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Two-photon excitation (TPE) spectra of Cr3+:K2NaScF6, excited by a Raman- shifted, Nd:YAG-pumped tunable dye laser, exhibit several unexpected features. A weak TPE spectrum of the symmetry-forbidden 4A2g → 4T2g transition is observed without a zero-phonon line. The symmetry-allowed TPE spectrum of the 4A2g → 4T1ag transition has a multi-phonon side band with anomalously extended vibrational progressions, and an anomalously weak, split zero- phonon line with anomalous polarization anisotropy. These observations are explained, respectively, in terms of theoretical models involving phonon assistance, departures from the closure approximation that permit electron- lattice coupling in intermediate states and a low-temperature phase transition involving librational instability. Hypothetical line-shape simulations are compared with observed TPE spectra.

Two-photon excitation spectroscopy of Cr3+:K2NaScF6elpasolite: II. Theoretical models

Two-photon excitation spectroscopy of Cr³⁺:K₂NaScF₆elpasolite: II. Theoretical models