Ultrafast electron diffraction (UED), also known as femtosecond electron diffraction (FED), is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. UED provides information on the dynamical changes of the structure of materials. It is very similar to time resolved crystallography, but instead of using X-rays as the probe, it uses electrons. In the UED technique, a femtosecond (fs) laser optical pulse excites (pumps) a sample into an excited, usually non-equilibrium, state. The pump pulse may induce chemical, electronic or structural transitions. After a finite time interval, a fs electron pulse is incident upon the sample. The electron pulse undergoes diffraction as a result of interacting with the sample. The diffraction signal is, subsequently, detected by an electron counting instrument such as a CCD camera. Specifically, after the electron pulse diffracts from the sample, the scattered electrons will form a diffraction pattern (image) on a CCD camera. This pattern contains structural information about the sample. By adjusting the time difference between the arrival (at the sample) of the pump and probe beams, one can obtain a series of diffraction patterns as a function of the various time differences. The diffraction data series can be concatenated in order to produce a motion picture of the changes that occurred in the data. UED can provide a wealth of dynamics on charge carriers, atoms, and molecules.

History

The design of early ultrafast electron diffraction instruments was based on x-ray streak cameras, the first reported UED experiment demonstrating an electron pulse length of 100 ps.^[1]

Electron Pulse Production

The electron pulses are typically produced by the process of photoemission in which a fs optical pulse is directed toward a photocathode.^[2] If the incident laser pulse has an appropriate energy, electrons will be ejected from the photocathode through a process known as photoemission. The electrons are subsequently accelerated to high energies, ranging from tens of kiloelectron-volts^[3] to several megaelectron-volts,^[4] using an electron gun.

Electron Pulse Compression

Generally, two methods are used in order to compress electron pulses in order to overcome pulsewidth expansion due to Coulomb repulsion. Generating high-flux ultrashort electron beams has been relatively straightforward, but pulse duration below a picosecond proved extremely difficult due to space-charge effects. Space-charge interactions increase in severity with bunch charge and rapidly act to broaden the pulse duration, which has resulted in an apparently unavoidable trade-off between signal (bunch charge) and time-resolution in ultrafast electron diffraction (UED) experiments. Radio-frequency (RF) compression has emerged has an leading method of reducing the pulse expansion in UED experiments, achieving temporal resolution well below 50 femtoseconds.^[5] Shorter electron beams below 10 femtoseconds are ultimately required to probe the fastest dynamics in solid state materials and observe gas phase molecular reactions.^[6]

Single shot

Schematic of Ultrafast Electron Diffraction showing the pump pulse (red) exciting the sample prior to the arrival of the electron probe pulse (blue)

For studying irreversible process, a diffraction signal is obtained from a single electron bunch containing ${\displaystyle 10^{5}}$ or more particles.^[7]

Stroboscopic

When studying reversible process, especially weak signals caused by, e.g., thermal diffuse scattering, a diffraction pattern is accumulated from many electron bunches, as many as ${\displaystyle 10^{8}}$.^[8]

Resolution

The resolution of an ultrafast electron diffraction apparatus can be characterized both in space and in time. Spatial resolution comes in two distinct parts: real space and reciprocal space. Real space resolution is determined by the physical size of the electron probe on the sample. A smaller physical probe size can allow experiments on crystals that cannot feasibly be grown in large sizes.^[9]

High reciprocal space resolution allows for the detection of Bragg diffraction spots that correspond to long periodicity phenomena. It can be calculated with the following equation:^[4]

${\displaystyle \Delta s={\frac {2\pi }{\lambda _{e}}}{\frac {\varepsilon _{n}}{\sigma _{x}}}}$,

where Δs is the reciprocal space resolution, λ_e is the Compton wavelength of the electrons, ϵ_n is the normalized emittance of the electrons, and σ_x is the size of the probe on the sample.

Temporal resolution is primarily a function of the bunch length of the electrons and the relative timing jitters between the pump and probe.^[4]

See also

• Ahmed Zewail
• R. J. Dwayne Miller
• Time resolved crystallography

References

Sources

• Srinivasan, Ramesh; Lobastov, Vladimir A.; Ruan, Chong-Yu; Zewail, Ahmed H. (2003). "Ultrafast Electron Diffraction (UED): A New Development for the 4D Determination of Transient Molecular Structures". Helvetica Chimica Acta. 86 (6): 1761. doi:10.1002/hlca.200390147.

• Sciani, Germain; Miller, R.J. Dwayne (2011). "Femtosecond electron diffraction: heralding the era of atomically resolved dynamics". Reports on Progress in Physics. 74 (9): 096101. Bibcode:2011RPPh...74i6101S. doi:10.1088/0034-4885/74/9/096101. S2CID 121497071.
• Chatelain, Robert P.; Morrison, Vance R.; Godbout, Chris; Siwick, Bradley J. (2012). "Ultrafast electron diffraction with radio-frequency compressed electron pulses". Applied Physics Letters. 101 (8): 081901. Bibcode:2012ApPhL.101h1901C. doi:10.1063/1.4747155.