J. Plasma Fusion Res. SERIES, Vol.7 (2006) 195-198

Total Electron Transfer Cross Sectionsfor Highly Charged Ion - Alkali Metal Atom Collisions
SAKAUE Hiroyuki A., HOSAKA Kazumoto1 , TAWARA Hiroyuki2 , YAMADA Ichihiro,NAKAMURA Nobuyuki3 , OHTANI Shunsuke3 , DANJO Atsunori4 , KIMURA Masahiro5 ,MATUMOTO Atsushi6 , SAKURAI Makoto7 and YOSHINO Masuhiro8
National Institute for Fusion Science, Toki 509-5292, Japan
1 National Physical Laboratory, United Kingdom
2 Max Planck Institute for Nuclear Physics, Heidelberg, Germany
3 The University of Electro-Communications, Chofu 182-8585, Japan
4 Department of Physics, Niigata University, Niigata 950-2181, Japan
5 Kochi University of Technology, Kochi 782-8502, Japan
6 Hiroshima Institute of Technology, Hiroshima 731-5193, Japan
7 Department of Physics, Kobe University, Kobe 657-8501, Japan
8 Shibaura Institute of Technology, Omiya 337-8570, Japan（Received: 5 October 2004 / Accepted: 21 December 2005）

Abstract
The cross sections for highly charged iodine ion (Iq+ , q = 6〜30, E = 1.5q keV) - alkali metal atom (Na, K,Rb, Cs) collisions have been measured on the basis of the initial growth-rate method. It is shown that the crosssections increase as the charge of the projectile ion increases. We compare our data with the calculations by
the extended classical over-barrier model and the classical-trajectory Monte Carlo method. The cross sections−13· q/IP2 (cm2 ), reasonably well, where IP (eV) is the ionization energy
are represented by σexp
q = 1.05 × 10of the target atom.
Keywords:
highly charged ion, charge exchange, electron transfer, electron capture, cross section
1．Introduction
The investigation of the electron transfer processes
in collisions of highly charged ions (HCIs) with neutral
atoms and molecules is of great importance not only inbasic atomic collision physics, e.g. for understanding
the interactions between charged particles, but also insuch diverse fields as controlled-thermonuclear-fusion
research, development of x-ray laser devices and astrophysics. In such processes, several electrons could
be simultaneously transferred to excited states of HCIs
with significantly large cross sections, and finally theproduct ions are stabilized by emission of photon(s) orelectron(s) or both,of the transferred electron states. To date, multiple electron transfer processes have been widely studied [1].
The electron transfer cross sections in HCIs-atom collisions have been expressed by the classical over-barriermodel (ECBM) [2-6] reasonably well. The energy deposition model has been successfully applied to the observed Auger decay processes and multiple ionization
processes [7-9]. Recent activities in those studies arereviewed by Cederquist et al. [10].
Previously, we measured the absolute cross sections
j(σq,q−i ) for i-electron capture (to the projectile) andj-electron removal (from the target) in collisions ofHCIs with rare gas atoms [11,12] at collision energyof 1.5q keV by the coincidence measurement method,and studied the decay processes from the excited states
produced by electron transfer processes [13]. We deduced a simple scaling relation for the partial and total multiple-electron transfer cross sections in HCIs
(q = 6 〜 30) - rare gas atom collisions from ECBM

Aq+ + B → A(q− j)+∗∗··· (n, n , · · · ) + B j+radiative decay → A(q− j)+ + B j+ + hν + hν + · · ·Auger decay→ A(q− j)+ + B j+ + ( j − i)e−(i ≤ j)where j and i are, respectively, the number of electrons
initially transferred from the target and the number ofelectrons eventually captured by the projectile ion afterstabilization, and n, n’ are principal quantum numbers

C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: sakaue@nifs.ac.jp

195Total Electron Transfer Cross Sections for Highly Charged Ion - Alkali Metal Atom Collisions[14]. Furthermore, we have shown that this scaling law
deduced for rare gas atoms can also reproduce our measured data for molecular targets [15], as well. For alkali metal atoms, since the ionization energies are fairly
small in comparison with those of rare gas atoms, it
is expected that electrons are transferred to HCIs withquite large cross sections.

ions, retarded between the first and second meshes andyet pass through the third mesh, are accelerated toward
the fourth mesh at − 2.6 kV and finally detected with amicro-channel plate (MCP). At a voltage slightly higher
than 1.5 kV applied to the retarding meshes, the incident Aq+ ions whose energy is 1.5q keV are reflected
and the scattered ions having lower charges are detectedby the MCP. By further increasing the retarding voltage
to about 1.5q/(q−1) kV, both the incident Aq+ and scattered A(q−1)+ ions are reflected and only the ions which
captured two or more electrons are detected.
The count rates of the scattered ions at the retarding
voltage between 1.5 kV and 1.5q/(q − 1) kV were confirmed to be almost constant. To determine the growthrate of the scattered ions A(q−i)+ , the retarding voltage
was fixed at the middle in this constant region. The linear growth curve of the count rate of the scattered ions
was obtained as a function of the target gas pressure,which is still suﬃciently low. From this growth curve,
the total electron transfer cross sections σq were determined in the usual way.

2．Experiments
Figure 1 shows the experimental apparatus, this isbasically the same as that of Tawara et al. [16].
In the present study, an electron beam ion source(EBIS) is used for the purpose of producing low energy (1.5q keV) HCIs. After the charge and mass selection by the magnet, the ions are directed into the collision chamber and enter into the target cell, which is
28 mm long with the entrance aperture of 0.5 mm andthe exit aperture of 2 mm in diameter. The target cellis connected to the oven, in which an alkali metal vapor is generated, and it’s temperature is measured bythermocouples and controlled by a programmable controller to within ± 0.1◦ C. The temperature of the collision cell is kept higher than that of the oven so as toprevent the alkali metal atoms in the target cell fromsticking on the wall. In order to judge whether thedensity of alkali atoms is suﬃciently stable, we employ a surface ionization type ion gauge which is located above the target cell and monitor the flux of alkali
atoms passing through a small hole on this target cellwall. The target density is estimated from the measured
oven temperature on the basis of the vapor pressure data[17]. The primary ions and charge-changed ions exiting
the target cell pass through four mesh electrodes. The
retarding positive voltage Vr is applied to the second
and third meshes which are connected together. The3．Results and discussion
In this paper, we consider the electron transfer cross
jof highly charged iodine ion collisions
section σq,q−iwith alkali metal atoms B (B = Na, K, Rb or Cs).
Here the j-electron transfer cross section is expressedj, the i-electron capture cross section isas σqj = Σσq,q−ii

jσq,q−i = Σσq,q−i
and the total electron transfer cross secj

j= Σσqj = Σσq,q−i .
tion is σq = Σ σq,q−ii, jjiIn Fig. 2 (a), (b), (c) and (d), we show the charge
dependence of the absolute total electron transfer crossjfor Na, K, Rb and Cs targets,section σq = Σ σq,q−ii, jrespectively, and we list them in table. We find that
σq increases in proportion to q for all the targets. Asimilar tendency had been observed in our previous experiment for HCI-rare gas atom or molecule collisions[11,12,18-20]. The present cross sections are very large
up to 10−13 cm2 . This may be ascribed to the reason
mentioned in Introduction. The calculated values by
ECBM are also shown in Fig. 2. We note the obvious diﬀerence between the experimental and calculated values. The calculated σq by ECBM are 2 〜 3times as large as our experimental ones. This is in contrast to the good agreement in the case of the rare gas
atom targets. We also compare in Fig. 2 our experimental values with the calculated ones by the classicaltrajectory Monte Carlo (CTMC) simulations. I. Yamada[21] have calculated the one electron capture cross sections (σq,q−1 ) in HCI-atomic hydrogen collision at thecollision energy (1.5q keV) by CTMC. In addition theFig. 1 A schematic experimental apparatus in which theretarding technique is combined with the growthrate method.

196

Total Electron Transfer Cross Sections for Highly Charged Ion - Alkali Metal Atom Collisions–14
2
Table 1 The absolute total electron transfer cross sections σq (10 cm ). The experimental uncertainties are estimatedto be about ± 30 %.

NaKRbCs6

1015

18

3.5
12.0
9.6
9.6

3.0

4.8
5.0

2.0
9.6
8.2
8.6

q20
5.8
16.0
14.0
12.0

232527

6.7

5.7
18.0
13.0
14.0

7.7

17.0

16.0

30

19.0

Fig. 2 Absolute total electron transfer cross sections for Na, K, Rb and Cs as a function of the projectile ions charge q.
The calculated values by the ECBM and CTMC are represented by the full line and the dotted line, respectively.

calculations for alkali metal targets (Na, K, Rb and Cs),
which can be regarded as quasihydrogenic at the collision energy of 1.5q keV, have been carried out, and theionization energy dependence of the cross sections isstudied. In HCI-alkali metal atom collisions, the one
electron capture processes are dominant in comparison
with the two or more electron capture processes. Actually, we confirmed experimentally that the ratio of theone electron capture cross section (σq,q−1 ) and the twoelectron capture one (σq,q−2 ) is larger than 10. Therefore σq is virtually equal to σq,q−1 and the total electron
capture cross section is virtually equal to the total electron transfer cross sections (Σσq,q−i = Σσqj = σq ). Asithe experimental uncertainties.
In Fig. 3, the total electron transfer cross sections
for alkali metal atoms are plotted over the scaling parameter, q/IP2 , where q is the initial charge of the projectile and IP (eV) is the ionization potential energy of
the target atom. It is found that the experimental crosssections are approximately proportional to the scaling
parameter, q/IP2 . Thus, a scaling formula for alkali
metal atom targets is obtained;−13· q/IP2 (cm2 ).
σexp
q = 1.05 × 10

The scaling law for the rare gas targets [14] is also
shown in Fig. 3. The gradient of the rare gas scaling
is about 2.5 times as large as the gradient of the alkali
metal one.

jFigure 2 shows our experimental total electron transfer
cross sections agree with the CTMC calculations within

197

Total Electron Transfer Cross Sections for Highly Charged Ion - Alkali Metal Atom Collisions

Fig. 3 Comparison of the observed total electron transfer cross sections with the scaling law for the rare gas atom targets.
The scaling law for rare gas atom targets and ﬁtted line to our data for the alkali metal atom targets are represented
by the full lines.

Acknowledgements
This work was performed as a part of the Guest Research Programme at the National Institute for Fusion
Science.

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