Upconversion in Doped Transition Metal Ion Systems

Cs2ZrCl6. A series of sharp transitions are observed that reflect the undistorted octahedral nature of these excited states. Excitation into these features produces the upconversion luminescence spectrum shown in Fig. 19 a. The 10 K excitation scan of this luminescence is compared to the absorption spectrum in Fig. 19b. The upconversion excitation scan closely follows the absorption profile over the full energy range. This observation leads to the conclusion that at 10 K the dominant mechanism for upconversion in 2.5% Re + Cs2ZrCl6 under these conditions is GSA/ETU. Time-dependent measurements confirm this conclusion (Fig. 19 a, inset), showing the characteristic delayed maximum and a 10 K decay constant (/Cdec = 1400 s ) approximately two times that of the excited 

Despite their similar electronic structures, upconversion processes in Mo + and Re + thus show dramatically different properties that can be related to differences in oscillator strengths due to spin selection rules. The observation of an energetically-resonant but spin-blocked upconversion pathway in Mo + differs from what is normally observed in rare earth upconversion materials. In the latter, spin-orbit coupling is usually so strong that spin selection rules are no longer relevant. 

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