Ultracold Electrons via Near-Threshold Photoemission from Single-Crystal Cu(100)

Karkare, Siddharth; Adhikari, Gowri; Schroeder, W. Andreas; Nangoi, J. Kevin; Arias, T. A.; Maxson, Jared; Padmore, H. A.

doi:10.1103/physrevlett.125.054801

Cited by 46 publications

(24 citation statements)

References 30 publications

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“…[17] Metallic photocathodes such as Cu and Mg are commonly used because of their long operational lifetime, fast response times, ease of synthesis, and reasonably low intrinsic emittance. [18][19][20] However, the high reflectivity and small electron-electron scattering mean free path in metals fundamentally limits their possible QE, with a peak QE of 0.3% observed in Mg. [21] Semiconducting alkali antimonide photocathodes such as Na 2 KSb and Cs 3 Sb were first discovered from pioneering photoemission studies by Sommer in the 1950s and remain among the most popular choice of photocathode materials today, primarily because of their impressively high QE (reaching over 40%) and relatively short response times. [22,23] However, alkali antimonide semiconductor photocathodes are extremely sensitive to oxidation.…”

Section: Identifying Semiconducting Photocathodesmentioning

confidence: 99%

Novel Ultrabright and Air‐Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening

et al. 2021

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The high brightness, low emittance electron beams achieved in modern X‐ray free‐electron lasers (XFELs) have enabled powerful X‐ray imaging tools, allowing molecular systems to be imaged at picosecond time scales and sub‐nanometer length scales. One of the most promising directions for increasing the brightness of XFELs is through the development of novel photocathode materials. Whereas past efforts aimed at discovering photocathode materials have typically employed trial‐and‐error‐based iterative approaches, this work represents the first data‐driven screening for high brightness photocathode materials. Through screening over 74 000 semiconducting materials, a vast photocathode dataset is generated, resulting in statistically meaningful insights into the nature of high brightness photocathode materials. This screening results in a diverse list of photocathode materials that exhibit intrinsic emittances that are up to 4x lower than currently used photocathodes. In a second effort, multiobjective screening is employed to identify the family of M2O (M = Na, K, Rb) that exhibits photoemission properties that are comparable to the current state‐of‐the‐art photocathode materials, but with superior air stability. This family represents perhaps the first intrinsically bright, visible light photocathode materials that are resistant to reactions with oxygen, allowing for their transport and storage in dry air environments.

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Section: Identifying Semiconducting Photocathodesmentioning

confidence: 99%

Novel Ultrabright and Air‐Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening

et al. 2021

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“…As a means for beam generation, photoemission affords fine control of the electron distribution both in space and time via laser shaping [31][32][33][34] . To increase spatial beam quality, in this work we employ photocathodes with high intrinsic brightness and further tune the photoemission wavelength 35,36 . The choice of photo-excitation energies, when approaching the photoemission threshold, involves a trade-off between (i) maximizing the ratio of emitted electrons to incident photons -the quantum efficiency (QE), and; (ii) minimizing the momentum spread of emitted photoelectrons, summarised in the mean transverse energy (MTE) 37 of the beam.…”

Section: Introductionmentioning

confidence: 99%

A kiloelectron-volt ultrafast electron micro-diffraction apparatus using low emittance semiconductor photocathodes

Li¹,

Duncan²,

Andorf³

et al. 2021

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We report the design and performance of a time-resolved electron diffraction apparatus capable of producing intense bunches with < 200 fs rms duration, single digit micron probe sizes, and simultaneously long coherence length. To generate high brightness electron bunches, the system employs high efficiency, low emittance semiconductor photocathodes driven with a wavelength near the photoemission threshold at a repetition rate up to 250 kHz. We characterize spatial, temporal, and reciprocal space resolution of the apparatus and perform proof-of-principle measurements of ultrafast heating in single crystal Au samples.

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“…Typical photocathodes used in accelerator facilities today have a MTE of a few hundred meV [10][11][12] whereas near threshold emission at room temperature has demonstrated electron beams with an MTE of ∼ 25 meV [13]. Furthermore, cryocooled photocathodes near threshold have shown the capability to go down even further to an MTE of ∼ 5 meV [14]. At these low temperatures, point-to-point interactions play an increasingly important role in the overall beam dynamics, as shown by the following argument.…”

Section: Introductionmentioning

confidence: 99%

Point-to-Point Coulomb Effects in High Brightness Photoelectron Beamlines for Ultrafast Electron Diffraction

Gordon,

van der Geer,

Maxson

et al. 2021

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In an effort to increase spatial and temporal resolution of ultrafast electron diffraction and microscopy, ultra-high brightness photocathodes are actively sought to improve electron beam quality. Beam dynamics codes often approximate the Coulomb interaction with mean-field space charge, which is a good approximation in traditional beams. However, point-to-point Coulomb effects, such as disorder induced heating and the Boersch effect, can not be neglected in cold, dense beams produced by such photocathodes. In this paper, we introduce two new numerical methods to calculate the important effects of the photocathode image charge when using a point-to-point interaction model. Equipped with an accurate model of the image charge, we calculate the effects of point-topoint interactions on two high brightness photoemission beamlines for ultrafast diffraction. The first beamline uses a 200 keV gun, whereas the second uses a 5 MeV gun, each operating in the singleshot diffraction regime with 10 5 electrons/pulse. Assuming a zero photoemission temperature, it is shown that including stochastic Coulomb effects increases the final emittance of these beamlines by over a factor of 2 and decreases the peak transverse phase space density by over a factor of 3 as compared to mean-field simulations. We then introduce a method to compute the energy released by disorder induced heating using the pair correlation function. This disorder induced heating energy was found to scale very near the theoretical result for stationary ultracold plasmas, and it accounts for over half of the emittance growth above mean-field simulations.

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Ultracold Electrons via Near-Threshold Photoemission from Single-Crystal Cu(100)

Cited by 46 publications

References 30 publications

Novel Ultrabright and Air‐Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening

Novel Ultrabright and Air‐Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening

A kiloelectron-volt ultrafast electron micro-diffraction apparatus using low emittance semiconductor photocathodes

Point-to-Point Coulomb Effects in High Brightness Photoelectron Beamlines for Ultrafast Electron Diffraction

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