3.14
Fragmentation of H2 O Molecules Following the Interaction with Slow, Highly
Charged Ne Ions
Z.D. Pešić a) , J.-Y. Chesnel b) , R. Hellhammer a) , B. Sulik, P. Sobocinski a) and N. Stolterfoht a)
The fragmentation of molecules induced by
the interaction of highly charged ions (HCI)
has been investigated intensively in the past
decade. Most of these studies have been
performed with diatomic molecules, especially
with the simplest molecules H2 and D2 . In addition to Coulomb explosion (CE), the energy
of the H+ fragments is influenced by the collisional momentum transfer to the molecule, as
well as by the post-collision field of the scattered slow highly charged ion [1,2]. In this
work, we investigate the molecular fragmentation of H2 O molecules, whose applications
are numerous. These experiments were performed using slow, highly charged Neq+ ions
produced by the 14.5-GHz Electron Cyclotron
Resonance (ECR) ion source facility at the Ionenstrahllabor (ISL) [3]. The energy of the
projectile was varied from 2 to 90 keV, while its
charge state ranged from 1 up to 9. The experimental chamber with a base pressure below 2
x 10−7 mbar contains an electrostatic parallelplate spectrometer, which can be rotated from
18o to 135o with respect to the incident ion
beam direction.
Figure 1. Energy spectra of ions from collisions of
5 keV Ne+ with H2 O, measured at the observation
angles 30o , 40o , 50o and 60o . The lines are drawn
to guide the eye.
Fig. 1 presents the energy distributions of
ions produced in collisions of 5 keV Ne+ ions
with H2 O molecules. The observation angle
was varied from 30o to 60o in steps of 10o .
Two groups of peaks are energetically separated: a low energy group (below 100 eV)
which does not show a significant energy shift,
and a group of peaks whose positions are angular dependent. In Fig.1, the peaks with energies above 100 eV can be associated to binary
collisions between the projectile and a single
target atom. The scattering of Neq+ projectiles up to 40◦ is also a signature of collisions
at small impact parameters with the oxygen
atoms. On the contrary, the presence of slow
species (energies ¡ 100 eV) can be explained by
means of a CE model. In most of cases, these
slow fragments are identified as H+ ions.
We have also remarked that increasing the
charge state of the projectile, the energy difference of the peak positions at the largest
forward and backward observation angle increases. Furthermore, the experiment shows
that the angular dependence is more pronounced for decreasing energy of the projectile. Therefore, it is attributed to the influence of the post-collision field of the scattered
ion. The role of the post-collision interaction
for lighter projectiles has previously been revealed [4]. In the present case, fragment ions
emitted in forward direction are decelerated,
while those emitted in backward direction are
accelerated. The deceleration or acceleration
is stronger if the interaction time and/or the
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charge of the projectile increases.
In Fig. 2 (a) we show the projectile charge
state dependence of the differential cross sections for fragmentation of H2 O. The energy
of Neq+ (q=3, 5, 7 and 9) projectiles is 21
keV, and the detection angle is 25o . The lines
are drawn to guide the eye. The peak labeled Oxygen is due to OQ+ ions, whereas
the peak Q=0 corresponds to H+ ions originating from the H+ +H+ +O0 fragmentation.
The differential cross sections for the production of H+ ions, following fragmentation into
H+ +H+ +OQ+ and/or H+ +H0 +OQ+ , are labeled Q=1, 2 and 3 in Fig. 2 (b).
capture. Here we did not plot data where the
charge state Q of the oxygen fragments exceeds
3. Fragments from this channel are present for
Neq+ projectiles and this a signature for the
capture of 4 or 5 electrons.
For the projectiles with charge states q=37 prediction of the COB model agrees well
with measured cross sections. In fact, doubleelectron capture calculated using the extended
COB model for projectile with charge state
q=3 nearly coincides with the measured value,
while for higher charge states of the projectile
(q=5,7) better agreement of the model with
the experiment is achieved when taking the
sum of double and triple electron capture.
In conclusion, two regions, due to the binary collisions and Coulomb explosion, are separated in the energy spectra. Furthermore,
a strong charge-state dependence was found.
The CE model indicates that for low projectile charge states fragmentation to two charged
particles and one neutral particle is dominant, while for high charge states of the neon
ions, fragmentation to three charged particles
is more probable.
The experiments have been performed at
the ECR beamline of the ISL facility in HahnMeitner Institute Berlin. Work was supported
in part by the Hungarian-German S&T Collaboration (D17/99).
Figure 2. Differential cross sections for fragmentation of H2 O molecules by 21 keV Neq+ at 25o as
a function of charge state of the projectile.
Fig. 2 shows a strong monotonic increase of
the differential cross section for fragmentation
with increasing projectile charge state, which
is especially pronounced for the peaks labeled
Oxygen and Q=0, as well as for peak Q=1. In
particular, the intensity of peak Q=3 (explosion which produces oxygen ions with charge
state Q=3) increases strongly when the incident charge of the projectile increases from
q=7 to q=9, while its intensity shows only a
slight increase in the range from q=3 to q=7.
This indicates that an open projectile K-shell
significantly influences the multiple electron
a) Hahn-Meitner Institute, Berlin, Germany
b)
CIRIL, Unité Mixte CEA-CNRS-EnsiCaenUniversité de Caen Caen, France
[1] DuBois R, Schlathölter T, Hadjar O, Hoekstra R,
Morgenstern R, Doudna C M, Feeler R and Olson R E, Europhys. Lett. 49 (2000) 41.
[2] Frémont F, Bedouet C, Tarisien M, Adoui L, Cassimi A, Dubois A, Chesnel J-Y and Husson X, J.
Phys. B: At. Mol. Opt. Phys. 33 (2000) L249.
[3] Sobocinski P, Rangama J, Laurent G, Adoui L,
Cassimi A, Chesnel J-Y, Dubois A, Hennecart
D, Husson X and Frémont F, J. Phys. B: At.
Mol. Opt. Phys. 35 (2002) 1353.
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