How do electrons absorb photons?

Question

I understand that atom-bound electrons can absorb photons (explanation for why they are typically atom-bound). However, what is the particular mechanism for this occurring? I am familiar with the idea that there is an energy-state lifetime, but would like to know exactly how the electron is able to store, bind, or capture this energy even for a short duration of time, and then release it.

Answer 1

Imagine you have two clouds of charge, Like two spheres with uniform charge distributions where the shape of each cloud is fixed. Also suppose the clouds can pass through eachother. The ground state is for the two charge clouds to be perfectly overlapped. In this case there will be no $E$ field and certainly no radiation. Now manually displace one of the charges. This will introduce electrostatic energy into the system. When you release the charge cloud it will move towards the other charge cloud. When it overlaps it will still have velocity and it will move past the equilibrium point. The charge clouds will oscillate in position with respect to each other. If this were the whole story the charges would oscillate forever like a mass on a frictionless spring.

However, the oscillating of the charges causes fluctuations in the electromagnetic field. These fluctuations carry away energy in the form of radiation. It costs kinetic energy for this radiation to be created so the charge oscillation slows down. Eventually all the energy is lost and the charge clouds over lap again. The system has decayed to the ground state by emitting electromagnetic radiation/energy.

Now play this picture in reverse and you have the story of how the charge cloud can absorb electromagnetic radiation. Basically the radiation illuminating the charge clouds imparts kinetic and electrostatic energy to the two clouds of charge, this comes at reducing the total energy of the electromagnetic field. Hence radiation energy is converted into kinetic and electrostatic energy (the energy of the bound state).