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If I have an hydrogen atom at $T=0K$ from Boltzmann distribution I can have only the G.S populated, so if I send to this atom photons at all energies is impossible to excite the atom, is this right? If this is right, the energy spectrum of this atom would be flat?

Qmechanic
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Salmon
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    What is "G.S"? Ground state? Why should it make it impossible to excite the atom? And what do you mean by "flat energy spectrum"? – Ruslan Jul 04 '20 at 18:11
  • You can't have one hydrogen atom at 0 K. Temperature is a statistical phenomenon, requiring many atoms before it makes any sense. – David White Jul 04 '20 at 18:31
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    Normally we don't describe a single atom as having a temperature. Can a single molecule have a temperature ? might be a relevant question here. – StephenG - Help Ukraine Jul 04 '20 at 18:32
  • Ok I've understand, so I will try another different question. If I have a system with discrete energy levels at $T=0$, and this system could be excited by EM waves, even if the only occupied state of this system is the ground state becaise of Boltzmann distribution, a photon of the right energy could excite this system so that it could have an absorption spectrum for example? – Salmon Jul 05 '20 at 01:44
  • The problem is that Boltzmann says the probability of population of a state different from the ground state is 0, so if, with an energetic photon, I get an excited state, would this be in contrast with Boltzmann distribution? – Salmon Jul 05 '20 at 01:46

1 Answers1

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If I have a single hydrogen atom with an electron in the ground state, can light of the correct energy excite this electron? The answer to this question is yes.

To bring in your discussion of equilibrium statistical mechanics phenomena, it is best to consider an ensemble of hydrogen atoms at zero temperature. Then your question may be recast as: can light excite electrons in the hydrogen atoms of this ensemble, which will lead to an energy distribution that is different to that predicted by the equilibrium statistical distribution? The answer again is yes. Your are driving the system out-of-equilibrium by using light to excite electrons, so your equilibrium distribution no longer applies.

ProfM
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