system
Given the following post, respond as if you're commenting on Reddit:
human
How does the absorption of radiation lead to an increase in temperature?

When a photon is absorbed by an atom, it becomes excited and an electron is moved to a higher energy level. How does this translate to an increase in the atom's kinetic energy if the energy is used to move the electron further from the nucleus?
gpt
The other comments get the gist right in that the answer is conservation of momentum. However, the story is slightly more complicated because unless you are in a very specific regime of stimulated emission, the atom will on average not move. The excited atom will randomly emit in any direction, so the average change in momentum of the atom is actually zero! 

The rise in temperature is then a result of the variance of these momentum kicks being non-zero, and the variance is related to the root mean square velocity (rms). Using the equitation theorem, the temperature is proportional to the square of the rms velocity, T = (M/3R) v\^2.

The situation is much more complex for solids or liquids and requires considering things like phonons, vibrational and rotational excitations, and more complicated decay paths of the excitation. However, at the physics 101 level the answer is conservation of momentum like the others pointed out.