Rami Arieli: "The Laser Adventure" Chapter 2.1 page 3

Energy transfer to and from the atom
Energy transfer to and from the atom can be performed in two different ways:

1. Collisions with other atoms, and the transfer of kinetic energy as a result of the collision. This kinetic energy is transferred into internal energy of the atom.
2. Absorption and emission of electromagnetic radiation.

Since we are now interested in the lasing process, we shall concentrate on the second mechanism of energy transfer to and from the atom (The first excitation mechanism is used in certain lasers, like Helium-Neon, as a way to put energy into the laser, and will be discussed in chapter 6 about the different kinds of lasers). 

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