# Polarization of an emitted photon

1. Dec 27, 2014

### naima

Bonnes fetes de Noel.

When an excited electron returns to its ground state a photon is emitted.
Suppose that the electron's spin skip from +1/2 to -1/2 (around z)and that the photon has a momentum parallel to z.
I suppose that this photon has a circular polarization around z (Is it true?)
The problem is that there is no spin alone conservation but total angular momentum conservation.
Knowing all about |e> and |g> what can we calculate about the polarization of the emitted photon?

Last edited: Dec 27, 2014
2. Dec 27, 2014

### Staff: Mentor

Merci !

To first order, the spin of the electron is not coupled to the electromagnetic field, so spin-flip doesn't occur during transtions. Look up selection rules.

You need to consider the total angular momentum of the electron, $j$. For an emission in which you have $j = +1/2 \rightarrow j' = -1/2$, that would correspond to a $\sigma^+$ photon, which means that if the photon is travelling along z, it has right-handed circular polarization. A recent discussion on this subject can be found here.

3. Dec 27, 2014

### DrDu

You should also take in mind that a photon in a momentum eigenstate is not in an eigenstate of photon angular momentum, which are the vector spherical harmonics.

4. Dec 28, 2014

Thank you