Temperature and Excitation Energy Dependence of the

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Temperature and Excitation Energy Dependence of the Photoionization of the F2 Center in α‑Al2O3 Shogo Ikeda and Takashi Uchino* Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan ABSTRACT: Photoionization spectra of the F2 center in α-Al2O3 have been determined in the temperature region from 77 to 400 K by using reversible photoinduced interconversion phenomena of the Fand F2-type centers. It has been shown that photoionization of the F2 center is most effectively initiated by resonant photoexcitation of the F2 center or by excitation with a ∼4.1 eV photon. The efficiency of photoionization increases with increasing temperature, demonstrating that a thermal process is involved for the electron to be promoted from the F2-center excited states to the conduction band under the resonant (or near-resonant) excitation condition. Under excitation in the energy region higher than 4.8 eV, however, the photoionization efficiency of the F2 center is hardly dependent on temperature, indicating that the direct photoionization of the F2 center is possible in this excitation energy region. It has also been demonstrated that the temperature has a substantial effect on lowering the photoionization onset energy, which can be varied from ∼3.85 eV at 77 K to ∼3.60 eV at 400 K. This temperature dependence of the photoionization onset energy can be interpreted in terms of the strong coupling between the photoexited electron and the phonon modes of the crystal.

1. INTRODUCTION Defects in crystalline materials have been a topic of extensive research for decades because imperfections in crystals play a vital role in determining the structure and properties of crystals. This is especially true for semiconductors, where the carrier density and carrier type are strongly influenced by the defect density. On the other hand, vacancies and interstitials are not usually incorporated in insulating refractory oxides, e.g., αAl2O3 (Tm = 2054 °C) and MgO (Tm = 2825 °C) under normal synthetic conditions because the energy to form such defects scales generally with the melting temperature.1 It should be noted, however, that when cation and/or oxygen vacancies are somehow incorporated in such refractory oxides by, for example, high-energy irradiation or thermochemical reduction, the resulting materials have been shown to exhibit interesting optical,2−4 electrical,3−5 and magnetic6,7 properties. Recently, much renewed attention has been paid to oxygen vacancies in pure and doped α-Al2O3.8−11 This is primarily because the oxygen vacancies in α-Al2O3 give rise to a variety of luminescence phenomena, including photoluminescence (PL),3,12,13 thermoluminescence (TL),14 and optically stimulated luminescence (OSL),15 over a wide spectral range from the ultraviolet (UV) to visible regions. Among other oxygen vacancies, the F center (an oxygen vacancy occupied by two electrons) is the commonly observed oxygen vacancy in αAl2O3. The excited state of the F center is achieved by absorbing light with an energy of ∼6 eV. Some of the photoexcited electrons of the F center recombine radiatively and nonradiatively to the ground state, and others are further excited into the conduction band to form the F+ centers (an © 2014 American Chemical Society

oxygen vacancy occupied by one electron) since the excited state of the F center is very close to or even in the conduction band.3,12,13 The photoexcited electrons in the conduction band can move through the crystal and subsequently be captured by a certain trapping site in the lattice.12,16 The trapped electrons can be optically released from the trapping site by ∼5-eV light irradiation, and some of them are recaptured again by the F+ centers to form the F centers.16 Thus, the interconversion of the F and F+ centers in α-Al2O3 can be performed solely by optical means. In addition to the F-type centers, the F2 center (two associated oxygen vacancies occupied by four electrons) and the F2+ center (two associated oxygen vacancies occupied by three electrons) in α-Al2O3 can be converted to each other by alternate irradiation of ∼4 eV and ∼6 eV light at room temperature.17,18 The observed interconversion was interpreted in terms of the photoionization of the F2 center by irradiation of ∼4 eV light, followed by the electron capture of the positively charged F+ center, as described by the following equations17,18 F2 + hv( ∼4 eV) → F2+ + e−

(1a)

e− + F + → F

(1b)

Received: December 16, 2013 Revised: January 31, 2014 Published: February 1, 2014 4346

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The reverse reactions are initiated by irradiation of ∼6 eV light, leading to the photoionization of the F center and the electron capture by the F2+ center, as given by F + hv( ∼6 eV) → F + e−

(2a)

e− + F2+ → F2

(2b)

controlled in a closed-cycle N2 cryostat in the temperature region from 77 to 450 K.

3. RESULTS As demonstrated in our previous paper,18 the thus-prepared αAl2O3 sample exhibit a variety of PL bands ascribed to the F, F+, F2, and F2+ centers. Since the goal of this paper is to estimate the temperature and excitation energy dependence of the photoionization of the F2 center in terms of the changes in the PL spectra, we will first show the underlying PL characteristics of the F2 and F2+ centers in sections 3.1 and 3.2. We should note that the F2 and F2+ centers can be independently excited and spectroscopically separated by photoirradiation with different light wavelengths of ∼300 and ∼360 nm, respectively, as will be shown below. However, the absolute intensity especially of the F2 center depends strongly on the irradiation history because of the photoionization process. Thus, it is a prerequisite to return the sample to the same initial condition before the respective photoirradiation processes. The details of the initialization procedure will be given in section 3.3. 3.1. Photoluminescence Characteristics of the F2 Center. Figure 1 shows the PL characteristics of the F2 center

Considering that the excited state of the F2 center as well as that of the F center are located just below the bottom of the conduction band,3,17,19 one would expect that the efficiency of this interconversion depends somehow on temperature owing to the expected electron−phonon coupling or other thermal effects. However, much less is known about the effect of temperature especially on the photoionization process of the F2 center. To make matters worse, the introduction of the F2-type centers into α-Al2O3 is much more difficult than that of the F-type centers. Usually, a high dose (∼1017 n/cm2) of fast-neutron irradiation has been used to obtain α-Al2O3 with observable amounts of F2-type center.17,20,21 Such a high dose of neutron irradiation often accompanies radiation-damage processes20 and will not be desirable for practical applications. Recently, we have presented a simple but effective method to introduce a number of oxygen vacancies in refractory oxides, including αAl2O3 and MgAl2O4 spinel, using a high-frequency induction heating unit.18,22 For example, when a polycrystalline α-Al2O3 powder is heated at ∼1900 °C under vacuum using a graphite crucible equipped with a high-frequency induction heating unit, substantial amounts (∼1017 cm−3) of the F, F+, F2, and F2+ centers are introduced in the resultant sample. The the positive charge of the F+ and F2+ centers is likely to be compensated by negatively charged cation vacancies, which are also introduced during the vacuum heating process.18 Since the thus-prepared α-Al2O3 samples exhibit rather efficient photoluminescence (PL) emission bands characteristic of the F- and F2-type color centers,18 these samples are suitable for exploring the possible effect of temperature on their photoionization process in terms of the PL characteristics. In this work, we therefore measure the changes in the PL spectra of the F2-type color centers at different temperatures before and after photoexcitation to obtain the temperature dependent photoionization spectra of the F2 center. We then discuss how the temperature, as well as the excitation energy, impacts the ionization processes of the F2 center.

2. EXPERIMENTAL SECTION We prepared oxygen-deficient α-Al2O3 crystals under vacuum using the same high-frequency induction heating unit reported previously.18,22 High-purity polycrystalline α-Al2O3 powders (purity 99.999%, Kojundo Chemical Laboratory Co., LTD) were used as starting materials. We heated the α-Al2O3 powders (∼0.3 g) at ∼1900 °C for ∼3 min in a graphite crucible by using the high-frequency induction heating unit, which is rated at 4 kW at a maximum frequency of 420 kHz, under continuous evacuation with a turbo molecular pump down to ∼10−3 Pa. Then, the sample was allowed to cool down to room temperature naturally, yielding a translucent sphere-shaped crystal with a diameter of ∼3 mm. We have previously demonstrated from the X-ray diffraction measurements that the resulting sample consists of a single phase of α-Al2O3.18 PL spectra were recorded on a spectrofluorometer (JASCO, FP 6600) by using a continuous wave xenon lamp (150 W). During the PL measurements, the sample temperature was

Figure 1. (a) Photoluminescence (PL) spectra of the F2 center in the α-Al2O3 sample measured at a temperature of 200 K under excitation of a 300 nm light. The inset shows the intensity-normalized PL excitation (PLE) spectra of the PL bands monitored at 330 nm (PLE1) and 500 nm (PLE2) measured at 200 K. Note that the x axis is not scaled in the normalization process. (b) The temperature dependence of the PL spectra of the F2 center is shown as a contour plot. The temperature of the sample was changed from 77 to 450 K in steps of 25 K, and the excitation wavelength was kept constant at 300 nm. 4347

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Figure 2. Temporal change in the peak intensity of (a) PL1 and (b) PL2 under continuous excitation at 300 nm measured at temperatures T = 77, 100, 150, 200, and 250 K. The PL intensity is normalized to unity at t = 0 s. Solid lines show the best fit of the data to a double exponential function, eq 3. The fitted time constants of the fast (τ1) and slow (τ2) components are shown in (c) and (d), respectively.

measured under excitation of a 300-nm light. Since the F2 center shows the photoionization and related photobleaching behaviors, the resulting PL intensity and the spectral shape could be more or less affected by the PL measurement itself. Thus, the PL spectra shown in Figure 1 are given only for semiquantitative purposes. As shown in Figure 1a, the present sample exhibits two principal PL bands peaking at ∼330 nm (∼3.76 eV) and ∼500 nm (∼2.48 eV), in agreement with the previous observations on the F2 center in α-Al2O3.3,21 In what follows, the ∼330 and ∼500 nm PL bands will be referred to as PL1 and PL2, respectively. The inset in Figure 1a shows the intensity-normalized PL excitation (PLE) spectra monitored at 330 nm (PLE1) and 500 nm (PLE2) measured at 200 K. The resulting PLE spectral features are almost identical to each other, implying that the same emission center, or the F2 center, is responsible for PL1 and PL2. The temperature dependence of the PL characteristics of PL1 and PL2 is illustrated as contour plot in Figure 1b. As the temperature of the sample is increased from 77 K, the intensity of PL1 decreases simultaneously with the increase of the intensity of PL2. Thus, the intensities of PL 1 and PL 2 show an anticorrelated temperature dependence, as has also been reported previously.21 This suggests that a thermally activated energy transfer process is involved between the excited energy levels of PL1 and PL2 after absorption of an incident photon.21 It has been demonstrated that PL2 has a quite long decay time, which is on the order of ∼50 ms.18,20,21 Thus, a likely scenario of the energy transfer is the thermally activated intersystem crossing from the first excited singlet state to an excited triplet state. That is, PL1 and PL2 are assumed to be fluorescence- and

phosphorescence-like emissions, respectively. To corroborate the assumption, however, the decay time of PL1 along with its temperature dependence needs to be evaluated. Provided that the F2 center is responsible both for PL 1 and PL 2, these PL bands will show a similar photobleaching phenomenon as a result of photoionization of the F2 center and the subsequent formation of the F2+ center, according to eq 1a. To confirm this we measured the intensity of PL1 and PL2 during continuous excitation at 300 nm at a different temperature, in the range from 77 to 250 K, as shown in Figure 2. Before measuring the temporal change in the PL intensity at respective temperatures, the intensity of the F2 center in the sample was maximized according to the procedure being described in section 3.3. As expected, a comparable PL bleaching behavior is observed for PL1 (Figure 2a) and PL2 (Figure 2b). This indicates that both PL1 and PL2 result from the same F2 center. We also found that the observed bleaching profile of PL1 and PL2 can be well represented by the following double exponential function I(t ) = A1exp( −t /τ1) + A 2 exp(−t /τ2)

(3)

where I(t) is the PL intensity at time t, and τi and Ai (i = 1, 2) are the bleaching time constant and initial intensity of the ith component, respectively. The fitted values of τ1 (fast component) and τ2 (slow component), which differed by an order of magnitude, are given in Figure 2c,d. The values of τ1 and τ2 obtained for PL 1 are very similar to those for PL 2, further confirming that these two PL emissions exhibit similar photobleaching phenomena. Note also that both τ1 and τ2 show a general decrease in magnitude with increasing temperature, 4348

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Before starting a series of PL measurements, the sample was preirradiated with 220 nm (5.6 eV) light from a monochromated xenon lamp for more than 10 min to induce the photoionization reaction of the F center (see eq 2a) and the subsequent electron capture of the F2+ center (see eq 2b). This procedure, which is called procedure 1, will ensure that the concentrations of the F2 and F2+ centers in the sample are maximized and minimized, respectively. In what follows, we refer to the sample state attained by procedure 1 as the “initial” state. After procedure 1, we measured the PL peak intensity of the F2 and F2+ centers under 300 and 360 nm light excitations, respectively. The resulting PL peak intensities of the F2 and F2+ centers measured at a temperature T are denoted as I0(T, F2) and I0(T, F2+), respectively. As for the F2 center, we monitored the PL peak intensity of both PL1 and PL2 just after (within a few seconds) irradiation of 300 nm light to minimize the effect of photobleaching. Next, ultraviolet (UV) light with a certain photon energy E was irradiated onto the sample for 3 min to promote the photoionization reaction of the F2 center (see eq 1a) and the accompanying electron capture of the F+ center (see eq 1b). This procedure is called procedure 2. After UV irradiation for 3 min, we then measured the PL intensities of the F2 and F2+ centers. When UV light with a photon energy of E is irradiated at a temperature T, the resultant PL intensities of the F2 and F2+ centers are referred to as I(T, E, F2) and I(T, E, F2+), respectively. We varied the irradiation energy E from 3.5 to 5.4 eV to investigate the possible excitation energy dependence on the photoionization process of the F2 center. Before moving on to the next irradiation energy in procedure 2, we carried out procedure 1 again to initialize the status of the F2 and F2+ centers in the sample. We indeed confirmed that, after performing procedure 1, the PL intensities of the F2 and F2+ centers return to almost the same initial levels irrespective of the irradiation history performed in procedure 2. We then can evaluate a relative decrease (or increase) of the F2 (or F2+) center at a temperature T before and after procedure 2 as follows:

indicating that the photobleaching rate is thermally activated. The bleaching kinetics of the F2 center will be discussed again in section 4.1. 3.2. Photoluminescence Characteristics of the F2+ Center. The PL spectra of the F2+ center were measured by changing temperature from 77 to 450 K. Typical examples are shown in Figure 3. In agreement with the previous observations

Figure 3. (a) Photoluminescence (PL) spectra of the F2+ center in the α-Al2O3 sample measured at a temperature of 300 K under excitation of a 358 nm light. The inset shows the temporal emission behavior of the PL peak intensity of the F2+ center under continuous excitation at 358 nm measured at 300 K. (b) The temperature dependence of the PL spectra of the F2+ center is shown as a contour plot. The temperature of the sample was changed from 77 to 300 K in steps of 25 K. +

a relative decrease of the F2 center: R(T , E , F2) =

I(T , E , F2) − I0(T , F2) I0(T , F2)

(4)

a relative increase of the F2+ center: R(T , E , F2+) =

3,23

the PL spectra show the emission band of the F2 center, peaking at ∼380 nm under excitation of a 358 nm light and also exhibit the thermal quenching behavior. In contrast to the case of the F2 center, the excited states of the F2+ center are located well below the bottom of the conduction band.3,19,23 It is hence expected that the photoionization and the related photobleaching of the F2+ center will not occur under normal photoexcitation conditions. As shown in the inset of Figure 3a, we indeed confirmed that the PL intensity of the F2+ center is almost constant during continuous excitation by 380 nm light for 10 min. 3.3. Temperature and Excitation Energy Dependence of the Photoionization of the F2 Center. In this work, we evaluated the temperature and excitation energy dependence of the photoionization of the F2 center according to the following procedures.

I(T , E , F2+) − I0(T , F2+) I(T , E , F2+)

(5)

As a result of the photoionization of the F2 center, R(T, E, F2) and R(T, E, F2+) result in a negative value in the range of −1 to 0 and a positive value in the range of 0 to 1, respectively. The whole procedures above were carried out by changing the measurement temperature in a stepwise manner to investigate the effect of temperature on the photoionization process of the F2 center. In this work, the measurement temperature was varied from 77 to 450 K in steps of 25 K. As mentioned earlier, PL1 (PL2) is a major PL band of the F2 center in the lower (higher) temperature region. Thus, at temperatures below (above) ∼150 K, the results measured for PL1 (PL2) were used to obtain R(T, E, F2). We confirmed that at 150 K the value of R(T, E, F2) obtained by using the data of PL1 is almost the same as that of PL2. 4349

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Figure 4 shows the values of R(T, E, F2) and R(T, E, F2+) measured at a different temperature as a function of excitation

4. DISCUSSION 4.1. Kinetics of Photobleaching of the F2 Center. As shown Figure 2, the F2 center PL emission shows a substantial decrease in intensity under continuous excitation at 300 nm. Since the photobleaching time constants, which are on the order of several hundreds or thousands of seconds, are much longer than the time constants of the photoexcitation (∼ps to ∼ns) and relaxation (∼ps to ∼μs) processes, it can safely be assumed that the kinetics of the photobleaching is governed mostly by the net rate constants of the photoionization of the F2 center and the subsequent electron capture reaction. It should be noted, however, that the PL bleaching curve shows a double-exponential behavior. This implies that the relevant process cannot be described as simple consecutive reactions, which will result in a single-exponential photobleaching dynamics, but may involve a reverse reaction to feed the F2 center, as will be shown in detail below. To explain the double exponential photobleaching behavior as simply as possible, we assumed that the photoionization of the F2 center can be viewed as a reversible-consecutive two-step scheme, as illustrated in Scheme 1. The assumed scheme consists of three states (states I, II, and III) and two simple reactions. In the first, which is reversible, the F2 center in state I is transiently ionized to form the F2+ center and the electron in the extended state of the conduction band (state II), and they can be converted back to the F2 center in state I. In the second reaction, which is irreversible, the F2 center in state I is permanently ionized, i.e., the photogenerated electron is permanently captured by a certain trapping site (state III). Let the rate constants for these reactions be denoted, respectively, by k1, k2, and k3, and then the rate equations will have the form

Figure 4. Values of R(T, E, F2) (the lower half of the panel) and R(T, E, F2+) (the higher half of the panel) as a function of excitation energy E. The measurement temperature T was changed from 77 to 450 K in steps of 25 K. Only typical results are shown here. The excitation energy range is classified into three energy regions: region I (E < ∼4.4 eV), region II (∼4.4 < E < ∼4.8 eV), and region III (E > ∼4.8 eV). The lines are guides for the eye.

energy E. One sees from Figure 4 that there exists a mirrorimage-like relationship between R(T, E, F2) and R(T, E, F2+), demonstrating that the photoionization of the F2 center is accompanied by the corresponding formation of the F2+ center, as expected from eq 1a. Thus, R(T, E, F2) and R(T, E, F2+) will yield complementary and consistent information about the photoionization process of the F2 center. It should be noted that the values of R(T, E, F2) and R(T, E, F2+) depend strongly on excitation energy as well as on temperature. First, we would like to point out that at the lowest temperature employed (T = 77 K), R(T, E, F2) and R(T, E, F 2 + ) already have a broad minimum and maximum, respectively, at 4.1 eV (∼300 nm), which coincides well with the peak energy of the absorption band (or the PLE band) of the F2 center. This illustrates that the photoionization of the F2 center is most effectively initiated by resonant photoexcitation of the F2 center. Upon increasing temperature, the absolute values of R(T, E, F2) and R(T, E, F2+) at ∼4 eV become larger, demonstrating that the photoionization of the F2 center is thermally activated. Note also that the absolute values of R(T, E, F2) and R(T, E, F2+) develop especially in the energy region below ∼4 eV with increasing temperature. This indicates that photoionization of the F2 center can possibly occur by absorption of lower energy ( ∼4.8 eV). Region I covers almost the entire energy range of the 4.1 eV absorption band of the F2 center. Figure 4 demonstrates that the photoionization of the F2 center occurs mostly in this energy region. Note also that the absolute values of R(T, E, F2) and R(T, E, F2+) show a substantial increase with increasing temperature, showing the thermally assisted photoionization of the F2 center, as mentioned in section 3.3. The temperature dependence of photoionization can be more clearly visible when the data of R(T, E, F2) and R(T, E, F2+) are replotted as a function of temperature (see Figure 5). From the data shown in

Eg (T ) = Eg (0) − S ℏω [coth( ℏω /2kT ) − 1]

(13)

where Eg(0) is the band gap at zero temperature, S is a dimensionless coupling constant, and ⟨ℏω⟩ is an average phonon energy. The value of S can vary, in principle, from 0 (a weak coupling case) to more than 10 (a strong coupling case). If this scheme is applicable to the photoionization process of the F2 center, EPI(T) is expected to vary in a similar manner with temperature as Eg(T); that is E PI(T ) = E PI(0) − S ℏω [coth( ℏω /2kT ) − 1]

(14)

We hence tried to fit the data shown in Figure 6 to eq 14 under the constraint that ⟨ℏω⟩ is 50 meV, which is a typical value of bulk α-Al2O3 acoustic modes.27 The constraint on ⟨ℏω⟩ was imposed because if we assume that all the three parameters in eq 14, i.e., EPI(0), S, and ⟨ℏω⟩, are free parameters, the fitted parameter values were not uniquely determined. Under this constraint on phonon energy, we obtained a reasonable fit, as shown in Figure 6, although one sees a small deviation from the fitted curve especially in the temperature region below ∼100 K. It should also be noted that the fitted values of S (S = ∼7 to ∼8) are relatively large compared with, for instance, GaAs (S = ∼3) and ZnO (S = ∼5),26,28 implying a strong electron− phonon interaction behind the temperature-induced change of EPI. It is hence quite likely that the excited states of the F2 center are coupled with the phonon modes in the crystal, probably allowing an energetic overlap with the conduction band to exhibit a strong thermal effect on the photoionization process of the F2 center. We next turn to the changes in the values of R(T, E, F2) or R(T, E, F2+) in region II (∼4.4 < E < ∼4.8 eV). Figure 4 shows that in this excitation energy region, the absolute values of R(T, E, F2) or R(T, E, F2+) are rather small at temperatures below ∼150 K. However, these values show a substantial increase with increasing temperature above ∼200 K especially for R(T, E, F2+). It is hence probable that there exists a phonon assisted photoionization channel in this energy region as well. It has previously been demonstrated that in addition to the wellrecognized excitation band at 4.1 eV, the F2 center has higherorder excited states, giving rise to several excitation bands on the higher energy side of the principal excitation band at 4.1 eV.20 We suggest that these higher-order excited states are responsible for the temperature dependent photoionization process in region II. Considering that the photoionization efficiency in region II is quite small as compared with that in region I, the density of states of these higher-order excited states will be much smaller than that of the first excited state. Finally, we focus on the behavior of R(T, E, F2) and R(T, E, F2+) in region III. Figure 4 shows that even at 77 K the absolute values of R(T, E, F2) and R(T, E, F2+) in region III are ∼0.1 and ∼0.25, respectively, and these values show only a slight increase with increasing temperature. This temperature-independent

Figure 5. Values of R(T, E, F2) (the lower half of the panel) and R(T, E, F2+) (the higher half of the panel) in region I as a function of temperature T. The lines are guides for the eye.

Figures 4 and 5, we can estimate the onset energy of photoionization, EPI, for a given temperature. In this work, EPI was defined as the excitation energy below which the absolute values of R(T, E, F2) or R(T, E, F2+) are less than 0.05. The uncertainty of the estimated value of EPI is ±0.05 eV. Figure 6 shows the values of EPI obtained independently from the curves of R(T, E, F2) and R(T, E, F2+) as a function of temperature. The value of EPI increases by ∼0.25 eV with increasing temperature from ∼100 K to ∼400 K, yielding an almost linear decrease with temperature with a coefficient of ∼−8 × 10−4 eV/K. Such a large temperature-induced variation of EPI cannot be ascribed solely to the change in thermal energy kT (∼0.03 eV) in this temperature region. 4351

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Figure 6. Temperature dependence of the onset energy of photoionization, EPI, obtained from (a) R(T, E, F2) and (b) R(T, E, F2+) shown in Figure 5. EPI is defined as the excitation energy below which the absolute values of R(T, E, F2) or R(T, E, F2+) are less than 0.05. The lines are the best fit of the data to eq 14 by assuming that ⟨ℏω⟩ is 50 meV. The fitted values of S and EPI(0) are also shown.

behavior suggests that in region III the photoionization process of the F2 center can occur even with phonon assisted processes. In other words, direct photoionization of the F2 center is possible to occur by pumping with light with an energy greater than ∼4.8 eV. Although it has previously been suggested that pumping with 4.8 eV light can directly photoionize the F2 center in α-Al2O3,17 the present results provide a firm piece of evidence for the phenomenon.

5. CONCLUSIONS We have shown through the use of reversible photoinduced interconversion phenomena of the F- and F2-type centers that the photoionization efficiency of the F2 center depends strongly on temperature as well as on excitation energy. The proposed photoexcitation and the accompanying thermal excitation processes are schematically shown in Figure 7. The excitation energy range can be classified into three energy regions depending on the temperature dependence of R(T, E, F2) and R(T, E, F2+): region I (E < ∼4.4 eV), region II (∼4.4 < E < ∼4.8 eV), and region III (E > ∼4.8 eV). In region I, the electron in the ground state of the F2 center is excited to the first excited state (see Figure 7a), and the electron−phonon coupling and the expected energetic overlap with the conduction band allow to complete the photoionization process of the F2 center. Consequently, the onset energy of photoionization can be decreased by ∼0.25 eV as the temperature of the system is increased from 100 to 400 K. Region II corresponds to the photoexcitation to the higherorder excited states (see Figure 7b). In region II, the photoionization efficiency of the F2 center is likely to be enhanced by phonon assisted processes as well; however, the efficiency is rather small because of the probability of small density of states of the relevant higher-order excited levels. Excitation in region III shows almost temperature-independent photoionization efficiency. This indicates that the direct photoionization of the F2 center occurs by pumping with light with an energy greater than ∼4.8 eV (see Figure 7c). The above pieces of information will not only shed new light on the photoionization process of the F2 center, but also be useful for the practical application of α-Al2O3 in luminescent and optical devices.

Figure 7. Schematic energy diagram for the photoionization process of the F2 center depending on the excitation energy E: (a) indirect photoionization, in which phonons assist the promotion of photoexcited electrons from the first excited state to the conduction band (region I; E < ∼4.4 eV), (b) indirect photoionization, in which phonons assist the promotion of photoexcited electrons from the higher-order excited states to the conduction band (region II; ∼4.4 < E < ∼4.8 eV), and (c) direct photoionization (region III; E > ∼4.8 eV).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Chiang, Y.-M.; Birnie III, D. P.; Kingery, W. D. Physical Ceramics: Principles for Ceramic Science and Engineering; Wiley: New York, 1996; pp 101−135. (2) Bridges, F.; Davis, G.; Robertson, J.; Stoneham, A. M. The Spectroscopy of Crystal Defects: A Compendium of Defect Nomenclature. J. Phys.: Condens. Matter 1990, 2, 2875−2928. (3) Evans, B. D. J. A Review of the Optical Properties of Anion Lattice Vacancies, And Electrical Conductivity in α−Al2O3: Their

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dx.doi.org/10.1021/jp412270f | J. Phys. Chem. C 2014, 118, 4346−4353