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 (2)E <-- (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 (2)E 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-spinelectron-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.