Two-photon excitation spectroscopy of Cr:K2NaScF6 elpasolite: I. Experimental aspects

Two-photon excitation experiments were performed to improve understanding of electron–lattice coupling and its effects on intra-3d3 transitions. Cr3+ occupies a scandium octahedral site in K2NaScF6. The transitions studied were A2g → T2g and A2g → T1ag . Complete spectra were recorded at a temperature of 10 K with the polarization vector η̂ parallel to the 〈100〉 or 〈110〉 crystallographic direction. The two bands exhibit different polarization anisotropies and phonon couplings. The electric-dipole-forbidden A2g → T2g band appears to be built on an eg-mode false origin and contains Fano antiresonances. This broad transition band lacks a zero-phonon line or any other sharp structure. The A2g → T1ag transition zero-phonon line is evident and shows a 163 cm−1 low-temperature phase-transition-induced splitting. It also contains an extended progression of 35 phonon peaks corresponding to a lattice mode with phonon energy 106 cm−1, and a second progression with phonon energy 310 cm−1. The very asymmetric phonon side band displays a polarization anisotropy that differs from that of the zero-phonon line. To facilitate analysis of the data, measurements of low-temperature T2g → A2g emission spectra with one-photon excitation are also reported and interpreted in the present paper.