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The Journal of Physical Chemistry A

Photoinduced Electron Transfer and Persistent Spectral Hole-Burning in Natural Emerald

Hans Riesen

Published May 6, 2011• 2 citations

Abstract

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.

DOI: 10.1021/jp2021769

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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.

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.

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 has been observed on the zero-phonon lines of the main site and several perturbed sites of

{\mathrm{Eu}}^{2+}

in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable

{4f}^{6}5d

excited state to the conduction band followed by electron transport to

{\mathrm{Eu}}^{3+}

centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features ($\ensuremath{\approx}5$ and $\ensuremath{\approx}100\mathrm{GHz}$) is observed. A mechanism is described for the occurrence of these unusually broad features. Time-varying internal electric fields which occur during the hole burning due to photoionization and trapping can lead to burning of holes at frequencies non-resonant with that of the laser. In addition, a mechanism for hole erasure, tunneling between Eu ions, is demonstrated. This mechanism is identified from the frequency dependence of the hole erasure which follows the

{\mathrm{Eu}}^{2+}

absorption, and the linear dependence of the photoconductivity and hole erasure efficiency on irradiation power, both of which indicate erasure in a single-photon process.

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.

In the first experimental search for persistent spectral hole burning in an elemental semiconductor, we examined a variety of electronic defects in Si and in a Si:Ge alloy. No persistent spectral changes have been observed for Be-Li, Se0, or thermal donors in Si; however, spectral holes which persist for ~5 ms have been produced in the 1s-2p absorption lines of a sulfur-hydrogen complex in Si. The addition of 0.7% Ge to the Si host ensures that an inhomogeneously broadened system is being studied. Measurements combining a CO2 laser pump with a Fourier transform spectrometer probe reveal that the hole burning is due to migration of electrons from the S-H centers to shallow traps. The large observed hole width (4.6 cm−1) is attributed to spectral diffusion by resonant excitation transfer. The hole burning behavior of the 2p± line is temperature independent below 30 K, whereas the hole burning of the 2p0 line exhibits strong temperature dependence, suggesting that the ionization of the laser-excited centers in the 2p± hole burning process occurs via energy transfer between neighboring excited centers rather than by thermal ionization.

Spectral hole burning in the 4A2–2E(2A) zero-phonon line for Cr3+ in Be3Al2(SiO3)6 is reported. Persistent holes with a linewidth of 0.5 GHz were observed when the crystal temperature was below 4 K. The burning is attributed to photoionization of Cr3+ from the 2E state and trapping of the released charge at other sites. The changes in the Coulomb field are the main factors contributing to the hole linewidth. High-resolution axial and transverse Zeeman measurements of the 4A2–2E(2A) transition performed by using the spectral hole are found to be consistent with the known g values.