Hyperfine structure

The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron spin and the electrons' orbital angular momentum. Hyperfine structure, with energy shifts typically orders of magnitude smaller than the fine structure, results from the interactions of the nucleus (or nuclei, in molecules) with internally generated electric and magnetic fields.

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:

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

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, $\scriptstyle {\phi _{i}^{l}}$, where $\scriptstyle {\sum _{i}\mathbf {l} _{i}=\sum _{i}\phi _{i}^{l}\mathbf {L} }$. For states with a well defined projection of the orbital angular momentum, L_z, we can write $\scriptstyle {\phi _{i}^{l}={\hat {l}}_{z_{i}}/L_{z}}$, 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:

$\mathbf {B} _{\text{el}}^{s}={\dfrac {\mu _{0}}{4\pi r^{3}}}\left(3({\boldsymbol {\mu }}_{\text{s}}\cdot {\hat {\mathbf {r} }}){\hat {\mathbf {r} }}-{\boldsymbol {\mu }}_{\text{s}}\right)+{\dfrac {2\mu _{0}}{3}}{\boldsymbol {\mu }}_{\text{s}}\delta ^{3}(\mathbf {r} )$.

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

${\begin{aligned}{\hat {H}}_{D}&=2g_{\text{I}}\mu _{\text{N}}\mu _{\text{B}}{\dfrac {\mu _{0}}{4\pi }}{\dfrac {1}{L_{z}}}\sum _{i}{\dfrac {{\hat {l}}_{zi}}{r_{i}^{3}}}\mathbf {I} \cdot \mathbf {L} \\&+g_{\text{I}}\mu _{\text{N}}g_{\text{s}}\mu _{\text{B}}{\dfrac {\mu _{0}}{4\pi }}{\dfrac {1}{S_{z}}}\sum _{i}{\dfrac {{\hat {s}}_{zi}}{r_{i}^{3}}}\left\{3(\mathbf {I} \cdot {\hat {\mathbf {r} }})(\mathbf {S} \cdot {\hat {\mathbf {r} }})-\mathbf {I} \cdot \mathbf {S} \right\}\\&+{\frac {2}{3}}g_{\text{I}}\mu _{\text{N}}g_{\text{s}}\mu _{\text{B}}\mu _{0}{\dfrac {1}{S_{z}}}\sum _{i}{\hat {s}}_{zi}\delta ^{3}(\mathbf {r} _{i})\mathbf {I} \cdot \mathbf {S} .\end{aligned}}$

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). It has been argued that one may get a different expression when taking into account the detailed nuclear dipole moment distribution.

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 $\scriptstyle {\mathbf {N} =\mathbf {l} -(g_{s}/2)\mathbf {s} +3(\mathbf {s} \cdot {\hat {\mathbf {r} }}){\hat {\mathbf {r} }}}$.

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:

${\hat {H}}_{\text{D}}=2g_{I}\mu _{\text{B}}\mu _{\text{N}}{\dfrac {\mu _{0}}{4\pi }}{\dfrac {\mathbf {N} \cdot \mathbf {J} }{\mathbf {J} \cdot \mathbf {J} }}{\dfrac {\mathbf {I} \cdot \mathbf {J} }{r^{3}}}$.

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 $\scriptstyle {I\geq 1}$ have an electric quadrupole moment. In the general case this is represented by a rank-2 tensor, $\scriptstyle {\underline {\underline {Q}}}$, with components given by:

$Q_{ij}={\dfrac {1}{e}}\int \left(3x_{i}^{\prime }x_{j}^{\prime }-(r^{\prime })^{2}\delta _{ij}\right)\rho (\mathbf {r} ^{\prime })d^{3}r^{\prime }$,

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:

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

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:

$q_{ij}={\dfrac {\partial ^{2}V}{\partial x_{i}\partial x_{j}}}$.

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:

$T_{0}^{2}(q)={\dfrac {\sqrt {6}}{2}}q_{zz}$

$T_{+1}^{2}(q)=-q_{xz}-iq_{yz}$

$T_{+2}^{2}(q)={\frac {1}{2}}(q_{xx}-q_{yy})+iq_{xy}$,

where:

$T_{-m}^{2}(q)=(-1)^{m}T_{+m}^{2}(q)^{*}$.

The quadrupolar term in the Hamiltonian is thus given by:

${\hat {H}}_{Q}=-eT^{2}(Q)\cdot T^{2}(q)=-e\sum _{m}(-1)^{m}T_{m}^{2}(Q)T_{-m}^{2}(q)$.

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.

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 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.

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, $\scriptstyle {{\hat {H}}_{II}}$.

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

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.

${\hat {H}}_{\text{IR}}={\dfrac {e\mu _{0}\mu _{\text{N}}\hbar }{4\pi }}\sum _{\alpha \neq \alpha ^{\prime }}{\dfrac {1}{R_{\alpha \alpha ^{\prime }}^{3}}}\left\{{\dfrac {Z_{\alpha }g_{\alpha ^{\prime }}}{M_{\alpha }}}\mathbf {I} _{\alpha ^{\prime }}+{\dfrac {Z_{\alpha ^{\prime }}g_{\alpha }}{M_{\alpha ^{\prime }}}}\mathbf {I} _{\alpha }\right\}\cdot \mathbf {T}$

Measurements

Hyperfine interactions can be measured, among other ways, in atomic and molecular spectra and in electron paramagnetic resonance spectra of free radicals and transition-metal ions.

Applications

Astrophysics

As the hyperfine splitting is very small, the transition frequencies usually are not optical, but in the range of radio- or microwave frequencies.

Hyperfine structure gives the 21 cm line observed in H I regions in interstellar medium.

Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record.

In radio astronomy, heterodyne receivers are widely used in detection of the electromagnetic signals from celestial objects. The separations among various components of a hyperfine structure are usually small enough to fit into the receiver's IF band. Because optical depth varies with frequency, strength ratios among the hyperfine components differ from that of their intrinsic intensities. From this we can derive the object's physical parameters.

Nuclear technology

The AVLIS process uses the hyperfine splitting between optical transitions in uranium-235 and uranium-238 to selectively photo-ionize only the uranium-235 atoms and then separate the ionized particles from the non-ionized ones. Precisely tuned dye lasers are used as the sources of the necessary exact wavelength radiation.

Use in defining the SI second and meter

The hyperfine structure transition can be used to make a microwave notch filter with very high stability, repeatability and Q factor, which can thus be used as a basis for very precise atomic clocks. Typically, the hyperfine structure transition frequency of a particular isotope of caesium or rubidium atoms is used as a basis for these clocks.

Due to the accuracy of hyperfine structure transition-based atomic clocks, they are now used as the basis for the definition of the second. One second is now defined to be exactly 9,192,631,770 cycles of the hyperfine structure transition frequency of caesium-133 atoms.

Since 1983, the meter is defined by declaring the speed of light in a vacuum to be exactly 299,792,458 metres per second. Thus:

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.

Precision tests of quantum electrodynamics

The hyperfine splitting in hydrogen and in muonium have been used to measure the value of the fine structure constant α. Comparison with measurements of α in other physical systems provides a stringent test of QED.

Qubit in ion-trap quantum computing

The hyperfine states of a trapped ion are commonly used for storing qubits in ion-trap quantum computing. They have the advantage of having very long lifetimes, experimentally exceeding ~10 min (compared to ~1 s for metastable electronic levels).

The frequency associated with the states' energy separation is in the microwave region, making it possible to drive hyperfine transitions using microwave radiation. However, at present no emitter is available that can be focused to address a particular ion from a sequence. Instead, a pair of laser pulses can be used to drive the transition, by having their frequency difference (detuning) equal to the required transition's frequency. This is essentially a stimulated Raman transition.

See also

• Dynamic nuclear polarisation
• Electron paramagnetic resonance

External links

• Nuclear Structure and Decay Data - IAEA Nuclear Magnetic and Electric Moments