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9107
Yttrium Oxide Upconverting Phosphors. 3. Upconversion Luminescent Emission from Europium-Doped Yttrium Oxide under 632.8 nm Light Excitation J. Silver,* M. I. Martinez-Rubio, T. G. Ireland, G. R. Fern, and R. Withnall* Centre for Phosphors and Display Materials, Chemical and Life Sciences, UniVersity of Greenwich, London SE18 6PF, United Kingdom ReceiVed: March 27, 2001; In Final Form: June 11, 2001
Anti-Stokes and Stokes emissions have been observed from cubic Y2O3:Eu3+ under 632.8 nm excitation. All the emission features exhibited a marked thermal dependence, decreasing in intensity as the temperature was lowered. Arrhenius plots of this thermal behavior indicated that the Eu3+ ions were thermally excited to a low-lying level ca. 1300 cm-1 above the ground state; this low-lying excited state was assigned to the 7F2 level. Subsequent absorption of a 632.8 nm photon by the thermally excited Eu3+ ion promoted it to its 5D0 level. Anti-Stokes emission bands at wavelengths longer than 580 nm exhibited a one photon dependence on the 632.8 nm exciting light and are assigned to the 5D0 f 7F0, 5D0 f 7F1, and 5D0 f 7F2 transitions of the Eu3+ ion. Stokes emission bands also showing a one photon dependence were assigned to the 5D0 f 7F3 and 5 D0 f 7F4 transitions. Upconversion anti-Stokes emission bands were observed at wavelengths shorter than 580 nm and showed a two photon dependence on the 632.8 nm exciting light. These emission bands are assigned to Eu3+ ions on the C2 sites of cubic Y2O3 with the only exception being a band at 582.2 nm. Although this emission band showed a thermal behavior similar to that of the other emission bands, it is assigned to the Eu3+ ion on the S6 site of cubic Y2O3, in keeping with previous assignments of others.
1. Introduction Er3+, Pr3+, Ho3+, and Tm3+ ions doped into heavy metal fluoride or oxide materials have been used to fabricate optical amplifiers that operate at the wavelengths of 1.55, 1.46, and 1.31 µm; in addition these materials have been utilized for solidstate upconversion lasers and the detection of infrared radiation.1-5 Anti-Stokes emission from materials containing Er3+ ions has been widely investigated because infrared lasers emitting radiation at 980 nm (excitation to the 4I11/2 level) and 800 nm (excitation to the 4I9/2 level) are readily available as excitation sources. In addition Yb3+ ions can be used as sensitizers; these are excited to their 4F5/2 level and can then transfer energy to promote 4I15/2 f 4I11/2 transitions of the Er3+ ions. A second photon can then be absorbed to pump the rare earth ion into higher excited states. Recently we confirmed that the Er3+ cation can be easily excited into its 4F9/2 level with 632.8 nm laser light.6 It had previously been reported that the absorption cross section or oscillator strength of the 4I15/2 f 4F9/2 transition is larger than those of the 4I15/2 f 4I11/2 and 4I15/2 f 4I9/2 transitions.7 For this reason, red light (in the 620-650 nm range) excitation results in more efficient pumping compared to that possible with 974 or 800 nm light.6 This finding arose from our recent studies of Er3+ luminescence from cubic Y2O3:Er3+ materials.6 It is well established that there are two Y3+ sites in cubic Y2O3; 75% of these sites are noncentrosymmetric having C2 symmetry, and the remaining 25% are centrosymmetric having S6 symmetry (see Figure 1).8-12 The Er3+ ions can be located on both of these sites in the cubic Y2O3:Er3+ lattice, as was made evident by the temperature-dependent behavior of the hot emission bands * Corresponding author. E-mail:
[email protected];
[email protected]. Fax: 44 208 331 8405.
Figure 1. The two Y3+ crystallographic sites in cubic Y2O3.
due to the Er3+ ions.6,9 As a result of previous emission studies of cubic Y2O3: Eu3+, it was reported that lines at 582.2 and 592.5 nm originate from the 5D0-7F1 transitions of Eu3+ ions on the S6 sites.13,14 Moreover it was found that nearly all of the other features in the spectrum were due to Eu3+ on the C2 sites. In light of these findings for Y2O3:Eu3+ and our capability of assigning transitions to Er3+ on the two different crystal sites, the present investigation of Y2O3:Eu3+ using the 632.8 nm laser line as an excitation source was undertaken. This important cathodoluminescent phosphor was used for some years as the red emitter in color television screens.15-17 It should be noted that there are, to our knowledge, no previous reports of studies of Y2O3:Eu3+ (anti-Stokes) upconversion luminescence emission excited with red laser light, although a number of studies have been carried out using laser light of shorter wavelength.18-20 2. Experimental Section 2.1. Chemical Preparation. The chemicals described throughout this section are yttrium oxide (99.99%), europium oxide (99.99%) (both Rhone Poulenc, France), urea, and nitric acid (BDH AnalaR).
10.1021/jp011143q CCC: $20.00 © 2001 American Chemical Society Published on Web 08/21/2001
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Figure 3. TEM of fired Y2O3:Eu3+ (magnification 50000×). Figure 2. TEM of unfired Y2O3:Eu3+ (magnification 20000×).
The urea homogeneous precipitation method21-23 was used to prepare spherical Y2O3:Eu3+ hydroxycarbonate submicrometer phosphor precursor powders. Yttrium nitrate stock solution (56.4 g/L) was prepared by dissolving Y2O3 in dilute nitric acid until the solution reached pH 3. Europium nitrate (0.02 g) and urea (15.0 g) were dissolved in 500 mL of the Y(NO3)3 stock solution after it had been diluted 20-fold with deionized water. The solution was kept boiling on a hot plate until turbidity was observed, and then it was left for 1 h. The precipitates were filtered and washed twice with deionized water. The precipitates were dried at 60 °C giving soft, white powders which were converted to the oxide by firing at 980 °C for 6 h. The final Y2O3:Eu3+ phosphor contained 2 mol % Eu3+. 2.2. Characterization of Physical Properties. The morphologies and the particle sizes of the samples were determined by scanning electron microscopy (SEM) using a Cambridge Instruments, Stereoscan 90, and transmission electron microscopy (TEM) using a JEOL JEM-200CX. The average diameter (and its deviation) of the spherical particles was estimated from measuring ∼50 particles per picture. Luminescence and Raman spectra were obtained using a Labram Raman spectrometer equipped with an 1800 g/mm holographic grating, a holographic supernotch filter, and a Peltier-cooled CCD detector. Samples were excited using a helium-neon laser with an output of 8 mW of power at the sample on the 632.8 nm line, unless an attenuation filter was used. Samples were also excited with the 514.5 nm line of an argon ion laser in order to study the emission bands in the region of 614-651 nm, as this region was masked by the notch filter when using 632.8 nm excitation. 3. Results and Discussion 3.1. Structural Investigations. The TEM micrograph presented in Figure 2 shows unfired particles of the amorphous material. The diameters of the particles shown in Figure 2 are in the range 250-350 nm. A typical TEM micrograph of fired particles is presented in Figure 3; the particle diameters are in the range 300-400 nm and the crystallite sizes range from 75 to 200 nm. X-ray powder diffraction data of all fired materials showed only cubic Y2O3. 3.2. Upconverting Properties. Luminescence spectroscopy was used to investigate the anti-Stokes and the Stokes emissions of these Y2O3:Eu3+ materials using 632.8 nm excitation. A strong Raman band typical of the cubic Y2O3 lattice is seen on both the Stokes and anti-Stokes sides of the spectrum at a
Figure 4. Energy level diagram of the low-lying states of the Eu3+ ion in Y2O3:Eu3+. The wavenumbers of the levels reached after one photon (15803 + 1430 ) 16233 cm-1) and two photon ((15803 × 2) + 1430 ) 32036 cm-1) excitation from the thermally excited level at 1430 cm-1 are indicated on the left-hand side of the diagram. †Radiative transitions 5D2 f 7FJ and 5D3 f 7FJ also occur but are not shown in this diagram.
wavenumber shift of 375 cm-1 from the 632.8 nm (15 803 cm-1) helium-neon laser line. Examination of our energy level diagram (in general agreement with that of Carnall et al.24) indicates that there are no transitions from the 7F0 ground state of Eu3+ to upper levels which come close in energy to a photon of the 632.8 nm exciting laser light (see Figure 4). This situation is different from that of Er3+, for which the transition from the 4I15/2 ground state to the 4F9/2 level does have an energy similar to that of a photon of 632.8 nm light.6 Nevertheless, Stokes and anti-Stokes emission spectra can be obtained from Y2O3:Eu3+ using 632.8 nm excitation (Figures 5 and 6). This is because the 7F0 ground state of Eu3+ lies close to the 7F1, 7F2, and 7F3 states and at room temperature these low-lying states may be thermally populated. The result of this is that the 5D0 level can be populated by absorption of a photon of 632.8 nm light by a thermally excited Eu3+ ion. This explanation can be verified by examining the temperature dependence of the spectra. Figures 5 and 6 show that the intensities of all of the anti-Stokes and Stokes luminescence lines manifest temperature-dependent
Yttrium Oxide Upconverting Phosphors
Figure 5. (a, top) Stokes luminescence spectra, obtained from Y2O3: Eu3+ under 632.8 nm excitation, for the region 700-750 nm showing the temperature dependence of the emission bands. (b, bottom) Stokes luminescence and Raman spectra, obtained from Y2O3:Eu3+ under 632.8 nm excitation, in the region 635-700 nm showing both the temperature dependence of the emission bands and the temperature independence of the Raman bands.
behavior, as expected. Indeed the only features which do not exhibit temperature-dependent intensities are the Stokes Raman bands which are seen in Figure 5b (although they do sharpen as the temperature decreases). In contrast, the anti-Stokes Raman band at -375 cm-1, which is seen in Figure 7, shows a temperature dependence in intensity which is different from that of the emission bands. To compare the relative intensities of all of the bands, the complete emission spectrum from 637.8 to 870 nm on the Stokes side and from 425 to 618.1 nm on the anti-Stokes side is shown in Figure 8. All of the bands observed in this emission spectrum are listed and assigned in Table 1. No bands are reported in the 618.1-637.8 nm region in this work since they were attenuated by the supernotch filter which is used to block the intense 632.8 nm exciting line. However, weak bands are known to occur in this region,15 which are assigned here to Stark split levels of the 5D0 f 7F2 transition. The bands in Table 1 are assigned on the basis of their temperature-dependent behavior and laser power dependence. The bands have been categorized according to whether they originate from one photon or two photon excitation processes.
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9109
Figure 6. (a, top) Anti-Stokes luminescence spectra, obtained from Y2O3:Eu3+ under 632.8 nm excitation, in the range 570-630 nm showing the temperature dependence of the emission bands. (b, bottom) Anti-Stokes luminescence spectra, obtained from Y2O3:Eu3+ under 632.8 nm excitation, in the range 530-570 nm showing the temperature dependence of the emission bands.
As previously mentioned, the energy of a 632.8 nm photon is insufficient to bridge the 5D0-7F0 energy gap, but the lowlying 7F1, 7F2, and 7F3 states can be thermally populated. When the sum of the thermal energy and the energy of the incoming 632.8 nm photon is equal to the 5D0-7F0 energy gap, excitation to the 5D0 level can occur. It is possible to determine the energy of the thermally excited Eu3+ ions, which undergo excitation to the 5D0 level, from the temperature dependence of the intensities of their emission lines originating from transitions down from the 5D0 level. The intensities of these emission bands (see Figures 5 and 6) are proportional to the populations of the thermally excited level from which absorption of a 632.8 nm photon occurs. The relative populations of the ground and thermally excited states obey a Boltzmann distribution as given by
(
I ) A exp -
∆E kT
)
where I is the integrated intensity of any emission band due to a transition down from the 5D0 level which is populated following optical excitation of thermally excited Eu3+ ions, ∆E is the thermal energy, and k is Boltzmann’s constant. The
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TABLE 1: Wavenumber Locations and Assignments of Y2O3:Eu3+ Emissions Excited by 632.8 nm Light at 293 K transition
wavelength (nm)
wavenumber (cm-1)
transition
wavelength (nm)
wavenumber (cm-1)
5D
5
f 7F6? D0 f 7F5
5
D0 f 7F4
5D
0
5D
0
f 7F2
5D
0
f 7F1
5D
1
f 7F3
5D
1
f 7F2
5D
1
f 7F1
5D
1
f 7F0 (C2)
5D
2
f 7FJ and 5D3 f 7FJc
Stokes Luminescence from 0 5 12 434 D0 f 7F4 12 937 13 210 13 284 5D f 7F 13 328 0 3 13 398 13 470 14 035 14 048 14 106 14 145 14 210 Stokes Raman (s) 375a (w) 325a (w) 189a (w) 175a Anti-Stokes Raman (w) -375a Anti-Stokes Luminescence 5D f 7F 631.1 (s) 15 845 0 1 b b 614.2 (broad) 16 282 612.7 (w) 16 320 5D f 7F (S ) 611.1 (vvs) 16 365 0 1 6 5D f 7F 599.4 (s) 16 683 0 0 Upconversion Luminescence Excited by 2 × 632.8 nm Photons 5D f 7F and 5D f 7F c 568.5 (w) 17 589 2 J 3 J 562.4 (broad) 17 781 554.4 (w) 18 039 553.6 (w) 18 062 553.0 (w) 18 084 552.1 (w) 18 113 550.7 (w) 18 160 548.9 (vw) 18 219 546.1 (vw) 18 313 538.1 (w) 18 584 537.5 (w) 18 603 533.4 (mw) 18 749 532.6 (vw) 18 772 527.7 (vw) 18 951 527.0 (vw) 18 976 525.5 (vw) 19 029 520.4 (vw) 19 218 519.5 (vw) 19 249
804.2 (w) 773.0 (broad) 757.0 (w) 753.0 (w) 750.0 (w) 746.0 (w) 742.0 (w) 712.5 (s) 711.8 (s) 708.9 (vs) 707.0 (s) 703.7 (mw)
695.6 (w) 693.4 (s) 687.3 (s) 683.3 (w) 667.1 (w) 663.6 (w) 662.3 (s) 657.8 (mw) 652.9 (w) 651.3 (w) 650.4 (s)
(w) (w) (w)
14 377 14 422 14 550 14 634 14 990 15 070 15 099 15 203 15 317 15 354 15 375
156a 131a 124a
593.0 (vs) 587.2 (vs) 585.7 (w) 584.9 (w) 582.1 580.3 (vs)
16 864 17 030 17 073 17 097 17 179 17 233
518.6 (vw) 516.6 (vw) 513.8 (vw) 512.5 (vw) 509.0 (vw) 500.0 (vw) 488.6 (vw) 473.8 (vw) 472.8 (vw) 472.1 (vw) 471.4 (vw) 467.6 (vw) 466.9 (vw) 464.9 (vw) 452.9 (vw) 450.2 (vw) 445.8 (vw) 436.6 (vw)
19 284 19 359 19 462 19 512 19 648 20 002 20 465 21 107 21 149 21 181 21 215 21 385 21 417 21 510 22 082 22 210 22 432 22 903
a Raman band locations are given as relative wavenumber displacements from the laser line at 632.8 nm (15 803 cm-1). b A fifth component of the 5D0 f 7F2 transition has been predicted in this region for cubic Y2O3:Eu3+ (see ref 25), but was not observed in the previous work or this.c Due to higher transitions from 5D2 f 7FJ and 5D3 f 7FJ, and these will overlap.
preexponential factor, A, comprises a number of parameters including the radiative transition probabilities, the degeneracies of the levels involved in the absorption and emission steps, and the photon energies of the 5D0 f 7FJ transitions. The ∆E values of the emission bands originating from the 5D0 levels are given in Table 2 and are derived from the slopes of plots of ln I versus 1/T. A typical plot is presented for the very intense 611.1 nm emission line, and on the same plot the temperature dependence of the anti-Stokes Raman band of Y2O3 at -375 cm-1 is shown for comparison in Figure 9. It is noteworthy that the temperature dependence of the emission bands observed in this work would not be expected if the photon energy of the exciting light matched the 5D0 f 7F0 energy gap. The ∆E values lie around 1300 cm-1, which is approximately the same magnitude as the 7F -7F energy difference. The spread in wavenumbers around 2 0 this value arises from the experimental error in the measurements of the peak areas, as some of the lines are weak. An additional source of error may be due to cross relaxation processes. The ∆E values for 19 emission lines are presented in Table 2, though in some of these cases the integrated intensities of two or more emission lines were used for the determination of
the ∆E values. As can be seen from Table 2, the ∆E values are reasonably constant for bands covering the whole range of 465708 nm. This is interpreted as evidence that all features of the spectra show the same temperature-dependent behavior. Thus even the bands due to upconversion are dependent on the thermal population of the low-lying 7FJ states. It is noteworthy that the thermally excited state has a population of only 0.2% of that of the ground state at room temperature, based on a value of 1300 cm-1 above the ground state. Thus only approximately 1 in 500 of the doping Eu3+ ions present is thermally activated at room temperature. The observation of the emissions in these experiments reflects the efficient optical absorption of the 632.8 nm light by the thermally excited Eu3+ ions. This is because the excitation takes place via the electric dipole allowed 7F2 f 5D0 transition of the Eu3+ ion on the C2 site which is seen in the emission spectrum at 611 nm.14 The mechanism for the thermal activation is explained by a coupling between the lattice phonon mode of cubic Y2O3 and the Stark split levels of the Eu3+ ion. To determine the number of photons of 632.8 nm light required to excite the Stokes and anti-Stokes emissions, the
Yttrium Oxide Upconverting Phosphors
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9111 TABLE 2: Emission Band Assignments and Initial Thermal Energies (Determined from Arrhenius Plots described in the Text) of Eu3+ Ions prior to 632.8 nm Laser Excitation transition 5D 0
f 7F4
5
D0 f 7F4 D0 f 7F4 5D f 7F 0 3 5D f 7F 0 3 5 D0 f 7F2 5
5D 0
5
D0 5D 0 5 D1 Figure 7. Anti-Stokes luminescence and Raman spectra, obtained from Y2O3:Eu3+ under 632.8 nm excitation, for the range 615-626 nm showing the temperature dependence of the Raman band at -375 cm-1.
f 7F1 7
f F1 (S6) f 7F0 f 7F1
5
D1 f 7F1
due to higher transitions a
}
wavelength (nm) wavenumber (cm-1) 712.5 (s) 711.8 (s) 708.9 (vs) 707.0 (s) 703.7 (mw) 693.4 (s) 687.3 (s) 662.3 (s) 651.1 (s) 614.2 (broad) 612.7 (w) 611.1 (vvs) 599.4 (s) 593.0 (vs) 587.2 (vs) 582.1 580.3 (vs) 538.1 (w) 537.5 (w) 533.4 (mw) 532.6 (vw) 500.0 (vw) 464.9 (vw)
14 035 14 048 14 106 14 145 14 210 14 422 14 550 15 099 15 359 16 282 16 320 16 365 16 683 16 864 17 030 17 179 17 233 18 584 18 603 18 749 18 772 20 002 21 510
} } } }
∆Ea/cm-1
1425 ( 60 1296 ( 27 1310 ( 22 1416 ( 53 1439 ( 35 1284 ( 45 1235 ( 41 1232 ( 40 1267 ( 42 1176 ( 55 1194 ( 55 1183 ( 90 1468 ( 129
Average ∆E value is 1302 cm-1.
Figure 8. Stokes (635-870 nm) and anti-Stokes (420-626 nm) emission spectra obtained from Y2O3:Eu3+ under 632.8 nm excitation. Band assignments are given in Table 1.
power of the exciting light was varied and the corresponding emission intensities were measured. Plots of the logarithm of the laser power versus the logarithm of the emission intensity were plotted for 13 emission bands in the range 580-712 nm (see Figure 10a). All the plots for these bands gave slopes ranging between 0.998 and 1.08, indicating a one photon absorption dependence, as assigned in Table 1. In contrast, 13 bands between 466 and 555 nm (see Figure 10b) gave slopes ranging between 1.73 and 1.99, suggesting that they originate from a two photon absorption process. The bands between 466 and 555 nm due to upconversion emission are very weak compared to the bands between 580 and 712 nm; thus it is clear that, of the small percentage of Eu3+ ions which are optically excited, only a small fraction give an upconversion emission. Emission bands have been reported at 582.2, 587.5, 593.2, and 599.8 nm in a previous luminescence study of cubic Y2O3: Eu3+ excited by 253.7 nm light.14 The first of these was assigned to the 5D0 f 7F1 emission of Eu3+ on the S6 site, and the other three were assigned to the 5D0 f 7F1 emissions of Eu3+ on the C2 sites.14 In this work we observe an identical temperature dependence of the luminescence due to Eu3+ ions on both sites. In addition, it has been observed in this work that the emission
Figure 9. Plot of the natural logarithm of the band intensity versus the reciprocal of the absolute temperature for (a) the intense 611.1 nm emission line (∆E ) 1284 ( 45 cm-1) and (b) the anti-Stokes Raman band of cubic Y2O3 at -375 cm-1 (∆E ) 397 ( 43 cm-1).
band at 582.1 nm and those in the range 584.9-599.4 nm due to Eu3+ on S6 and C2 sites, respectively, show a one photon dependence (of the 632.8 nm exciting light). This is consistent with the assignments of these bands given by Forest and Ban14 and shown in Table 1. This different behavior of Eu3+ compared to that of Er3+ on the C2 and S6 sites in the cubic Y2O3 lattice6,9 is most probably due to (a) the larger Eu3+ (0.95 Å diameter) ion filling the lattice sites better than the smaller Er3+ (0.88 Å diameter) ion and (b) the electronic factors arising from the different numbers of unpaired electrons on these ions. In this work no new features were observed in the spectra which could be assigned to other emissions from Eu3+ ions on the S6 sites of the cubic Y2O3 lattice. It should be noted that multiphonon excitations are known to play an active role in nonresonant upconversion exhibiting an avalanche effect.26 These excitations are dependent both on the coupling of lattice phonons to the Stark levels and on the
9112 J. Phys. Chem. B, Vol. 105, No. 38, 2001
Silver et al. appreciable thermal population of low-lying excited states of Eu3+ at room temperature but not at temperatures below -100 °C. In fact, all of the features in the emission spectrum excited by 632.8 nm light, on both the Stokes and anti-Stokes sides, show hot band behaVior. We have studied the luminescence spectrum as a function of Eu3+ ion concentration as a test for the presence of Eu3+ in inorganic lattices.27 Even in the most dilute Y2O3:Eu3+ solids (10-6 mol % Eu3+), there is still an excitation to the 5D0 state, as made evident by the observation of the 611 nm emission line due to the 5D0 f 7F2 transition. Thus, although we cannot rule out multiion effects, they are not responsible for the gross features of the spectra. The emission line at 582.2 nm, arising from Eu3+ ions on the S6 site, and the five lines between 584.9 and 599.4 nm, which are due to emission from Eu3+ ions on the C2 site, all show the same thermal behavior. This indicates that the thermally populated low-lying states are approximately equal in energy for both sites. From the work reported here it is apparent that we are removing thermal energy from the Y2O3 lattice. This has implications for laser cooling effects which will be discussed elsewhere. Acknowledgment. We express our gratitude to the EPSRC (Grant Ref Nos. GR/L85176, GR/M78847, and GR/N28535). Thanks are also due to Prof. F. Auzel for useful discussions and a referee for helpful comments. References and Notes Y2O3:Eu3+
Figure 10. (a, top) Luminescence spectra, obtained from under 632.8 nm excitation, for the region 680-768 nm. The band intensities increase with the power of the exciting laser (0.8, 2, 4, and 8 mW). The band intensities show a one photon dependence on laser power as shown by the logarithmic plot in the inset. (b, bottom) Luminescence spectra, obtained from Y2O3:Eu3+ under 632.8 nm excitation, for the region 490-540 nm. The band intensities increase with the power of the exciting laser (0.8, 2, 4, and 8 mW). The band intensities show a two photon dependence on laser power as shown by the logarithmic plot in the inset.
concentration of the rare earth dopant. However, the effect shows a photon dependence higher than unity, whereas the emission bands reported here at wavelengths longer than 580.3 nm exhibit a single photon dependence on the excitation. In common with the nonresonant upconversion with avalanche effect, the present work involves phonon-assisted excitation. 4. Conclusions We have observed anti-Stokes luminescence emission in Y2O3:Eu3+ phosphor materials. This is the first report of antiStokes luminescence from Eu3+ materials under 632.8 nm light excitation. The anti-Stokes emission bands which are shorter in wavelength than the 5D0-7F0 transition (at 580.3 nm) all show a two photon dependence on the 632.8 nm exciting laser light. In contrast, the anti-Stokes emission bands due to the 5D0-7F0 transition (at 580.3 nm) and emission bands at longer wavelengths all exhibit a one photon dependence. These anti-Stokes emission bands, showing a one photon dependence on the 632.8 nm laser light, originate from Eu3+ ions which have been thermally excited to the 7F2 low-lying level prior to absorption of the 632.8 nm photons. The temperature dependence of the anti-Stokes luminescence spectrum of cubic Y2O3:Eu3+ is due to the fact that there is an
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