If I understand correctly:
An atom acts a potential well for the electrons -- and particles in a potential well have discrete energy levels. There is a non-zero minimum to this energy called the ground state. Electrons may move between energy states by either absorbing or emitting energy. If the energy is supplied via a photon, this photon must have exactly the energy between the current state of the electron and the one after that. If it has too much or not enough energy, it moves through that atom without interaction.
if the energy is not supplied via photon, then how does this work? Suppose you are heating the substance. Will only certain temperatures cause transitions?
is there a limit to the possible states? I know of ionization energy -- but suppose that each time you supply exactly the energy needed for the electron to change states. Can it theoretically go to extremely high values of n without "ionizating"?
when an atom is excited, how long does it take for it to go back to its ground state? Why does it go back to its ground state? I can only find "because nature tends to go to the lowest energy"; is there a more scientific explanation?
if I point a monochromatic laser (visual spectrum) on a concrete wall, nothing will go through because the photon energy does not equal the energy required for the first transition, right? Suppose an electron therein is in some excited state, say n, and that the energy required to go to n+1 is now equal to the laser's, will it go through?
suppose the photons don't go through -- they are absorbed. Photons with equal energy will then be emitted when the electron returns to the ground state. Why don't we see those?