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Multiple Photoionization of CMultiple Photoionization of C6060
K. A. Barger, R. Wehlitz, and P. JuranicK. A. Barger, R. Wehlitz, and P. Juranic
Synchrotron RadiationSynchrotron Radiation Electro Magnetic Radiation emitted Electro Magnetic Radiation emitted
by charged particles that are that are by charged particles that are that are traveling at relativistic speeds and traveling at relativistic speeds and that are accelerated by magnetic that are accelerated by magnetic fieldsfields
– The source of this radiation was the The source of this radiation was the Aladdin electron storage ring at the Aladdin electron storage ring at the Synchrotron Radiation Center (SRC) in Synchrotron Radiation Center (SRC) in Stoughton, Wisconsin.Stoughton, Wisconsin.
Schematic of the Aladdin ringSchematic of the Aladdin ring
Port 0426m TGM
Bending Magnets
Undulators
Flux vs. the Aladdin ring photon Flux vs. the Aladdin ring photon energy for SRC's bending energy for SRC's bending magnets and undulatorsmagnets and undulators
PhotoionizationPhotoionization
Photo-effect: Usually thought of as one photon being Photo-effect: Usually thought of as one photon being absorbed by the atom/molecule and one electron is emittedabsorbed by the atom/molecule and one electron is emitted
This is when a This is when a photon interacts photon interacts with a particle with a particle causing it to lose causing it to lose one or more one or more electrons and electrons and become positively become positively chargedcharged
Simultaneous emissionSimultaneous emission One photon comes One photon comes
in and causes two in and causes two electrons to be electrons to be simultaneously ejected simultaneously ejected through electron through electron correlationcorrelation
Coulomb Dipole Coulomb Dipole interactions occur interactions occur between the:between the:– Emitted electronsEmitted electrons– Remaining electronsRemaining electrons– Nucleus of the atomNucleus of the atom
+-
The Cross Section The Cross Section σσ
The ionization The ionization cross section is cross section is a measure of a measure of the probability the probability that the particle that the particle will become will become ionized.ionized.
dparticles sca ttered o d tim e
particles inciden t tim e
t et nuclei encoun tered
area o f beam
# in t /
# /
# arg
Example of Rutherford Scattering Cross SectionsExample of Rutherford Scattering Cross Sections
History of Double PhotoionizationHistory of Double Photoionization
Other experiments included oxygen & sodium, but had:Other experiments included oxygen & sodium, but had:Large error bars Large error bars Few photon energies Few photon energies
In 1988, In 1988, the first near the first near threshold threshold experiment experiment was done was done on He.on He.
Wannier Theory: α=1.056
Experimental: α=1.05 ± 0.02
79
He measured the He measured the double-to-single double-to-single photoionization photoionization ratio with high ratio with high accuracy near the accuracy near the threshold energy threshold energy and has found and has found oscillations in the oscillations in the double double photoionization photoionization cross sectioncross section
Ralf Wehlitz has studied Li and BeRalf Wehlitz has studied Li and BeRecent YearsRecent Years
BeBe2+2+’s relative cross section as ’s relative cross section as a function of excess energya function of excess energy
Excess Energy = Photon Energy – Threshold EnergyExcess Energy = Photon Energy – Threshold Energy
Be
Double-Photoionization Cross Double-Photoionization Cross Section of BerylliumSection of Beryllium
)(4/5excexc EMCE
])ln(sin[)( excexc EDEM
Coulomb Dipole TheoryCoulomb Dipole Theory
Δσ is the Difference between our DPI cross section data and smooth theoretical Wannier curve
Photoionization of CPhotoionization of C6060
Experimental SetupExperimental SetupPP PP - Pusher- Pusher PlatePlate
EPEP - Extractor - Extractor PlatePlate
CP - CP - Condenser plateCondenser plate
MCP MCP - - Microchannel Microchannel PlatePlate
CFDCFD - Constant Fraction - Constant Fraction DiscriminatorDiscriminator
TAC TAC - Time to Amplitude- Time to Amplitude ConverterConverter
MCB – Multichannel MCB – Multichannel BufferBuffer
PP-Pushes all ions through the extractor plate by creating a localized electric field. The pulse applied to the pusher plate serves as the stare pulse
of the Time-to-Flight measurementEP-a grounded plate marking the boundaryof the localized electric field
CFD-used to cut off noise and it also gives pulse positions that are independent of the
height of the pulses
TAC-measures the time difference between the PP and the time for the C60
ions to reach the MCP
MCP-an array of three detector plates that have voltages between 2800-3000 Volts.
These Plates are designed to convert ionized particles into electric pulses, which can be used to count C60 ions
CP-improves the vacuum by freezing unwanted gases and un-ionized C60 to the
surface of the plate
MCB-sorts the pulse heights into channels which creates a spectrum
Time-of-Flight Mass SpectrometerTime-of-Flight Mass Spectrometer
This spectrum was taken using photons at This spectrum was taken using photons at an energy of 154eV and with the oven set to an energy of 154eV and with the oven set to a temperature of 324°C.a temperature of 324°C.
Measures Measures mass-to-charge mass-to-charge ratio (m/q) which ratio (m/q) which forms separate forms separate peaks for each peaks for each charge statecharge state
This can be This can be used to find the used to find the Relative Relative Ionization Ionization Cross-SectionCross-Section
(atomic mass units/charge)
Ratio of Ionization Charge States Ratio of Ionization Charge States as a Function of Excess Energyas a Function of Excess Energy
Work done by Ralf Wehlitz in March of 2004Work done by Ralf Wehlitz in March of 2004
Oscillations in the COscillations in the C60602+2+/ C/ C6060
++
Cross-Section ratioCross-Section ratio
Work done by Ralf Wehlitz in March of 2004Work done by Ralf Wehlitz in March of 2004
)(4/5excexc EMCE
])ln(sin[)( excexc EDEM
Δσ is the Difference between our DPI cross section data and smooth theoretical Wannier curve
Ratio of Ionization Charge States Ratio of Ionization Charge States as a Function of Excess Energyas a Function of Excess Energy
The ratio of the integrated peak areas The ratio of the integrated peak areas CC6060
2+2+/C/C60601+1+ versus the excess energies versus the excess energies
The The ratio of theratio of the integrated peak areas integrated peak areas CC6060
3+3+/ C/ C60601+1+ versus the excess energiesversus the excess energies
New
Problems with TheoriesProblems with Theories
The Wannier Theory & Coulomb The Wannier Theory & Coulomb Dipole Theory:Dipole Theory:
– Only apply to near thresholdOnly apply to near threshold
– They do not apply to moleculesThey do not apply to molecules
Strangely Coulomb Dipole Theory does correctly Strangely Coulomb Dipole Theory does correctly predict the oscillations in the cross sections for predict the oscillations in the cross sections for
CC6060, but the theory applies to atoms, but the theory applies to atoms
SummarySummary
Using a Time-of-Flight mass spectrometer we are able Using a Time-of-Flight mass spectrometer we are able to studying the 1to studying the 1++ to 3 to 3++ charge states as a function of charge states as a function of excess energy excess energy
This information can be used to determine the relative This information can be used to determine the relative cross sections of each charge state cross sections of each charge state
We have observed that the double ionization cross We have observed that the double ionization cross section ratio does not change linearly, and that the section ratio does not change linearly, and that the amplitude and wave length of the oscillations change amplitude and wave length of the oscillations change with excess energywith excess energy
The theories available only apply to atoms and not The theories available only apply to atoms and not moleculesmolecules
AcknowledgmentsAcknowledgments
I would like to thank the REU program at I would like to thank the REU program at University of Wisconsin-Madison, and the University of Wisconsin-Madison, and the
staff of the Synchrotron Radiation Center for staff of the Synchrotron Radiation Center for their support. I would also like to thank my their support. I would also like to thank my mentor at the SRC Ralf Wehlitz, and Pavle mentor at the SRC Ralf Wehlitz, and Pavle
Juranic as well as my advisor Jim Stewart at Juranic as well as my advisor Jim Stewart at WWU for all their help and guidance. WWU for all their help and guidance.
This work is based upon research This work is based upon research conducted at the Synchrotron Radiation conducted at the Synchrotron Radiation
Center, University of Wisconsin-Madison, Center, University of Wisconsin-Madison, which is supported by the NSF under which is supported by the NSF under
Award No. DMR-0084402Award No. DMR-0084402
References:References:[1] D. Lukić, J. B. Bluett, and R. Wehlitz, Phys. Rev. Lett. 93, 023003 [1] D. Lukić, J. B. Bluett, and R. Wehlitz, Phys. Rev. Lett. 93, 023003
(2004).(2004).[2] R. Wehlitz, J. B. Bluett, and S. B. Whitfield, Phys. Rev. Lett. 89, [2] R. Wehlitz, J. B. Bluett, and S. B. Whitfield, Phys. Rev. Lett. 89,
093002 (2002). 093002 (2002). [3] A. Reinköster, S. Korica, G. Prümper, J. Viefhaus, K. Godehusen, O. [3] A. Reinköster, S. Korica, G. Prümper, J. Viefhaus, K. Godehusen, O.
Schwarzkopf, M Mast, and U. Becker, Rev. Phys. B 37, 2135-2144 Schwarzkopf, M Mast, and U. Becker, Rev. Phys. B 37, 2135-2144 (2004). (2004).
[4] H. Steger, J. de Vries, B. Kamke, W. Kamke, and T. Drewello, Chem. [4] H. Steger, J. de Vries, B. Kamke, W. Kamke, and T. Drewello, Chem. Phys. Lett. 194, 452-456 (1992).Phys. Lett. 194, 452-456 (1992).
[5] R. K. Yoo, B. Ruscic, and J. Berkowitz, J. Chem. Phys. 96, 911-918 [5] R. K. Yoo, B. Ruscic, and J. Berkowitz, J. Chem. Phys. 96, 911-918 (1992).(1992).
[6] R. Wehlitz, D. Lukić, C. Koncz, and I. A. Sellin, Rev. Sci. Instrum. 73, [6] R. Wehlitz, D. Lukić, C. Koncz, and I. A. Sellin, Rev. Sci. Instrum. 73, 1671-1673 (2002).1671-1673 (2002).
[7] J. B. Bluett, D. Lukić, and R. Wehlitz, Phys. Rev. A 69, 042717 (2004).[7] J. B. Bluett, D. Lukić, and R. Wehlitz, Phys. Rev. A 69, 042717 (2004).[8] J. M. Rost, Priv. Comm. [8] J. M. Rost, Priv. Comm. (2004).(2004).[9] M. J. Seaton, J. Rev. Phys. B 20, 6363-6378 (1987).[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).[10] S. Petrie, and D. K. Bohme, Rev. ApJ 540, 869-885 (2000).[11] SRC http://www.src.wisc.edu/ (2004).[11] SRC http://www.src.wisc.edu/ (2004).[12] J. J. Brehm, and W. J. Mullin, Introduction to the Structure of Matter [12] J. J. Brehm, and W. J. Mullin, Introduction to the Structure of Matter
(1989).(1989).[13] C. R. Nave, Rutherford scattering [13] C. R. Nave, Rutherford scattering
hyperphysics.phy-astr.gsu.edu/hbase/rutsca (2003).hyperphysics.phy-astr.gsu.edu/hbase/rutsca (2003).
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