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Related Concept Videos

¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.

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A Basic Positron Emission Tomography System Constructed to Locate a Radioactive Source in a Bi-dimensional Space
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Coupling between positron-atom scattering channels above the first inelastic threshold.

P G Coleman1, N Cheesman, E R Lowry

  • 1Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom.

Physical Review Letters
|June 13, 2009
PubMed
Summary

Positron scattering on argon and xenon shows a step-like increase at positronium formation energy. This suggests a virtual positronium state, differing from theoretical predictions of cusp-like behavior.

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Area of Science:

  • Atomic and Molecular Physics
  • Quantum Scattering Theory
  • Positron-Atom Interactions

Background:

  • Elastic scattering cross sections are fundamental in atomic physics.
  • Theoretical models predicted cusp-like behavior in positron scattering near inelastic thresholds.
  • Positronium formation is a key inelastic process.

Purpose of the Study:

  • To experimentally determine positron scattering cross sections for argon and xenon.
  • To investigate the behavior of elastic scattering near the positronium formation threshold.
  • To compare experimental findings with theoretical predictions.

Main Methods:

  • Experimentally measured elastic scattering cross sections.
  • Utilized positron scattering experiments on noble gas targets (argon, xenon).
  • Analyzed scattering data at energies corresponding to the first inelastic threshold.

Main Results:

  • Observed a distinct steplike increase in elastic scattering cross sections.
  • This feature was more pronounced in xenon than in argon.
  • The observed behavior deviates from theoretically predicted cusp-like features.

Conclusions:

  • The steplike increase suggests the presence of an intermediate virtual positronium state.
  • This virtual state appears to enhance the elastic interaction probability.
  • Experimental results challenge existing theoretical models for positron scattering near thresholds.