Diffusion and Reactions of Hydrogen in ${\mathrm{F}}_2$-Laser-Irradiated ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ Glass

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 ${\mathrm{H}}_2$.

Figure 1

Thermal NBOHC decay, $C(T)$, of ${\mathrm{H}}_2$-free and ${\mathrm{H}}_2$-impregnated samples. The latter contained $\sim 1\times {10}^{18}{\mathrm{c}\mathrm{m}}^{-3}$ of ${\mathrm{H}}_2$, which was determined by the relative intensity of the ${\mathrm{H}}_2$ Raman band ($4135{\mathrm{c}\mathrm{m}}^{-1}$) shown in the inset to the $800{\mathrm{c}\mathrm{m}}^{-1}$ band of ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glass [16]. Samples were irradiated with a single ${\mathrm{F}}_2$ laser pulse at 10 K, followed by warming at a rate of $3\mathrm{K}{\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$. Open symbols are simulated values calculated by solving the rate equations of Eqs. (4, 5, 6). This procedure provided a distribution function of the activation energy, $w(E_{\mathrm{a}})$, as shown in the inset of Fig. 3.

Figure 2

Isothermal NBOHC decay, $C_T(t)$, of ${\mathrm{H}}_2$-free samples after irradiating with a single ${\mathrm{F}}_2$ laser pulse at constant temperatures of 60, 70, 80, and 100 K for region I; 110, 120, 140, and 160 K for region II; and 200, 230, 240, 260, and 300 K for region III.

Figure 3

Arrhenius plot of NBOHC decay rates, $1/{\tau }_T$, in regions I, II, and III derived from the isothermal NBOHC decay shown in Fig. 2. The inset shows the distribution of $E_{\mathrm{a}}$, $w(E_{\mathrm{a}})$, obtained by simulating the experimental thermal NBOHC decay shown in Fig. 1.

Figure 4

Effect of the number of ${\mathrm{F}}_2$ laser pulses on the thermal NBOHC decay, $C(T)$, of the ${\mathrm{H}}_2$-free samples. The inset shows the isochronal decay of ${\mathrm{H}}^0$ measured by EPR for the sample irradiated at 77 K. Annealing at each temperature for 600 s was performed followed by an EPR measurement at 77 K. The normalized NBOHC concentrations after 600 s of isothermal annealing [$C_T(t=600\mathrm{s})$, taken from Fig. 2] are shown for comparison.