I read about the process of inverse bremsstrahlung where a free electron can gain kinetic energy by absorbing a photon.
However, I'm having some trouble to understand why exactly a heavy particle must take part in this process, c.f.
The momentum conservation law requires this process can proceed only in the presence of an ion, which carries the extra momentum...
from google books
Which extra momentum is the author talking about? The photon's?
btw: sorry for the useless tags, I'm not a physicist..
EDIT: I kind of get it, I guess:
before absorption: electron energy : $0.5 m_e v_1^2$
electron momentum : $m_e v_1$
photon energy:$\hbar\omega$
photon momentum: $\hbar\omega/c$
total momentum: $\hbar\omega/c + m_e v_1$
After absorption:
electron energy: $1/2 m_e v_2^2$ where $v_2^2=v_1^2 + 2 \hbar\omega/m_e$
electron momentum: $m_e v_2=m_e \sqrt{v_1^2 + 2 \hbar\omega/m_e}$
which is probably more than $\hbar\omega/c + m_e v_1$ ? or isn't it?