Electron spin change and photon absorption/emission I came across this document and would like to verify that I understand it correctly:
My understanding is that only an electron with a negative electron spin can absorb a photon and thereby acquire a positive electron spin. In such a scenario, the circular polarization of the photon must be positive. Similarly, when a photon is emitted from an electron as its spin changes from positive to negative, the circular polarization of that photon can only be positive.
 A: The spin of an electron was derived from the phenomenon of the deflection of them in an external magnetic field. In the case, that electrons and anti-protons get deflected say to the right, than positrons and protons get deflected to the left.$^*$
You call the spin negative and positive. In use are the attributes „up“ and „down“ and these attributes  are related to opposite orientations of two electrons in an atomic orbital. $^*$$^*$
With the spin of a photon it is a bit tricky. The electric and the magnetic field component of a photon could have a positive or an negative isospin. $^*$$^*$$^*$ It turns out that electrons emit photons all with the same isospin. $^*$$^*$$^*$$^*$
Resume. Electrons emit photons always with the same isospin.
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$^*$ The electric field of subatomic particles seems to be equaly distributed and furthermore doesn‘t interact with magnetic fields. But these particles have a also magnetic dipole and this interact with the external magnetic field and could be the reason for the deflection.
$^*$$^*$ For your imagination only, connect the spin with the magnetic dipoles of the electrons again. Two of them could interact perfectly like two bar magnets wich are connected south by north and north by south.
$^*$$^*$$^*$ The isospin was introduced to describe unsymmetrical situations from the point of motion. If you see on the photon in the direction of its motion and the electric field shows say to the left than the magnetic field could be directed up OR down.
$^*$$^*$$^*$$^*$ The evidence for this is the emission of photons in an antenna rod. The electric field of the emitted photons is polarized (because all the electron get accelerated only forth and back in the rod). The clue is, that a receiving magnetic loop antenna wouldn‘t receive any signal if the magnetic field component of the emitted photons could be directed to the left for half of the photons and to the right for the others.
