Transfer Ionization in MeV p-He Collisions Studied by
Pulsed Recoil-Ion-Momentum Spectroscopy in a Storage
Ring/Gas Target Experiment
H. T. Schmidta, C. L. Cockeb, A. Fardia, J. Jensena, H. Schmidt-Böckingc,
L. Schmidtc, R. Schucha, H. Zettergrena, and H. Cederquista
a
Department of Physics, Stockholm University, SE-106 91, Stockholm, Sweden
Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA
c
Institute of Nuclear Physics, Frankfurt University, DE 60486, Germany
b
Abstract. We have investigated the transfer ionization process in fast collisions between protons, stored in the heavy-ion
storage and cooler ring CRYRING, and a cold supersonic helium gas-jet target: H+ + He → H + He2+ + e-. We have
refined the COLTRIMS (COLd Target Recoil-Ion-Momentum Spectroscopy) technique by applying time-varying fields
in the recoil-ion-momentum spectrometer to block random singly charged helium ions from single-ionization events.
In processes such as transfer ionization (TI), where
electrons are emitted to the continuum, these will also
carry kinetic energy and momentum and eq. 1 will in
general not apply. Nevertheless, the determination of
the longitudinal recoil momentum can serve as a
signature of the detailed mechanism of the process as
first demonstrated in p-He TI by Mergel et al. [5,6].
The present work may be viewed as an extension of
the work of Mergel et al. [5,6] to a new regime of
higher collision velocities, where the TI cross section
becomes very small. This requires high luminosity,
which is available at the heavy-ion storage ring
CRYRING. However, the high luminosity itself
becomes a problem due to the high production rate of
singly charged recoil ions from single ionization
events. This process has a cross section, which exceeds
that of TI by nine orders of magnitude for our highest
velocities. In order to overcome this problem we have
developed a time-switched recoil-ion-momentum
spectrometer, which we present together with the first
TI cross sections for p-He at 2.5-4.5 MeV [7].
INTRODUCTION
For the past ten years the technique of Recoil-IonMomentum Spectroscopy (RIMS) has been used in
studies of electron transfer in ion-atom collisions [1].
In the special case of pure (single or multiple) electron
transfer, the conservation laws of energy and
momentum lead to a well-defined longitudinal recoil
momentum for a given Q-value of the process (in
atomic units):
p|| = − qv p / 2 − Q/v p ,
(1)
where vp is the projectile velocity, q is the number
of transferred electrons and Q is the change in total
electronic binding energy. The final-state selectivity
was achieved in electron transfer in slow collisions
between highly charged ions and thermal He atoms
effusing from a nozzle [2]. Two aspects were
important in this context; the large energy separation
between neighboring capture states and the low
projectile velocity vp (cf. eq. 1). By the introduction of
the COLTRIMS technique (RIMS using COLd
supersonic gas-jet Targets), the resolution was greatly
improved and state selectivity in single-electron
capture (SC) became possible also for fast collisions
involving projectiles in low charge states. [3,4].
When an electron in a helium atom is captured by a
fast proton the He+ ion, which is left behind, will not
necessarily be in the He+ ground state – but can be
found in any state with appropriate properties. The He+
excitation probabilities are determined by the overlaps
between the corresponding He+-wave functions and
the wave function of the neutral He(1s2) state. Of
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
44
particular interest here is the finite probability for the
second electron to be emitted to the continuum, which
leads to a transfer ionization (TI) process. This TI
mechanism is referred to as Kinematic Transfer
Ionization (KTI) as the dominating SC mechanism at
these energies is kinematic capture [7]. The
corresponding experimental signatures are longitudinal
recoil momenta close to the values for single-electron
capture close to vp/2 for large vp (cf. eq 1) and a slow
free electron.
supersonic gas-jet. Neutral hydrogen atoms formed in
the collisions continued straight in the following
dipole magnet of the ring and were detected. A
transverse electric field was applied across the
interaction region, extracting the recoil-ions from
electron transfer processes and projecting them onto a
position-sensitive detector. Their time of arrival
relative to the detection of a neutral hydrogen atom
from the same collision event gave their charge states,
and their longitudinal recoil momenta were determined
from the impact positions along the beam direction.
An alternative TI mechanism was proposed in 1979
by several authors [8,9]. This mechanism is a two-step
process in which an electron after being scattered off
the projectile interacts with the other electron so that
one of the two ends up in a bound state of the
projectile while the other electron is emitted as a highvelocity continuum electron. This mechanism is
similar to the Thomas single-capture mechanism
proposed in 1927 [10] and demonstrated
experimentally by Horsdal-Pedersen et al. in 1983
[11], in which an electron scatters first on the
projectile and then on the target nucleus to end up in a
bound projectile state. Due to the similarity between
these two processes also the TI mechanism is referred
to as a Thomas scattering process (TTI, Thomas
Transfer Ionization, in the following). Three
characteristics of this mechanism are available for
experimental separation of TTI from the KTI
mechanism described above: (i) The characteristic
projectile scattering angle of 0.55 mrad. (ii) The
characteristic velocity vector of the continuum electron
equal to the projectile velocity at 90° with respect to
the projectile beam direction. (iii) Longitudinal recoilion momentum close to zero (p ≅0).
The proton beam was produced in a plasma ion
source, extracted, accelerated to 300 keV by the RFQ
accelerator and injected in CRYRING. After
accumulation at this energy, the ring was switched to
synchrotron operation and the ions were accelerated to
the desired collision energy. At this energy a velocitymatched cold intense electron beam from the electron
cooler cooled the proton beam both longitudinally and
transversely. The transverse cooling lead to a strong
reduction of the beam width, which for the present
experiment was of the order of 1 mm half width at half
maximum at beam currents of 20-60 µA.
THE EXPERIMENTAL TECHNIQUE
Helium gas at 3 bar pressure and a temperature of
85 K expanded through a φ=30 µm nozzle into a
chamber with a vacuum in the 10-3 mbar-range
maintained by a 1000 l/s turbomolecular pump. The
gas was accelerated to supersonic velocity (~550 m/s)
when it flowed through the nozzle where random
thermal energy was transformed into directed kinetic
energy. The gas expanded isentropically, leading to
cooling to a sub-K temperature. The transversal jet
size as well as temperature distribution was then
limited by means of a φ=100 µm skimmer at a variable
distance of 0–20 mm below the nozzle. A second
skimmer (φ=300 µm) was placed 8 cm further
downstream and provided the final jet collimation. The
jet then passed two more differential pump stages,
separated by apertures of φ=1 mm and φ=1.5 mm
which did not touch the jet. After the jet crossed the
stored proton beam in the collision chamber, it was
dumped in a set of three differential pump stages
separated by conductance-limiting tubes. At the point
of intersection with the ion beam the jet had a diameter
of 1.0 mm and a density of about 1011 cm-3. This target
density was achieved without any measurable increase
in the CRYRING background pressure in the
10-12 mbar range [13].
The present experiment was performed at the
heavy-ion storage and cooler ring CRYRING at the
Manne Siegbahn Laboratory in Stockholm. An
electron-cooled, intense, beam of protons with 2.5-4.5
MeV kinetic energy was intersected by a cold
Helium recoil ions were extracted from the
interaction region by a homogeneous transverse
electric field and projected on a microchannel plate
detector with a resistive anode for 2D position
sensitivity as shown in fig.1. The momentum imaging
The first clear evidence of the TTI process was
based on angle and energy analysis of the emitted
electrons observed in coincidence with neutral
hydrogen [12], while more recent quantitative results
were obtained by the COLTRIMS technique with
coincident determination of the projectile scattering
angle and the recoil-ion momentum [5]. At the higher
projectile energies considered in the present work, the
two mechanisms are separated directly by their
different longitudinal recoil momenta [7].
45
FIGURE 1. The setup as described in the text.
was optimized by applying a negative voltage to one
of a series of ring electrodes through which the recoil
ions traveled to obtain the function of an Einzel lens.
The longitudinal recoil-ion momentum was deduced
from the position of impact along the beam direction
with a momentum resolution of 0.3 a.u.
FIGURE 2. Three schematic snapshots illustrating the
technique to avoid randoms from single ionization at the
time corresponding to TI. The singly charged ions are
shown as open circles whereas the He2+ ion from the TI
event is a filled circle. a) The moment of switching off of
the deflectors. A neutral hydrogen atom has been detected
and the delay before switching is chosen so that the He2+
ion is not deflected. b) The He2+ ion has passed the
deflector and is continuing towards the detector together
with the first undeflected He+ ions. The last of the deflected
He+ ions are hitting the chamber wall. c) Because of its
higher velocity the He2+ ion has overtaken the undeflected
He+ ions and reaches the detector first.
The high luminosity in the 1024 cm-2s-1 range
allowed for the TI process to be investigated at higher
collision velocities than what has previously been
attained. The TI cross section at 4.5 MeV (the highest
energy considered here) is in fact three orders of
magnitude smaller than at the highest proton energy at
which this process had been considered prior to this
work (1.4 MeV [5]). However, the high luminosity
poses an experimental problem as the cross sections
for pure ionization processes are much larger than for
processes involving electron transfer. Therefore the
production rate of helium ions from ionization was
very high and produced a strong contribution of
random coincidences. To strongly reduce the random
level at the time of arrival of the 2+ recoil ions from
TI, we applied a recoil-charge state selection prior to
detection based on a time-switching technique. As
indicated in fig.2 an electric field can be applied
transverse to the spectrometer axis. With the deflector
voltage on no ions reached the detector. In the event of
detection of a neutral projectile from a SC or TI event
the deflecting field was switched off after a preset
delay and the spectrometer was open for recoil-ion
detection. The delay was chosen such that the
deflection field was off when the He2+ ion reached the
deflector. Due to the high He+ production rate, many
He+ ions also passed the deflector while it was
switched off. These He+ ions are, however, slower than
the He2+ ions and will therefore reach the detector well
after the arrival of the He2+ ion. In this way, we were
able to completely avoid the influence from the He+
randoms from single ionization and have only randoms
from the much weaker double-ionization process (see
fig.3). The two insets of fig.3 show the distributions of
recoil ions on the detector for flight times
corresponding to SC (a) and TI (b). The data in inset b
are used to separate the contributions from the KTI
and TTI mechanisms. The separate cross sections are
extracted by normalizing to the SC peak and using the
SC cross section values of Schwab et al. [14].
KTI
TTI
FIGURE 3. The time-of-flight spectrum recorded in the
time-switched mode with 2.5 MeV protons. The SC peak
rides on the large single ionization (SI) random level,
which is absent for the TI peak. Inset a) and b) are contour
plots of the recoil-positions recorded within narrow time
intervals around the SC and TI peaks.
46
RESULTS AND DISCUSSION
CONCLUSIONS
We have demonstrated a new time-switched
COLTRIMS setup capable of efficient random
suppression by charge selection prior to detection. The
cross sections for the Thomas and Kinetic Transfer
Ionization mechanisms in p-He collisions were
determined separately at 2.5, 3.5, and 4.5 MeV proton
energy. The TTI data were found to be consistent with
the v-11 velocity dependence predicted for the highvelocity limit. The probability for emission of the
second target electron upon transfer of the first
electron to the projectile was found to decrease with
increasing velocity in this range and appear to
approach the 1.63% asymptotic limit also found in the
case of photoionization.
TABLE 1: Measured TI cross sections
Energy
Total TI
TTI
(MeV)
(10-26cm2)
(10-26cm2)
2.5
123 ± 14
43 ± 7
3.5
18.1 ± 1.8
6.7 ± 1.1
4.5
3.7 ± 0.7
1.2 ± 0.5
The results of the present work are shown in table
1, where the measured total TI and TTI cross sections
are given. These results are also illustrated in fig. 4 as
functions of the projectile velocity together with
previous results of Mergel et al. [5] and of Shah and
Gilbody [15]. We find that unlike the situation at lower
velocities, the TTI cross section at the present high
velocities scales with the expected σ∝v-11 power law
behavior [8,9]. From the KTI and total capture
cross sections it is possible to extract the probability
for emission of the second electron when the first
electron is transferred to the projectile as a function of
projectile velocity. With this approach we find that
after a maximum at velocities between 5 and 10 a.u.
this shake-off probability decreases towards the value
1.63% also found in photoionization [16]. This
observed behavior is reproduced in a recent calculation
by Shi and Lin [17] where the shake-off probability as
a function of the velocity of the first removed electron
is considered.
ACKNOWLEDGMENTS
This work has been supported by the Knut and
Alice Wallenberg Foundation, the Swedish Research
Council, the Swedish Foundation for International
collaboration in Research and Higher Education, and
the Royal Swedish Academy of Sciences.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
FIGURE 4. The total TI (▲ [15], „ [5], z present work [7]), and
Thomas TI († [5],  present work [7]) cross sections as functions
of the projectile velocity. The line through the present Thomas p-ee TI data points () has a slope corresponding to σ∝v-11.
13.
14.
15.
16.
17.
47
R. Ali et al., Phys. Rev. Lett. 69, 2491 (1992).
W. Wu et al., Phys. Rev. A 51, 3718 (1995).
V. Mergel, et al., Phys. Rev. Lett. 74, 2200 (1995).
H. Zhang et al., Phys. Rev. A 60, 3694 (1999).
V. Mergel et al., Phys. Rev. Lett. 79, 387 (1997).
V. Mergel et al., Phys. Rev. Lett. 86, 2257 (2001).
H. T. Schmidt et al., Phys. Rev. Lett.89, 163201 (2002).
J. S. Briggs and K. Taulbjerg, J.Phys. B 12, 2565 (1979).
R. Shakeshaft and L. Spruch, Rev. Mod. Phys. 51, 369
(1979).
L. H. Thomas, Proc. Roy. Soc. London, Ser. A 114, 561
(1927).
E. Horsdal-Pedersen, C. L. Cocke, and M. Stockli, Phys.
Rev. Lett. 50, 1910 (1983).
J. Pálinkás, R. Schuch, H. Cederquist, and O. Gustafsson
Phys. Rev. Lett. 63, 2464 (1989).
H. T. Schmidt et al., Hyperfine Interact. 108, 339
(1997).
W. Schwab et al., J. Phys. B 20, 2825 (1987).
M. B. Shah and H. B. Gilbody, J.Phys. B 18, 899 (1985).
L. Spielberger et al., Phys. Rev. Lett. 76, 4685 (1995)
T. Y. Shi and C. D. Lin, Phys. Rev. Lett. 89, 163202
(2002).