7.2: Representation of Angular Momentum

Now, we saw earlier, in Section [s7.2], that the operators, \(p_i\), which represent the Cartesian components of linear momentum in quantum mechanics, can be represented as the spatial differential operators \(-{\rm i}\,\hbar\,\partial/\partial x_i\). Let us now investigate whether angular momentum operators can similarly be represented as spatial differential operators.

It is most convenient to perform our investigation using conventional spherical polar coordinates: that is, \(r\), \(\theta\), and \(\phi\). These are defined with respect to our usual Cartesian coordinates as follows:

\[\begin{align} \label{e8.21} x &= r\,\sin\theta\,\cos\phi,\\[4pt] y&= r\,\sin\theta\,\sin\phi,\\[4pt] z&= r\,\cos\theta.\label{e8.23}\end{align} \]

We deduce, after some tedious analysis, that

\[\begin{align} \frac{\partial}{\partial x} &= \sin\theta\,\cos\phi\,\frac{\partial}{\partial r} + \frac{\cos\theta\,\cos\phi}{r}\,\frac{\partial}{\partial\theta} - \frac{\sin\phi}{r\,\sin\theta}\,\frac{\partial}{\partial\phi},\label{e8xx}\\[4pt] \frac{\partial}{\partial y} &= \sin\theta\,\sin\phi\,\frac{\partial}{\partial r} + \frac{\cos\theta\,\sin\phi}{r}\,\frac{\partial}{\partial\theta} + \frac{\cos\phi}{r\,\sin\theta}\,\frac{\partial}{\partial\phi},\label{e8yy}\\[4pt] \frac{\partial}{\partial z} &= \cos\theta\,\frac{\partial}{\partial r} -\frac{\sin\theta}{r}\,\frac{\partial}{\partial \theta}.\label{e8zz}\end{align} \]

Making use of the definitions \ref{e8.1}–\ref{e8.3}, \ref{e8.9}, and \ref{e8.13}, the fundamental representation \ref{e6.12}–\ref{e6.14} of the \(p_i\) operators as spatial differential operators, Equations \ref{e8.21}–\ref{e8zz}, and a great deal of tedious analysis, we finally obtain

\[\begin{align} L_x &= - {\rm i}\,\hbar\left(-\sin\phi\,\frac{\partial}{\partial\theta} -\cos\phi\,\cot\theta\,\frac{\partial}{\partial\phi}\right),\\[4pt] L_y &= - {\rm i}\,\hbar\left(\cos\phi\,\frac{\partial}{\partial\theta} -\sin\phi\,\cot\theta\,\frac{\partial}{\partial\phi}\right),\\[4pt] L_z &= -{\rm i}\,\hbar\,\frac{\partial}{\partial\phi},\label{e8.26}\end{align} \]

as well as

\[L^2 = -\hbar^2\left[\frac{1}{\sin\theta}\frac{\partial}{\partial\theta}\left( \sin\theta\,\frac{\partial}{\partial\theta}\right) + \frac{1}{\sin^2\theta}\frac{\partial^2}{\partial\phi^2}\right], \nonumber \]

and

\[ L_\pm = \hbar\,\rm e^{\pm{\rm i}\,\phi}\left(\pm\frac{\partial}{\partial\theta} +{\rm i}\,\cot\theta\,\frac{\partial}{\partial\phi}\right). \label{e8.28} \]

We, thus, conclude that all of our angular momentum operators can be represented as differential operators involving the angular spherical coordinates, \(\theta\) and \(\phi\), but not involving the radial coordinate, \(r\).

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