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triplet state : ウィキペディア英語版
triplet state

In quantum mechanics, a triplet is a quantum state of a system with a spin of 1, such that there are three allowed values of the spin component, −1, 0 and +1.
In physics, spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. In quantum mechanics, spin is particularly important for systems at atomic length scales, such as individual atoms, protons, or electrons. Such particles and the spins of quantum mechanical systems ("particle spin") possess several unusual or non-classical features, and for such systems, spin angular momentum is not associated with rotation in the geometric sense, but instead refers to an abstract kind of "internal" angular momentum.
Almost all molecules encountered in daily life exist in a singlet state, but molecular oxygen is an exception. At room temperature, O2 exists in a triplet state, which would require the forbidden transition into a singlet state before a chemical reaction could commence, which makes it kinetically nonreactive despite being thermodynamically a strong oxidant. Photochemical or thermal activation can bring it into singlet state, which is strongly oxidizing also kinetically.
== Two spin-1/2 particles ==
In a system with two spin-1/2 particles - for example the proton and electron in the ground state of hydrogen, measured on a given axis, each particle can be either spin up or spin down so the system has four basis states in all
:\uparrow\uparrow,\uparrow\downarrow,\downarrow\uparrow,\downarrow\downarrow
using the single particle spins to label the basis states, where the first and second arrow in each combination indicate the spin direction of the first and second particle respectively.
More rigorously
:
|s_1,m_1\rangle|s_2,m_2\rangle=|s_1,m_1\rangle\otimes|s_2,m_2\rangle,

where s_1 and s_2 are the spins of the two particles, and m_1 and m_2 are their projections onto the z-axis. Since for spin-1/2 particles, the |1/2,m\rangle basis states span a 2-dimensional space, the |1/2,m_1\rangle|1/2,m_2\rangle basis states span a 4-dimensional space.
Now the total spin and its projection onto the previously defined axis can be computed using the rules for adding angular momentum in quantum mechanics using the Clebsch–Gordan coefficients. In general
:|s,m\rangle = \sum_C_^|s_1m_1\rangle|s_2m_2\rangle
substituting in the four basis states
: |1/2,+1/2\rangle\;|1/2,+1/2\rangle\ (\uparrow\uparrow)
: |1/2,+1/2\rangle\;|1/2,-1/2\rangle\ (\uparrow\downarrow)
: |1/2,-1/2\rangle\;|1/2,+1/2\rangle\ (\downarrow\uparrow)
: |1/2,-1/2\rangle\;|1/2,-1/2\rangle\ (\downarrow\downarrow)
returns the possible values for total spin given along with their representation in the |1/2,\ m_1\rangle|1/2,\ m_2\rangle basis. There are three states with total spin angular momentum 1
:
\left.\begin
|1,1\rangle & =\;\uparrow\uparrow\\
|1,0\rangle & =\;(\uparrow\downarrow + \downarrow\uparrow)/\sqrt2\\
|1,-1\rangle & =\;\downarrow\downarrow
\end\right\}\quad s=1\quad\mathrm

and a fourth with total spin angular momentum 0
:\left.|0,0\rangle=(\uparrow\downarrow - \downarrow\uparrow)/\sqrt2\;\right\}\quad s=0\quad\mathrm
The result is that a combination of two spin-1/2 particles can carry a total spin of 1 or 0, depending on whether they occupy a triplet or singlet state.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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