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Multiple Photoionization of Carbon-60
K. A. Barger, P. Juranic, and R. Wehlitz
University of Wisconsin-Madison, Synchrotron Radiation Center
Abstract: In this experiment, C60 clusters were ionized using synchrotron radiation. The relative photoionization cross section of the C60 was studied using
photons with specified energies range from 37 to 160eV. The 1+ to 3+ charge states were investigated as a function of energy using a time-of-flight mass
spectrometer. From this information, the relative ionization cross sections can be studied for each charge state. The current theoretical models available for
predicting the relative ionization cross sections are not valid for molecules or for energies that are greater then a few eV above threshold energy. The
photoionization cross section for C60 has not been closely studied until now and the data collected in this experiment can be used as benchmark data to find
a model for the C60 molecules’ relative cross sections. The results show that relative ionization cross sections do not change linearly, and oscillations in its
size occur with the change in photon energy. This research is still being continued at higher energies to determine when the oscillations discontinue.
In this experiment, C60 was photoionized with single photons with specified energies between
the energy range of 37 to 160eV. The photons used came from synchrotron radiation that occurs when
charged particles are accelerated in a magnetic field. The source of this radiation was the Aladdin
electron storage ring at the Synchrotron Radiation Center (SRC) in Stoughton, Wisconsin.
When these C60 molecules are ionized near threshold energies, the electrons that are emitted will
leave the C60 relatively slowly compared to the ones ionized at higher energies. The threshold energy
is just the minimum energy required to ionize the particle. When two or more electrons are emitted
with different kinetic energies, coulomb dipole interactions may occur. As a result, the electron with
the greatest momentum “sees” the positively ionized C60 and the e- with less momentum as a dipole.
These interactions can cause oscillations in the C602+/C601+ cross section ratio. The ionization cross
section is a measure of the probability that the particle will become ionized. In past experiments, this
oscillating behavior was observed in beryllium and lithium close to threshold. However, past
experiments on ionization C60 do not show this oscillating behavior. This might be the result of large
error bars and insufficient data in those experiments.
In order to measure the ionization cross section of C60, a ion time-of-flight (TOF) spectrometer
was used, schematic shown in figure 1. The time-of-flight spectrometer measures the time a particle
takes to travel down the drift tube. The time is proportional to the mass-to-charge ratio of the particle.
This can be seen by analyzing the force acting on the system. Force is equal to mass times
acceleration which is also equal to the electric field times the charge of the particle. The acceleration
electric field  charge of particle
of the particle is given by:
accelerati on 
mass of particle
This means that if all the particles have the same mass, as in the case of the C60, then the greater
the charge the faster the particle. By comparing these times, the charge state can be determined. Once
the travel time of the ionized C60 is measured, the data is used to create a spectrum of the events, like
the one shown in figure 2. Each channel corresponds to a different travel time down the drift tube
which can be used to calculate the energy and the level of ionization of each particle.
The data from spectra were then plotted as ratios of the integrated peak areas, shown in figure 4.
This figure shows that the ionization ratios do not behave linearly and that they change with photon
energy. To analyze this nonlinear behavior, a smooth polynomial curve was subtracted from the data.
Once this was done, the oscillating behavior, which is believed to be due to coulomb dipole
interactions, became apparent and is shown in figure 5.
*
Figure 3: Photograph of the experimental equipment used. This figure shows the Time-of-Flight spectrometer,
the oven where the C60 is placed, the filters used to filter out second-order light, the Condenser plate that is
cooled by liquid nitrogen, and the beam line which leads to the source of the photons.
Figure 4: The ratio of the ionization charge states of the C602+/ C601+ as a function of excess energy. The excess
energy is the energy of the photons with the double-ionization threshold energy subtracted. The threshold energy
used here is 19.02eV, the amount of energy required to doubly ionize C60. Each data point corresponds to a
spectrum like the one shown in figure 2. This figure shows that the behavior of the C602+/ C601+ ratio does not
behave linearly as the photon energy increases. The non linear behavior of this graph is analyzed in figure 5.
Figure by R. Wehlitz, used with permission.
PP - Pusher Plate
EP - Extractor Plate
CP - Condenser plate
MCP - Microchannel Plate
TAC - Time to Amplitude Converter
* - Interaction Region
Figure 1: Schematic diagram of experimental setup. The C60 in the oven are heated to ~ 310°C. At this
temperature, the C60 will sublimate and turn into a gas. The C60 will then enter the interaction region where
photons enter perpendicular to the plane of the schematic shown above. Once the C60 becomes ionized, it is
accelerated by an electric field created between PP and EP. This will cause the ions to travel down the drift tube.
The drift tube and the MCP makeup the TOF spectrometer. From the C60 travel time, the mass to charge ratio can
be determined. Figure modified from R. Wehlitz, D. Lukić, C. Koncz, and I. A. Sellin, Rev. Sci. Instrum. 73, 1671-1673
(2002).
Figure 5: This plot shows the difference between the doubly photoionized cross section data shown in figure 4 a
smooth polynomial curve. The gray fit corresponds to the Coulomb Dipole Theory. This figure shows that the
doubly ionized cross section of C60 does not behave linearly but instead oscillates. The amplitude and the
wavelength of this oscillations increases with photon energy. Figure by R. Wehlitz, used with permission.
(atomic mass units/charge)
Figure 2: This spectrum was taken using photons at an energy of 154eV and with the oven set to a temperature of
324°C. Each peak consists of ionized C molecules as a measure of the mass-to-charge ratio. The amount of time
the ions travel down the drift tube is measured by the MCP. The MCB then categorizes the measured times and
creates a spectrum like the one shown above. Each one of these spectra corresponds to one data point in figure 4.
Conclusion: The Coulomb Dipole theory predicts oscillations in the
ionization cross section near threshold energies. In this experiment, these
oscillations where observed in the photoionization cross section ratios of
C602+/C60+ not only near threshold but also at energies far above. These
oscillations have been found to have an increasing wavelength and amplitude
between the energies of 19 to 160eV. This research is currently being
continued to 300eV to determine when these oscillations discontinue.
Acknowledgments
K. A. Barger would like to thank the NSF REU program at University of Wisconsin-Madison, and the staff of the Synchrotron Radiation Center for their support. K. A. Barger would also like to thank her mentor at the SRC Ralf Wehlitz, and Pavle Juranic as well as
her advisor Jim Stewart at WWU 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