The temperature dependence of transient and persistent spectral holes is reported for the R-lines ( 2 E ← 4 A 2 transitions) in 4% chromium(III) doped NaMgAl(oxalate) 3 ·9H 2 O. 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 2 E 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 R 2 line provides a value for the relaxation rate within the 2 E multiplet, k (R 2 → R 1 ) = 3.5 × 10 9 s -1 at 2.5 K. The observed optical line width of Γ = 46 MHz for the R 1 -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 R 1 and R 2 -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.
The temperature dependence of transient and persistent spectral holes is reported for the R-lines ( 2 E ← 4 A 2 transitions) in 4% chromium(III) doped NaMgAl(oxalate) 3 ·9H 2 O. 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 2 E 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 R 2 line provides a value for the relaxation rate within the 2 E multiplet, k (R 2 → R 1 ) = 3.5 × 10 9 s -1 at 2.5 K. The observed optical line width of Γ = 46 MHz for the R 1 -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 R 1 and R 2 -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.
Persistent spectral hole-burning in the R-lines of tetravalent manganese in X-irradiated submicron corundum powder is reported for the first time. Two samples were investigated, namely, nominally 0.4 atom % Mn 3+ -doped and 0.4 atom % Mn 4+, Mg 2+ -codoped α-Al 2 O 3 . The samples were produced by a simple combustion reaction method at 600 °C. Spectral hole-burning properties of Mn 4+ ions burned via the R 1 and R 2 lines were measured either by the photoluminescence excitation mode via monitoring the luminescence signal of a vibrational sideband at ∼694 nm or in luminescence, with blue light excitation after the burn period. The hole widths were studied as a function of burn fluence and temperature in the range from 2 to 90 K. We propose a photochemical hole-burning mechanism for both samples based on photoionization and photoreduction of the tetravalent manganese ion. Interestingly, the 0.4 atom % Mn 4+, Mg 2+ sample displayed a higher hole-burning efficiency than the Mn 3+ sample. The hole widths obtained for the persistent spectral holes in the present powder samples were approximately 300 times broader at 2.2 ± 0.1 GHz compared to the ∼6 MHz width observed in transient hole-burning in a macroscopic crystal [ Riesen, H. Chem. Phys. Lett. 2011, 515 (44), 241−244]. We assign this effect to the disruption of the crystal lattice upon the valence state change of the Mn 4+ ion in the hole-burning process. From the hole width, we obtained a decay time of 0.72 ns for the 2 A̅ → E̅ one-phonon relaxation process within the split 2 E excited state. The stability of the spectral holes burned via the R 1 and R 2 lines was investigated for both samples, and we observed a decay of about 20% within 2 h, indicating slow spontaneous hole-filling. A hole area recovery of ∼70 and ∼64% was observed after a temperature cycle from 2.5 to 70 and 90 K for the 0.4 atom % Mn 3+ and 0.4 atom % Mn 4+, Mg 2+ samples, respectively.
Wavelength-selective excited-state lifetime measurements and absorption, luminescence, and hole-burning spectra of a natural African emerald crystal are reported. The (2)E excited-state lifetime displays an extreme wavelength dependence, varying from 190 to 37 μs within 1.8 nm of the R(1)-line. Overall, the excited state is strongly quenched, in comparison to laboratory-created emerald (τ=1.3 ms), with an average quenching rate of ∼6 × 10(3) s(-1) at 2.5 K. This quenching is attributed to photoinduced electron transfer caused by a relatively high concentration of Fe(2+) ions. The forward electron-transfer rate, k(f), from the nearest possible Fe(2+) sites at around 5 Å is estimated to be ∼20 × 10(3) s(-1) at 2.5 K. The photoreductive quenching of the excited Cr(3+) ions by Fe(2+) is followed by rapid electron back-transfer in the ground state upon deactivation. The exchange interaction based quenching can be modeled by assuming a random quencher distribution within the possible Fe(2+) sites with the forward electron-transfer rate, k(f), given as a function of acceptor-donor separation R by exp[(R(f)-R)/a(f)]; R(f) and a(f) values of 13.5 and 2.7 Å are obtained at 2.5 K. The electron transfer/back-transfer reorganizes the local crystal lattice, occasionally leading to a minor variation of the short-range structure around the Cr(3+) ions. This provides a mechanism for spectral hole-burning for which a moderately high quantum efficiency of about ∼0.005% is observed. Spectral holes are subject to spontaneous hole-filling and spectral diffusion, and both effects can be quantified within the standard two-level systems for non-photochemical hole-burning. Importantly, the absorbance increases on both sides of broad spectral holes, and isosbestic points are observed, in accord with the expected distribution of the "photoproduct" in a non-photochemical hole-burning process.
Persistent and time-resolved transient holes were burnt into the 1D2${\ensuremath{\leftarrow}}^{3}$H4 transition of Pr3+ and the 5D0${\ensuremath{\leftarrow}}^{7}$F0 transition of Eu3+ in silicate glasses. The holewidths were studied as a function of burning-fluence density, wavelength, and temperature between 0.4 and 12 K. Transient holes arising from optical pumping of ground-state nuclear hyperfine levels were found for both ions at T3 K. Their lifetime decreases with increasing temperature. They are accompanied by symmetrically placed antiholes. For Eu3+, two pairs of antiholes were found which are attributed to the two europium isotopes, 151Eu and 153Eu. For Pr3+, an additional mechanism leading to persistent hole burning dominates above 3 K. The holewidth follows a ${\mathit{T}}^{(1.3\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for Pr3+ and a ${\mathit{T}}^{(1.0\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for Eu3+. The temperature dependence is the same as that of the contribution of the two-level systems to the specific heat measured on the same samples and in the same temperature range. The quadrupole splittings of the 7F0 ground state and the crystal-field splittings of the 7F1 and 7F2 levels of Eu3+ as a function of excitation wavelength indicate that the strength of the noncubic component of the crystal field varies monotonically for excitation across the 5D0${\ensuremath{\leftarrow}}^{7}$F0 absorption band, falling to zero at the long-wavelength edge. The hole width shows a similar variation with excitation frequency. No influence of the experimental time scale on the hole width was detected for either sample between ${10}^{\mathrm{\ensuremath{-}}2}$ and 102 s.
Time-resolved transient spectral hole-burning experiments in zero field and in low magnetic fields B( parallelc) are reported for the chromium(III) R(1)-line, 2A((2)E)<-- 2A((4)A(2)) of Chatham lab created emerald, Be(3)Al(2)Si(6)O(18)ratio Cr(III)(0.0017% per weight), in the temperature range of 3 to 12 K. In low magnetic fields and temperatures >5 K conversion of side-holes to anti-holes is observed with progressing time. Anti-holes are due to the population stored in ground state levels. The dynamics of the hole pattern can be well modelled by a set of coupled differential equations for the levels of the (4)A(2) and (2)E multiplets. The measurements allow the simultaneous determination of g-factors and spin-lattice relaxation rates in the excited state and the ground state. At 6 K the relaxation times between the split +/-1/2 2A((2)E) excited state levels and +/-3/2 2A((4)A(2)) levels of the ground state are about 0.16 ms and 9 ms, respectively. From the temperature dependence it follows that the spin-lattice relaxation rates are dominated by Orbach processes in the experimental temperature range.
We report transient spectral hole-burning experiments conducted in the R1(±1/2) line of 53-chromium(III) doped Al2O3 in zero field and low magnetic fields B∥c at 3 K. These facile experiments enable, in principle, the simultaneous determination of the hyperfine coupling parameters for both the 4A2 ground state and the Ē(E2) excited state, and in both directions to the crystal caxis. In order to determine these parameters we have applied a global fit by simulated spectra to the experimental data and good agreement between calculated and measured hole-burning patterns is achieved. The relevant resulting parameters for the Ē(E2) excited state are A∥=18±4 MHz and A⊥=40±4 MHz.
Laser induced hole burning in optical absorption bands of low temperature solids has been investigated with emphasis on mechanisms for population hole burning, the interactions with the environment which limit the hole widths and the use of hole burning for ultra high resolution spectroscopy. Hole burning was observed in a wide range of solids including organic and organometallic molecules in crystals and rare earth ions in crystals and glasses. The mechanisms for hole burning also varied widely and a new mechanism involving optically induced nuclear spin flips of near neighbor nuclei was discovered. Examples of hole burning in new classes of materials are presented here for the first time, namely inorganic glasses (silicate glasses containing Nd(3+), Pr(3+) or Eu(3+)) and undoped stoichiometric compounds (e.g., EuP5O14). Hole recovery was used to determine nuclear spin lattice relaxation rates in praseodymium doped crystals. For allowed optical transitions (e.g., singlet states of zinc porphyrin, dibromoindigo and color centers in alkali halides) the hole widths at 2K are limited by population decay to approximately 50 MHz. For longer lived optical levels in crystalline materials, e.g., those containing rare earth ions, the hole widths are narrower (less than 10 MHz) and are limited by dynamical interactions with surrounding nuclei. In amorphous materials such as europium doped silicate glass, hole widths are much broader, being limited by low frequency tunneling motions. Hole burning measurements were complemented by time domain coherent transient (photon echo, and optical free decay) measurements of homogeneous linewidths which enabled us to measure optical resonances as narrow as 760 Hz in Eu(3+):Y2O3.