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Related Concept Videos

Predicting Reaction Outcomes02:24

Predicting Reaction Outcomes

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Kinetics describes the rate and path by which a reaction occurs. In contrast, thermodynamics deals with state functions and describes the properties, behavior, and components of a system. It is not concerned with the path taken by the process and cannot address the rate at which a reaction occurs. Although it does provide information about what can happen during a reaction process, it does not describe the detailed steps of what appears on an atomic or a molecular level. On the other hand,...
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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
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The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. This premise is the basis for a theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates.
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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Reaction Mechanisms03:06

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Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
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Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving
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Inducing thermodynamically blocked atomic ordering via strongly driven nonequilibrium kinetics.

Chulho Jung1,2, Yungok Ihm2,3, Do Hyung Cho1,2

  • 1Department of Physics, POSTECH, Pohang 37673, Korea.

Science Advances
|December 22, 2021
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Summary

Scientists used ultrafast lasers to observe hidden material phases in bismuth nanoparticles. This nonequilibrium process revealed kinetically controlled surface ordering, demonstrating a new route for material discovery.

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Ultrafast Phenomena

Background:

  • Ultrafast light-matter interactions can access novel material phases via nonequilibrium kinetics.
  • Understanding intermediate states in driven systems is crucial for material discovery but remains limited.

Purpose of the Study:

  • To investigate intermediate states in photoexcited bismuth nanoparticles using ultrafast time-resolved imaging.
  • To explore the role of electron-mediated free-energy modification in inducing exotic material phases.

Main Methods:

  • Single-pulse time-resolved imaging utilizing X-ray Free-Electron Lasers (XFELs).
  • Molecular dynamics simulations to analyze observed kinetic pathways.

Main Results:

  • Observed kinetically reversed surface ordering in photoexcited bismuth nanoparticles during ultrafast melting.
  • Identified an entropy-lowering reaction that is thermodynamically inaccessible in equilibrium.
  • Demonstrated electron-mediated ultrafast free-energy modification as key to inducing exotic phases.

Conclusions:

  • Ultrafast photoexcitation of electrons offers an efficient strategy to access hidden material phases.
  • Nonequilibrium reaction pathways can overcome thermodynamic barriers, enabling the discovery of new material properties.