 2l2l'2l'' (the
latter only in C3+) configurations. For
C3+ and O5+ ions no 2l3l' hollow states
were observed at these velocities. It is
noted that 2lnl' states correspond to Kshell ionization plus K-shell excitation
of the Li-like ion, while the 2l2l'2l''
states correspond to double-K-shell
excitation.
Experiment
The measurements were carried out at
Western Michigan University (some
measurements were also conducted at
Kansas State University) using the
tandem Van de Graaff accelerator.
Beams of Li-like Be+, B2+, C3+ and O5+
ions were incident on neutral helium
targets at different velocities. Singlyand doubly-K-shell excited ions were
investigated using high-resolution Auger
electron spectroscopy. Auger electrons
emitted at 00 to the beam direction were
detected using a tandem parallel-plate
electron spectrometer [6,7]. Target gas
pressures were adjusted to ensure that
single-collision conditions prevailed in
the interaction region.
To identify individual excited states
formed in the collision, the Augerelectron emission spectra were measured
in high resolution. To accomplish this,
electrons emerging from the first stage
of the tandem spectrometer were
decelerated to 50 eV prior to entering the
second stage.
For an inherent
spectrometer resolution of about 3%, this
resulted in spectra with a resolution of
about 1.5 eV, thereby permitting
78
2s3p 3P-
2s3s 3S +2s3p 1P
0.020
1.1 MeV/u
2s2p 1P
0.040
2s2p 3P
0.060
+
Be
2s2 1S
0.080
sections for 1s2s2p 2P state vary with the
predicted (1/v2) ln(v) dependence of the
Born approximation for these allowed
4
120
125
130
135
140
145
2
1s2s2p P
Born Approx.
150
1 MeV/u
2+
B
3+
0.00
2s2p2 2P
2s2p 1P
265
270
275
280
5+
O
0.04
285
290
230
2+
B
1.2
0.8
0.4
0.0
3+
C
0.9
1.5 MeV/u
0.6
2s2p 1P
2s2 1S 3
2s2p P
0.01
0
1.2
295
0.03
0.02
1
2
cm /sr)
2s3p 3P
225
1 MeV/u
C
0.12
0.04
220
-19
210
o
0.16
200
2s2p2 2P
2s 2p2 2S
190
dσ/dΩ(θ=0 )(x10
180
2s2p2 2S
0.00
2s2p 1P
2s2 1S 3
2s2p P
0.10
+
Be
2
X10
0.20
0.08
2s3s 3S+2s3p 1P
3
0.30
2s2 1S
2s2p 3P
d2σ/dΩdε (x 10-19 cm2/eV.sr)
0.000
0.3
0.0
0.00
460
480
500
520
3
4
5
6
7
8
9
10
Collision Velocity (a.u.)
540
Projectile Frame Electron Energy (eV)
Fig. 1. High-resolution spectra for Be+,
B2+, C3+ and O5+ Li-Like ions. The
energy ranges shown correspond mainly
to electrons emitted from doubly vacant
K-shell states.
Fig. 2. Single differential cross sections
in the projectile frame of reference for
the 1s2s2p 2P single-K-shell excited
states. Closed symbols represent the
measured cross sections and the dashed
lines represent calculated Born cross
sections.
To interpret the present data, an
important consideration is the previous
finding that inner-shell excitation of Lilike ions by neutral helium in the
collision velocity range studied here
leaves the outer shell, i.e., the 2s
electron, undisturbed [8].
This
phenomenon is referred to as “needle
excitation”, for which the excitation of
the K-shell by the helium atom is
expected to be much the same as if
excited by a structureless particle of
charge Z=2. Fig. 2 shows the measured
and calculated K-shell excitation cross
sections for the 1s2s2p 2P (1s→2p
transition) states in Be+, B2+ and C3+
ions, respectively, as functions of the
collision velocity v. The measured cross
transitions [9]. The good agreement
between the measured and calculated
Born approximation [10] cross sections
indicates that perturbation theory is valid
over the velocity range investigated, and
shows the validity of the needle
approximation in interpreting the singleK-shell excitation process for the
systems studied here.
To gain insight into the role of the e-e
interaction in producing hollow Li-like
ions, the absolute differential cross
sections of the observed hollow states
were examined as functions of the
collision velocity. For the TS1
mechanism, where the hollow state is a
79
residual ion must change and a second
electron can be
result of a single encounter with the
projectile nucleus, the cross section can
be written as σTS1=const*σionization, and
so the cross section varies as (1/v2) ln(v)
[4,5]. On the other hand, if the hollow
state is produced by TS2, the cross
section is expected to have a velocity
dependence proportional to 1/v4. For
example, in Fig. 3 the absolute single
differential cross sections for 2s2 1S,
2s2p 3P, 2s2p 1P, and 2s2p2 2P hollow
states observed in C3+ ions are plotted as
functions of the collision velocity.
To better exhibit the velocity
dependence of the cross sections for
these states, the functions (1/v2) ln (v)
and 1/v4 are also plotted. As seen from
the graph, the measured cross sections of
the hollow states deviate from the 1/v4
dependence, which characterizes the
TS2 transitions, and tend toward a
velocity dependence of the form (1/v2)
ln(v) at the higher collision energies.
This latter behavior of the cross sections
indicates that TS1 is the responsible
mechanism in the high velocity range,
giving evidence for the significant role
played by the e-e interaction in this
velocity regime.
The e-e interaction may contribute to
the formation of hollow states in
different forms depending on how fast
the first K-shell electron is promoted
(ionized or excited). If the first electron
is promoted slowly, it can interact with
another electron through their mutual
repulsion,
such
that
subsequent
excitation or ionization takes place and a
hollow state is formed. This process is
referred to as dielectronic correlation
[11], and may lead to the formation of
both S and P hollow states. On the other
hand, if the second electron is promoted
suddenly, thereby “freezing” the
remaining electrons in their initial states,
the wave function of the electrons in the
0.01
2s 2 1 S
3+
C
2
1/v ln(v)
4
1/v
3
-19
2
dσ/dΩ (x10
cm /sr)
2 s2 p P
0.01
1
2 s2 p P
0.01
2s2p 2 2 P
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
C ollision E nergy (a.u.)
Fig. 3. Absolute differential cross
sections for 2s2 1S, 2s2p 3P, 2s2p 1P and
2s2p2 2P hollow states formed in C3+
ions vs. collision velocity v.
The
symbols represent the measured cross
sections and the dashed and solid curves
represent (1/v2) ln(v) and 1/v4
dependences, respectively (see text).
excited or ionized due to the subsequent
electronic rearrangement. This process
is described in terms of shake dynamics
[12,13] and may only lead to hollow S
states due to angular momentum
conservation.
It should be noted that similar velocity
dependences were observed for the
hollow-state cross sections of the other
Li-like ions investigated in this work.
However, the magnitude of the electron
correlation effects decreases strongly
80
[3] Safronova, U. I., and Bruch, R.,
Physics Scripta, 57, 519,1998.
[4] McGuire, J. H., Berrah, N., Bartlett,
R. J., Samson, J. A. R., Tanis, J. A.,
Cocke, C. L., and Schlachter, A. S., J.
Phys. B, 28, 913-940 (1995).
[5] McGuire, J. H., Electron Correlation
Dynamics in Atomic Collisions.
Cambridge Monographs, UK.(1997).
[6] Zouros, T. J. M., and Lee, D. H,.
Zero-degree
Auger
electron
spectroscopy of projectile ions, in
Accelerator-Based Atomic Physics
Technique and Applications; edited by
S. M. Shafroth and J. C. Austin, (AIP,
NY), Chap 13, 427-477 (1997).
[7] Lee, D. H., Richard, P., Zouros, T. J.
M., Sanders, J. M., Shinpaugh, J. L., and
Hidmi, H.,Phys. Rev. A 41, 4816-4823.
(1990).
[8] Stolterfoht, N., Physics Reports, 146,
317-425 (1987).
[9] Kim, Y.-K., and Inokuti, M., Phys.
Rev. A, 1, 1132-1137(1970).
[10] The PWBA code was provided by
A. Salin.
[11]Stolterfoht, N., Nuclear Instruments
and Methods in Physics Research, B 53,
477- 503.
[12] Ishihara, T., Mizuno, J., and
Watanabe, T., Phys. Rev. A., 22, 15521557 (1980).
[13] McGuire, J. H., Phys. Rev, A 36,
1114-1123 (1982).
with increasing atomic number of the Lilike ion. More discussion of these
correlation effects will be presented in
upcoming publications.
Conclusion
Single and double-K-shell vacancies
produced in Be+, B2+, C3+ and O5+ Lilike ions have been investigated using
zero-degree projectile Auger-electron
spectroscopy. The velocity dependence
of the cross sections for production of
the double-K-shell vacancy states are
used to determine the electron-electron
interaction contributions to the formation
of the measured hollow states. These e-e
contributions are found to decrease
strongly as the atomic number of the Lilike ion increases. Further discussion of
variation of the e-e interaction in the
present Li sequence will be published in
the near future.
Acknowledgement
This work was supported in part by
the Chemical Sciences, Geosciences, and
Biosciences Division, Office of Basic
Energy Sciences, U.S. Department of
Energy.
References
[1] Diehl, S., Cubaynes, D., Kennedy, E.
T., Wuilleumier, F. J., Bizau, J.-M.,
Journel, C., VoKy, L., Faucher, P.,
Hibbert, L., Blancard, A., Berrah, N.,
Morgan, T. J., Bozek, J., and Schlachter,
A. S., J. Phys. B, 30, L595-L605 (1997).
[2] Tanis, J. A., Chesnel, J.-Y., Frémont,
F., Grether, M., Skogvall, B., Sulik, B.,
Tschersich, M., and Stolterfoht, N.,
Phys. Rev. A, 57, 3154-3157 and Tanis,
J.A., et al, Phys. Rev. Lett. 83, 11311134.
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