Diffusion and Reactions of Hydrogen inF2-Laser-IrradiatedSiO2Glass

The diffusion and reactions of hydrogenous species generated by single-pulsed ${\mathrm{F}}_2$ laser photolysis of SiO-H bond in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glass were studied in situ between 10 and 330 K. Experimental evidence indicates that atomic hydrogen (${\mathrm{H}}^0$) becomes mobile even at temperatures as low as $\sim 30\mathrm{K}$. A sizable number of ${\mathrm{H}}^0$ dimerize by a diffusion-limited reaction into molecular hydrogen (${\mathrm{H}}_2$) that may migrate above $\sim 200\mathrm{K}$. Activation energies for the diffusion, inherently scattered due to the structural disorder in glass, are separated into three bands centered at $\sim 0.1\mathrm{e}\mathrm{V}$ for free ${\mathrm{H}}^0$, $\sim 0.2\mathrm{e}\mathrm{V}$ presumably for shallow-trapped ${\mathrm{H}}^0$, and $\sim 0.4\mathrm{e}\mathrm{V}$ for ${\mathrm{H}}_2$.