Transcript electron diffraction

Probing Lattice Dynamics
Pampa Devi
April 29,2009
Go Through….
Bragg Law
Pump Probe Set Up
Velocity Mismatch Problem
Way to overcome it
Bragg Law
W. L. Bragg presented a simple explanation of the
diffracted beams from a crystal based on a
reflection from planes of atoms.
The difference in the paths
traversed by the two beams
shown in the figure is:


dsin
2dsin = nl
d
X-Ray diffraction
Electron Diffraction
Pump Probe
Photography
Powder Diffraction
Au
Seeing is believing ..
Committee Meeting # 2
Hard to see …
Ruan et al.
8
Ultrafast Electron Diffraction Groups Around the World
Velocity Mismatch Picture
Pump (Laser):
c
Probe (Electron Beam):
0.34 c
Why not use laser as
probe ???
No excitation
Accessories :
Excitation
After Excitation
Peter Baum and Ahmed H. Zewail
AC AB
t 

vel
c
AC AB '

vel
c
1
v
(
Cos

)
Sin

1
el
   tan (
)
1
2
c  vel (Cos ) Cos
Passing through a grating does the job…
l
1
r  Cos (
1
d tan 
l
)
i  Sin (  Sin ( r ))
d
Implementation