The temperature of a fixed volume of a gas is increased when it interacts with radiation. Why does the temperature increase (i.e. why does the velocity of a gas molecule increase) when a photon is absorbed?
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Photons carry energy, as given by the Planck-Einstein relation: $$E=h\nu$$ When a molecule absorbs a photon, an electron undergoes an electronic energy transition, "jumping" to a higher energy level. When this happens, the kinetic energy of the molecule can increase by $h\nu$. Since temperature is really just a measure of kinetic energy at the microscopic level, the average temperature of the gas increases whenever a molecule absorbs a photon.
Note that photons do not have a temperature.
HDE 226868
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4The energy goes in to the electronic states, which than can transfer in to rotational or translational levels through intra-molecular relaxation or collisional processes. – Jon Custer Oct 19 '15 at 22:39
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@JonCuster Thanks for pointing that out. I've edited to make that clear. – HDE 226868 Oct 19 '15 at 22:42
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It exist a technology of laser cooling. So a photon can cool down an atom too. – HolgerFiedler Oct 20 '15 at 06:07
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Do not forget momentum conservation too. The photon carries momentum that has to be conserved, so the atom/molecule has to move . Two things at least have to happen 1) absorption of the photon to satisfy energy conservation 2)additional momentum to satisfy momentum conservation, and thus increase in kinetic energy. – anna v Nov 08 '15 at 05:23
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@JonCuster How are intra-molecular movements further transferred to collisions between molecules? The mass of electrons seems too small to "draw" a molecule. – Ziyuan Jan 23 '22 at 14:27
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@ziyuang - the orbitals of the molecules are where the interaction takes place, so an electron being in a different orbital changes the ‘shape’. Consider why a HeNe laser needs helium to actually work. – Jon Custer Jan 23 '22 at 16:19
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According to foundational principles in Quantum Mechanics, a photon cannot directly heat a gas molecule.
A photon adds orbital energy to an electron, which rapidly re-releases an identical photon in a random direction.
A molecule's temperature is almost completely due to the kinetic energy of the atomic nuclei, as that is where almost all of the mass resides.
In the short time that the electron is in a higher orbit, the molecule may bump into another molecule and increase the next molecule's kinetic energy. This is called thermalization.
Thermalization readily occurs in solids and liquids, but is extremely unlikely in gasses except at very high pressure.
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A molecule in the gas may absorb the photon with energy $E=\hbar \omega$. Now, the electrons typically want to fall down to lower energy states when they are excited. When they do this they either release a photon with the same energy as before, OR the electron may give its energy to rotational and/or vibrational stages. This process is referred to as non-radiative recombinaiton, and this is how the tmeperature of the gas is increased.
DakkVader
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