There’s an excellent question on Stack Exchange regarding the radiolysis of water here but I have one simple outstanding question that perhaps someone might now; in radiobiology, a common DNA damaging free radial is caused by the radiolysis of water. This reaction is something like
$$\ce{H2O + $h\nu$ -> H2O+ + e- }$$
$$\ce{H2O+ + H2O -> H3O+ + OH. }$$
which produces a hydroxyl radical which can prove fatal to cells. I follow the logic here, but given the massive energy of the ionizing radiation, how come a cleaving reaction like the following isn’t common?
$$\ce{H2O + $h\nu$ -> H. + OH. }$$
I’ve seen the energy quoted to break water this way at around $\pu{493.4 kJ/mol}$, which is way below the energy ($h\nu$) of an x-ray photon, even after compton scattering. Is there a reason why this reaction isn’t valid or common?