ARTICLE pubs.acs.org/JPCA
Photoinduced Electron Transfer and Persistent Spectral Hole-Burning in Natural Emerald Hans Riesen* School of Physical, Environmental, and Mathematical Sciences, University of New South Wales, ADFA, Canberra, ACT 2600, Australia 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 R1-line. Overall, the excited state is strongly quenched, in comparison to laboratory-created emerald (τ = 1.3 ms), with an average quenching rate of ∼6 103 s1 at 2.5 K. This quenching is attributed to photoinduced electron transfer caused by a relatively high concentration of Fe2þ ions. The forward electron-transfer rate, kf, from the nearest possible Fe2þ sites at around 5 Å is estimated to be ∼20 103 s1 at 2.5 K. The photoreductive quenching of the excited Cr3þ ions by Fe2þ 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 Fe2þ sites with the forward electron-transfer rate, kf, given as a function of acceptordonor separation R by exp[(Rf R)/ af]; Rf and af 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 Cr3þ 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 nonphotochemical 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.
I. INTRODUCTION The optical spectroscopy of emerald has been of interest for decades.110 Emerald is the green variety of the ring silicate beryl, which has the idealized formula of Be3Al2Si6O18. The color is caused by ca. 0.10.5% Cr3þ substituting for the Al3þ ions in sites of D3 point symmetry. However, chrome-free emerald with V3þ as the main impurity is very similar in color.5 Beryl crystallizes in the hexagonal space group P6/mcc.11,12 Hexagonal Si6O18 rings are the dominant feature of the crystal structure. These rings are stacked above one another in a staggered arrangement and thus form channels parallel the hexagonal c-axis. The channels are joined laterally by the Be2þ and Al3þ ions, and they can accommodate a variety of impurities such as alkali ions, water, and CO2 molecules.4,5,13 Natural emerald often contains Fe2þ and Fe3þ ions in the Al3þ positions and in the channels provided by the Si6O18 rings.4,5,1417 Electronic transitions in the solid state are subject to inhomogeneous broadening due to imperfections of the crystal lattice and isotope distributions.1823 Spectral hole-burning is one of the laser-based techniques that can overcome this broadening, and the homogeneous line width can be approached under favorable conditions.23 In particular, spectral hole-burning allows an increase in the resolution by many orders of magnitude. Depending on the mechanism of the hole-burning process, transient and persistent spectral holes can be observed. Whereas r 2011 American Chemical Society
transient holes are based on the population storage in metastable excited states or in some fine or hyperfine level of the ground state, persistent hole-burning is caused by photochemical reactions in the excited state, such as photoreduction or photooxidation, or by some minor rearrangement of the local environment.1922 (The latter mechanism is called non-photochemical hole-burning.) Spectral hole-burning is a very powerful technique and has been applied to a very wide range of systems and problems in physics, chemistry, and biology.1929 We have previously reported transient spectral hole-burning in the R-lines (2Er4A2 excitations) of laboratory-created (Chatham) pale green and very low chrome emerald (beryl).9,10 Emerald crystals grown by the Chatham laboratories are of high optical quality and are free of water, alkalis, and Fe2þ/Fe3þ impurities. These crystals display only transient spectral hole-burning based on population storage in the 2E excited state or the spin levels of the 4A2 ground state,9,10 with the latter being only the case for very low Cr3þconcentrations. Persistent spectral hole-burning has been observed for the R-lines (2Er4A2 excitations) of Bironlaboratory emerald, and the mechanism has been assigned to photoreduction or photooxidation of the Cr3þ centers.8 Received: March 7, 2011 Revised: April 13, 2011 Published: May 06, 2011 5364
dx.doi.org/10.1021/jp2021769 | J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
ARTICLE
III. RESULTS AND DISCUSSION Polarized absorption spectra at 298 and 2.5 K are shown in Figure 1. The transitions at around 683, 640, and 430 nm are readily assigned to the 4A2f2E, 4A2f4T2, and 4A2f4T1 dd electronic excitations of Cr3þ in the D3 sites of aluminum in the beryl structure. The absorption bands above 700 nm are due to the 5T2f5E transition of Fe2þ in channels of the beryl structure and, possibly, aluminum sites. It appears that the R-lines and the
Figure 1. Polarized absorption spectra of natural emerald (π = E c; σ = E^c) at room temperature and 2.5 K. The pronounced dd transition of Cr3þ are denoted.
A2f4T2 ligand field transition of the Cr3þ ions are strongly π = E c polarized. The predominantly σ = E^c polarized transition at 810 nm has been assigned to Fe2þ in channel sites,14 although there is still no final agreement as to the exact nature of the Fe2þ sites.1517 Figure 2 shows details of luminescence and absorption spectroscopy of the R-lines. The measured 2E splitting of 69 cm1 is significantly larger than the value of 63 cm1 observed for laboratory-created (Chatham) emerald. Moreover, the peak maximum of the R1-line is shifted to the red by about 8 and 13 cm1 for the luminescence and absorption spectra, respectively. The larger 2E splitting and the red shift are due to dielectric variations, a change of crystal field parameters, dipolar interactions, and some contribution by exchange interactions with nearby Fe2þ ions, i.e., the interaction of the S = 3/2 system of the Cr3þ ion with the S = 2 system of the Fe2þ ions. Since the Cr3þFe2þ distance is subject to a (random) distribution, the inhomogeneous width of the R-lines is significantly larger than the one observed for laboratory-created emerald that has a low secondary (tertiary) impurity concentration. An important and peculiar feature observed in the spectra of Figure 2 is the difference in peak wavelength/wavenumber of the R1-line between the luminescence and absorption spectra. The maximum occurs some 5 cm1 higher in energy in luminescence. Also, the R1-line is narrower in luminescence. As is shown below, 4
)
II. EXPERIMENTAL METHODS A natural African emerald was cut and polished parallel to the c-axis, with a final a thickness of 4 mm. From an analysis by X-ray fluorescence and absorption spectroscopy (visible and nearinfrared region) Cr3þ and overall iron (Fe2þ, Fe3þ) concentrations of 1.5 105 and 3 104 mol/cm3 were determined. Polarized absorption spectra at room temperature were measured on a Cary 50 spectrometer equipped with a Glan-Thompson polarizer. For transmission spectra, the light of a 50 W halogen lamp was passed through a 650 nm long pass interference filter and the sample, then dispersed by a Spex 1404 monochromator (1200 grooves/mm holographic grating), and detected by a Hamamatus R 928 photomultiplier. The signal was processed by a currentto-voltage preamplifier (Femto DLPCA-200) and a lock-in amplifier (Stanford Research Systems SR810 DSP). For luminescence experiments, a current- and temperaturestabilized 635 nm laser diode (Hitachi HL6320G) or a highpower LED (light-emitting diode; Thorlabs M617L2; 617 nm center wavelength) were used as excitation sources and the emitted light was analyzed and processed in the same way as that for the transmission experiments. Excited-state lifetimes were measured by modulating the light of the 635 nm laser diode with an acoustooptic modulator (Isomet AOM 1205C-2), and observing the luminescence decay at constant wavelength through the monochromator with a bandpass of 0.1 nm. These data were also used for extracting time-resolved luminescence spectra of the R1-line. Spectral holes were burnt and read out by a stabilized diode laser (Thorlabs TEC2000 temperature controller with TCLDM9 thermoelectric mount; ILX ultralow noise current controller LDX-3620, Hitachi 6738MG laser diode). After burning a spectral hole at constant frequency for a set period of time, the laser frequency was tuned by modulating the injection current with a triangular waveform (2500 Hz) generated by a waveform synthesizer (Stanford Research Systems SRS DS345). The frequency scans were calibrated by a 300 MHz FabryPerot etalon (Coherent 300 MHz confocal spectrum analyzer). The laser light was measured in transmission by a Si photodiode (Thorlabs PDA36A-EC). The signal was averaged on a digital storage oscilloscope (LeCroy waveSurfer 422) and subsequently on a PC. Typically 512 waveforms were averaged. Laser power densities of 7 and 260 mW/cm2 were used for the readout and burn period. The crystals were embedded using Crycon grease on a sapphire window, which was mounted on the coldfinger of a closed-cycle cryostat (Janis/Sumitomo SHI-4.5).
)
In the present paper we report on the optical spectroscopy and the mechanism of persistent hole-burning in natural emeralds. As stated above, most natural emerald contains Fe2þ/Fe3þ as an impurity, and it appears that the efficiency of persistent spectral holeburning is directly correlated with the concentration of Fe2þ.30
5365
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
ARTICLE
)
Figure 2. Polarized absorption (π = E c, solid line; σ = E^c, dashed line) and unpolarized luminescence (dashed line normalized to the R1 absorption) spectra of natural emerald at 2.5 K in the region of the 4A2f2E electronic origins (R-lines). The narrow solid trace is the spectrum, as measured by the monochromator (with limited resolution), of the laser used to burn spectral holes.
R1-line. A very pronounced variation of the excited-state lifetime is observed. Clearly, the sites at the red tail of the inhomogeneous distribution of the R1-line are subject to significantly enhanced quenching in comparison with the sites at the blue tail. Naturally, because the Cr3þFe2þ separation is subject to a distribution, the decay curves are not single exponentials. The effective lifetime, τe, i.e., the time at which the initial luminescence intensity has decayed to the value of 1/e, varies by a factor of 5 (190 to 37 μs) from the blue (682 nm) to the red tail (683.8 nm) of the R1-line (see also Figure 4); this is a very pronounced wavelength dependence over a very narrow wavelength range (1.8 nm). Laboratory-created (Chatham) emerald exhibits a single-exponetial decay of the 2Ef4A2 luminescence with a lifetime of τ = 1.3 ms. If we make the crude approximation given in 1 1 þ kq τe τ
Figure 3. Normalized luminescence decay after pulsed excitation (635 nm, 10 μs pulse with 5 mW peak power) as a function of the wavelength within the R1-line at 2.5 K. The wavelength from the top to the bottom trace is 682.2683.8 nm in steps of 0.2 nm. The insert shows the averaged decay curve (average of decay data from 681.7 to 683.8 nm in steps of 0.1 nm). The dashed lines are best fits by eq 3.
this behavior is based on the interaction of the Cr3þ centers with nearby Fe2þ ions. The Cr3þ centers at the red tail of the R1-line are subject to stronger interactions with Fe2þ ions; i.e., they have Fe2þ centers in close proximity. These centers are thus subject to electron transfer and/or electric dipoledipole excitation energy transfer, and hence the 2E excited state is quenched. This is indeed experimentally observed as is illustrated in Figure 3, where excited-state lifetime measurements are shown as a function of wavelength within the inhomogeneously broadened
ð1Þ
approximate quenching rates kq between 4600 (682 nm) and 26 000 s1 (683.8 nm) are calculated. This quenching can be due to nonradiative excitation energy transfer based on electric dipoledipole coupling between the 2Ef4A2 transition of Cr3þ and the 5T2f5E excitation of Fe2þ or, more likely, due to electron transfer from Fe2þ to Cr3þ upon excitation of Cr3þ, i.e., photoreductive quenching. Most likely, the electron undergoes back-transfer to the Fe3þ center upon deactivation to the ground state. We believe this to be the main mechanism because of the following facts: the actual spectroscopy of the R-lines is affected by the interaction with the Fe2þ centers indicating some degree of exchange coupling (see above and hole-burning results below); the quenching is extremely wavelength dependent within the inhomogeneous distribution of the R1-line; i.e., the electronic energy is correlated with the quenching constant, indicating some strong electronic interaction with nearby Fe2þ centers; the standard reduction potentials E of Cr3þ þ e f Cr2þ and Fe3þ þ e f Fe2þ are 0.41 and þ0.77 V, yielding an estimated excited-state reduction potential E* for (Cr3þ)* þ e f Cr2þ of þ1.4 V; the efficiency of the spectral hole-burning mechanism appears to be strongly correlated with the concentration of Fe2þ.30 The rate constant for forward electron transfer, as proposed by Inokuti and Hirayama,31 is given in eq 2, where τ is the excited-state 5366
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
ARTICLE
Figure 4. Time-resolved luminescence spectra in the region of the R1-line at 2.5 K. The solid triangles and circles show the R1-luminescence line measured within the first 10 μs and between 350 and 360 μs after pulsed excitation (635 nm, 10 μs pulse with 5 mW peak power), respectively. The effective lifetime τe is shown as a function of wavelength (crosses).
lifetime and Rf and af parametrize the distance scale and the falloff of the transfer rate between the electron donor and acceptor and R is the distance between donor and acceptor. 1 Rf R kf ðRÞ ¼ exp ð2Þ τ af Following the work of Blumen and Manz,32 the ensemble averaged decay of the Cr3þ luminescence, φ, is given by eq 3 for a discrete lattice with randomly distributed quenchers, where p is the probability that one of the N lattice sites is occupied by Fe2þ and kf is defined by eq 2. φðp, tÞ ¼ expð t=τÞ
N Y ½1 p þ p expð tkf ðRi ÞÞ
ð3Þ
i¼1
In the case of very low quencher concentrations and in the thermodynamic limit for an isotropic medium, eq 3 can be approximated by eq 4, where Rm is the radius of the excluded acceptor volume (closest distance between acceptor and donor).31,33,34 Z ¥ φðtÞ ¼ expð t=τÞ expð4πC ½1 expð kf ðRÞtÞR 2 dRÞ Rm
ð4Þ The inset of Figure 3 shows a fit of eq 3 to the average decay curve of the R1-line. The product in eq 3 was extended over all possible Al3þ and channel sites in the beryl crystal structure within 25 Å of the Cr3þ ion, accounting for 796 possible sites for Fe2þ. The averaged waveform was obtained by adding 22 individual experimental waveforms at 681.7, 681.8, 681.9, ..., and 683.8 nm with subsequent normalization. With fixed τ = 1.3 ms and p = 0.016 (calculated from the Fe2þ concentration of 3 104 mol/cm3) the fit yields the paramaters Rf = 13.5 and af = 2.7 Å. Using eq 4 with Rm = 4.5 Å results in very similar values of Rf = 13.3 and af = 2.5 Å with a comparable value for the χ2 test. Not surprisingly the parameters and the quality of the fits are very similar; this is due to the very low quencher concentration. The dashed traces in the main part of Figure 3 are based on fits by eq 3 with fixed values of Rf = 13.5 Å, af = 2.7 Å, and τ = 1.3 ms and adjustable parameter p. The quality of these fits is of lesser quality compared to the averaged waveform; by selectively monitoring the luminescence decay within the inhomogeneous distribution of the R1-line, subsets of sites are selected for which the distribution of Fe2þ in
the near neighborhood deviates significantly from the average value; i.e., the distribution of quenchers is not uniform. For example, the Cr3þ centers at the red and blue tails of the inhomogeneous distribution will have a higher and lower Fe2þ concentration, respectively, in their proximity. As a consequence, eq 3 cannot reproduce the data very accurately over long periods of time although it still simulates the trend well. In particular, at long times, when the Fe2þ ions at larger distances become important, the agreement gets worse since, most likely, the distribution of Fe2þ centers becomes more uniform again. Nevertheless, a fit of eq 3 to the data in Figure 3 allows one to conclude that the “average” Fe2þ concentration in the vicinity of the Cr3þ subset of sites increases by a factor of ∼4 from the blue (682 nm) to the red tail (683.8 nm) of the transition. From the Rf and af values of the averaged waveform (see inset of Figure 3), we can defer a photoreductive electron-transfer rate at 2.5 K of ≈20 000 s1 at a Cr3þFe2þ separation of 5 Å. Figure 4 shows time-resolved luminescence spectra for the R1-line; the spectrum observed within the first 10 μs after the excitation pulse resembles very closely the absorption spectrum, shown in Figure 2, with a similar peak wavelength and line width. At longer delays, the spectrum shifts to the blue and narrows; i.e., it becomes similar to the continuous-wave (CW) luminescence spectrum displayed in Figure 2. This unusual behavior is caused by the fact that sites at the red edge of the R1-line are more efficiently quenched. Shallow holes burnt into the R1-line at 683 nm are shown together with the dependence of the hole width on hole depth in Figure 5. A hole width Γhole = 340 MHz is obtained by extrapolation to zero fluence. This hole width is limited by fast spectral diffusion (see below) due to structural reorganization of the crystal lattice and electronelectron and electronnuclear spin spin interactions. The dependences of hole depth and hole width on the laser fluence are shown in the upper panel of Figure 6. The hole depth data appear to follow dispersive first-order kinetics and can be well-described by eq 5, which has been developed for two-level systems (TLSs) in amorphous hosts.35,36
1 pffiffiffiffiffiffi 2π
Z
ΔA ¼1 ΔAlim þ¥ ¥
expð x2 =2Þ expf Σ0 exp½ 2ðλ0 þ σ λ xÞtg dx
ð5Þ 5367
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
ARTICLE
Figure 5. Shallow spectral holes burnt in π polarization at 683 nm into the R1-line (upper panel) at 2.5 K and dependence of hole width on hole depth (lower panel).
In eq 5 Σ 0 = PσΩ 0 τ is given by the product of the photon flux, P, the peak absorption cross-section, σ, the excitedstate lifetime, τ, and Ω 0 . Ω 0 is defined in eq 6, where λ is the tunnelling parameter, which is assumed to be subject to a Gaussian distribution with a standard deviation σλ, and R is the phonon-assisted relaxation rate in a double-well potential. R ¼ Ω0 expð 2λÞ
ð6Þ
The hole depth data in the upper panel of Figure 6 is welldescribed by eq 5. We note here that eq 5 neglects the intrinsic dispersion in hole-growth kinetics due to the Lorentzian line shape.21 The hole width is a function of the hole depth but also increases with time (fluence) as a consequence of spectral diffusion (see below). The dashed trace in the upper panel of Figure 6 is thus only a guide to the eye for the hole width data. The initial quantum efficiency for hole-burning can be estimated by using21 η ¼ lNA cðΓh Þhν
ð dA=dF Þlim F ¼ 0 A0 ð1 10A0 Þ
Figure 6. Hole-burning properties of the R1-line of natural emerald at 683 nm in π polarization and at 2.5 K. The upper panel shows the dependence of hole depth (solid triangles) and hole width (solid circles) on the burn fluence. The lower panel shows the spontaneous decay (solid triangles) and broadening (solid circles) of the spectral hole as a function of wait time tw. Spectral holes were burnt at 683 nm in π polarization. Solid lines are best fits to eqs 5, 8, and 9. The dashed line is a guide to the eye only.
and readout period. In ∼3000 s, the hole has decayed to about 50% of its initial depth and a significant increase in width is observed. Clearly the spectral holes are subject to spontaneous hole-filling and spectral diffusion. Again borrowing from the extensive work on TLS based non-photochemical hole-burning,35,36 the decay of the hole depth can be quantized by ΔA ΔA0 Z þ¥ 1 ¼ pffiffiffiffiffiffi expð x2 =2Þ expf Ω0 exp½ 2ðλ0 þ σλ xÞtg dx 2π ¥
ð7Þ
where NA is Avogadro’s number, c(Γh) is the concentration of Cr3þ centers within one homogeneous line width in units of mol/cm3, l is the optical path length in cm, h is Planck’s constant, ν is the laser frequency, F is the laser fluence, and A0 is the initial absorbance at the burn wavelength. From data, such as those shown in Figure 6, we estimate a moderately high quantum efficiency of ≈0.005% at 683 nm. The lower panel of Figure 6 shows the dependence of the hole width and hole depth on waiting time between the burn
ð8Þ A fit to the spontaneous hole decay data yields the parameters Ω0 exp(λ0) = 2.2 104 s1 and σλ = 1.4. The hole width data in the lower panel of Figure 6 imply that spectral holes in the current system are subject to relatively rapid spectral diffusion; the width increases significantly with time. Again the data can be well-described by a TLS-based model36,37 that assumes a Gaussian distribution of 5368
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
ARTICLE
Figure 7. Burning of broad spectral holes in natural emerald at 683 nm in π polarization and at 2.5 K. Isosbestic points and an increase on of ΔA on both sides of the wide hole are observed with increasing hole depths.
tunnelling rates R. The resulting equation is given in eq 9, where σ0 = 2σλ,
¼
A pffiffiffiffiffiffi σ 0 2π
Z
ΓSD ðtÞ þ¥
¥
expf½lnðR=R0 Þ2 =2ðσ 0 Þ2 gf1 expð Rtw Þg d½lnðRÞ
ð9Þ We have conducted nonselective bleaching experiments using 635 nm laser light and 617 nm light from a 390 mW LED with long exposure times; within the experimental accuracy, no change can be detected in the R1-line, both in transmission and luminescence. If the hole-burning mechanism were based on a persistent reduction or oxidation of the Cr3þ centers, a decrease of the R1-line would be observed. (This is for example the case for Mn4þ-doped Al2O3, where the Mn4þ gets photoreduced upon exposure to blue light.) These observations, together with the fact that spectral diffusion and hole decay can be modeled well within the TLS model, indicate that the spectral hole-burning mechanism is most likely non-photochemical. To test this hypothesis, we have burnt very wide spectral holes into the R1-line as is illustrated in Figure 7: Indeed, we observe isosbestic points and a slight increase in absorbance on both sides of the wide hole with increasing hole depth ΔA. Thus, most of the “photoproduct” appears to lie within the inhomogeneous distribution for the R1line confirming a non-photochemical hole-burning mechanism. Together with the observation of rapid quenching at the red tail of the R1-line, this points to a (non-photochemical) hole-burning mechanism that is based on photoinduced electron transfer, followed by rapid back-transfer in the ground state, from the Fe2þ to the Cr3þ upon excitation of the Cr3þ. The transfer and back-transfer of an electron leads to a major reorganization of the local lattice geometry, and hence, after going through this process for ∼20 000 times the local environment (bond angles, bond lengths) of the Cr3þ ions may undergo some minor change, yielding a shift of the transition frequency, i.e., providing a mechanism for spectral hole-burning. Naturally, it is possible that for a minority of chromium centers electron back-transfer is very slow; this may affect the spontaneous hole-filling kinetics, and some of the hole depth remaining at long times may then be due to photochemical hole burning.
IV. CONCLUSIONS Conventional luminescence and absorption spectroscopy, including selective excited-state lifetime measurements within the inhomogeneously broadened R1-line of natural emerald, establish that the 2Er4A2 transition is severely affected by the presence of Fe2þ centers. In particular, compared to laboratorycreated emerald, a larger 2E splitting and severe inhomogeneous broadening are observed. Moreover, the excited state is strongly quenched, and this is most likely due to photoreductive electron transfer, with subsequent fast back-transfer, from the Fe2þ ions to the excited Cr3þ optical centers. The quenching is extremely wavelength dependent within the inhomogeneous distribution of the R1-line, establishing that the red-shifted Cr3þ centers are closer to Fe2þ impurities. The decay kinetics is reasonably well described by a model based on the exchange interactions between the Cr3þ and the Fe2þ centers with the assumption of a random distribution over all possible sites for the quencher ions. The spectral hole-burning mechanism is likely based on this electron transfer/back-transfer that, by reorganization of the local host lattice, provides an effective non-photochemical spectral hole-burning mechanism. Each time chromium centers cycle through the reductionoxidation process, it is possible that the local environment, e.g., bond lengths and bond angles, undergoes some minor changes leading to a shift of the optical transition frequency. In accord with this interpretation, the hole-burning kinetics is well-described by models developed for the so-called two level systems in amorphous hosts. FLN, and in particular time-resolved FLN measurements, may provide further insight into the mechanism and kinetics of the photoreductive quenching of the R1-line in the title system. Such experiments have previously been applied to related problems in Cr3þ systems.38,39 The present paper illustrates that combining conventional spectroscopy with hole-burning measurements (including the burning of wide spectral holes) can reveal subtle details of the electronic structure and kinetics in condensed matter. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: þþ61 (0)2 6268 8679. 5369
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
The Journal of Physical Chemistry A
’ ACKNOWLEDGMENT I thank Captain Brendan Hayward for some preliminary work and his interest in the spectroscopy of natural emerald. The Australian Research Council (ARC Discovery Project DP0770415; ARC Linkage Project LP110100451) and the University of New South Wales are acknowledged for financial support of our research into radiation-induced changes in the solid state.
ARTICLE
(36) Jankowiak, R.; Hayes, J. M.; Small, G. J. Chem. Rev. 1993, 93, 1471. (37) Bai, Y.; Fayer, M. D. Chem. Phys. 1988, 128, 135. (38) Hauser, A.; Riesen, H.; Pellaux, R.; Decurtins, S. Chem. Phys. Lett. 1996, 261, 313. (39) von Arx, M. E.; Hauser, A.; Riesen, H.; Pellaux, R.; Decurtins, S. Phys. Rev. B 1996, 54, 15800.
’ REFERENCES (1) Deutschbein, O. Z. Physik 1932, 77, 489. (2) Wood, D. L.; Ferguson, J.; Knox, K.; Dillon, J. F., Jr J. Chem. Phys. 1963, 39, 890. (3) Wood, D. L. J. Chem. Phys. 1965, 42, 3404. (4) Wood, D. L.; Nassau, K. J. Chem. Phys. 1967, 47, 2220. (5) Wood, D. L.; Nassau, K. Am. Mineral. 1968, 53, 777. (6) Edgar, A.; Hutton, D. R. J. Phys. C: Solid State Phys. 1978, 11, 5051. (7) Hasan, Z.; Keany, S. T.; Manson, N. B. J. Phys. C: Solid State Phys. 1986, 19, 6381. (8) Rigby, N. E.; Manson, N. B.; Dubicki, L.; Troup, G. J.; Hutton, D. R. J. Opt. Soc. Am. B 1992, 9, 775. (9) Riesen, H. Chem. Phys. Lett. 2003, 382, 578. (10) Hayward, B. F.; Riesen, H. Phys. Chem. Chem. Phys. 2005, 7, 2579. (11) Deer, W. A.; Howie, R. A.; Zussman, J. Rock Forming Minerals, Vol. 1; Longmans: London, 1962. (12) Morosin, B. Acta Crystallogr. 1972, B28, 1899. (13) Edgar, A.; Vance, E. R. Phys. Chem. Miner. 1977, 1, 165. (14) Goldman, D. S.; Rossman, G. R.; Parkin, K. M. Phys. Chem. Miner. 1978, 3, 225. (15) Blak, A. R.; Isotani, S.; Watanabe, S. Phys. Chem. Miner. 1982, 8, 161. (16) Viana, R. R.; Jordt-Evangelista, H.; Magela de Costa, G.; Stern, W. B. Phys. Chem. Miner. 2002, 29, 668. (17) Spinolo, G.; Fontana, I.; Galli, A. Phys. Status Solidi B 2007, 244, 4660. (18) Yen, W. M.; Selzer, P. M. High Resolution Laser Spectroscopy of Ions in Crystals. In Topics in Applied Physics, Vol. 49, 2nd ed.;Yen W. M., Selzer P. M., Eds; Springer: Berlin, 1981; p 141. (19) Moerner, W. E., Ed. Persistent Spectral Hole-Burning: Science and Applications. Topics in Current Physics, Vol. 44; Springer-Verlag: Berlin, 1988. (20) Riesen, H. Struct. Bonding (Berlin) 2004, 107, 179. (21) Riesen, H. Coord. Chem. Rev. 2006, 250, 1737. (22) V€olker, S. Annu. Rev. Phys. Chem. 1989, 40, 499. (23) Macfarlane, R. M.; Shelby, R. M. Coherent Transient and Holeburning Spectroscopy of Rare Earth Ions in Solids. In Spectroscopy of Solids Containing Rare Earth Ions; Kaplyanskii, A. A., Macfarlane, R. M., Eds.; Elsevier: Amsterdam, 1987; p 51. (24) Riesen, H.; Krausz, E. J. Chem. Phys. 1992, 97, 7902. (25) Krausz, E.; Riesen, H. Coord. Chem. Rev. 1997, 159, 9. (26) Riesen, H.; Wallace, L.; Krausz, E. Inorg. Chem. 2000, 39, 5044. (27) Hughes, J. L.; Prince, B. J.; Krausz, E.; Smith, P. J.; Pace, R. J.; Riesen, H. J. Phys. Chem. B 2004, 108, 10428. (28) Macfarlane, R. M. J. Lumin. 2002, 100, 1. (29) Rebane, K. K. J. Lumin. 2000, 86, 167. (30) Hayward, B. F. B.Sc. thesis, University of New South Wales, ADFA, 2004. (31) Inokuti, M.; Hirayama, F. J. Chem. Phys. 1965, 43, 1978. (32) Blumen, A; Manz, J. J. Chem. Phys. 1979, 71, 4694. (33) Swallen, S. F.; Weidemaier, K.; Fayer, M. D. J. Phys. Chem. B 1995, 99, 1856. (34) Lin, Y.; Dorfman, R. C.; Fayer, M. D. J. Chem. Phys. 1989, 90, 159. (35) Shu, L.; Small, G. J. J. Opt. Soc. Am. B 1992, 9, 733. 5370
dx.doi.org/10.1021/jp2021769 |J. Phys. Chem. A 2011, 115, 5364–5370
