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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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When an object is in equilibrium, it is either at rest or moving with a constant velocity. There are two types of equilibrium: static and dynamic. Static equilibrium occurs when an object is at rest, while dynamic equilibrium occurs when an object is moving with a constant velocity. In both cases, there must be a balance of forces acting on the object.
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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Dynamic coupling between particle-in-cell and atomistic simulations.

Mihkel Veske1, Andreas Kyritsakis1, Flyura Djurabekova1

  • 1Department of Physics and Helsinki Institute of Physics, University of Helsinki, P.O. Box 43 (Pietari Kalmin katu 2), 00014 Helsinki, Finland.

Physical Review. E
|June 25, 2020
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Summary
This summary is machine-generated.

We developed a new simulation method for metal surfaces under high electric fields. Narrow field emitters exhibit cyclical thermal runaway with intense evaporation, potentially igniting plasma arcs.

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Area of Science:

  • Computational physics
  • Materials science
  • Plasma physics

Background:

  • Simulating metal surface behavior under extreme electric fields is crucial for applications like field emission and plasma generation.
  • Understanding thermal runaway and space-charge effects is key to predicting device performance and failure.

Purpose of the Study:

  • To develop and validate a novel computational method for simulating metal surface response to high electric fields.
  • To investigate the thermal runaway process in field-emitting tips, considering three-dimensional space-charge effects.
  • To compare the runaway dynamics between field emitters of different geometries.

Main Methods:

  • Coupling molecular dynamics, finite element method, and particle-in-cell techniques for comprehensive simulation.
  • Simulating the evolution of a field-emitting tip under thermal runaway conditions.
  • Analyzing space-charge effects and neutral evaporation dynamics in three dimensions.

Main Results:

  • Identified cyclical thermal runaway in narrow field emitters, characterized by alternating intensive neutral evaporation and cooling periods.
  • Demonstrated statistically significant differences in runaway processes between tip geometries.
  • Showed that the evaporation rate during intensive phases is sufficient to trigger plasma arc formation.

Conclusions:

  • The proposed multi-physics simulation approach accurately captures complex phenomena in field emitters.
  • Narrow field emitters exhibit unique cyclical behavior under high electric fields, impacting their stability and performance.
  • The findings provide critical insights into the mechanisms of plasma arc ignition above field emitters.