Two-photon excitation spectroscopy of Cr³⁺:K₂NaScF₆elpasolite: I. Experimental aspects

Abstract

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. 1.