Transcript Slide 1

Ultrafast Electron Diffraction from
Molecules in the Gas Phase
Martin Centurion
University of Nebraska – Lincoln
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Outline
Recent progress in Gas Phase diffraction:
• UED from aligned molecules.
Opportunities and challenges ahead:
• Phase retrieval algorithms.
• Pulse parameters
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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 subAngstrom resolution.
(groundbreaking picosecond experiments were done by the Zewail
group at Caltech)
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Gas Electron Diffraction
Total Scattering
Itot (s)  I at (s)  I mol (s)
s
4

sin( / 2)
Molecular Scattering
I mol (s)  Fij  fi f j
sin(s  rij )
s  rij
rij are the interatomic distances
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Gas Electron Diffraction
Azimuthally averaged sM(s)
Modified scattering intensity
s  I mol
sM ( s) 
I at
Theory
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Experiment
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0.8
0.6
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Experiment
0.4
s (1/Å)
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Sine Transform
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0
0
Radial Distribution function
I-…
-0.2
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-0.4
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F-F
C-F
I-I
-0.6
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Theory
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0
s (1/Å)
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-0.8
-1
C2F4I2
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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.
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Diffraction from Aligned Molecules
Non-adiabatic (field-free) alignment
Random orientation:
limited to 1D information
Aligned molecules:
3D structure accessible
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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).
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2D. Saldin, et. al. Acta Cryst. A66, 32–37 (2010).
Experiment – Target Interaction Region
100 µm diameter interaction region
Overall resolution 850 fs
(first gas phase experiment with sub-ps resolution)
Target:
CF3I
Supersonic
seeded gas jet
(helium)
Simple molecule
with 3D structure
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Experimental Setup
Third Harmonic
generation
Cathode
Anode
Magnetic
Lens
Gas Nozzle
40fs, 1mJ, 800nm
Imaging
Detector
Turbo
pump
Electron pulses
• 25 keV
• 500 fs
• 2000 electrons/pulse
Diffusion
pump
Alignment laser pulses
• 800 nm
• 300 fs
• 2.2 x 1013 W/cm2
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Anisotropic Diffraction Patterns
p3405
p34201refNO0x2E5
p3400
p34151refNO0x2E5
p34451refNO0x2E3
p3430
p3500
p3440
p3490
p3475
p3455
p3470
p3460
p3480
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Δ𝑠𝑀 𝑡 = 𝑠𝑀 𝑡 − 𝑠𝑀𝐺𝑟𝑜𝑢𝑛𝑑
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delay = -0.2
-1.7
-1.2
-0.7
0.3
0.8 ps
1.3
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2.3
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Laser
5 min integration time
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Revivals can also be measured
Data collection
Revival
Non-zero background
after initial alignment
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Experimental Data
Δ𝑠𝑀
90° projection
Δ𝑠𝑀
60° projection
𝑠𝑀
random
orientation
electrons
No laser
Laser
polarization
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Theory – Reconstruction
Phase
retrieval
algorithm
Diffraction
with Perfect
Alignment
Molecular Structure
New path
Diffraction
with Partial
Alignment
There is no
algorithm for
partial
alignment
Molecular Structure
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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.
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Retrieving Perfect Alignment from Multiple
Diffraction Patterns
Partial alignment 90°
60°
Random orientation
Combine multiple diffraction patterns to build the
pattern corresponding to perfect alignment
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Genetic Algorithm for Retrieving Perfect Alignment
Rotation and
averaging
uniform
guess
small
change
Difference with
data
defines error
partial
aligned
retain change
error locally
minimized?
yes
discard change
no
error
reduced?
reconstruct
object
no
yes
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Retrieval Result from Data
100k iterations
2 hours
The algorithm also optimizes for the
degree of alignment.
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Reconstruction of CF3I
Structure from
experimental data
The image is retrieved form the data
without any previous knowledge of the
structure
Experiment
rCI
rFI
z (Å)
I-C-F Angle
Literature
2.19±0.07Å 2.14 Å
2.92±0.09Å 2.89 Å
120±90
1110
r (Å)
C. J. Hensley, J. Yang and M. Centurion, Phys. Rev. Lett. 109, 133202(2012)
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Outline
Recent progress in Gas Phase diffraction:
• 3D structure determination with aligned
molecules.
Opportunities and challenges ahead:
• Phase retrieval algorithms.
• Pulse parameters
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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
Constraints applied on object plane.
Algorithm: Fienup Hybrid Input-Output + Flip-Charge
1D.
2D.
Starodub, J. Spence, D. Saldin, Proc. SPIE Conf., 7800, 7800 (2010).
Saldin, et. al. Acta Cryst. A66, 32–37 (2010).
3D objects
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Temporal Resolution
Ideal parameters:
Pulse duration: ~ 20 fs
Charge: as high as possible
With RF Gun:
100 fs, 1 million e
System was purchased from AccTec in Eindhoven
Group velocity mismatch
Laser with a tilted pulse
front
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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.
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Acknowledgements
Group Members
• Chris Hensley (postdoc)
• Jie Yang (grad student)
• Ping Zhang (postdoc)
• Omid Zandi (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
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