Direct singlet-to-triplet optical absorption and luminescence excitation band of the twofold-coordinated silicon center in oxygen-deficient glassy SiO 2

A direct singlet-to-triplet optical absorption transition (S0→T1) in the twofold-coordinated silicon center (‘B2(Si) center’) has been detected by time-resolved photoluminescence techniques. The peak of the excitation band is at 3.15 eV, the peak of emission is at 2.75 eV, decay time is 10.5 ms at 293 K. The oscillator strength of this transition is ∼ 10−6. The newly detected band is similar to the low energy absorption/excitation bands in the Ge- and Sn-based isoelectronic analogues of the twofold-coordinated silicon center in doped silicas. The differences in the emission spectra obtained under this excitation and under excitation in the singlet-to-singlet ‘B2’-band at 5.03 eV are due to correlated effects of glassy disorder both on transition energy and on intercombinational singlet-to-triplet conversion. The spin conversion goes to the second excited triplet state (T2) and is thermally activated with an activation energy of 0.13 eV. Competing structural models for the B2(Si) center are discussed.