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

Electron diffraction experiment. r = Å r = Å I 2 ground state I 2 excited state

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

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

Electron Bunch Parameters

GTF (gun test facility) beam line at SLAC

Simulated Single-Shot Diffraction Theoretical scattering image, and radially averaged scattering signal of aluminum foil 2 pC (1.2x10 7 ) No aperture

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.

Effect of Charge and Laser Pulse on Electron Pulse Duration

First MeV results 1600 Ångstrom Foil in Foil out Total bunch charge: 3 pC = 2 · 10 7 electrons Aluminum foil thickness: 160 nm Drift tube length: 3.95 m Beam Energy: 5.5 MeV kinetic Pulse duration: 500 fs Important parameters: Single Shots! Dark current image subtracted

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

Comparison of electron probe techniques UED (10’s of kV) MeV-UED ApplicationSmall Molecules Phase transitions Time scales≈ 1 ps≈ 100 fs LimitationsSpace charge Scattering angle resolution?

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

Acknowledgments Peter Weber David Dowell John Schmerge Jerome Haistings

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!

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

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?

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

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

Electron Bunches Characterization: D. Dowell, J. Schmerge RMS Bunch Length (ps) Bunch Charge (pC) Time (ps) Energy (keV) Electron Bunch Length vs. Charge

Simulation of the MeV RF Gun RF amplitude:

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

MeV-UED simulations Program: GTP (General Particle Tracer) Realistic geometries Includes AC & DC fields Charge per pulse 2pC No Collimator Total number of particles in the simulation – Question: are the beam parameters sufficient to resolve diffraction patterns? Conclude: Divergence is sufficiently small 2 pC = 1.2x10 7 electrons within the pulse is okay