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 non photochemical 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.

Energy migration within the 2E state of Cr3+

“…A least squares fit with a sum of two Gaussians to both R lines give a value of 1.29(7) cm −1 for the ground state ZFS and a inhomogeneous line width of 1.1(1) cm −1 . This value of the ZFS is in line with values determined by EPR spectroscopy [28] and high-resolution optical methods [29], which also identified the Ms = ±3/2 component of the 4 A 2 multiplet as the ground state. Fig.…”

“…This is due to phonon-assisted relaxation processes such as direct absorption or emission of phonons, and Raman and Orbach processes [6] basically within the two components of 2 E state. As shown by Riesen, for [Cr(ox) 3 ] 3− [29] and similar chromium(III) complexes [34], both the direct process as well as Raman processes contribute to the homogeneous line width above 4.2 K. In contrast, for ruby Raman processes dominate at higher temperatures [35,22]. The key difference between the otherwise very similar chromphores lies in the density of phonon states at the energies of the 2 E ZFS, which is much higher for the softer oxalate systems with a Debye temperature of around 50 K [36] as compared to 950 K for ruby [37].…”

The Native Reaction Centre of Photosystem II: A New Paradigm for P680

“…The current and temperature of the 690-nm laser diode were maintained and controlled by Thorlabs LDC500 and TEC2000 current and temperature controllers, respectively. [27,28] To burn holes, the laser was kept at constant current (and thus at constant wavelength). The beam was attenuated by an initial neutral-density (ND) filter, before being defocussed at the sample to a beam size ∼0.6 cm 2 .…”

Massive enhancement of persistent spectral hole-burning in the R-lines of NaMgAl(oxalate)3·9H2O:Cr(III) by partial deuteration

“…We have recently reported the temperature dependence and magnetic field effects in persistent non-photochemical spectral hole-burning of the Rlines in NaMgAl(oxalate) 3 Á 9H 2 O:Cr(III) [9][10][11]. We have shown that the temperature dependence of the homogeneous line width is governed by a direct process between the two 2 E levels and twophonon Raman scattering by two pseudo-local vibrations [9]. Also, we have demonstrated that spectral holes in this system can be used as a permanent record of magnetic fields [11].…”

“…A more recent paper reported a refinement to P 3 3c1 with 10 water molecules of crystallization Chemical Physics Letters 372 (2003) 563-568 www.elsevier.com/locate/cplett [13]. However, elementary analysis performed by several groups over the years has been in best agreement with nine water molecules [9]. A detailed knowledge of the hydrogen positions is important in order to understand the extraordinary hole-burning properties of this material.…”