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