Photostimulated electron transfer between coactivator ions in alkali silicate glasses

Abstract

Silicate glasses of the composition (mol%) 22 R 2 O·3 CaO·75 SiO 2 (where R=Li, Na, K) have been studied. They were doped either with Ce 3+, or Tb 3+ ions, or with pairs (Ce 3+ + Fe 3+, Tb 3+ + Fe 3+, Tb 3+ + Eu 3+). The samples were melted in reducing (adding starch into the batch) or oxidizing (blowing oxygen through the glass liquid) conditions. Rare earth oxide concentration did not exceed 0.5 mol%, and that of Fe 2O 3 was varied from 0.004 to 0.025 mol% (the iron content in glasses without doping was at the level of 10 −4 wt%). The activators act as extrinsic (impurity) electron donors (D n +) and acceptors (A m +) in processes of their photooxidation (photoionization: Ce 3+, Tb 3+) and photoreduction (Fe 3+, Eu 3+) to form extrinsic (impurity) color centers (CCs), (D n +) + and (A m +) −. To create them, glasses were excited optically in the limits of UV absorption bands of a D n + (3.8 to 5.9 eV for Tb 3+, and 3.3 to 3.9 eV for Ce 3+). Spectral dependencies of the efficiency, η( E), of intrinsic electron CCs formation due to D n + photoionization were measured. While varying the energy, E e, of excitation quanta, two regions of η( E) were resolved for each donor. In each region, η( E e) was approximately invariant. Such spectral dependence is explained by the fact that the largest energy donor states are in the glasses' forbidden bands. Hence, an overbarrier mechanism and a tunneling mechanism are each activated with changing E e. There is a transition region for η between the two regions in which η decreases with decreasing E e and is caused by the inhomogeneous broadening of the donor energy levels. Decay curves of the donor phosphorescence, and photoinduced absorption spectra in glasses with D n + and with (D n + + A m +) at various mechanisms of the D n + ionization were studied and analyzed.