Is the Rotation of Spherical Harmonics Using Wigner Matrices Correct?

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SUMMARY

The discussion centers on the application of Wigner matrices for the rotation of spherical harmonics, specifically addressing the equation involving the spherical harmonic functions \(Y_{2; m'}\) and the Wigner \(d\)-matrix \(d^{(2)}_{1m'}\). The user expresses uncertainty regarding the use of the Euler angle \(\beta\) in conjunction with the angles \(\theta\) and \(\phi\). It is concluded that \(\beta\) will be eliminated in the final function, which will depend solely on \(\theta\) and \(\phi\), thus confirming the correctness of the approach.

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Homework Statement
Rotate the spherical harmonic $$\ket{l=2, m=1}=Y_{2, 1}$$ an angle of π/4 about the y-axis.
Relevant Equations
$$\sum_{m'=-l}^{l} {d^{(l)}}_{m, m'} Y_{l, m'}$$
I tried using the Wigner matrices:

$$\sum_{m'=-2}^{2} {d^{(2)}}_{1m'} Y_{2; m'}={d^{(2)}}_{1 -2} Y_{2; -2} + {d^{(2)}}_{1 -1} Y_{2; -1} + ...= -\frac{1-\cos(\beta)}{2} \sin(\beta) \sqrt{\frac{15}{32 \pi}} \sin^2(\theta) e^{-i \phi} + ...$$

where $$\beta=\frac{\pi}{4}$$. But I don't know if this is ok since $$\beta$$ is an Euler angle while $$\theta$$ and $$\phi$$ are not. If this is not right, what should I do?
 
Last edited:
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The angle ##\beta## will go away as it is replaced by the value of the rotation, leaving a function of ##(\theta,\phi)##, which is what you want.
 
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