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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.

Some questions:

  1. 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?

  2. 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"?

  3. 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?

  4. 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?

  5. 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?

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closed as too broad by Chris White, Manishearth Sep 8 '13 at 8:18

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

    
Welcome to Physics.SE! This question is probably too broad, and you have better chances of getting good answers by splitting it into three or four different questions. –  Emilio Pisanty Sep 1 '13 at 20:52
    
@EmilioPisanty Thank you! I will keep that in mind in the future. –  jns Sep 2 '13 at 20:19
    
I suggest you split this into multiple questions, each question containing a maximum of two very-similar subquestions. –  Manishearth Sep 8 '13 at 8:19

1 Answer 1

up vote 2 down vote accepted

First of all, no transition has a precise frequency, each transition has at least the so-called "natural width". 1. No, for any non-zero temperature, there is some distribution over energy, for example, for electromagnetic radiation that is in a thermal equilibrium with the substance, but transitions can be very rare for low temperatures. 2.Theoretically (and experimentally), n can be very high without ionization. 3. If no external factors affect the atom, the time it will take it to go to the ground state will depend on the intensities of the relevant (sequential) transitions. You may wish to look up "spontaneous emission" - it's an effect of quantum electrodynamics. 4. I cannot give a short answer. 5. Sometimes it does happen, but sometimes the excitation is removed through different mechanisms, and the substance can radiate at a different (usually, lower) frequency.

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Very interesting! Thank you for your answer. I am not sure why I am being downvoted, though? I did attempt to research this but I could not find a clear answer. –  jns Aug 31 '13 at 22:51
1  
I don't know why you were downvoted, but my guess is you put too many questions in your question - that may look irritating:-) –  akhmeteli Aug 31 '13 at 23:31

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