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Sub-picosecond Megavolt Electron Diffraction

Sub-picosecond Megavolt Electron Diffraction. International Symposium on Molecular Spectroscopy June 21, 2006 . Stanford Linear Accelerator : J. Hastings D. Dowell J. Schmerge. Brown University : Peter Weber Job Cardoza. Fedor Rudakov Department of Chemistry,

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Sub-picosecond Megavolt Electron Diffraction

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  1. Sub-picosecond Megavolt Electron Diffraction International Symposium on Molecular Spectroscopy June 21, 2006 • Stanford Linear Accelerator: • J. Hastings • D. Dowell • J. Schmerge • Brown University: • Peter Weber • Job Cardoza Fedor Rudakov Department of Chemistry, Brown University, Providence, R.I, USA. Funding: Department of Energy Army Research Office

  2. Electron diffraction experiment. r = 2.667 Å I2 ground state r = 3.027 Å I2 excited state

  3. Time resolution limitations: • Space charge effect • Laser pulse and electron pulse velocity mismatch • Initial electron velocity spread.

  4. Megavolt electron diffraction. Advantages of relativistic electron beams for ultrafast electron diffraction: • Shorter electron bunches • AC field allows electron pulse compression • Velocity spread for highly relativistic particles becomes becomes negligible even though the energy spread can be large. •  Higher charge per pulsepossibility to obtain diffraction patterns with a single electron pulse. Problem: scattering angles of relativistic electrons are very small

  5. Electron Bunch Parameters

  6. GTF (gun test facility) beam line at SLAC

  7. Simulated Single-Shot Diffraction Theoretical scattering image, and radially averaged scattering signal of aluminum foil 2 pC (1.2x107) No aperture

  8. Space-Charge Effects: Spatial Patterns Calculated diffraction pattern of a 1500 nm aluminum foil: 5 pC electron pulse 2 pC electron pulse Both images obtained with optimal focusing conditions.

  9. Effect of Charge and Laser Pulse on Electron Pulse Duration

  10. First MeV results Single Shots! 1600 Ångstrom Foil in Foil out Dark current image subtracted Important parameters: Total bunch charge: 3 pC = 2·107 electrons Aluminum foil thickness: 160 nm Drift tube length: 3.95 m Beam Energy: 5.5 MeV kinetic Pulse duration: 500 fs

  11. Comparison to a theoretical pattern (111) Theory: calculation with GPT; inclusion of quadrupole and all elements (311) (200) (220) Experiment

  12. Comparison of electron probe techniques

  13. Summary on MeV-UED • MeV-UED is a feasible tool for measuring structural dynamics! • We obtained diffraction patterns with single shots … • … of femtosecond electron pulses! • This opens the door for: • Electron diffraction with 100 fs time resolution

  14. Acknowledgments • Peter Weber • David Dowell • John Schmerge • Jerome Haistings

  15. Differential Scattering Cross Sections • The differential cross section increases with increasing energy • This just balances the loss of signal from the small scattering angles! •  Overall: there is no signal penalty in going to relativistic electrons!

  16. Relativistic Scattering Cross Section Rutherford differential scattering cross section of a single point charge:

  17. Total Scattering Cross Section Total Scattering Cross Section F. Salvat, Phys. Rev. A, 43, 578 (1991) • The total scattering cross section is largely unchanged • The diffraction signal is highly centered at small scattering angles •  Does the signal decrease dramatically?

  18. The case for MeV Advantages of relativistic electron beams for ultrafast electron diffraction: • Shorter electron bunches • AC field allows electron pulse compression • Velocity spread for highly relativistic particles becomes becomes negligible even though the energy spread can be large. •  Higher charge per pulsepossibility to obtain diffraction patterns with a single electron pulse. •  Larger Penetration Depth •  Smaller Scattering Angles

  19. Electron Wavelength • Experiments • at SLAC: • 5 MeV • = 230 fm  = v/c =0.995

  20. Electron BunchesCharacterization: D. Dowell, J. Schmerge 2 1.5 20 10 1 RMS Bunch Length (ps) 0 Energy (keV) 0.5 -10 -20 0 0 50 100 150 200 250 300 Bunch Charge (pC) -1 -0.5 0 0.5 1 Time (ps) Electron Bunch Length vs. Charge

  21. Simulation of the MeV RF Gun RF amplitude:

  22. Scattering Angles Bragg’s law: B=Bragg angle d = lattice constant Example: 5 MeV kinetic energy for the electrons λ=0.00223Å 2.34Å d-spacing for Al (111)  Bragg angle: 476 micro-radians • Conclude: • Detector can be far separated from sample: 5 - 10 m • MeV-ED is useful to make structural measurements on samples that are far from the detector!

  23. MeV-UED simulations Question: are the beam parameters sufficient to resolve diffraction patterns? • Program: GTP (General Particle Tracer) • Realistic geometries • Includes AC & DC fields • Charge per pulse 2pC • No Collimator • Total number of particles in the simulation – 300.000 Conclude: • Divergence is sufficiently small • 2 pC = 1.2x107 electrons within the pulse is okay

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