Transient and Persistent Spectral Hole-Burning in the R-lines of Chromium(III) in NaMgAl(Oxalate)3·9H2O

Abstract

The temperature dependence of transient and persistent spectral holes is reported for the R-lines (2E ← 4A2 transitions) in 4% chromium(III) doped NaMgAl(oxalate)3·9H2O. The temperature dependence is well described by taking into account two pseudolocal vibrations in a weak electron−phonon coupling model and the direct process between the two 2E levels. The persistent spectral hole-burning is based on a nonphotochemical mechanism involving mainly a rearrangement of a water molecule of crystallization. The 1.12 GHz width of the transient hole in the R2 line provides a value for the relaxation rate within the 2E multiplet, k(R2 → R1) = 3.5 × 109 s-1 at 2.5 K. The observed optical line width of Γ = 46 MHz for the R1-line at 2.5 K is most likely dominated by chromium(III)−chromium(III) spin−spin interactions. Spectral diffusion on the ms time scale causes persistent spectral holes to be significantly broader and holewidths of 260 MHz and 1.63 GHz for the R1 and R2-line, respectively, are observed. Spontaneous hole-filling is thermally activated and spectral diffusion on the minute time scale is slow in the investigated temperature range of 2.5−30 K.