Selection rule

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See also angular momentum coupling

In physics and chemistry, a selection rule (or transition rule) is a constraint on the possible transitions of a system from one state to another. They are encountered most often in spectroscopy. For example, an electron excited by a photon can only jump from one state to another in integer steps of angular momemtum (δJ = 0).

[edit] Overview

In general, electric (charge) radiation or magnetic (current, magnetic moment radiation) can be classified into multipoles Eλ (electric) or Mλ (magnetic) of order 2^λ, e.g. E1 for electric dipole, E2 for quadrupole, or E3 for octupole. In transitions where the change in angular momentum between the initial and final states makes a several multipole radiations possible, usually the lowest order multipoles are overwhelmingly more likely, and dominate the transition.

The emitted particle carries away an angular momentum λ, which for the photon must be at least 1, since it is a vector particle (i.e., it has J^P = 1⁻). Thus there is no E0 (electric monopoles) or M0 (magnetic monopoles, which can not seem to exist) radiation.

Since the total angular momentum has to be conserved during the transition, we have that

$\mathbf J_{\mathrm{i}} = \mathbf{J}_{\mathrm{f}} + \boldsymbol{\lambda}$

where $\Vert \boldsymbol{\lambda} \Vert = \sqrt{\lambda(\lambda + 1)} \, \hbar$ , and its z-projection is given by $\lambda_z = \mu \, \hbar.$ The corresponding quantum numbers λ, μ must satisfy

$| J_{\mathrm{i}} - J_{\mathrm{f}} | \le \lambda \le J_{\mathrm{i}} + J_{\mathrm{f}}$

and

$\mu = M_{\mbox{i}} - M_{\mbox{f}}\,.$

Parity is also preserved. For electric multipole transitions

$\pi(\mathrm{E}\lambda) = \pi_{\mathrm{i}} \pi_{\mathrm{f}} = (-1)^{\lambda}\,$

while for magnetic multipoles

$\pi(\mathrm{M}\lambda) = \pi_{\mathrm{i}} \pi_{\mathrm{f}} = (-1)^{\lambda+1}\,.$

Thus, parity does not change for E-even or M-odd multipoles, while it changes for E-odd or M-even multipoles.

These considerations generate different sets of transitions rules depending on the multipole order and type. The expression forbidden transitions is often used; this does not mean that these transitions cannot occur, only that they are electric-dipole forbidden. These transitions are perfectly possible, they merely occur at a lower rate. If the rate for an E1 transition is non-zero, the transition is said to be permitted; if it is zero, then M1, E2, etc. transitions can still produce radiation, albeit with much lower transitions rates. These are the so-called forbidden transitions. The transition rate decreases by a factor of approximately 10³ from one multipole to the next one, so the lowest multipole transitions are most likely to occur.

Semi-forbidden transitions (resulting in so called intercombination lines) are electric dipole (E1) transitions for which the selection rule that the spin does not change is violated. This is a result of the failure of LS coupling.

[edit] Summary table

		Electric dipole (E1)	Magnetic dipole (M1)	Electric quadrupole (E2)	Magnetic quadrupole (M2)	Electric octupole (E3)	Magnetic octupole (M3)
Rigorous rules	(1)	$\begin{matrix} \Delta J = 0, \pm 1 \\ (J = 0 \not \leftrightarrow 0)\end{matrix}$		$\begin{matrix} \Delta J = 0, \pm 1, \pm 2 \\ (J = 0 \not \leftrightarrow 0, 1;\ \begin{matrix}{1 \over 2}\end{matrix} \not \leftrightarrow \begin{matrix}{1 \over 2}\end{matrix})\end{matrix}$		$\begin{matrix}\Delta J = 0, \pm1, \pm2, \pm 3 \\ (0 \not \leftrightarrow 0, 1, 2;\ \begin{matrix}{1 \over 2}\end{matrix} \not \leftrightarrow \begin{matrix}{1 \over 2} \end{matrix}, \begin{matrix}{3 \over 2}\end{matrix};\ 1 \not \leftrightarrow 1) \end{matrix}$
	(2)	$\Delta M_J = 0, \pm 1$		$\Delta M_J = 0, \pm 1, \pm2$		$\Delta M_J = 0, \pm 1, \pm2, \pm 3$
	(3)	$\pi_{\mathrm{f}} = -\pi_{\mathrm{i}}\,$	$\pi_{\mathrm{f}} = \pi_{\mathrm{i}}\,$		$\pi_{\mathrm{f}} = -\pi_{\mathrm{i}}\,$		$\pi_{\mathrm{f}} = \pi_{\mathrm{i}}\,$
LS coupling	(4)	One electron jump Δl = ±1	No electron jump Δl = 0, Δn = 0	None or one electron jump Δl = 0, ±2	One electron jump Δl = ±1	One electron jump Δl = ±1, ±3	One electron jump Δl = 0, ±2
LS coupling	(5)	If ΔS = 0 $\begin{matrix}\Delta L = 0, \pm 1 \\ (L = 0 \not \leftrightarrow 0)\end{matrix}$	If ΔS = 0 $\Delta L = 0\,$	If ΔS = 0 $\begin{matrix}\Delta L = 0, \pm 1, \pm 2 \\ (L = 0 \not \leftrightarrow 0, 1)\end{matrix}$		If ΔS = 0 $\begin{matrix}\Delta L = 0, \pm 1, \pm 2, \pm 3 \\ (L=0 \not \leftrightarrow 0, 1, 2;\ 1 \not \leftrightarrow 1)\end{matrix}$
Intermediate coupling	(6)	If ΔS = ±1 $\Delta L = 0, \pm 1, \pm 2\,$		If ΔS = ±1 $\begin{matrix}\Delta L = 0, \pm 1, \\ \pm 2, \pm 3 \\ (L = 0 \not \leftrightarrow 0)\end{matrix}$	If ΔS = ±1 $\begin{matrix}\Delta L = 0, \pm 1 \\ (L = 0 \not \leftrightarrow 0)\end{matrix}$	If ΔS = ±1 $\begin{matrix}\Delta L = 0, \pm 1, \\ \pm 2, \pm 3, \pm 4 \\ (L = 0 \not \leftrightarrow 0, 1)\end{matrix}$	If ΔS = ±1 $\begin{matrix}\Delta L = 0, \pm 1, \\ \pm 2 \\ (L = 0 \not \leftrightarrow 0)\end{matrix}$

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Selection rule

From Wikipedia, the free encyclopedia

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