Oxygen Exchange at the Internal Surface of Amorphous ${\mathrm{SiO}}_2$ Studied by Photoluminescence of Isotopically Labeled Oxygen Molecules

The exchange between lattice and interstitial oxygen species in an oxide was studied by the ${}^{16}\mathrm{O}\mathrm{\text{−}}{}^{18}\mathrm{O}$ isotope shift of the $a^1{\Delta }_g(v=0)\to X^3{\Sigma }_g^{-}(v=1)$ infrared photoluminescence band of the oxygen molecules (${\mathrm{O}}_2$) incorporated into the interstitial voids of amorphous ${\mathrm{SiO}}_2$ ($a\mathrm{\text{−}}{\mathrm{SiO}}_2$) by thermal annealing in ${}^{18}\mathrm{O}_2$ gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of $a\mathrm{\text{−}}{\mathrm{SiO}}_2$, is found. The average exchange rate has an activation energy of $\sim 2\mathrm{eV}$, which is much larger than that for the diffusion of interstitial ${\mathrm{O}}_2$ ($\sim 0.8–1.2\mathrm{eV}$). The average exchange-free diffusion length of interstitial ${\mathrm{O}}_2$ exceeds $\sim 1\mu \mathrm{m}$ below $900°\mathrm{C}$, providing definite evidence that oxygen diffuses as interstitial molecules in $a\mathrm{\text{−}}{\mathrm{SiO}}_2$.

Figure 1

PL spectra of interstitial ${\mathrm{O}}_2$ in samples thermally annealed in ${}^{16}\mathrm{O}_2$ for 72 h at $700°\mathrm{C}$ or ${}^{18}\mathrm{O}_2$ for 12 h at $500°\mathrm{C}$. Schematic diagram of interstitial ${\mathrm{O}}_2$ energy levels is also shown. The spectra were normalized to the amplitude of the pure electronic band (PEB) at $\sim 7855{\mathrm{cm}}^{-1}$. The vibrational sideband (VSB) appears at $\sim 6300–6400{\mathrm{cm}}^{-1}$. Small peaks located between 6500 and $7500{\mathrm{cm}}^{-1}$ are due to interactions with phonons of the $a\mathrm{\text{−}}{\mathrm{SiO}}_2$ network.

Figure 2

Experimental and simulated VSB spectra normalized to the PEB amplitude for samples thermally annealed at $700°\mathrm{C}$ in ${}^{16}\mathrm{O}_2$ or ${}^{18}\mathrm{O}_2$.

Figure 3

Variation of the fractions of (a) interstitial ${}^{16}\mathrm{O}_2$ and (b) ${}^{18}\mathrm{O}_2$ ($f_{66}$ and $f_{88}$, respectively) with time and temperature of the thermal annealing in ${}^{18}\mathrm{O}_2$ gas. The filled symbols were derived by peak decomposition of VSB under the restriction condition $f_{66}+f_{68}+f_{88}=1$. The solid and dashed lines were drawn by directly simulating the VSB spectra from the solutions of the rate equations Eq. (3, 4, 5, 6). The simulation was performed both with and without considering the distribution in the exchange rate constant $k$. The error bars represent the experimental uncertainties.

Figure 4

Temperature dependence of the average exchange rate constant $k_a$ and the average exchange-free diffusion length $l$ of an interstitial ${\mathrm{O}}_2$ molecule [Eq. (7)]. The error bars represent the experimental uncertainties.