Ultrafast Electron Diffraction from Molecules in the Gas Phase Martin Centurion

1. Ultrafast Electron Diffraction from Molecules in the Gas Phase Martin Centurion University of Nebraska – Lincoln

2. Outline • Recent progress in Gas Phase diffraction: • UED from aligned molecules. • Opportunities and challenges ahead: • Phase retrieval algorithms. • Pulse parameters

3. Ultrafast Gas Phase Electron Diffraction Structure and Dynamics of Isolated Molecules • Determine the 3D structure of molecules without crystallization. • Investigate photoreactions of isolated molecules. Image intermediate states with femtosecond and sub-Angstrom resolution. • (groundbreaking picosecond experiments were done by the Zewail group at Caltech)

4. Gas Electron Diffraction Total Scattering Molecular Scattering rijare the interatomic distances

5. Gas Electron Diffraction 1 Theory 6 0.8 0.6 4 0.4 2 0.2 Experiment 0 0 I-… F-F -0.2 2 C-F I-I -0.4 4 -0.6 -0.8 6 -1 6 4 2 0 2 4 6 Azimuthally averaged sM(s) Modified scattering intensity Experiment Sine Transform s (1/Å) Radial Distribution function Theory s (1/Å) C2F4I2

6. Gas Electron Diffraction • Advantages • High Scattering Cross Section. • Compact Setup. • Limited by random orientation of molecules: • 1D Information. • Structure is retrieved by iteratively comparing the data with a theoretical model. • Low contrast diffraction patterns.

7. Diffraction from Aligned Molecules Non-adiabatic (field-free) alignment Random orientation: limited to 1D information Aligned molecules: 3D structure accessible

8. From diffraction pattern to structure Perfect alignment — <cos2α> = 1 r Fourier-Hankel Transform1,2 z Partial alignment — <cos2α> = 0.50 α Fourier-Hankel Transform1,2 1P. Ho et. al. J. Chem. Phys. 131, 131101 (2009). 2D. Saldin, et. al. ActaCryst. A66, 32–37 (2010).

9. Experiment – Target Interaction Region 100 µm diameter interaction region Overall resolution 850 fs (first gas phase experiment with sub-ps resolution) Supersonic seeded gas jet (helium) Target: CF3I alignment laser electron pulse Simple molecule with 3D structure

10. Experimental Setup Magnetic Lens CathodeAnode Gas Nozzle Third Harmonic generation 40fs, 1mJ, 800nm Imaging Detector Turbo pump Diffusion pump • Electron pulses • 25 keV • 500 fs • 2000 electrons/pulse • Alignment laser pulses • 800 nm • 300 fs • 2.2 x 1013 W/cm2

11. Anisotropic Diffraction Patterns Laser delay = -1.7 ps delay = 4.8 ps delay = 4.3 ps delay = 3.8 ps delay = 3.3 ps delay = 2.8 ps delay = 2.3 ps delay = 1.3 ps delay = 0.8 ps delay = 0.3 ps delay = -0.2 ps delay = -0.7 ps delay = -1.2 ps delay = 1.8 ps 5 min integration time

12. Revivals can also be measured Data collection Revival Non-zero background after initial alignment

13. Experimental Data 90° projection 60° projection random orientation electrons No laser Laser polarization

14. Theory – Reconstruction Molecular Structure Phase retrieval algorithm Diffraction with Perfect Alignment New path There is no algorithm for partial alignment Molecular Structure Diffraction with Partial Alignment

15. Retrieving Perfect Alignment from Multiple Diffraction Patterns Perfect alignment Perpendicular Partial alignment Any orientation Rotation and averaging • Transformation requires knowledge of the degree of alignment (angular distribution), but not the structure. • There is no inverse transformation.

16. Retrieving Perfect Alignment from Multiple Diffraction Patterns Partial alignment 90° Random orientation 60° Combine multiple diffraction patterns to build the pattern corresponding to perfect alignment

17. Genetic Algorithm for Retrieving Perfect Alignment Rotation and averaging Difference with datadefines error partial aligned uniformguess smallchange retain change error locally minimized? yes no error reduced? discard change no yes reconstruct object

18. Retrieval Result from Data 100k iterations 2 hours The algorithm also optimizes for the degree of alignment.

19. Reconstruction of CF3I The image is retrieved form the data without any previous knowledge of the structure Structure from experimental data z (Å) r (Å) C. J. Hensley, J. Yang and M. Centurion, Phys. Rev. Lett.109, 133202(2012)

20. Outline • Recent progress in Gas Phase diffraction: • 3D structure determination with aligned molecules. • Opportunities and challenges ahead: • Phase retrieval algorithms. • Pulse parameters

21. Work in progress: Modified iterative phase retrieval algorithm for molecules of unknown symmetry Benzotrifluoride (C7H5F3) 2D object Simulated pattern in cylindrical coordinates Inputs: Diffraction Pattern Constraintsapplied on object plane. Algorithm: Fienup Hybrid Input-Output + Flip-Charge 3D objects 1D. Starodub, J. Spence, D. Saldin, Proc. SPIE Conf., 7800, 7800 (2010). 2D. Saldin, et. al. ActaCryst. A66, 32–37 (2010).

22. Temporal Resolution Ideal parameters: Pulse duration: ~ 20 fs Charge: as high as possible With RF Gun: 100 fs, 1 million e Group velocity mismatch Laser with a tilted pulse front System was purchased from AccTec in Eindhoven

23. Summary • 3D imaging of molecules possible with laser-aligned molecules. • Molecular dynamics can be probed in a field free environment. • We are working to apply this method to larger molecules. • RF gun will greatly improve the experimental conditions.

24. Acknowledgements • Group Members • Chris Hensley (postdoc) • Jie Yang (grad student) • Ping Zhang (postdoc) • OmidZandi (grad student) • Walter Bircher (undergrad) • Former members • Cory Baumgarten (undergrad) • James Ferguson (undergrad) • Funding • Department of Energy, Basic Energy Sciences • Air Force Office of Scientific Research