Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF₂ and SrF₂ crystals

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF₂ and Yb/Sr pairs in SrF₂ crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4f^N−15d excited states of Y b²⁺: these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b²⁺ + Ca²⁺ (Sr²⁺) → Y b³⁺ + Ca⁺ (Sr⁺) electron phototransfer. This mechanism applies to all the observed Y b²⁺ 4f–5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF₂:Y b²⁺ because the Y b³⁺–Ca⁺ states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF₂:Y b²⁺ at the wavelengths of the first 4f–5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b²⁺ active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF₂ host, associated with the lowest 4f–5d band.