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.