ALBERT

Description: In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb 2+ –Yb 3+ mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb 2+ -doped CaF 2 and SrF 2 , usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb 2+ –Yb 3+ mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4 f 14 –1 A 1g → 4 f 13 ( 2 F 7/2 )5 de g –1 T 1u absorption in the Yb 2+ part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb 3+ moiety is in the higher 4 f 13 ( 2 F 5/2 ) multiplet. The Yb 2+ –Yb 3+ → Yb 3+ –Yb 2+ IVCT emission consists of an Yb 2+ 5 de g → Yb 3+ 4 f 7/2 charge transfer accompanied by a 4 f 7/2 → 4 f 5/2 deexcitation within the Yb 2+ 4 f 13 subshell: [ 2 F 5/2 5 de g , 2 F 7/2 ] → [ 2 F 7/2 ,4 f 14 ]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations explain the different luminescent properties of Yb-doped CaF 2 , SrF 2 , BaF 2 , and SrCl 2 : the presence of IVCT luminescence in Yb-doped CaF 2 and SrF 2 ; its coexistence with regular 5 d -4 f emission in SrF 2 ; its absence in BaF 2 and SrCl 2 ; the quenching of all emissions in BaF 2 ; and the presence of additional 5 d –4 f emissions in SrCl 2 which are absent in SrF 2 . They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb 2+ -doped CaF 2 and SrF 2 , the appearance of Yb 2+ 4 f –5 d absorption bands in the excitation spectra of the IR Yb 3+ emission in partly reduced CaF 2 :Yb 3+ samples, and to identify the broadband observed in the excitation spectrum of the so far called anomalous emission of SrF 2 :Yb 2+ as an IVCT absorption, which corresponds to an Yb 2+ 4 f 5/2 → Yb 3+ 4 f 7/2 electron transfer.

Description: Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF 2 and Yb/Sr pairs in SrF 2 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 4 f N −1 5 d excited states of Y b 2+ : these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b 2+ + Ca 2+ (Sr 2+ ) → Y b 3+ + Ca + (Sr + ) electron phototransfer. This mechanism applies to all the observed Y b 2+ 4 f –5 d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF 2 :Y b 2+ because the Y b 3+ –Ca + states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF 2 :Y b 2+ at the wavelengths of the first 4 f –5 d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b 2+ 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 SrF 2 host, associated with the lowest 4 f –5 d band.