I know that light is emitted when an electron is excited to a higher energy state and then decays back to its ground state, emitting a photon whose energy corresponds to the difference in energy of the two energy states.

I also learned that if a H2 molecule absorbs a photon with an energy equivalent to the energy difference between the bonding and anti-bonding orbital, then the electron will be promoted to the anti-bonding orbital, causing the bond between the H atoms to break (since there will be no stabilizing interaction).

My question is how can a H2 molecule emit light if the only possible higher energy state that the electron can go to is the anti-bonding oribtal, and that will break the bond (which doesnt release photons)? What am I missing here?

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  • $\begingroup$ The light could be emitted when the dissociated atoms recombine, or perhaps tge electron returns to the ground state before the molecule can actually dissociate. $\endgroup$ Dec 11, 2023 at 2:37
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    $\begingroup$ When an electron is raised to a higher level, that absorbs energy. When it falls back to a lower energy state, it releases energy. $\endgroup$ Dec 11, 2023 at 5:20
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    $\begingroup$ Experiments show that there are several electronically excited states of increasing energy, each of these also has vibrational & rotational levels from which emission can occur. $\endgroup$
    – porphyrin
    Dec 11, 2023 at 8:28
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    $\begingroup$ As 1s is not the only orbital in H atoms. transition to the antibonding sigma orbital is not the only option. $\endgroup$
    – Poutnik
    Dec 11, 2023 at 9:59
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    $\begingroup$ Gerhard Herzberg (Nobel Prize in Chemistry, 1971) literally wrote the book on the spectra of diatomic molecules: en.wikipedia.org/wiki/Gerhard_Herzberg. So molecular spectroscopy is complex and the little first year chemistry MO diagram is not particularly useful in this regard. $\endgroup$
    – Ed V
    Dec 11, 2023 at 14:47


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