Two-photon excitation (TPE) spectra of Cr 3+ :K 2 NaScF 6 , excited by a Ramanshifted, Nd:YAG-pumped tunable dye laser, exhibit several unexpected features. A weak TPE spectrum of the symmetry-forbidden 4 A 2g → 4 T 2g transition is observed without a zero-phonon line. The symmetry-allowed TPE spectrum of the 4 A 2g → 4 T 1ag transition has a multi-phonon side band with anomalously extended vibrational progressions, and an anomalously weak, split zerophonon 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 electronlattice 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 (TPE) spectra of Cr 3+ :K 2 NaScF 6 , excited by a Ramanshifted, Nd:YAG-pumped tunable dye laser, exhibit several unexpected features. A weak TPE spectrum of the symmetry-forbidden 4 A 2g → 4 T 2g transition is observed without a zero-phonon line. The symmetry-allowed TPE spectrum of the 4 A 2g → 4 T 1ag transition has a multi-phonon side band with anomalously extended vibrational progressions, and an anomalously weak, split zerophonon 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 electronlattice coupling in intermediate states and a low-temperature phase transition involving librational instability. Hypothetical line-shape simulations are compared with observed TPE spectra.

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