Multiple Photoionization of Carbon-60
K. A. Barger, P. Juranic, and R. Wehlitz
University of Wisconsin-Madison, Synchrotron Radiation Center
In this experiment, C60 clusters were ionized using
monochromatized synchrotron radiation. The relative cross section of the
photoionized C60 was studied using photons that had an energy range of
37-240eV. The 1+ to 3+ charge states where investigated as a function of
excess energy using a Time-of-Flight mass spectrometer. From this
information, the relative cross sections can be studied for each charge
state. The current theories available for predicting the relative cross
sections not molecules; the data collected in our experiment can be used as
benchmark data to model molecule’s relative cross sections. Our data
shows that relative ionization cross sections do not change linearly, and
that oscillations in its size occur with the change in photon energy.
1. Introduction
In this experiment, we will
photoionize C60, a cluster of 60 carbon
molecules, with single photons of
variable wavelength. The photons used
have a specified energy that ranges from
37-240eV, which is controlled by a
monochromator.
Near the threshold energy, the
electrons leave the C60 relatively slowly.
We believe that they may have time to
interact with the remaining electrons in
the C60. These interactions will cause
oscillations in the C602+/C601+ and
C603+/C601+ cross section ratios to occur.
In past experiments, this oscillating
behavior was observed in beryllium and
lithium [1-2]. Experiments on the
ionization of C60 have been done in the
past [3-5], but due to large error bars or
insufficient data the oscillating behavior
has not been observed.
Once our data on the ionized C60
are collected, we will compare the
doubly and triply ionized molecules with
the singly ionized molecules. When this
is completed, the cross sections of the
ionized C60 will be calculated and
compared with theoretical models, and
any oscillating behavior will be
investigated.
2. Experimental Method and
Setup
The
measurements
were
preformed the 6m TGM beam line at the
Synchrotron Radiation Center (SRC) in
Stoughton Wisconsin. The photon beam
has an energy range of 8-200eV.
The C60 powder used had a purity
of 99.5%. In preparing for the
experiment, the C60 was evaporated in a
resistively heated oven. This enables the
impurities to be outgased. Once the oven
is cool, the C60 is placed in the oven. The
vacuum in the chamber was kept at
approximately 10-9 Torr during the
experiment. Once the proper vacuum has
been achieved, the temperature is raised
slowly until the solvents, that are used to
create C60, are evaporated and a
sufficient vapor pressure of C60 is
achieved.
During the experiment, the oven
temperature is ≈315 °C. At this
temperature, unwanted thermal electrons
will be created inside the oven. It is
important to remove these electrons so
that only photons are responsible for
ionizing the C60, and not electrons. To
ensure the electrons do not interfere with
the experiment, a positive bias voltage is
applied, which attracts them to the walls
of the oven.
Figure 1: Schematic diagram of experimental setup
At ≈315 °C the C60 starts to
sublimates, causing some of the C60 to
leave the crucible and enter the area that
is located between the pusher plate (PP)
and the extractor plate (EP). This area is
where the photons interact with the C60
(interaction
region).
Above
the
interaction region, there is a condenser
plate (CP) that is cooled by liquid
nitrogen (LN2). The function of the CP is
to improve the vacuum by freezing
unwanted gases and un-ionized C60 to
the surface of the plate, removing them
from the interaction region.
When the photons enter the
interaction region, they have a specified
energy above the threshold energy
required to ionize the C60. When an
interaction occurs with an energetic
photon and the C60, the C60 can become
singly, doubly, or triply ionized. The
threshold energy required for C60+, C602+,
and C603+ is 7.53eV, 19.02eV, and
35.7eV
respectively.
Most
monochromators contribute secondorder light in addition to the desired
energy. Filters are used to reduce second
order light. An aluminum filter was used
for the range of 37-72eV and a Si3N4 for
51-98eV a Si3N4.
During this process, a periodic
+50 Volt pulse is introduced to the PP.
This pulse creates an electric field
between the PP and the grounded EP.
The electric field causes the positively
charged C60 ions to accelerate to the drift
tube and travel through it.
Once the C60 has traveled
through the drift tube, it travels to the
Microchannel Plate (MCP) detector. The
MCP is an array of three detector plates.
These plates have voltages that are set
between 2800-3000 Volts. The MCP is
designed to convert ionized particles into
electric pulses, which can be used to
count C60 ions.
The signal then travels to the
Constant Fraction Discriminator (CFD).
The CFD is used to cut off noise and it
also gives pulse positions that are
independent of the height of the pulses.
After that, the information is then sent to
the Time to Amplitude Converter
(TAC), which measures the time
difference between the pulse across the
interaction region and the time for the
C60 ions to reach the MCP.
3. Data Taking
By using a program called
Maestro, the intensity of the C60 charge
states versus TOF can be graphed as
shown in figure 2. The integrated peak
areas represent the different ionization
intensities. The TOF measures the mass-
to-charge ratio (m/q) [6], making the
travel time different for each charge state
of C60. This creates separate peaks for
each ionization state of C60. We then
separately take the area of the C602+ and
the area of the C603+ peak and divide
them by the area of the C601+ peak; the
C602+/C601+ and C603+/ C601+ ratios are
later analyzed as a function of photon
energy. The photon energy, the
beginning and ending electron storage
ring current, and the current incident on
the nickel mesh, which monitors the
photon beam, are all recorded for each
spectrum.
Figure 2: This spectrum was taken using photons at
energy of 38eV and a Si3N4 filter for 30 min.
These data will enable us to
determine the relative ionization cross
section σR1+ of C60. This can be done by:
 1R  I 1 f p g 
(1)
Here I 1 is the intensity of the C601+, g 
is the gas density, f p is the photon flux
that is a result of photons hitting and
ionizing nickel mesh. The photon flux is
used to determine the detection
efficiency where f p = current/efficiency.
Once the C60+ cross section is
determined, the other relative ionization
cross sections can be calculated by:
 x  ( A x A1 )  1
(2)
Where x = 2 or 3 and A is the integrated
peak area.
4. Problems with Current
Theories
In past experiments, the results of
the photoionization were compared with
the Wannier theory (WT). The WT is
used to predict the ionization cross
section of atoms. This theory predicts
that the double-photoionization (DPI)
cross section rises at threshold according
to a power law [1]:

   0 Eexc
(3)
Where  is the cross section near
threshold, the excess energy given
by Eexc  hv  E0 , E0 is the threshold
energy,  0 is a constant, and α is the
Wannier exponent. The value of the
Wannier exponent is not the same for
ionization that is due to electrons and
due to photons. The value also changes
for each charge state.
The WT assumes that both of the
emitted electrons have the same
momentum and maintain an equal
distance from the nucleus [1]. This
assumption limits the theory. This theory
is limited further because it is only for
modeling the effects of the relative
ionization cross section of atoms near
threshold energies, in our experiment we
are using molecules and going to
energies that are not near threshold, for
this reason this theory does not apply.
Another theory that was used to
predict cross sections near threshold is
the Coulomb-dipole (CD) theory. This
theory is based on the interactions
between at least two emitted electrons
and the remaining nucleus. For this
scenario, one of the emitted electrons
has less momentum then the other. The
faster electron is subjected to a dipole
potential formed by the residual ion and
the slower electron [1]. This electron is
subject to a Coulomb and a dipole field.
This dipole interaction caused a
characteristic oscillatory behavior.
This theory predicts that the
cross section oscillates while increasing
monotonically near threshold [1]. The
theory states:
14
  Eexc [1  CEexc
 M ( Eexc )]
can be investigated. We first analyzed
the C60 with excess energy ranging from
0-24eV. As in Beryllium and Lithium
cases [1-2], oscillating behavior in the
cross section ratio of the C602+/C601+
charge state was observed.
(4)
The  is the cross section near
threshold, Eexc is the excess energy, M is
a modulation factor given by:
M ( E )  sin[ D ln( Eexc )   ]
(5)
Where C, D, and  are all suitable
constants. In contrast to the WP theory,
the CDT does predict oscillations in the
cross sections near threshold due to the
dipole interaction [1]. However, this
theory does not apply to our current
experiment either because the CD theory
only applies to atoms, not molecules.
Nevertheless, first indications for
oscillations in the C602+ cross sections
have been observed recently.
Other than the aforementioned
theories, a new theory is being
developed that could correctly predict
the ionization cross sections for C60. J.
M. Rost at the Max Planck Institute for
the Physics of Complex Systems is
developing this theory [8]. Once this
theory is complete, we will compare our
results with this theory.
5. Results and Discussion
The ratios of the integrated peak
areas were graphed versus the excess
energy. From this graph, the data can be
closely analyzed and any abnormalities
Figure 3: The ratio of the ionization charge states of
the C602+/ C601+ as a function of excess energy.
The behavior of the cross
sectional ratio, as shown in figure 3,
does not exhibit a linear quality as the
excess energy increases. To investigate
this behavior we subtracted out a
seventh-order Wannier curve from data
to provide a smooth curve as in [1]. We
found that the data closely fit the
equation:
5/ 4
  CEexc
 M ( Eexc )
(6)
Where M ( Eexc ) is the same modulation
factor found in Eq. (5).
Figure 4: The Difference between our DPI cross
section data and a seventh-order Wannier curve. The
gray fit corresponds to the modulation term of the CD
Theory [Eq. (6)].
This data in figure 4 was graphed
with Eq. (6). When the parameters are
chosen correctly for C, D, and , the
oscillating behavior in the data matches
the behavior of Eq. (6). The double
ionization cross section ratio also has an
increasing amplitude and wave length as
the excess energy increases. This is very
strange because Eq. (6) is predicted from
the CDT, and this theory does not apply
to molecules, but the data seems to agree
with this theory. To completely
investigate this behavior, we continued
the experiment and concentrated on the
higher energies.
Once this data was collected, the
program Taurus was used to separate our
data points into various categories. The
data taken with different filters shown in
figures 5 and 6 are now being analyzed.
Once the graphed data are carefully
analyzed, the oscillating behavior will be
investigated.
Figure 5: The ratio of the ionization charge states of
the C602+/ C601+ as a function of excess energy.
Aluminum and a Si3N4 filters were used to filter out
second order light.
Figure 6: The ratio of the ionization charge states of
the C603+/ C601+ as a function of excess energy.
Aluminum and a Si3N4 filters were used to filter out
second order light.
As the excess energy increases,
the data shows that the C602+/C601+ and
C603+/C601+ ratios start to decrease past a
certain 65eV and 55eV respectively.
This behavior was not observed in a
previous C60 experiment [3]. For this
reason, we are now closely investigating
the cross section of the C60 between the
energies of 85-240eV. We are also
trying to determine if this behavior is a
result of using a Si3N4 filter or if this
could be due to temperature dependence.
The Si3N4 filter used could have been
oxidized. The spectrum taken with Si3N4
filter was repeated without that
particular
filter
to
check
for
inconsistencies.
To check for temperature
dependence in the spectrum, we
compared our data with previous
experiments. In a previous experiment
on C60, the oven was kept at a higher
temperature of ≈425 °C and the
temperature range of 420-500 °C was
checked for temperature dependences,
but none was observed [3]. In our
experiment, we are also checking for
temperature dependence but for the
temperature range of 232-344 °C for
several photon energies. The results on
the temperature dependence are still
pending.
6. Applications
Once the relative cross sections
of the ionized C60 are measured, they can
then be used as benchmark information
for theoretical models. The cross section
can also be used to find the opacity of
the different ionization stages of C60.
The opacity is a quantity that is used to
determine the transport of radiation
through matter, which is useful in both
physics and astronomy [9]. Astronomers
have found naturally formed C60 the
interstellar medium [10], as well as
ionized C60.
Acknowledgments
I would like to thank the REU
program at University of WisconsinMadison, and the staff of the
Synchrotron Radiation Center for their
support. I would also like to thank my
mentor Ralf Wehlitz, and Pavle Juranic
for all their help and guidance. This
work is based upon research conducted
at the Synchrotron Radiation Center,
University
of
Wisconsin-Madison,
which is supported by the NSF under
Award No. DMR-0084402
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