# Multiphoton momentum trasfer

## Main Question or Discussion Point

Hi everybody,

I'm reading P. Gibbon book "Short Pulse Laser Interaction with Matters", and I'm having a problem.
Describing the ejection of an electron from a focused laser beam the book says, on pag 39, that "the kinetic energy of the electron is given by:

$\Delta$U=($\gamma$-1)mc$^{2}$

This energy is extracted from electromagnetic field via multiphoton momentum transfer."

I would like to know the demostration of that formula, since no clue is given in the book.

Thanks for helping

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