fine structure

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microstructural、microstructure

WordNet

1. thin in thickness or diameter; "a fine film of oil"; "fine hairs"; "read the fine print"
2. money extracted as a penalty (同)mulct, amercement
3. characterized by elegance or refinement or accomplishment; "fine wine"; "looking fine in her Easter suit"; "a fine gentleman"; "fine china and crystal"; "a fine violinist"; "the fine hand of a master"
4. free from impurities; having a high or specified degree of purity; "gold 21 carats fine"
5. minutely precise especially in differences in meaning; "a fine distinction"
6. of textures that are smooth to the touch or substances consisting of relatively small particles; "wood with a fine grain"; "fine powdery snow"; "fine rain"; "batiste is a cotton fabric with a fine weave"; "covered with a fine film of dust"
7. give a structure to; "I need to structure my days"
8. a thing constructed; a complex entity constructed of many parts; "the structure consisted of a series of arches"; "she wore her hair in an amazing construction of whirls and ribbons" (同)construction
9. the manner of construction of something and the arrangement of its parts; "artists must study the structure of the human body"; "the structure of the benzene molecule"
10. a particular complex anatomical part of a living thing; "he has good bone structure" (同)anatomical structure, complex body part, bodily structure, body structure
11. the complex composition of knowledge as elements and their combinations; "his lectures have no structure"
12. propel oneself through the water in a finning motion
13. a stabilizer on a ship that resembles the fin of a fish
14. organ of locomotion and balance in fishes and some other aquatic animals
15. show the fins above the water while swimming; "The sharks were finning near the surface" (同)break water
16. equip (a car) with fins
17. having definite and highly organized structure; "a structured environment"

PrepTutorEJDIC

1. 罰金 / 〈人〉‘に'罰金を科する:
2. (物・事が)『りっぱな』,みごとな;けっこうな.満足できる / (人が)『優れた』,りっぱな / (天気が)『晴れた』,気持ちのよい / 《補語にのみ用いて》《話》『元気な』,健康な(well) / (人・衣服・態度などが)『洗練された』,いきな,上品な(refined) / (粒・きめなどの)細かい;細い(thin) / 《名詞の前にのみ用いて》微妙な(delicate) / 精巧な / (物が)混じりけのない,(金・銀などが)純度…の / (言葉が)華美な / 《名詞の前にのみ用いて》《皮肉に》けっこうな,ごりっぱな / 細かく / 《話》りっぱに,うまく
3. {C}(建物・橋などの)建造物 / {U}(…の)構造,誠成,組織《+of~名》 / {C}構造(構成)体,組織体 / 〈思想など〉‘を'組み立てる,組織化する
4. (魚などの)ひれ / (形・働きが)ひれに似た物 / (飛行機の)垂直安定板 / 《話》手,腕

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/08/13 02:44:10」(JST)

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Interference fringes, showing fine structure (splitting) of a cooled deuterium source, viewed through a Fabry-Pérot étalon.

In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to electron spin and relativistic corrections to the non-relativistic Schrödinger equation.

Background

Gross structure

The gross structure of line spectra is the line spectra predicted by the quantum mechanics of non-relativistic electrons with no spin. For a hydrogenic atom, the gross structure energy levels only depend on the principal quantum number n. However, a more accurate model takes into account relativistic and spin effects, which break the degeneracy of the energy levels and split the spectral lines. The scale of the fine structure splitting relative to the gross structure energies is on the order of (Zα)², where Z is the atomic number and α is the fine-structure constant, a dimensionless number equal to approximately $1/137$.

Relativistic corrections

The fine structure energy corrections can be obtained by using perturbation theory. To do this one adds three corrective terms to the Hamiltonian: the leading order relativistic correction to the kinetic energy, the correction due to the spin-orbit coupling, and the Darwinian term.

These corrections can also be obtained from the non-relativistic limit of the Dirac equation, since Dirac's theory naturally incorporates relativity and spin interactions.

Kinetic energy relativistic correction

Classically, the kinetic energy term of the Hamiltonian is

$T={\frac {p^{2}}{2m}}$

where $p$ is the momentum and $m$ is the mass of the electron.

However, when considering a more accurate theory of nature via. special relativity, we must use a relativistic form of the kinetic energy,

$T={\sqrt {p^{2}c^{2}+m^{2}c^{4}}}-mc^{2}$

where the first term is the total relativistic energy, and the second term is the rest energy of the electron. ($c$ is the speed of light) Expanding this in a Taylor series ( specifically a binomial series ), we find

$T={\frac {p^{2}}{2m}}-{\frac {p^{4}}{8m^{3}c^{2}}}+\cdots$

Then, the first order correction to the Hamiltonian is

$H_{\mathrm {kinetic} }=-{\frac {p^{4}}{8m^{3}c^{2}}}$

Using this as a perturbation, we can calculate the first order energy corrections due to relativistic effects.

$E_{n}^{(1)}=\left\langle \psi ^{0}\right\vert H'\left\vert \psi ^{0}\right\rangle =-{\frac {1}{8m^{3}c^{2}}}\left\langle \psi ^{0}\right\vert p^{4}\left\vert \psi ^{0}\right\rangle =-{\frac {1}{8m^{3}c^{2}}}\left\langle \psi ^{0}\right\vert p^{2}p^{2}\left\vert \psi ^{0}\right\rangle$

where $\psi ^{0}$ is the unperturbed wave function. Recalling the unperturbed Hamiltonian, we see

${\begin{aligned}H^{0}\left\vert \psi ^{0}\right\rangle &=E_{n}\left\vert \psi ^{0}\right\rangle \\\left({\frac {p^{2}}{2m}}+V\right)\left\vert \psi ^{0}\right\rangle &=E_{n}\left\vert \psi ^{0}\right\rangle \\p^{2}\left\vert \psi ^{0}\right\rangle &=2m(E_{n}-V)\left\vert \psi ^{0}\right\rangle \end{aligned}}$

We can use this result to further calculate the relativistic correction:

${\begin{aligned}E_{n}^{(1)}&=-{\frac {1}{8m^{3}c^{2}}}\left\langle \psi ^{0}\right\vert p^{2}p^{2}\left\vert \psi ^{0}\right\rangle \\E_{n}^{(1)}&=-{\frac {1}{8m^{3}c^{2}}}\left\langle \psi ^{0}\right\vert (2m)^{2}(E_{n}-V)^{2}\left\vert \psi ^{0}\right\rangle \\E_{n}^{(1)}&=-{\frac {1}{2mc^{2}}}\left(E_{n}^{2}-2E_{n}\langle V\rangle +\left\langle V^{2}\right\rangle \right)\end{aligned}}$

For the hydrogen atom, $V(r)={\frac {-e^{2}}{4\pi \varepsilon _{0}r}}$, $\left\langle {\frac {1}{r}}\right\rangle ={\frac {1}{a_{0}n^{2}}}$, and $\left\langle {\frac {1}{r^{2}}}\right\rangle ={\frac {1}{(l+1/2)n^{3}a_{0}^{2}}}$ where $a_{0}$ is the Bohr Radius, $n$ is the principal quantum number and $l$ is the azimuthal quantum number. Therefore, the first order relativistic correction for the hydrogen atom is

${\begin{aligned}E_{n}^{(1)}&=-{\frac {1}{2mc^{2}}}\left(E_{n}^{2}+2E_{n}{\frac {e^{2}}{4\pi \varepsilon _{0}}}{\frac {1}{a_{0}n^{2}}}+{\frac {1}{16\pi ^{2}\varepsilon _{0}^{2}}}{\frac {e^{4}}{(l+{\frac {1}{2}})n^{3}a_{0}^{2}}}\right)\\&=-{\frac {E_{n}^{2}}{2mc^{2}}}\left({\frac {4n}{l+{\frac {1}{2}}}}-3\right)\end{aligned}}$

where we have used:

$E_{n}=-{\frac {e^{2}}{8\pi \varepsilon _{0}a_{0}n^{2}}}$

On final calculation, the order of magnitude for the relativistic correction to the ground state is $-9.056\times 10^{-4}\ {\text{eV}}$.

Spin-orbit coupling

Main article: Spin-orbit interaction

For a hydrogen-like atom with $Z$ protons, orbital momentum ${\vec {L}}$ and electron spin ${\vec {S}}$, the spin-orbit term is given by:

$H_{so}={\frac {1}{2}}\left({\frac {Ze^{2}}{4\pi \varepsilon _{0}}}\right)\left({\frac {g_{s}}{2m_{e}^{2}c^{2}}}\right){\frac {{\vec {L}}\cdot {\vec {S}}}{r^{3}}}$

$m_{e}$ is the electron mass, $\varepsilon _{0}$ is the vacuum permittivity and $g_{s}$ is the spin g-factor. $r$ is the distance of the electron from the nucleus.

The spin-orbit correction can be understood by shifting from the standard frame of reference (where the electron orbits the nucleus) into one where the electron is stationary and the nucleus instead orbits it. In this case the orbiting nucleus functions as an effective current loop, which in turn will generate a magnetic field. However, the electron itself has a magnetic moment due to its intrinsic angular momentum. The two magnetic vectors, ${\vec {B}}$ and ${\vec {\mu }}_{s}$ couple together so that there is a certain energy cost depending on their relative orientation. This gives rise to the energy correction of the form

$\Delta E_{SO}=\xi (r){\vec {L}}\cdot {\vec {S}}$

Notice that there is a factor of 2, called the Thomas precession, which comes from the relativistic calculation that changes back to the electron's frame from the nucleus frame.

Since

${\begin{aligned}\left\langle {\frac {1}{r^{3}}}\right\rangle &={\frac {Z^{3}}{n^{3}a_{0}^{3}}}{\frac {1}{l\left(l+{\frac {1}{2}}\right)(l+1)}}\\\left\langle {\vec {L}}\cdot {\vec {S}}\right\rangle &={\frac {\hbar ^{2}}{2}}[j(j+1)-l(l+1)-s(s+1)]\end{aligned}}$

the expectation value for the Hamiltonian is:

$\left\langle H_{SO}\right\rangle ={\frac {E_{n}{}^{2}}{m_{e}c^{2}}}~n~{\frac {j(j+1)-l(l+1)-{\frac {3}{4}}}{l\left(l+{\frac {1}{2}}\right)(l+1)}}$

Thus the order of magnitude for the spin-orbital coupling is ${\frac {Z^{4}}{n^{3}(j+1/2)}}10^{-5}{\text{ eV}}$.

Darwin term

There is one last term in the non-relativistic expansion of the Dirac equation. Because it was first derived by Charles Galton Darwin it is referred to as the Darwin term, and is given by:

${\begin{aligned}H_{\mathrm {Darwinian} }&={\frac {\hbar ^{2}}{8m_{e}^{2}c^{2}}}\,4\pi \left({\frac {Ze^{2}}{4\pi \varepsilon _{0}}}\right)\delta ^{3}\left({\vec {r}}\right)\\\langle H_{\mathrm {Darwinian} }\rangle &={\frac {\hbar ^{2}}{8m_{e}^{2}c^{2}}}\,4\pi \left({\frac {Ze^{2}}{4\pi \varepsilon _{0}}}\right)|\psi (0)|^{2}\\\psi (0)&=0{\text{ for }}l>0\\\psi (0)&={\frac {1}{\sqrt {4\pi }}}\,2\left({\frac {Z}{na_{0}}}\right)^{\frac {3}{2}}{\text{ for }}l=0\\H_{\mathrm {Darwinian} }&={\frac {2n}{m_{e}c^{2}}}\,E_{n}^{2}\end{aligned}}$

The Darwin term affects only the s-orbit. This is because the wave function of an electron with $l>0$ vanishes at the origin, hence the delta function has no effect. For example, it gives the 2s-orbit the same energy as the 2p-orbit by raising the 2s-state by 6977145109124427590♠9.057×10⁻⁵ eV.

The Darwin term changes the effective potential at the nucleus. It can be interpreted as a smearing out of the electrostatic interaction between the electron and nucleus due to zitterbewegung, or rapid quantum oscillations, of the electron. This can be demonstrated by a short calculation

Quantum fluctuations allow for the creation of virtual electron-positron pairs with a lifetime estimated by the uncertainty principle $\Delta t\approx \hbar /\Delta E\approx \hbar /mc^{2}$. The distance the particles can move during this time is $\xi \approx c\Delta t\approx \hbar /mc=\lambda _{c}$, the Compton wavelength. The electrons of the atom interact with those pairs. This yields a fluctuating electron position ${\vec {r}}+{\vec {\xi }}$. Using a Taylor expansion, the effect on the potential $U$ can be estimated:

$U({\vec {r}}+{\vec {\xi }})\approx U({\vec {r}})+\xi \cdot \nabla U({\vec {r}})+{\frac {1}{2}}\sum _{ij}\xi _{i}\xi _{j}\partial _{i}\partial _{j}U({\vec {r}})$

Averaging over the fluctuations ${\vec {\xi }}$

${\overline {\xi }}=0,\quad {\overline {\xi _{i}\xi _{j}}}={\frac {1}{3}}{\overline {{\vec {\xi }}^{2}}}\delta _{ij},$

gives the average potential

${\overline {U\left({\vec {r}}+{\vec {\xi }}\right)}}=U\left({\vec {r}}\right)+{\frac {1}{6}}{\overline {{\vec {\xi }}^{2}}}\nabla ^{2}U\left({\vec {r}}\right).$

Approximating ${\overline {{\vec {\xi }}^{2}}}\approx \lambda _{c}^{2}$, this yields the perturbation of the potential due to fluctuations:

$\delta U\approx {\frac {1}{6}}\lambda _{c}^{2}\nabla ^{2}U={\frac {\hbar ^{2}}{6m^{2}c^{2}}}\nabla ^{2}U$

To compare with the expression above, plug in the Coulomb potential:

$\nabla ^{2}U=-\nabla ^{2}{\frac {Ze^{2}}{4\pi \varepsilon _{0}r}}=4\pi \left({\frac {Ze^{2}}{4\pi \varepsilon _{0}}}\right)\delta ({\vec {r}})\quad \Rightarrow \quad \delta U\approx {\frac {\hbar ^{2}}{6m^{2}c^{2}}}4\pi \left({\frac {Ze^{2}}{4\pi \varepsilon _{0}}}\right)\delta ({\vec {r}})$

This is only slightly different.

Another mechanism that affects only the s-state is the Lamb shift, a further, smaller correction that arises in quantum electrodynamics that should not be confused with the Darwin term. The Darwin term gives the s-state and p-state the same energy, but the Lamb shift makes the s-state higher in energy than the p-state.

Total effect

The full Hamiltonian is given by

$H=H_{0}+H_{\mathrm {kinetic} }+H_{\mathrm {so} }+H_{\mathrm {Darwinian} },\!$

where $H_{0}$ is the Hamiltonian from the Coulomb interaction.

The total effect, obtained by summing the three components up, is given by the following expression:

$\Delta E={\frac {E_{n}(Z\alpha )^{2}}{n}}\left({\frac {1}{j+{\frac {1}{2}}}}-{\frac {3}{4n}}\right)\,,$

where $j$ is the total angular momentum ($j=1/2$ if $l=0$ and $j=l\pm 1/2$ otherwise). It is worth noting that this expression was first obtained by A. Sommerfeld based on the old Bohr theory; i.e., before the modern quantum mechanics was formulated.

The total effect can also be obtained by using the Dirac equation. In this case, the electron is treated as non-relativistic. The exact energies are given by

$E_{j\,n}=-m_{\text{e}}c^{2}\left[1-\left(1+\left[{\dfrac {\alpha }{n-j-{\frac {1}{2}}+{\sqrt {\left(j+{\frac {1}{2}}\right)^{2}-\alpha ^{2}}}}}\right]^{2}\right)^{-1/2}\right].$

This expression, which contains all higher order terms that were left out in the other calculations, expands to first order to give the energy corrections derived from perturbation theory. However, this equation does not contain the hyperfine structure corrections, which are due to interactions with the nuclear spin. Other corrections from quantum field theory such as the Lamb shift and the anomalous magnetic dipole moment of the electron are not included.

See also

• Angular momentum coupling

References

• Griffiths, David J. (2004). Introduction to Quantum Mechanics (2nd ed.). Prentice Hall. ISBN 0-13-805326-X. 
• Liboff, Richard L. (2002). Introductory Quantum Mechanics. Addison-Wesley. ISBN 0-8053-8714-5. 

External links

• Hyperphysics: Fine Structure
• University of Texas: The fine structure of hydrogen

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fine structure、microstructural

「microstructural」

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• 微細構造の

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fine structure、microstructure

「structure」

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• n.

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「fine」

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• adj.

• 微細な、細密な

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refined