Probing the R Lines in Tris(acetylacetonato) Chromium(III) and Tris(3-bromo-acetylacetonato) Chromium(III) by Luminescence and Excitation Line Narrowing Spectroscopy

Site-selective and narrowed luminescence and excitation spectra in the region of the E-2 <- (4)A(2) transitions are reported for single crystals of Al(acac)(3)/Cr(III) and Al(3-Br-acac)(3)/Cr(III) (where acac is acetylacetonate). The R-2 line is pronounced in the brominated system and displays a comparable oscillator strength as the R-1 line. The E-2 splitting is found to be 138 cm(-1), and the (4)A(2) ground-state splitting is 1.39 cm(-1). However, in the case of the Al(acac)3/Cr(III) system the R-2 line is not a distinct feature. We propose that vibronic coupling via a second-order Jahn-Teller effect leads to a redistribution of R-2 intensity over several vibrational sidelines. An upper limit for the R-1 line width Gamma(h) = 15 MHz is deduced for the Al(acac)(3)/Cr(III) 1% system at 1.5 K. This line width is limited by direct and indirect electron-spin-electron-spin interactions. Accurate zero-field splittings (1.20, 1.19, 1.17 cm(-1)) for the three sites in Al(acac)(3)/Cr(III) are determined and compared with previously published electron paramagnetic resonance (EPR) data.