I know free electron cannot absorb or emit photon as it cannot satisfy both the conservation of energy and momentum at once but how about in Bremsstrahlung process? The presence of a positively charged particle is used to deflect the trajectory of an unbounded electron causing it to decelerate/accelerate depending if it's inwards or outwards, then it should emit photon no?

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    $\begingroup$ Yes, in the presence of an external force the momentum is not conserved at all, so the processes you mention become possible. $\endgroup$ Commented Jan 26, 2021 at 6:58

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Bremsstrahlun photons were used in order to identify electrons when studyig elementary particle interactions in cloud chambers and bubble chambers.

Strong magnetic fields were imposed so that charged particles would bend and their momentum measured. The photon from the bending electrons pair-produces in the chamber and thus identify the photon

In this paper one such example is given in page 19,

This unassuming picture is one of the bubble chamber ‘greats’: a tiny electromagnetic shower demonstrates the existence of a very simple, but rare, collision between an anti-muon neutrino ̄νμbeam particle and an atomic electrone−in the bubble chamber liquid


The first e− is the one that was knocked out of an atom by the ̄νμ. The two little e+e− pairs are produced by bremsstrahlung photons emitted from this first electron, thus identifying it.


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