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I have came up with a weird doubt: photon absorption by atoms causes heat? I mean, I was always told that if the photon's frequency is the magic one, the atoms absorbs the photon and goes to an excited state. So I have to suppose that heating (increasing in kinetic energy) happens when the frequency is outside the set of permitted transitions. Is it correct?

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This is true. If for example you subject Hydrogen gas to a perfectly monochromatic 121.57 nm laser, then all that will happen is that the gas will scatter the light in all directions, glowing without increasing the temperature.

Otherwise there are many different phenomena that are involved in the heat transfer of energy by radiation. For example in solids, photons are absorbed and turn into phonons which are waves that when they are numerous lead to thermalization, and for molecules you have photon absorption that leads to molecular vibration which again increases the root mean square speed, etc...

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  • $\begingroup$ "If for example you subject Hydrogen gas to a perfectly monochromatic 121.57 nm laser" Well, even this will cause heat, as due to the conservation of momentum and natural line width the line will be shifted off the resonance by the repeated absorption/emission processes, depositing energy in the gas during this process. $\endgroup$ May 23 '15 at 0:27
  • $\begingroup$ true, but for a dilute enough gas this can be neglected. $\endgroup$
    – Ali Moh
    May 23 '15 at 0:29
  • $\begingroup$ Sure, but still, the answer to question will be no in general, for several reasons. (Unless it is directed at a single isolated atom, for which speaking of a temperature is pointless). (Although there are obviously exceptions, for example laser cooling). $\endgroup$ May 23 '15 at 0:30

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