I'm going to go way out on a limb and say that this is a hypothetical experiment that has never been carried out. I believe it is impossible to prepare a hydrogen atom in the ground state, expose it to light energy, and then observe it in the excited state. Never been done, as far as I know.
EDIT: This posting has been accumulating anonymous downvotes since it went up yesterday. I don't believe I've said anything controversial. Certainly, everyone "knows" that this it what happens when you expose hydrogen to light energy. I'm just questioning whether it's actually observable in an experimental sense.
I happen to believe that nothing of the sort happens. I think that when hydrogen is in the presence of an oscillating electromagnetic field of the right frequency, it is driven into a superposition of the s and p states. In this state, the charge distribution of the atom oscillates like a tiny antenna; and it interacts with the external field exactly as would a classical antenna with the same parameters. There is nothing in the mathematics of the interaction which requires it to end up in a pure p state, and nothing that requires the energy to be exchanged with the external field in specific quantities.
I know that my opinions would be considered to be verging on "crackpot", but that is not the point. I am simply asking people who think they know better than me: just what is the experimental evidence that their model of quantized energy exchange is superior to my model. I have already said that I do not believe that observation of the s-p transition of hydrogen provides any such evidence.