# Wigner D-matrix

The Wigner D-matrix is a matrix in an irreducible representation of the groups SU(2) and SO(3). The complex conjugate of the D-matrix is an eigenfunction of the Hamiltonian of spherical and symmetric rigid rotors. The matrix was introduced in 1927 by Eugene Wigner.

## Definition of the Wigner D-matrix

Let Jx, Jy, Jz be generators of the Lie algebra of SU(2) and SO(3). In quantum mechanics these three operators are the components of a vector operator known as angular momentum. Examples are the angular momentum of an electron in an atom, electronic spin, and the angular momentum of a rigid rotor. In all cases the three operators satisfy the following commutation relations,

$[J_x,J_y] = i J_z,\quad [J_z,J_x] = i J_y,\quad [J_y,J_z] = i J_x,$

where i is the purely imaginary number and Planck's constant $\hbar$ has been put equal to one. The operator

$J^2 = J_x^2 + J_y^2 + J_z^2$

is a Casimir operator of SU(2) (or SO(3) as the case may be). It may be diagonalized together with $J_z$ (the choice of this operator is a convention), which commutes with $J^ 2$. That is, it can be shown that there is a complete set of kets with

$J^2 |jm\rangle = j(j+1) |jm\rangle,\quad J_z |jm\rangle = m |jm\rangle,$

where j = 0, 1/2, 1, 3/2, 2,... and m = -j, -j + 1,..., j. For SO(3) the quantum number j is integer.

A rotation operator can be written as

$\mathcal{R}(\alpha,\beta,\gamma) = e^{-i\alpha J_z}e^{-i\beta J_y}e^{-i\gamma J_z},$

where α, β, γ are Euler angles (characterized by the keywords: z-y-z convention, right-handed frame, right-hand screw rule, active interpretation).

The Wigner D-matrix is a square matrix of dimension 2j + 1 with general element

$D^j_{m'm}(\alpha,\beta,\gamma) \equiv \langle jm' | \mathcal{R}(\alpha,\beta,\gamma)| jm \rangle = e^{-im'\alpha } d^j_{m'm}(\beta)e^{-i m\gamma}.$

The matrix with general element

$d^j_{m'm}(\beta)= \langle jm' |e^{-i\beta J_y} | jm \rangle$

is known as Wigner's (small) d-matrix.

## Wigner (small) d-matrix

Wigner[1] gave the following expression

$\begin{array}{lcl} d^j_{m'm}(\beta) &=& [(j+m')!(j-m')!(j+m)!(j-m)!]^{1/2} \sum\limits_s \left[\frac{(-1)^{m'-m+s}}{(j+m-s)!s!(m'-m+s)!(j-m'-s)!} \right.\\ &&\left. \cdot \left(\cos\frac{\beta}{2}\right)^{2j+m-m'-2s}\left(\sin\frac{\beta}{2}\right)^{m'-m+2s} \right]. \end{array}$

The sum over s is over such values that the factorials are nonnegative.

Note: The d-matrix elements defined here are real. In the often-used z-x-z convention of Euler angles, the factor $(-1)^{m'-m+s}$ in this formula is replaced by $(-1)^s\, i^{m-m'}$, causing half of the functions to be purely imaginary. The realness of the d-matrix elements is one of the reasons that the z-y-z convention, used in this article, is usually preferred in quantum mechanical applications.

The d-matrix elements are related to Jacobi polynomials $P^{(a,b)}_k(\cos\beta)$ with nonnegative $a\,$ and $b\,$.[2] Let

$k = \min(j+m,\,j-m,\,j+m',\,j-m').$

$\hbox{If}\quad k = \begin{cases} j+m: &\quad a=m'-m;\quad \lambda=m'-m\\ j-m: &\quad a=m-m';\quad \lambda= 0 \\ j+m': &\quad a=m-m';\quad \lambda= 0 \\ j-m': &\quad a=m'-m;\quad \lambda=m'-m \\ \end{cases}$

Then, with $b=2j-2k-a\,$, the relation is

$d^j_{m'm}(\beta) = (-1)^{\lambda} \binom{2j-k}{k+a}^{1/2} \binom{k+b}{b}^{-1/2} \left(\sin\frac{\beta}{2}\right)^a \left(\cos\frac{\beta}{2}\right)^b P^{(a,b)}_k(\cos\beta),$

where $a,b \ge 0. \,$

## Properties of the Wigner D-matrix

The complex conjugate of the D-matrix satisfies a number of differential properties that can be formulated concisely by introducing the following operators with $(x,\, y,\,z) = (1,\,2,\,3)$,

$\begin{array}{lcl} \hat{\mathcal{J}}_1 &=& i \left( \cos \alpha \cot \beta \, {\partial \over \partial \alpha} \, + \sin \alpha \, {\partial \over \partial \beta} \, - {\cos \alpha \over \sin \beta} \, {\partial \over \partial \gamma} \, \right) \\ \hat{\mathcal{J}}_2 &=& i \left( \sin \alpha \cot \beta \, {\partial \over \partial \alpha} \, - \cos \alpha \; {\partial \over \partial \beta } \, - {\sin \alpha \over \sin \beta} \, {\partial \over \partial \gamma } \, \right) \\ \hat{\mathcal{J}}_3 &=& - i \; {\partial \over \partial \alpha} , \end{array}$

which have quantum mechanical meaning: they are space-fixed rigid rotor angular momentum operators.

Further,

$\begin{array}{lcl} \hat{\mathcal{P}}_1 &=& \, i \left( {\cos \gamma \over \sin \beta} {\partial \over \partial \alpha } - \sin \gamma {\partial \over \partial \beta } - \cot \beta \cos \gamma {\partial \over \partial \gamma} \right) \\ \hat{\mathcal{P}}_2 &=& \, i \left( - {\sin \gamma \over \sin \beta} {\partial \over \partial \alpha} - \cos \gamma {\partial \over \partial \beta} + \cot \beta \sin \gamma {\partial \over \partial \gamma} \right) \\ \hat{\mathcal{P}}_3 &=& - i {\partial\over \partial \gamma}, \\ \end{array}$

which have quantum mechanical meaning: they are body-fixed rigid rotor angular momentum operators.

The operators satisfy the commutation relations

$\left[\mathcal{J}_1, \, \mathcal{J}_2\right] = i \mathcal{J}_3, \qquad \hbox{and}\qquad \left[\mathcal{P}_1, \, \mathcal{P}_2\right] = -i \mathcal{P}_3$

and the corresponding relations with the indices permuted cyclically. The $\mathcal{P}_i$ satisfy anomalous commutation relations (have a minus sign on the right hand side).

The two sets mutually commute,

$\left[\mathcal{P}_i, \, \mathcal{J}_j\right] = 0,\quad i,\,j = 1,\,2,\,3,$

and the total operators squared are equal,

$\mathcal{J}^2 \equiv \mathcal{J}_1^2+ \mathcal{J}_2^2 + \mathcal{J}_3^2 = \mathcal{P}^2 \equiv \mathcal{P}_1^2+ \mathcal{P}_2^2 + \mathcal{P}_3^2 .$

Their explicit form is,

$\mathcal{J}^2= \mathcal{P}^2 = -\frac{1}{\sin^2\beta} \left( \frac{\partial^2}{\partial \alpha^2} +\frac{\partial^2}{\partial \gamma^2} -2\cos\beta\frac{\partial^2}{\partial\alpha\partial \gamma} \right) -\frac{\partial^2}{\partial \beta^2} -\cot\beta\frac{\partial}{\partial \beta}.$

The operators $\mathcal{J}_i$ act on the first (row) index of the D-matrix,

$\mathcal{J}_3 \, D^j_{m'm}(\alpha,\beta,\gamma)^* = m' \, D^j_{m'm}(\alpha,\beta,\gamma)^* ,$

and

$(\mathcal{J}_1 \pm i \mathcal{J}_2)\, D^j_{m'm}(\alpha,\beta,\gamma)^* = \sqrt{j(j+1)-m'(m'\pm 1)} \, D^j_{m'\pm 1, m}(\alpha,\beta,\gamma)^* .$

The operators $\mathcal{P}_i$ act on the second (column) index of the D-matrix

$\mathcal{P}_3 \, D^j_{m'm}(\alpha,\beta,\gamma)^* = m \, D^j_{m'm}(\alpha,\beta,\gamma)^* ,$

and because of the anomalous commutation relation the raising/lowering operators are defined with reversed signs,

$(\mathcal{P}_1 \mp i \mathcal{P}_2)\, D^j_{m'm}(\alpha,\beta,\gamma)^* = \sqrt{j(j+1)-m(m\pm 1)} \, D^j_{m', m\pm1}(\alpha,\beta,\gamma)^* .$

Finally,

$\mathcal{J}^2\, D^j_{m'm}(\alpha,\beta,\gamma)^* = \mathcal{P}^2\, D^j_{m'm}(\alpha,\beta,\gamma)^* = j(j+1) D^j_{m'm}(\alpha,\beta,\gamma)^*.$

In other words, the rows and columns of the (complex conjugate) Wigner D-matrix span irreducible representations of the isomorphic Lie algebra's generated by $\{\mathcal{J}_i\}$ and $\{-\mathcal{P}_i\}$.

An important property of the Wigner D-matrix follows from the commutation of $\mathcal{R}(\alpha,\beta,\gamma)$ with the time reversal operator $T\,$,

$\langle jm' | \mathcal{R}(\alpha,\beta,\gamma)| jm \rangle = \langle jm' | T^{\,\dagger} \mathcal{R}(\alpha,\beta,\gamma) T| jm \rangle = (-1)^{m'-m} \langle j,-m' | \mathcal{R}(\alpha,\beta,\gamma)| j,-m \rangle^*,$

or

$D^j_{m'm}(\alpha,\beta,\gamma) = (-1)^{m'-m} D^j_{-m',-m}(\alpha,\beta,\gamma)^*.$

Here we used that $T\,$ is anti-unitary (hence the complex conjugation after moving $T^\dagger\,$ from ket to bra), $T | jm \rangle = (-1)^{j-m} | j,-m \rangle$ and $(-1)^{2j-m'-m} = (-1)^{m'-m}$.

## Orthogonality relations

The Wigner D-matrix elements $D^j_{mk}(\alpha,\beta,\gamma)$ form a complete set of orthogonal functions of the Euler angles $\alpha$, $\beta,$ and $\gamma$:

$\int_0^{2\pi} d\alpha \int_0^\pi \sin \beta d\beta \int_0^{2\pi} d\gamma \,\, D^{j'}_{m'k'}(\alpha,\beta,\gamma)^\ast D^j_{mk}(\alpha,\beta,\gamma) = \frac{8\pi^2}{2j+1} \delta_{m'm}\delta_{k'k}\delta_{j'j}.$

This is a special case of the Schur orthogonality relations.

## Kronecker product of Wigner D-matrices, Clebsch-Gordan series

The set of Kronecker product matrices

$\mathbf{D}^j(\alpha,\beta,\gamma)\otimes \mathbf{D}^{j'}(\alpha,\beta,\gamma)$

forms a reducible matrix representation of the groups SO(3) and SU(2). Reduction into irreducible components is by the following equation:

$D^j_{m k}(\alpha,\beta,\gamma) D^{j'}_{m' k'}(\alpha,\beta,\gamma) = \sum_{J=|j-j'|}^{j+j'} \sum_{M=-J}^J \sum_{K=-J}^J \langle j m j' m' | J M \rangle \langle j k j' k' | J K \rangle D^J_{M K}(\alpha,\beta,\gamma)$

The symbol $\langle j m j' m' | J M \rangle$ is a Clebsch-Gordan coefficient.

## Relation to spherical harmonics and Legendre polynomials

For integer values of $l$, the D-matrix elements with second index equal to zero are proportional to spherical harmonics and associated Legendre polynomials, normalized to unity and with Condon and Shortley phase convention:

$D^{\ell}_{m 0}(\alpha,\beta,0) = \sqrt{\frac{4\pi}{2\ell+1}} Y_{\ell}^{m*} (\beta, \alpha ) = \sqrt{\frac{(\ell-m)!}{(\ell+m)!}} \, P_\ell^m ( \cos{\beta} ) \, e^{-i m \alpha }$

This implies the following relationship for the d-matrix:

$d^{\ell}_{m 0}(\beta) = \sqrt{\frac{(\ell-m)!}{(\ell+m)!}} \, P_\ell^m ( \cos{\beta} )$

When both indices are set to zero, the Wigner D-matrix elements are given by ordinary Legendre polynomials:

$D^{\ell}_{0,0}(\alpha,\beta,\gamma) = d^{\ell}_{0,0}(\beta) = P_{\ell}(\cos\beta).$

In the present convention of Euler angles, $\alpha$ is a longitudinal angle and $\beta$ is a colatitudinal angle (spherical polar angles in the physical definition of such angles). This is one of the reasons that the z-y-z convention is used frequently in molecular physics. From the time-reversal property of the Wigner D-matrix follows immediately

$\left( Y_{\ell}^m \right) ^* = (-1)^m Y_{\ell}^{-m}.$

There exists a more general relationship to the spin-weighted spherical harmonics:

$D^{\ell}_{-m s}(\alpha,\beta,-\gamma) =(-1)^m \sqrt\frac{4\pi}{2{\ell}+1} {}_sY_{{\ell}m}(\beta,\alpha) e^{is\gamma}.$

## Relation to Bessel functions

In the limit when $\ell \gg m, m^\prime$ we have $D^\ell_{mm^\prime}(\alpha,\beta,\gamma) \approx e^{-im\alpha-im^\prime\gamma}J_{m-m^\prime}(\ell\beta)$ where $J_{m-m^\prime}(\ell\beta)$ is the Bessel function and $\ell\beta$ is finite.

## List of d-matrix elements

Using sign convention of Wigner, et al. the d-matrix elements for j = 1/2, 1, 3/2, and 2 are given below.

for j = 1/2

• $d_{1/2,1/2}^{1/2} = \cos (\theta/2)$
• $d_{1/2,-1/2}^{1/2} = -\sin (\theta/2)$

for j = 1

• $d_{1,1}^{1} = \frac{1+\cos \theta}{2}$
• $d_{1,0}^{1} = \frac{-\sin \theta}{\sqrt{2}}$
• $d_{1,-1}^{1} = \frac{1-\cos \theta}{2}$
• $d_{0,0}^{1} = \cos \theta$

for j = 3/2

• $d_{3/2,3/2}^{3/2} = \frac{1+\cos \theta}{2} \cos \frac{\theta}{2}$
• $d_{3/2,1/2}^{3/2} = -\sqrt{3} \frac{1+\cos \theta}{2} \sin \frac{\theta}{2}$
• $d_{3/2,-1/2}^{3/2} = \sqrt{3} \frac{1-\cos \theta}{2} \cos \frac{\theta}{2}$
• $d_{3/2,-3/2}^{3/2} = - \frac{1-\cos \theta}{2} \sin \frac{\theta}{2}$
• $d_{1/2,1/2}^{3/2} = \frac{3\cos \theta - 1}{2} \cos \frac{\theta}{2}$
• $d_{1/2,-1/2}^{3/2} = - \frac{3\cos \theta + 1}{2} \sin \frac{\theta}{2}$

for j = 2 [3]

• $d_{2,2}^{2} = \frac{1}{4}\left(1 +\cos \theta\right)^2$
• $d_{2,1}^{2} = -\frac{1}{2}\sin \theta \left(1 + \cos \theta\right)$
• $d_{2,0}^{2} = \sqrt{\frac{3}{8}}\sin^2 \theta$
• $d_{2,-1}^{2} = -\frac{1}{2}\sin \theta \left(1 - \cos \theta\right)$
• $d_{2,-2}^{2} = \frac{1}{4}\left(1 -\cos \theta\right)^2$
• $d_{1,1}^{2} = \frac{1}{2}\left(2\cos^2\theta + \cos \theta-1 \right)$
• $d_{1,0}^{2} = -\sqrt{\frac{3}{8}} \sin 2 \theta$
• $d_{1,-1}^{2} = \frac{1}{2}\left(- 2\cos^2\theta + \cos \theta +1 \right)$
• $d_{0,0}^{2} = \frac{1}{2} \left(3 \cos^2 \theta - 1\right)$

Wigner d-matrix elements with swapped lower indices are found with the relation:

$d_{m', m}^j = (-1)^{m-m'}d_{m, m'}^j = d_{-m,-m'}^j$.