sub-picosecond megavolt electron diffraction
Post on 11-Feb-2016
40 Views
Preview:
DESCRIPTION
TRANSCRIPT
Sub-picosecond Megavolt
Electron DiffractionInternational Symposium on Molecular Spectroscopy
June 21, 2006
Fedor RudakovDepartment 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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Electron diffraction experiment.
r = 3.027 Å
r = 2.667 Å I2 ground state
I2 excited state
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Problem: scattering angles of relativistic electrons are very small
Electron Bunch Parameters
Parameter Value Units Charge 16 pC
Number of electrons 108 - Energy 5.5 MeV
rms Energy Spread 36 keV rms Pulse Length 0.44 ps rms Beam Size 1.7 mm
rms Beam Divergence 45 rad Solenoid Field 1.7 kG Gun Gradient 110 MV/m
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
GTF (gun test facility) beam line at SLAC
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Simulated Single-Shot Diffraction
Theoretical scattering image, and radially averaged scattering signal of aluminum foil
2 pC (1.2x107) No aperture
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
Tota
l bun
ch c
harg
e: 3
pC
= 2
·107 e
lect
rons
Alu
min
um fo
il th
ickn
ess:
160
nm
Drif
t tub
e le
ngth
: 3.9
5 m
Bea
m E
nerg
y: 5
.5 M
eV k
inet
icPu
lse
dura
tion:
500
fs
Important parameters:
Single Shots!
Dark current image subtracted
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
Application Small MoleculesSmall MoleculesPhase transitions
Time scales ≈ 1 ps ≈ 100 fs
Limitations Space 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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Acknowledgments
• Peter Weber•David Dowell•John Schmerge•Jerome Haistings
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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 SectionRutherford
differential scattering cross section of a single point charge:
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
m m0
1 2
dd
s 2me2
2 s 2
2
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?
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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Electron Wavelength
Experimentsat SLAC:5 MeV
= 230 fm = v/c =0.995
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Electron BunchesCharacterization: D. Dowell, J. Schmerge
0 50 100 150 200 250 3000
0.5
1
1.5
2
RMS Bunch Length (ps)
Bunch Charge (pC)-1 -0.5 0 0.5 1
-20
-10
0
10
20
Time (ps)
Ene
rgy
(keV
)
Electron Bunch Length vs. Charge
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Simulation of the MeV RF Gun QuickTime™ and a
TIFF (LZW) decompressorare needed to see this picture.
mm0
1
1 2
vc
RF amplitude:
Scattering Angles
Bragg’s law:
2d sinBB = Bragg angle d = lattice constant
Example: 5 MeV kinetic energy for the electronsλ=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!
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
– 300.000
Question: are the beam parameters sufficient to resolve diffraction patterns?
Conclude:• Divergence is sufficiently small• 2 pC = 1.2x107 electrons within the
pulse is okay
top related