Transcript Photoionization of C60 - UW Madison Astronomy Department

Multiple Photoionization of C60
K. A. Barger, R. Wehlitz, and P. Juranic
Synchrotron Radiation
 Electro Magnetic Radiation emitted
by charged particles that are that are
traveling at relativistic speeds and
that are accelerated by magnetic
fields
– The source of this radiation was the
Aladdin electron storage ring at the
Synchrotron Radiation Center (SRC) in
Stoughton, Wisconsin.
Schematic of the Aladdin ring
Port 042
6m TGM
Flux vs. the Aladdin ring photon
energy for SRC's bending
magnets and undulators
Photoionization
This is when a
photon interacts
with a particle
causing it to lose
one or more
electrons and
become positively
charged
Photo-effect: Usually thought of as one photon being
absorbed by the atom/molecule and one electron is emitted
Simultaneous emission
 One photon comes
in and causes two
electrons to be
simultaneously
ejected through
electron correlation
 Coulomb Dipole
interactions occur
between the:
– Emitted electrons
– Remaining electrons
– Nucleus of the atom
-
+
The Cross Section σ
The ionization
cross section is
a measure of
the probability
that the particle
will become
ionized.
Example of Rutherford Scattering Cross Sections
d 
# particles scattered int o d / time
# particles incident / time
# t arg et nuclei encountered
area of beam
History of Double Photoionization
In 1988,
the first near
threshold
experiment
was done
on He.
79
Wannier Theory:
α=1.056
Experimental:
α=1.05 ± 0.02
Other experiments included oxygen & sodium, but had:
Large error bars
Few photon energies
Recent Years
Ralf Wehlitz has studied Li and Be
Be
Be2+’s relative cross section
as a function of excess energy
He measured the
double-to-single
photoionization
ratio with high
accuracy near the
threshold energy
and has found
oscillations in the
double
photoionization
cross section
Excess Energy = Photon Energy – Threshold Energy
Double-Photoionization Cross
Section of Beryllium
5/ 4
  CEexc
 M (Eexc )
Δσ is the Difference between our DPI cross
section data and smooth theoretical Wannier curve
M ( Eexc )  sin[D ln(Eexc )   ]
Coulomb Dipole Theory
Photoionization of C60
Experimental Setup
PP - Pusher Plate
TAC-measures
the
time
difference
MCB-sorts
the
into gives
CFD-used
to cut
offpulse
noiseheights
and it also
PP-Pushes
all
ions
through
the
EP
- Extractor
Plate
between
the
PP
and
the
time
for
the Cof60
channels
which
creates
a
spectrum
pulse positions that are independent
the
extractor
plate
by creating
a localized
ions
to
reach
the
MCP
CP - Condenser plate
height of the pulses
electric field. The pulse applied to the
pusher plate serves as the MCP
stare- Microchannel
pulse
Plate
EP-aCP-improves
plate
marking
thefreezing
boundary
ofgrounded
the Time-to-Flight
the vacuum
measurement
by
CFD
- Constant
Fraction
MCP-an
of
array
the
localized
of
three
electric
detector
field
plates
that
unwanted gases and un-ionized CDiscriminator
60 to the
have voltages
between
Volts.
surface
of the2800-3000
plate
- Time to Amplitude
These Plates are designedTAC
to convert
Converter
ionized particles into electric pulses,
MCB – Multichannel
which can be used to count C60Buffer
ions
Time-of-Flight Mass Spectrometer
Measures
mass-to-charge
ratio (m/q) which
forms separate
peaks for each
charge state
(atomic mass units/charge)
This spectrum was taken using photons at
an energy of 154eV and with the oven set to
a temperature of 324°C.
This can be
used to find the
Relative
Ionization
Cross-Section
Ratio of Ionization Charge States
as a Function of Excess Energy
Work done by Ralf Wehlitz in March of 2004
Oscillations in the C602+/ C60+
Cross-Section ratio
5/ 4
  CEexc
 M (Eexc )
Δσ is the Difference between our DPI cross
section data and smooth theoretical Wannier curve
M ( Eexc )  sin[D ln(Eexc )   ]
Work done by Ralf Wehlitz in March of 2004
Ratio of Ionization Charge States
as a Function of Excess Energy
New
The ratio of the integrated peak areas
C602+/C601+ versus the excess energies
The ratio of the integrated peak areas
C603+/ C601+ versus the excess energies
Problems with Theories
The Wannier Theory & Coulomb
Dipole Theory:
– Only apply to near threshold
– They do not apply to molecules
Strangely Coulomb Dipole Theory does correctly
predict the oscillations in the cross sections for
C60, but the theory applies to atoms
Summary
 Using a Time-of-Flight mass spectrometer we are able
to studying the 1+ to 3+ charge states as a function of
excess energy
 This information can be used to determine the relative
cross sections of each charge state
 We have observed that the double ionization cross
section ratio does not change linearly, and that the
amplitude and wave length of the oscillations change
with excess energy
 The theories available only apply to atoms and not
molecules
Acknowledgments
I would like to thank the REU program at
University of Wisconsin-Madison, and the
staff of the Synchrotron Radiation Center for
their support. I would also like to thank my
mentor at the SRC Ralf Wehlitz, and Pavle
Juranic as well as my 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
References:
[1] D. Lukić, J. B. Bluett, and R. Wehlitz, Phys. Rev. Lett. 93, 023003
(2004).
[2] R. Wehlitz, J. B. Bluett, and S. B. Whitfield, Phys. Rev. Lett. 89,
093002 (2002).
[3] A. Reinköster, S. Korica, G. Prümper, J. Viefhaus, K. Godehusen, O.
Schwarzkopf, M Mast, and U. Becker, Rev. Phys. B 37, 2135-2144
(2004).
[4] H. Steger, J. de Vries, B. Kamke, W. Kamke, and T. Drewello, Chem.
Phys. Lett. 194, 452-456 (1992).
[5] R. K. Yoo, B. Ruscic, and J. Berkowitz, J. Chem. Phys. 96, 911-918
(1992).
[6] R. Wehlitz, D. Lukić, C. Koncz, and I. A. Sellin, Rev. Sci. Instrum. 73,
1671-1673 (2002).
[7] J. B. Bluett, D. Lukić, and R. Wehlitz, Phys. Rev. A 69, 042717 (2004).
[8] J. M. Rost, Priv. Comm. (2004).
[9] M. J. Seaton, J. Rev. Phys. B 20, 6363-6378 (1987).
[10] S. Petrie, and D. K. Bohme, Rev. ApJ 540, 869-885 (2000).
[11] SRC http://www.src.wisc.edu/ (2004).
[12] J. J. Brehm, and W. J. Mullin, Introduction to the Structure of Matter
(1989).
[13] C. R. Nave, Rutherford scattering hyperphysics.phyastr.gsu.edu/hbase/rutsca (2003).