Multiple Photoionization of C 60 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 Bending Magnets Undulators

Flux vs. the Aladdin ring photon energy for SRC's bending magnets and undulators

Port 042-6m TGM

Photoionization Photo-effect: Usually thought of as one photon being absorbed by the atom/molecule and one electron is emitted This is when a photon interacts with a particle causing it to lose one or more electrons and become positively charged

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

History of Double Photoionization Other experiments included oxygen & sodium, but had: Large error bars Few photon energies In 1988, the first near threshold experiment was done on He. Wannier Theory: α=1.056 Experimental: α=1.05 ±

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 Ralf Wehlitz has studied Li and Be Recent Years Be 2+ ’s relative cross section as a function of excess energy Excess Energy = Photon Energy – Threshold Energy Be

Double-Photoionization Cross Section of Beryllium Coulomb Dipole Theory Δσ is the Difference between our DPI cross section data and smooth theoretical Wannier curve

Photoionization of C 60 Experimental Setup PP - Pusher Plate EP - Extractor Plate CP - Condenser plate MCP - Microchannel Plate CFD - Constant Fraction Discriminator TAC - Time to Amplitude Converter Converter MCB – Multichannel Buffer 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 measurement EP-a grounded plate marking the boundary of 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 C 60 ions to reach the MCP MCP-an array of three detector plates that have voltages between Volts. These Plates are designed to convert ionized particles into electric pulses, which can be used to count C 60 ions CP-improves the vacuum by freezing unwanted gases and un-ionized C 60 to the surface of the plate MCB-sorts the pulse heights into channels which creates a spectrum

Time-of-Flight Mass Spectrometer This spectrum was taken using photons at an energy of 154eV and with the oven set to a temperature of 324°C. Measures mass-to-charge ratio (m/q) which forms separate peaks for each charge state This can be used to find the Relative Ionization Cross-Section (atomic mass units/charge)

Ratio of Ionization Charge States as a Function of Excess Energy Work done by Ralf Wehlitz in March of 2004

Oscillations in the C / C 60 + Cross-Section ratio Work done by Ralf Wehlitz in March of 2004 Δσ is the Difference between our DPI cross section data and smooth theoretical Wannier curve

Ratio of Ionization Charge States as a Function of Excess Energy The ratio of the integrated peak areas C /C versus the excess energies The ratio of the integrated peak areas C / C versus the excess energies New

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 C 60, 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

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