Two-photon excitation spectroscopy of Cr3+: K2NaScF6 elpasolite: I. Experimental aspects

Two-photon excitation experiments were performed to improve understanding o f electron-lattice coupling and its effects on intra-3d(3) transitions. Cr3 + occupies a scandium octahedral site in K2NaScF6. The transitions studied were (4)A(2g) --> T-4(2g) and 4A(2g) --> T-4(1ag), Complete spectra were re corded at a temperature of 10 K with the polarization vector <(<eta>)over c ap> parallel to the (100) or (110) crystallographic direction. The two band s exhibit different polarization anisotropies and phonon couplings. The ele ctric-dipole-forbidden (4)A(2g) --> T-4(2g) band appears to be built on an e(g)-mode false origin and contains Fano antiresonances. This broad transit ion band lacks a zero-phonon line or any other sharp structure. The (4)A(2g ) --> T-4(1ag) transition zero-phonon line is evident and shows a 163 cm(-1 ) low-temperature phase-transition-induced splitting. It also contains an e xtended 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 anisot ropy that differs from that of the zero-phonon line. To facilitate analysis of the data, measurements of low-temperature T-4(2g) --> (4)A(2g) emission spectra with one-photon excitation are also reported and interpreted in th e present paper.