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

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