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Landé g-factor

1921
  • Alfred Landé
Laboratory scene with scientist analyzing spectrometer for quantum mechanics research.

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The Landé g-factor (\(g_J\)) is a dimensionless proportionality constant that relates an atom’s total magnetic moment to its total angular momentum in the weak-field limit. It is crucial for quantitatively explaining the anomalous Zeeman effect. Its value is given by the formula: \(g_J = 1 + \frac{J(J+1) + S(S+1) – L(L+1)}{2J(J+1)}\), where L, S, and J are the quantum numbers.

The Landé g-factor was introduced by Alfred Landé in 1921, even before the concept of electron spin was fully formulated, as an empirical way to fit the experimental data of the anomalous Zeeman effect. Its theoretical justification came later with the development of quantum mechanics. The formula arises from the vector model of the atom, where the orbital (\(\vec{L}\)) and spin (\(\vec{S}\)) angular momenta are considered to precess rapidly around their resultant total angular momentum vector (\(\vec{J}\)) due to spin-orbit coupling. The interaction with a weak external magnetic field is much slower. Therefore, the field effectively interacts with the time-averaged magnetic moment, which is the projection of the total magnetic moment (\(\vec{\mu}_L + \vec{\mu}_S\)) onto the direction of \(\vec{J}\).

The g-factor essentially accounts for the different ratios of magnetic moment to angular momentum for orbital motion (\(g_L=1\)) and spin (\(g_S \approx 2\)). When \(S=0\), then \(J=L\), and the formula correctly gives \(g_J=1\), corresponding to the normal Zeeman effect. When \(L=0\), then \(J=S\), and the formula gives \(g_J=2\) (using \(g_S=2\) instead of 1 in the derivation), corresponding to pure spin systems as in electron spin resonance. For all other cases, \(g_J\) takes on a rational value between 1 and 2, quantifying the complex interplay between spin and orbital contributions to the atom’s magnetism.

UNESCO Nomenclature: 2209
– Mechanics

Type

Theoretical Concept

Disruption

Foundational

Usage

Widespread Use

Precursors

  • the vector model of the atom
  • the concept of spin-orbit coupling
  • experimental data from the anomalous zeeman effect that required a quantitative explanation
  • the quantization of angular momentum in the bohr-sommerfeld model

Applications

  • quantitative prediction of anomalous zeeman splitting patterns in spectroscopy
  • electron spin resonance (esr) and nuclear magnetic resonance (nmr) analysis
  • calculation of magnetic susceptibility and other magnetic properties of materials
  • quantum information processing using atomic states
  • determination of atomic term symbols from experimental data

Patents:

NA

Potential Innovations Ideas

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Related to: Landé g-factor, g-factor, anomalous Zeeman effect, angular momentum, quantum numbers, spin-orbit coupling, magnetic moment, spectroscopy, atomic physics, vector model.

Historical Context

Landé g-factor

1919-05-29
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(if date is unknown or not relevant, e.g. "fluid mechanics", a rounded estimation of its notable emergence is provided)

Related Invention, Innovation & Technical Principles

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