Reactions in water radiolysis - producing hydroxyl radicals

Question

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?

Answer 1

H (atomic hydrogen radical) is in fact formed. It is 9% of the radial species formed by radiolysis.

See Reactivity of the Hydroxyl Radical in Aqueous Solution and references cited therein for further information.

Answer 2

For ionizing radiation, various mechanisms for interaction with matter are possible. Depending on the energy, the most important types for gamma radiation are photoelectric effect, Compton scattering, and pair production. Charged particles, such as alpha and beta particles, mainly interact through coulomb forces between their charge and the negative charge of the orbital electrons of the absorber atoms.

All these processes lead to partial or complete transfer of the radiation energy to electron energy; thus, they lead to electron excitation or ionization:

$$\begin{align} \ce{H2O &->[\gamma][\quad] H2O^{$*$} }\\ \ce{H2O &->[\gamma][\quad] H2O+ + e- } \end{align}$$

The interaction is very fast. The excitation or ionization event occurs within a time scale of $< 10^{-16}\ \mathrm{s}$. This process is too fast for immediate chemical reactions. By way of comparison, the frequencies of molecular vibrations range from less than $10^{12}\ \mathrm{s^{-1}}$ to approximately $10^{14}\ \mathrm{s^{-1}}$.

Accordingly, the first chemical reactions follow slightly later, within a time scale of $10^{-14}\ \mathrm{s}$ to $10^{-13}\ \mathrm{s}$:

$$\begin{align} \ce{H2O^+ + H2O &-> *OH + H3O+ }\\ \ce{H2O^{$*$} &-> H. +\, .OH }\\ \ce{H2O^{$*$} &-> H2 + O. } \end{align}$$

The radiolysis products proceed to diffuse, and further chemical reactions occur.