The average c=c double bond has somewhere around 614 kJ/mol of energy, if we convert it to eV, then the bond strength is roughly 6.4 eV. Additionally, Ethene absorbs light at 171 nm. However, if we calculate the energy of 171 nm it is around 7.3 eV, hence ethene absorbs energy that is higher than its bonding energy, which should lead to a complete disintegration of the double bond. So why does it not happen? I'm fully aware that this does not happen, otherwise most bonds would break at around 300-400nm light excitation, however, I have not seen the reason why discussed.

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    $\begingroup$ Who said all that energy must go into this one bond? $\endgroup$ – Ivan Neretin Aug 13 '19 at 9:34
  • $\begingroup$ Well, no one, however, the opposite is also true, I have not seen a discussion about such dissipation. It should also translate in Stokes shift change, however, in publications, I have never seen someone state or imply that the stokes shift or a part of the Stokes shift is caused by such mechanisms. $\endgroup$ – Ethene Aug 13 '19 at 10:55
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    $\begingroup$ On uv excitation an excited state will be produced so that reaction will not occur from the ground state. The dissociation of the excited state could should be less than that of the ground state but if, for example as a guess, it is found at 6 eV above the ground state it could be stable with a dissociation energy of say 2 eV and decay by fluorescence or isomerisation (by curve crossing/conical intersection) or other non-radiative pathways. $\endgroup$ – porphyrin Aug 13 '19 at 20:29
  • $\begingroup$ Yes, such mechanistic pathway is probably expected and is somewhat supported by the Vavilov's rule, however the ''probably'' is my main issue. Does anyone perhaps have any sources on mechanistic studies etc. on this? $\endgroup$ – Ethene Aug 14 '19 at 11:01

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