science

Ultrafast electron diffraction

Ultrafast electron diffraction is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ultrafast electron diffraction rather than just read about it. In short: Ultrafast electron diffraction (UED), also known as femtosecond electron diffraction, 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 in the structure of materials such as those undergoing phase transitions or chemical reactions.

Key takeaways

Ultrafast electron diffraction belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ultrafast electron diffraction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ultrafast electron diffraction from memory before moving on to harder problems.

Reference excerpt

Ultrafast electron diffraction (UED), also known as femtosecond electron diffraction, 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 in the structure of materials such as those undergoing phase transitions or chemical reactions. It is conceptually similar to time-resolved crystallography, but instead of using X-rays as the probe, it uses electrons. UED can provide a wealth of dynamics on charge carriers, atoms, and molecules. The technique uses a femtosecond (10–15 second) laser optical pulse to promote (pump) 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 short (with a duration in femtoseconds or picoseconds) electron pulse is incident upon the sample, which interacts with and diffracts off the underlying atomic structure. The diffraction signal is, subsequently, detected by an electron counting instrument such as a charge-coupled device camera or a direct electron detector similar to detection methods in electron microscopy. Specifically, the detectors measure diffraction patterns, which contain 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.

History The design of early UED instruments was based on X-ray streak cameras, the first reported UED experiment demonstrating an electron pulse length of 100 picoseconds (10–10 seconds). The temporal resolution of ultrafast electron diffraction has been reduced to the attosecond (10–18 second) time scale to perform attosecond electron-diffraction measurements which reveal electron motion dynamics.

Electron pulse production The electron pulses are typically produced by the process of photoemission, in which a femtosecond optical pulse is directed toward a photocathode. If the incident laser pulse has an appropriate energy, then electrons will be ejected from the photocathode. The electrons are subsequently accelerated to high energies, ranging from tens of kiloelectronvolts to several megaelectronvolts, 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 tradeoff between signal (bunch charge) and time-resolution in UED experiments. Radio-frequency (RF) compression has emerged has a leading method of reducing the pulse expansion in ultrafast electron-diffraction experiments, achieving temporal resolution well below 50 femtoseconds. Shorter electron beams below 10 femtoseconds are ultimately required to probe the fastest dynamics in solid-state materials and observe gas-phase molecular reactions.

Single shot

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

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 108.

Resolution The resolution of a UED 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. High reciprocal-space resolution allows for the detection of Bragg diffraction spots that correspond to long-periodicity phenomena. It can be calculated with the equation

Δ s = 2 π λ e ε n σ x , {\displaystyle \Delta s={\frac {2\pi }{\lambda _{e}}}{\frac {\varepsilon _{n}}{\sigma _{x}}},}

where Δs is the reciprocal-space resolution, λe is the Compton wavelength of the electrons, ϵn is the normalized emittance of the electrons, and σx 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.

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.

Worked examples

Example 1 — a first encounter with Ultrafast electron diffraction

Start with the simplest possible case. Write down what Ultrafast electron diffraction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ultrafast electron diffraction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ultrafast electron diffraction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ultrafast electron diffraction

In research: Ultrafast electron diffraction appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ultrafast electron diffraction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ultrafast electron diffraction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Diffraction, Laser applications, so understanding it makes those chapters shorter.
In everyday life: Look for Ultrafast electron diffraction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ultrafast electron diffraction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ultrafast electron diffraction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ultrafast electron diffraction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ultrafast electron diffraction in simple terms?

Why does Ultrafast electron diffraction matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ultrafast electron diffraction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ultrafast electron diffraction.