Orientation dependence of N< sub> 2</sub> and O< sub> 2</sub> multiple ionization in slow and fast collisions with highly charged Xe-ions

Abstract

The influence of the molecular orientation on the multiple ionization of N 2 and O 2 was studied in collisions with highly charged Xe-ions at velocities of 0.2–0.3 and %15 a.u. The applied technique is based on a position-and time-sensitive multi-particle detector which allows to determine the ionization cross-section as a function of the orientation of the molecular axis. At lower collision energies, e.g. in collisions with 360 keV Xe 18þ , no anisotropy was found, whereas in collisions with 770 MeV Xe 18þ significant orientation effects appear at higher degrees of ionization.

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Orientation dependence of N2and O2multiple ionization
in slow and fast collisions with highly charged Xe-ions
B. Siegmanna,*, U. Wernera, H. Lebiusb, B. Huberb, H.O. Lutza, R. Mannc
aFakult€ a at f€ u ur Physik, Universit€ a at Bielefeld, D-33615 Bielefeld, Germany
bCIRIL, Rue C. Bloch, BP 5133, 14070 Caen Cedex 05, France
cGesellschaft f€ u ur Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
Abstract
The influence of the molecular orientation on the multiple ionization of N2and O2was studied in collisions with
highly charged Xe-ions at velocities of 0.2–0.3 and ?15 a.u. The applied technique is based on a position- and time-
sensitive multi-particle detector which allows to determine the ionization cross-section as a function of the orientation
of the molecular axis. At lower collision energies, e.g. in collisions with 360 keV Xe18þ, no anisotropy was found,
whereas in collisions with 770 MeV Xe18þsignificant orientation effects appear at higher degrees of ionization.
? 2003 Elsevier Science B.V. All rights reserved.
PACS: 34.50.Gb
Keywords: Ion–molecule collision; Multiple ionization; Fragmentation; Orientation; Diatomic molecules; Highly charged ions
1. Introduction
The multiple ionization and fragmentation of
simple molecules have been the subject of many
recent experimental and theoretical investigations
(see e.g. [1,2] and references therein). An important
characteristic of these processes is the angular
distribution of ionized fragments. For the studied
collision energies the interaction time of <10?16s is
considerably shorter than the rotational periods of
the molecules and thus in a diatomic molecule the
fragments emerge practically along the direction of
the molecular axis. Therefore the initial orienta-
tion of the molecular axis can be derived from the
measured fragment velocity vectors and the ion-
ization cross-section can be studied as function of
the alignment of the molecular axis with respect to
the ion beam [3,4].
2. Experimental set-up
The experiment has been performed using the
highly charged ion beams of the AIM facility at
the CEA in Grenoble and the UNILAC accelera-
tor at the GSI in Darmstadt. Collimated beams of
Xeqþ(q ¼ 14;18;28;43) projectiles interact with a
N2or O2gas target. The slow ions and electrons
generated in the collision process are separated by
a weak homogenous electric field of E ¼ 333 V/cm
perpendicular to the detector plane (Fig. 1).
*Corresponding author. Fax: +49-521-106-6002.
E-mail address: siegmann@physik.uni-bielefeld.de (B. Sieg-
mann).
0168-583X/03/$ - see front matter ? 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0168-583X(03)00593-7
Nuclear Instruments and Methods in Physics Research B 205 (2003) 629–633
www.elsevier.com/locate/nimb

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Electrons are detected by a channeltron at one
side of the interaction region; positive ions are
accelerated towards the time- and position sensi-
tive multi-particle detector at the other side [5].
After passing a field-free time-of-flight region the
ions are post-accelerated to a few keV to increase
the detector efficiency. The detector is based on
microchannel plates in combination with an etched
crossed-wire anode structure consisting of inde-
pendent x- and y-wires: an electron cloud from the
microchannel plates hitting a wire-crossing will
result in coincident pulses on the corresponding x-
and y-wires. For each positive fragment ion the
position on the detector (xi;yi) and the time-of-
flight tirelative to the electron signal are recorded.
If both fragments from a particular Coulomb
fragmentation, as e.g. NðpþqÞþ!Npþ+Nqþ, are
detected these data yield a kinematically complete
description of the fragmentation process.
3. Results
The present experimental set-up allows the si-
multaneous measurement of all reaction channels
resulting in at least one electron and one or more
positive fragments. In collisions with fast highly
charged ions processes leading to correlated
Opþ+Oqþor Npþ+Nqþpairs with a total charge
04590135180
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dn/dθ
04590135180
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dn/dθ
04590135180
04590135180
(O2)
6+
(O2)
6+
(O2)
10+
(O2)
10+
Fig. 2. Alignment dependence for the multiple ionization of O2molecules in collisions with 360 keV Xe18þ(left) and 5.9 MeV/u Xe18þ
(right), (???) corresponds to an isotropic distribution.
θ
e
xytDetector
Xe q+
+U
Channeltron
E
Fig. 1. Principle of the experimental set-up.
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of p+q up to 12 were observed; in the slower
collisions fragment ion pairs with p+q 6 10 oc-
curred.
For each fragmentation channel the relative
cross-section was measured as a function of the
angle h between the molecular axis and the pro-
jectile beam. As an example, Fig. 2 shows the
distributions obtained for sixfold and tenfold
ionization of O2in collisions with 360 keV Xe18þ
and 5.9 MeV/u Xe18þ. The (O2)6þspectrum con-
tains contributions from the O3þ+O3þand the
O2þ+O4þchannels; the (O2)10þdata contain con-
tributions from O5þ+O5þand O4þ+O6þcoinci-
dences. Note, that in all cases the symmetric
charge distribution is clearly more abundant than
the asymmetric one. An isotropic fragmentation
would result in an angular distribution propor-
tional to sinh (dotted curves in Fig. 2). The spectra
measured by 360 keV Xe18þimpact are in good
agreement with an isotropic distribution. The same
holds for all observed Coulomb fragmentation
processes of O2 and N2 in collisions with highly
charged Xe ions in the 0.2–0.3 a.u. velocity range.
In collisions with 5.9 MeV/u Xe18þ, however, the
spectra for the highest degrees of ionization clearly
deviate from the isotropic distribution: the highest
degrees of ionization are more easily achieved if
the molecular axis is aligned along the projectile
beam than for the perpendicular orientation (Figs.
2 and 3). Similar effects have been found in colli-
sions with 100–300 keV Hþand Heþat consider-
ably lower degrees of ionization [4,6].
The origin of this orientation effect is directly
related to the anisotropy of the electron density
distribution in the target molecule. Qualitatively,
the achievable degree of ionization is correlated to
the energy deposited by the impinging ion which
depends on the integrated electron density along
the projectile path. Thus higher degrees of ion-
ization can be expected for trajectories which are
close and parallel to the molecular axis, whereas
less energy is deposited in case of a perpendicular
oriented molecule. An extended version [2,7] of the
statistical energy deposition model (SED) [8,9]
04590135180
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dn/dθ
0 4590135180
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dn/dθ
04590135180
045 90135180
(N2)
6+
(N2)
6+
(N2)
10+
(N2)
10+
Fig. 3. Orientation dependence of the cross-section for (N2)6þand (N2)10þionization channels in collisions with 5.9 MeV/u Xe18þ(left)
and Xe43þ(right), (---) SED-UCA model, (???) isotropic distribution.
B. Siegmann et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 629–633
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yields a quantitative description of the effect. For
collisions with fast highly charged ions the de-
posited energy was calculated using the unitary
convolution approximation (UCA) [10]. The de-
rived differential cross-sections are in good agree-
ment with the orientation dependence observed in
fast collisions (Fig. 3). The most distinct effect
occurs for 5.9 MeV/u Xe18þimpact. In collisions
with 5.9 MeV/u Xe43þthe contribution of larger
impact parameters to this ionization channel in-
creases and thus the sensitivity to the orientation
of the molecule is reduced.
The theoretical models are not applicable to
slow collisions (v < 1 a.u.) with highly charged
XeQþions. However, the absence of the orienta-
tion effect in these collisions may be qualitatively
understood. As a first approximation the interac-
tion strength may be expressed by the Sommerfeld
parameter j ¼ Q=v which is large for slow highly
charged ions. Thus the situation is similar to the
Xe43þcase discussed above and close collisions
which are sensitive to the orientation of the mol-
ecule are less important.
The theoretical models consider only the total
charge of the intermediate molecular ions. In our
experiment the contributions of individual frag-
mentation channels,as
N5þ+N5þin case of tenfold ionization, can be
resolved since they appear as separated structures
in the 2-dim. correlation map (see e.g. [11]). As an
example, Fig. 4 shows the main contributions to
the eightfold ionization of O2in collisions with 280
keV Xe14þand 560 keV Xe28þ. The individual
spectra are very similar and within statistics com-
patible with an isotropic distribution. Surprisingly
the situation changes for the highest degree of
ionization observed in fast collisions (Fig. 5). Al-
though the statistics for the N4þ+N6þchannel is
low (around 35 counts for Xe18þand 100 counts
for Xe43þin the maxima) it is clearly less asym-
metric than the N5þ+N5þdistribution. Conse-
quently the asymmetry of the pure N5þ+N5þ
channel is even more pronounced than in the
e.g.N4þ+N6þ
and
045 90135 180
θ [degree]
0.0
0.5
1.0
dn/dθ
0 4590135180
0.0
0.5
1.0
dn/dθ
Xe
14+
Xe
28+
Fig. 4. Orientation dependence of the cross-section for
O4þ+O4þ(???) and O3þ+O5þ(––) in collisions with 280 keV
Xe14þand 560 keV Xe28þ, (---) isotropic distribution.
0 4590135180
0.0
0.5
1.0
dn/dθ
04590135180
θ [degree]
0.0
0.5
1.0
dn/dθ
Xe
18+
Xe
43+
Fig. 5. Orientation dependence of the cross-section for
N5þ+N5þ(???) and N4þ+N6þ(––) in collisions with 5.9 MeV/u
Xe18þand 5.9 MeV/u Xe43þions, (---) isotropic distribution.
632
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spectra for tenfold ionization in Fig. 3. The reason
for this effect is still unclear.
4. Summary
We have reported on the influence of the mo-
lecular orientation on the multiple ionization of N2
and O2in collisions with fast (?15 a.u.) and slow
(0.2–0.3 a.u.) highly charged Xe-ions. In slow
collisions practically no orientation effects occur.
They appear, however, in fast collisions at the
highest observed degrees of ionization: the multi-
ple ionization cross-section shows a minimum if
the molecular axis is oriented perpendicular to the
projectile path. The observations are in agreement
with qualitative arguments and (for fast projec-
tiles) calculations in the SED-UCA model.
Acknowledgements
This work was supported by the Deutsche
Forschungsgemeinschaft in the project ‘‘Strah-
lungswechselwirkung’’ and by the EU Infrastruc-
ture Cooperation Network HPRI-CT-1999-40012
(LEIF).
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