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1 History
2 Theory
- 2.1 Atomic hyperfine structure
  - 2.1.1 Magnetic dipole
  - 2.1.2 Electric quadrupole
- 2.2 Molecular hyperfine structure
  - 2.2.1 Direct nuclear spin-spin
  - 2.2.2 Nuclear spin-rotation
3 Measurements
4 Applications
5 References
6 External links

In atoms, hyperfine structure occurs due to the energy of the nuclear magnetic dipole moment in the magnetic field generated by the electrons, and the energy of the nuclear electric quadrupole moment in the electric field gradient due to the distribution of charge within the atom. Molecular hyperfine structure is generally dominated by these two effects, but also includes the energy associated with the interaction between the magnetic moments associated with different magnetic nuclei in a molecule, as well as between the nuclear magnetic moments and the magnetic field generated by the rotation of the molecule.

Schematic illustration of fine and hyperfine structure in hydrogen.

History

The optical hyperfine structure was already observed in 1881 by Albert Abraham Michelson. It could, however, only be explained in terms of quantum mechanics when Wolfgang Pauli proposed the existence of a small nuclear magnetic moment in 1924.

In 1935, M. Schüler and Theodor Schmidt proposed the existence of a nuclear quadrupole moment in order to explain anomalies in the hyperfine structure.

Theory

The theory of hyperfine structure comes directly from electromagnetism, consisting of the interaction of the nuclear multipole moments (excluding the electric monopole) with internally generated fields. The theory is derived first for the atomic case, but can be applied to each nucleus in a molecule. Following this there is a discussion of the additional effects unique to the molecular case.

Atomic hyperfine structure

Magnetic dipole

Main article: Dipole

The dominant term in the hyperfine Hamiltonian is typically the magnetic dipole term. Atomic nuclei with a non-zero nuclear spin have a magnetic dipole moment, given by:

$\boldsymbol{\mu}_\text{I} = g_\text{I}\mu_\text{N}\mathbf{I}$ .

There is an energy associated with a magnetic dipole moment in the presence of a magnetic field. For a nuclear magnetic dipole moment, μ_I, placed in a magnetic field, B, the relevant term in the Hamiltonian is given by:[1]

$\hat{H}_\text{D} = -\boldsymbol{\mu}_\text{I}\cdot\mathbf{B}$ .

In the absence of an externally applied field, the magnetic field experienced by the nucleus is that associated with the orbital (l) and spin (s) angular momentum of the electrons:

$\mathbf{B} \equiv \mathbf{B}_\text{el} = \mathbf{B}_\text{el}^l + \mathbf{B}_\text{el}^s$ .

Electron orbital angular momentum results from the motion of the electron about some fixed external point that we shall take to be the location of the nucleus. The magnetic field at the nucleus due to the motion of a single electron, with charge -e at a position r relative to the nucleus, is given by:

$\mathbf{B}_\text{el}^l = \dfrac{\mu_0}{4\pi}\dfrac{(-e)\mathbf{v}\times(-\mathbf{r})}{r^3}$ ,

where -r gives the position of the nucleus relative to the electron. Written in terms of the Bohr magneton, this gives:

$\mathbf{B}_\text{el}^l = -2\mu_\text{B}\dfrac{\mu_0}{4\pi}\dfrac{1}{r^3}\dfrac{\mathbf{r}\times m_\text{e}\mathbf{v}}{\hbar}$ .

Recognizing that m_ev is the electron momentum, p, and that r×p/ħ is the orbital angular momentum in units of ħ, l, we can write:

$\mathbf{B}_\text{el}^l = -2\mu_\text{B}\dfrac{\mu_0}{4\pi}\dfrac{1}{r^3}\mathbf{l}$ .

For a many electron atom this expression is generally written in terms of the total orbital angular momentum, $\scriptstyle{\mathbf{L}}$ , by summing over the electrons and using the projection operator, $\scriptstyle{\phi^l_i}$ , where $\scriptstyle{\sum_i\mathbf{l}_i = \sum_i\phi^l_i\mathbf{L}}$ . For states with a well defined projection of the orbital angular momentum, L_z, we can write $\scriptstyle{\phi^l_i = \hat{l}_{z_i}/L_z}$ , giving:

$\mathbf{B}_\text{el}^l = -2\mu_\text{B}\dfrac{\mu_0}{4\pi}\dfrac{1}{L_z}\sum_i\dfrac{\hat{l}_{zi}}{r_i^3}\mathbf{L}$ .

The electron spin angular momentum is a fundamentally different property that is intrinsic to the particle and therefore does not depend on the motion of the electron. Nonetheless it is angular momentum and any angular momentum associated with a charged particle results in a magnetic dipole moment, which is the source of a magnetic field. An electron with spin angular momentum, s, has a magnetic moment, μ_s, given by:

$\boldsymbol{\mu}_\text{s} = -g_s\mu_\text{B}\mathbf{s}$ ,

where g_s is the electron spin g-factor and the negative sign is because the electron is negatively charged (consider that negatively and positively charged particles with identical mass, travelling on equivalent paths, would have the same angular momentum, but would result in currents in the opposite direction).

The magnetic field of a dipole moment, μ_s, is given by:[2]

$\mathbf{B}_\text{el}^s = \dfrac{\mu_0}{4\pi r^3}\left(3(\boldsymbol{\mu}_\text{s}\cdot\hat{\mathbf{r}})\hat{\mathbf{r}}-\boldsymbol{\mu}_s\right) + \dfrac{2\mu_0}{3}\boldsymbol{\mu}_\text{s}\delta^3(\mathbf{r})$ .

The complete magnetic dipole contribution to the hyperfine Hamiltonian is thus given by:

$\begin{align} \hat{H}_D &= 2g_\text{I}\mu_\text{N}\mu_\text{B}\dfrac{\mu_0}{4\pi}\dfrac{1}{L_z}\sum_i\dfrac{\hat{l}_{zi}}{r_i^3}\mathbf{I}\cdot\mathbf{L}\\ &+ g_\text{I}\mu_\text{N}g_\text{s}\mu_\text{B}\dfrac{\mu_0}{4\pi}\dfrac{1}{S_z}\sum_i\dfrac{\hat{s}_{zi}}{r_i^3}\left\{3(\mathbf{I}\cdot\hat{\mathbf{r}})(\mathbf{S}\cdot\hat{\mathbf{r}}) - \mathbf{I}\cdot\mathbf{S}\right\}\\ &+ \frac{2}{3}g_\text{I}\mu_\text{N}g_\text{s}\mu_\text{B}\mu_0\dfrac{1}{S_z}\sum_i\hat{s}_{zi}\delta^3(\mathbf{r}_i)\mathbf{I}\cdot\mathbf{S}. \end{align}$

The first term gives the energy of the nuclear dipole in the field due to the electronic orbital angular momentum. The second term gives the energy of the "finite distance" interaction of the nuclear dipole with the field due to the electron spin magnetic moments. The final term, often known as the "Fermi contact" term relates to the direct interaction of the nuclear dipole with the spin dipoles and is only non-zero for states with a finite electron spin density at the position of the nucleus (those with unpaired electrons in s-subshells).

For states with l ≠ 0 this can be expressed in the form

$\hat{H}_D = 2g_I\mu_\text{B}\mu_\text{N}\dfrac{\mu_0}{4\pi}\dfrac{\mathbf{I}\cdot\mathbf{N}}{r^3}$ ,

where $\scriptstyle{\mathbf{N} = \mathbf{l}-(g_s/2)\mathbf{s}+3(\mathbf{s}\cdot\hat{\mathbf{r}})\hat{\mathbf{r}}}$ .[1]

If hyperfine structure is small compared with the fine structure (sometimes called IJ-coupling by analogy with LS-coupling), I and J are good quantum numbers and matrix elements of $\scriptstyle{\hat{H}_\text{D}}$ can be approximated as diagonal in I and J. In this case (generally true for light elements), we can project N onto J (where J = L + S is the total electronic angular momentum) and we have:[3]

$\hat{H}_\text{D} = 2g_I\mu_\text{B}\mu_\text{N}\dfrac{\mu_0}{4\pi}\dfrac{\mathbf{N}\cdot\mathbf{J}}{\mathbf{J}\cdot\mathbf{J}}\dfrac{\mathbf{I}\cdot\mathbf{J}}{r^3}$ .

This is commonly written as

$\hat{H}_\text{D} = \hat{A}\mathbf{I}\cdot\mathbf{J}$ ,

with $\scriptstyle{\langle\hat{A}\rangle}$ determined by experiment. Since I.J = ½{F.F - I.I - J.J} (where F = I + J is the total angular momentum), this gives an energy of

$\Delta E_\text{D} = \frac{1}{2}\langle\hat{A}\rangle[F(F+1)-I(I+1)-J(J+1)]$ .

In this case the hyperfine interaction satisfies the Lande interval rule.

Electric quadrupole

Main article: Quadrupole

Atomic nuclei with spin $\scriptstyle{I\ge 1}$ have an electric quadrupole moment.[4] In the general case this is represented by a rank-2 tensor, $\scriptstyle{\underline{\underline{Q}}}$ , with components give by:[2]

$Q_{ij} = \dfrac{1}{e}\int\left(x_i^\prime x_j^\prime - (r^\prime)^2\delta_{ij}\right)\rho(\mathbf{r}^\prime)d^3r^\prime$ ,

where i and j are the tensor indices running from 1 to 3, x_i and x_j are the spatial variables x, y and z depending on the values of i and j respectively, δ_ij is the Kronecker delta and ρ(r) is the charge density. Being a 3-dimensional rank-2 tensor, the quadrupole moment has 3² = 9 components. From the definition of the components it is clear that the quadrupole tensor is a symmetric matrix (Q_ij = Q_ji) that is also traceless (Σ_iQ_ii = 0), giving only five components in the irreducible representation. Expressed using the notation of irreducible spherical tensors we have:[2]

$T^2_m(Q) = \sqrt{\dfrac{4\pi}{5}} \int \rho(\mathbf{r}^{\prime})(r^\prime)^2 Y^2_m(\theta^{\prime},\phi^{\prime})d^3r^\prime$ .

The energy associated with an electric quadrupole moment in an electric field depends not on the field strength, but on the electric field gradient, confusingly labelled $\scriptstyle{\underline{\underline{q}}}$ , another rank-2 tensor given by the outer product of the del operator with the electric field vector:

$\underline{\underline{q}} = \nabla\otimes\mathbf{E}$ ,

with components given by:

$q_{ij} = \dfrac{\partial^2V}{\partial x_i\partial x_j}$ .

Again it is clear this is a symmetric matrix and, because the source of the electric field at the nucleus is a charge distribution entirely outside the nucleus, this can be expressed as a 5-component spherical tensor, $\scriptstyle{T^2(q)}$ , with:[5]

$T^2_0(q) = \dfrac{\sqrt{6}}{2}q_{zz}$ $T^2_{+1}(q) = -q_{xz} - iq_{yz}$ $T^2_{+2}(q) = \frac{1}{2}(q_{xx} - q_{yy}) + iq_{xy}$ ,

where:

$T^2_{-m}(q) = (-1)^mT^2_{+m}(q)^*$ .

The quadrupolar term in the Hamiltonian is thus given by:

$\hat{H}_Q = -eT^2(Q)\cdot T^2(q) = -e\sum_m (-1)^mT^2_m(Q)T^2_{-m}(q)$ .

A typical atomic nucleus closely approximates cylindrical symmetry and therefore all off-diagonal elements are close to zero. For this reason the nuclear electric quadrupole moment is often represented by Q_zz.[4]

Molecular hyperfine structure

The molecular hyperfine Hamiltonian includes those terms already derived for the atomic case with a magnetic dipole term for each nucleus with $\scriptstyle{I>0}$ and an electric quadrupole term for each nucleus with $\scriptstyle{I\geq 1}$ . The magnetic dipole terms were first derived for diatomic molecules by Frosch and Foley[6] and the resulting hyperfine parameters are often called the Frosch and Foley parameters.

In addition to the effects described above there are a number of effects specific to the molecular case.[7]

Direct nuclear spin-spin

Each nucleus with $\scriptstyle{I>0}$ has a non-zero magnetic moment that is both the source of a magnetic field and has an associated energy due to the presence of the combined field of all of the other nuclear magnetic moments. A summation over each magnetic moment dotted with the field due to each other magnetic moment gives the direct nuclear spin-spin term in the hyperfine Hamiltonian, $\scriptstyle{\hat{H}_{II}}$ .[8]

$\hat{H}_{II} = -\sum_{\alpha\neq\alpha^\prime}\boldsymbol{\mu}_\alpha\cdot \mathbf{B}_{\alpha^\prime}$ ,

where α and α‘ are indices representing the nucleus contributing to the energy and the nucleus that is the source of the field respectively. Substituting in the expressions for the dipole moment in terms of the nuclear angular momentum and the magnetic field of a dipole, both given above, we have:

$\hat{H}_{II} = \dfrac{\mu_0\mu_\text{N}^2}{4\pi}\sum_{\alpha\neq\alpha^\prime}\dfrac{g_\alpha g_{\alpha^\prime}}{R_{\alpha\alpha^\prime}^3}\left\{\mathbf{I}_\alpha\cdot\mathbf{I}_{\alpha^\prime} - 3(\mathbf{I}_\alpha\cdot\hat{\mathbf{R}}_{\alpha\alpha^\prime})(\mathbf{I}_{\alpha^\prime}\cdot\hat{\mathbf{R}}_{\alpha\alpha^\prime})\right\}$ .

Nuclear spin-rotation

The nuclear magnetic moments in a molecule exist in a magnetic field due to the angular momentum, T (R is the internuclear displacement vector), associated with the bulk rotation of the molecule.[8]

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