Ultrafast electron diffraction
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. 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 produces 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]
Single shot
Stroboscopic
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 [6].
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]:
- ,
where \Delta s is the reciprocal space resolution, \lambda_e is the Compton wavelength of the electrons, \var_epsilon_n is the normalized emittance of the electrons, and sigma_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<ref name="weathersby_megavolt">.
Detectors
See also
References
- ^ Mourou, Gerard; Williamson, Steve (1982). "Picosecond electron diffraction". Applied Physics Letters. 41 (1): 44.
- ^ Srinivasan, R.; Lobastov, V.; Ruan, C.-Y.; Zewail, A. (2003). "Ultrafast Electron Diffraction (UED)". HCA. 86: 1761–1799. doi:10.1002/hlca.200390147.
- ^ Siwick, Bradley J.; Dwyer, Jason R.; Jordan, Robert E.; Miller, R. J. Dwayne (21 Nov 2003). "An Atomic-Level View of Melting Using Femtosecond Electron Diffraction". Science. 302 (5649): 1382–1385. doi:10.1126/science.1090052.
- ^ a b Weathersby, S. P. (2015). "Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory". Review of Scientific Instruments. 86 (7): 073702. doi:10.1063/1.4926994.
- ^ Qi, F. (2020). "Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor". Physical Review Letters. 124 (13): 134803. Bibcode:1902AnP...313..149L. doi:10.1103/PhysRevLett.124.134803.
- ^ Bie, Ya-Qing; Zong, Alfred; Wang, Xirui; Jarillo-Herrero, Pablo; Gedik, Nuh (2021). "A versatile sample fabrication method for ultrafast electron diffraction". Ultramicroscopy. 230: 113389. doi:10.1016/j.ultramic.2021.113389.
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. Dewayne (2011). "Femtosecond electron diffraction: heralding the era of atomically resolved dynamics". Reports on Progress in Physics. 74: 096101. doi:10.1088/0034-4885/74/9/096101.
- 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. doi:10.1063/1.4747155.