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I just wish to confirm whether my understanding is correct.

I know that photon absorption/emission brings about quantised changes in electron energy levels. Photons (infrared) also interact with chemical bonds to bring about quantised changes in rotational and vibrational kinetic energy in greenhouse gases for example. My understanding of internal energy is that it comprises vibrational, rotational, translational and potential energy. What constituent of internal energy does an electron excitation represent? Is it only lower frequency photons that interact with chemical bonds?

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What constituent of internal energy does an electron excitation represent?

You can think of electrons as just like planets orbiting the sun and get the correct answer to this question. An electron in a higher energy level has less kinetic energy, but more potential energy as it is (generally) farther from the nucleus. The net result is more energy.

Is it only lower frequency photons that interact with chemical bonds?

Vibrational transitions typically require an amount of energy that corresponds to a photon in the infrared region of the spectrum. Rotational levels are even closer, and are usually measured using combined rotational-vibrational transitions. This leads to the usual ladder of spectroscopy:

Vibration (with or without rotation): infrared
Electronic excitation (outer electrons, or small atoms) : visible / ultraviolet (UV)
Electronic excitation (inner electrons, large atoms): X-ray
Nuclear excitation: gamma-ray

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  • $\begingroup$ That's great thanks. The spectroscopy ladder really helps. $\endgroup$ – Seanosapien Sep 9 '14 at 21:53
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The change in electronic excitation represents both a potential and a kinetic energy term in classical physics, but there is no simple correspondence to classical physics terms, when you are looking at quantum systems. All we really care about is the total energy difference between electronic states. Those energy differences correspond to the energies at which photons can be absorbed and emitted. In the presence of magnetic fields there are also energy terms that stem from the spins and angular momenta of electrons and of the spins of the nucleons. The magnetic fields can be either external, or generated by the configuration of electrons inside the molecules and atoms, themselves. Finally, spins can interact with the dipole fields of other spins.

All of these spin interactions can add tiny variations to the frequencies of absorption and emission, and they give very important clues to the chemical composition of molecules, so much so, that magnetic resonance has become one of the chemist's workhorse methods for molecular structure determination.

In addition, the interaction between the photon spin and the molecules can lead to polarization effects. The polarization of photons scattering off these molecules can change or emitted light can be polarized.

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