When energy is added to the material, either by a laser «pump» or as an electrical current, it kicks some of the electrons orbiting the molecules into higher energy states. (sciencedaily.com)
Like a boulder perched at the top of a hill, with a bit of a nudge, the electron tumbles from higher energy states to lower, releasing energy along the way. (newscientist.com)
Each hydrogen atom, made up of just a single proton and electron, can be found in two slightly different states: a higher energy state in which the electron and proton essentially spin in the same direction, and a lower energy state in which they spin in opposite directions. (discovermagazine.com)