UV/Vis-Absorbtion and bond dissotiation

Sep 2018

I am currently studying for my analytics class at university and something provoked me. I learned, that in UV/Vis-Absorption electrons in the molecule get excited, so as to switch from a HOMO to a LUMO state.

I was wondering, if therefore the homolytic bond cleavage of halogens by light (but really any radiation induced bond cleavage) could be explained by that. When looking at a bromine molecule for example, it seemed logical to assume an excitation from the sigma to the sigma* molecular orbital, thus desintegrating the molecule (as there is no other bond between the atoms). Bylooking at some UV/Vis-Spectra of molecular bromine

e.g. https://pubs.acs.org/doi/abs/10.1021/j100863a025

I found the absorbing wavelengths to be about 430 nm for Br2, which resembles an photon energy of about 2.9 eV.

Wikipedia (i know...) lists the bond dissotiation energy of bromine to be 1.99 eV.
Why is that? I know there are other ways of destroying the bond for example by heat, but shouldn't the energies be the same?

Thanks in advance,
Sep 2018
I have to admit to guessing

I don't know the answer to your question,
however I suspect that while there probably is an association between the absorbed photon energies and the bond disassociation energies,
it is not as direct as you are assuming.
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