Kinetic energy releases of exploding C60 ions produced by
slow highly charged ions
J. Jensen∗† , H. Zettergren† , A. Fardi† , H.T. Schmidt† and H. Cederquist†
∗
†
Manne Siegbahn Laboratory, Frescativägen 24, S-104 05 Stockholm, Sweden
Physics Department, Stockholm University, SCFAB, S-106 91 Stockholm, Sweden
Abstract. We have studied electron transfer processes and collision-induced fragmentation in collisions between slow highly
charged ions and C60 molecules. Our newly developed reaction microscope provides information on the dynamics in the
charge transfer and the fragmentation processes. By using a position-sensitive detector, located after a time-of-flight tube, we
obtain recoil energies of fragments resulting from the disintegration of multiply charged C60 ions. Thereby we deduce kinetic
energy releases (KER) for the dissociation processes where multiple charged C60 ions emits either a single C+
2 ion or a neutral
ions
predominately
are
due
to
the
evaporation of
C2 -unit following ionization by slow highly charged ions. We find that C4+
58
a neutral C2 molecule.
INTRODUCTION
the mobilities of the valence electrons and the coupling
of electronic and vibrational degrees of freedom. The energetic and dynamics of dissociation reactions of neutral and charged fullerenes have received considerable attention over the past decade, and different experimental
methods have been used for these studies [4]. One way
to unravel cluster energetics is through the analysis of the
kinetic energy release (KER) [5, 6, 7, 8] in dissociation
processes.
Here we present a new method to deduce the KER of
various fragmentation modes of multiply C60 ions, which
have been created in collision with slow highly charged
atomic ions.
Highly excited finite systems, like clusters and large
biomolecules, can relax in a variety of ways such as by
emission of photons, electrons or fragmentation by neutral particles emission. Moreover, multiply charged systems may decay via fission reactions ejecting charged
rather than neutral fragments - analogous to nuclear
fission. Coulomb repulsion within a multiply charged
molecule or cluster leads to the destruction of the ion
if the corresponding binding energies are weaker than
the effective Coulomb force. C60 molecules show exceptional stability against fragmentation thus allowing
the production and observations of multiply charged C60
ions.
Slow highly charged ions have proven to be efficient
tools for multiple-ionisation of large molecules and clusters [1, 2]. At low collision energies electron capture
dominate the interaction, and the capture process occurs
at large distances. In this way, one may prepare relatively
"cold" multiply charged clusters with rather low internal energies and high stabilities. C60 ions have been observed in charge states up to at least ten [3] after collision
with slow highly charged ions. At lower projectile charge
states the interaction processes are dominated by small
impact parameters leading to higher target electronic and
vibrational excitations.
The production of multi-charged fullerenes and the investigation of subsequent decay processes is an interesting way of studying C60 ions, their structures, stabilities
and dynamics. Fragmentation of C60 is not only governed
by the dynamics of the cluster constituents, but also by
EXPERIMENTAL SET-UP
The highly charged atomic projectiles used in this study
were produced by the ECR ion source at the Manne Siegbahn Laboratory. The ions had energies of 3 keV/q and
their mass-to-charge-ratios were selected with a double
focusing analysing magnet. Behind the magnet, the ion
beam was collimated before entering the collision chamber, where it crosses with a collimated C60 -jet effusing
from a small oven at a temperature of about 600◦ C (Fig.
1). The interaction lies in the extraction stage of a linear
time-of-flight (TOF) mass spectrometer. Projectiles exiting the collision chamber in the (q − s)+ charge state
(i.e. stabilizing s electrons) were selected by means of a
180◦ electrostatic cylindrical analyser followed by a position sensitive detector (PSD1). The signal from the projectile detector triggers a transverse electric field, which
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
763
Time-of-flight
Vex
Cylindrical analyzer
TRIG
7+
6+
C60 C60
C60
3+
C60
4+
C60-2m
+
C3
+
C5
3+
C60-2m
A(q-s)+
START
STOP
PSD2
C60
5+
Intensity (arb. units)
Aq+
4+
Collimated C60 Jet
T=600 °C
5
PSD1
10
15
20
25
m/q (units of C)
Multi-hit TDC
FIGURE 2. The fragment distribution of recoiling intact and
fragmented Cr+
60 ions measured in coincidence with two stabilizing electrons (s = 2) in 50 keV Xe17+ - C60 collisions. Note
the broken vertical scales.
FIGURE 1. A schematic of the set-up used for coincidence
registration of final projectile and target charge states and the
fragment kinetic energies.
m=0
extract and accelerate recoil ions before entering the drift
region of our linear TOF spectrometer (length=1m) followed by a second position sensitive detector (recoil detector with a diameter of 50 mm). The multiply charged
fullerenes and their fragments are analysed with respect
to their mass-to-charge ratios in the TOF spectrometer.
The time-of-flight is determined by the ’start signal’ from
the projectile, which also triggers the extraction, and the
’stop signal’ from the recoil ions. The signals are treated
by means of multi-hit detection electronics and the data
are stored event by event in list mode.
m=1
m=2
RESULTS AND DISCUSSION
Fig. 2 shows the fragmentation patterns of C60 recorded
in coincidence with outgoing projectile stabilizing two
electrons (s = 2) after collision with 50 keV Xe17+ projectile ions. These C60 ions are produced in excited
states, which decay mainly by evaporation and asymmetrical fission processes. This is seen as the sequences of
4+
peaks to the left of C3+
60 and C60 . One observes: i) inr+
r+
tact C60 ions, ii) C60−2m (due to evaporation processes
r+
of the type Cr+
60 → C60−2m + C2m , which are known to
FIGURE 3. Position images on the recoil detector showing
kinetic energy releases in connection with the post-collisional
fragmentation of C60 following interaction with 50 keV Xe17+
ions. The dimension of the shown detector images are 15×15
mm2 .
bilized on the projectile.
By setting a gate around a certain peak in the TOF
spectra, we obtain the corresponding position distributions on the recoil detector. Fig. 3 shows examples of
measured distributions for intact and fragmented C60 ions. In the left row of Fig. 3 the position distributions
6+
associated with intact C60 ions (C4+
60 -C60 ) are shown.
The widths of the fragments become wider as the parent
charge state, and the numbers of emitted C2 -units (simultaneously or sequentially) increase. It is apparent that the
daughter-ion peaks, C58 -ions, are broader than their nondissociative counterparts indicating substantial energy
releases in the dissociation processes. Fig. 4 shows the
projection of the position images on the recoil detector
corresponding to C3+
60−2m ions for m = 0 − 4. It is clearly
seen that as the number of emitted C2 -units increases,
(r−1)+
dominate for r ≤ 3 [9, 10]), iii) C60−2m (due to asym(r−1)+
metric fission processes of the type Cr+
60 → C60−2m +
+
C2m , which is usually taken to be dominating for r > 3
[9, 10]), and v) C+
n (due to multi-fragmentation, occurring for even larger r [10]). Events corresponding to one
electron stabilized on the projectile mostly yield stable
C60 ions, due to distant collisions, transfering less energy to the C60 molecule than in s = 2 collisions [11].
In collisions with more than two electrons stabilized on
the projectile (s ≥ 2) hotter C60 parent ions are produced
and the C60 decay through multi-fragmentation giving
smaller carbon clusters. For the present study we have
focused those collisions in which two electrons are sta-
764
Collimators
Intensity (arb. units)
C523+
Ion beam
C543+
Ion beam
’Warm’ Xe target
’Cold’ C60-jet
(high v⊥(Xe))
(low v⊥(C60))
C60+
Xe+
Detector
C563+
~ 40 meV
C583+
Detector
~ 3 meV
FIGURE 6. Schematic picture of the callibration and resolution of our recoil spectrometer. Left is shown the distribution
following single electron capture from thermal Xe gas (used
for calibration). Right is shown the collimation of the C60 -jet,
yielding a target with a much lower transversal temperature
than in the Xe case.
C603+
100
200
300
400
Channel nr.
C58 recoils. In order to calibrate the measured width to a
kinetic energy scale, Xe-gas is introduced in the interaction region. Fig. 6 shows a comparison between the position distributions on the recoil detector following single
electron capture from room temperature (T=300 K) Xe
gas and the collimated C60 -jet. By means of the corresponding (300 K) three-dimensional Maxwellian kinetic
energy distribution of Xe we calibrate the fragment kinetic energy scale. The width of Xe+ corresponds to 40
meV. The velocity spread of the C60 -jet perpendicular to
the spectrometer axis can thus be determined to ≈3 meV.
Without collimation, the transversale temperature of the
jet would give a spot size on the recoil detector much
larger than that for Xe+ (partly due to a longer flight time
+
for C+
60 ). The recoil energy of Xe from the collision can
easily be shown to be negligible in comparison with its
initial thermal energy.
From the kinematics for a fragmentation process leading to two fragments the KER is determined by momentum and energy conservation [12, 13]. In the analyzis, the
width of the daughter ions have been corrected (deconvoluted) by the width of the parent ion, which represents
the instrumental broadening and energy distribution of
the intact C60 ions.
Fig. 7 shows the measured KER values for the process where a C60 -ion emits a single C+
2 or a neutral C2 unit as a function of the final fragment charge. This is
compared with published measurements using different
methods such as the MIKE and TOF techniques. In the
MIKE-scan technique (Mass-analyzed Ion Kinetic Energies) [5, 6, 14, 15] the energy distribution of the heavy
fragments (C58 -ion) are measured by means of an electrostatic analyzer, and from the widths of the resulting
distributions, the KER values are determined. In the TOF
technique [7, 16] the energy distribution of the small C+
2
Corrected width (channels)
FIGURE 4. Projections of the position images on the recoil
detector after selecting, in the TOF-spectra, events corresponding to C3+
60−2m ions, m = 0 − 4.
120
C58-fragment
C60
90
60
30
0
2
3
4
5
6
7
8
9
Charge state of ion
FIGURE 5. The full width at 22% maximum peak height of
projections of C60 - and C58 -ions for different charge states. All
widths are scaled to the flight time of C+
60 to correct for different
TOF of the recoil ions.
the widths of the projections increases. We have chosen to take the full widths at 22% of the maximum peak
height to represent a typical values of the corresponding kinetic energy releases. The width of intact C60 -ions
and C58 fragment ions are shown in Fig. 5. These values
have been corrected for the different flight times of the
recoil ions by scaling the measured widths with a factor
equal to the ratios between the flight times of C+
60 and the
various C58 - or C60 -ions. A clear difference between the
widths of intact and fragmented ions, due to the kinetic
energy released when emitting a single C2 -unit, is seen.
From the widths of the fragment ion peaks on the recoil
detector it is possible to deduce the kinetic energy of e.g.
765
Kinetic Energy Release (eV)
17+
Present expt. data for Xe
Scheier et al., MIKE
Senn et al., MIKE
Chen et al., TOF
Tomita et al., TOF
16
12
duced kinetic energy releases of the fragmentation process. The present KER-values for asymmetric fission
where a single C+
2 -unit is emitted, are in good agreement
with existing values for charge state r > 5. Model calculations of KER-values for such a fission process are in
progress [12, 13].
From the measured kinetic energy releases it is possible to obtain fission barriers, as a function of the C60
charge, for the decay process where C+
2 is emitted [?
]. This gives information on the stability limit for multiply charged C60 ions. Furthermore, the collimation of
the C60 -jet giving the low velocity spread of the target C60 , appears to open up possibilities for using the
method of Recoil Ion Momentum Spectroscopy (RIMS),
which yield direct information on the Q-values (stateselective electron capture cross sections) and scattering
angles (impact parameters).
C60r+ → C58(r-1)+ + C2+
8
C60r+ → C58r+ + C2
17+
4
Present expt. data for Xe
2+
Present expt. data for He
Matt et al., MIKE
0
1
2
3
4
5
6
7
8
9
Charge of final fragment C58
FIGURE 7. Measured kinetic energy releases for the process
where C60 emits a single C2 -unit (charged or neutral) compared
with existing measurements based on other methods, such as
the MIKE technique by Scheier et al. [5], Senn et al. [15], and
Matt et al. [6], and the TOF technique by Chen et al. [7] and
Tomita et al. [16].
ACKNOWLEDGMENTS
This work is supported by the Swedish Natural Research Council through Contract No. F650-19981278
and F5102-993/2001, and by The Swedish Foundation
for International Cooperation in Research and Higher education (STINT).
fragment were obtained from peak shape analyzis of the
C+
2 peaks in the TOF spectra. In those measurements the
primary C60 ions were produced by slow highly charge
ion impact, as in our case, whereas the MIKE-scan measurements used electron impact to ionize C60 molecules.
In the measurements by Scheier [5], Senn [15], Chen
[7], Tomita [16] the selected dissociation process is that
(r−1)+
of pure asymmetric fission, of the type Cr+
60 → C58
+
+ C2 , which dominates completely for charges of C58
larger than five [10]. Our values are in good agreement
with the fission data in this range. The measurements by
Matt [6] are only on evaporative dissociation processes
r+
of the type Cr+
60 → C58 + C2 (dominating for charges
of C58 lower than three [10]), having much lower KER
values. In our measurements we see large contributions
from evaporation of neutral C2 -units, for final charges
of C58 ions lower than five resulting in the observed
lower KER values. Shown are also our measurements obtained using 6 keV He2+ projectiles, giving KER-values
3+
for C2+
58 and C58 , the latter in agreement with the one
3+
obtained for C58 using Xe17+ as projectile. Both He2+
measurements are in accordance with the evaporation
values measured by Matt [6], within the experimental
error bars. The low KER-value of C4+
58 ions compared
with those obtained for asymmetrical fisson, suggest that
in our case these ions originate predominately from the
evaporation of a neutral C2 from C4+
60 .
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CONCLUSION AND OUTLOOK
15.
We have presented a new technique to obtain recoil energies of fragments created in collision between slow,
highly charged ions and C60 molecules. We have thus de-
16.
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