Is it necessary for electrons to always emit the energy back as photons after absorbing?
In: Physics
No. When an electron in an atom or molecule is excited, one of several different things can happen.
We can think of our electron as being like a ball sitting in dip in the ground. Our photon gives the ball a little push. Let’s first imagine that the photon has lots of energy, and it gives the ball a really big push, enough that it escapes from the dip and rolls away. In this case, the electron goes flying completely out of your material, and the energy of the photon is converted to the kinetic energy (motion) of the electron. This is called the [photoelectric effect](https://en.m.wikipedia.org/wiki/Photoelectric_effect) —it led to the development of quantum mechanics and is actually what Einstein won his Nobel Prize for, although it’s not even close to his most famous discovery.
Now let’s imagine the other possibility, where the photon doesn’t have enough energy to push the electron completely out of its dip—instead it now just starts rolling back and forth inside the dip. This is called an excited state. The electron doesn’t experience friction, so how can it get rid of this energy?
One possibility is what you suggested: it could drop back down to its initial state, and re-emit the energy as a photon. However, usually at least some of the energy is converted into heat, by causing the atoms in the material to start vibrating. These vibrations are called phonons. The most common occurrence is that all of the energy is converted into phonons—this is called [internal conversion](https://en.m.wikipedia.org/wiki/Internal_conversion_(chemistry)). However, sometimes, for reasons that are complicated, only a little of the energy can be easily converted to photons. In that case, the leftover energy will eventually be re-emitted as photons, via processes called [fluorescence](https://en.m.wikipedia.org/wiki/Fluorescence) (if the re-emission happens quickly) or [phosphorescence](https://en.m.wikipedia.org/wiki/Phosphorescence) (if the re-emission is slow). You’ve probably seen these before—fluorescent materials glow under UV lights, and glow in the dark paints are made of phosphorescent materials.
Yes, the nucleus attracts the electrons to be in the lowest possible orbitals (energy wise), similarly to how gravity attracts objects to settle down in the lowest points on Earth.
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