- Email: [email protected]
Spectroscopic properties of trivalent praseodymium in barium yttrium fluoride
Journal of Luminescence 102–103 (2003) 481–486
Spectroscopic properties of trivalent praseodymium in barium yttrium fluoride B. Di Bartolo*, B.E. Bowlby1 Department of Physics, Boston College, Chestnut Hill, MA 02467, USA
Abstract We have conducted a spectroscopic investigation of Pr3+ in barium yttrium fluoride (BaY2O8). Two doping concentrations were used: BaY2F8:Pr3+ (0.3%) and BaY2F8:Pr3+ (1%). The measurements included absorption, luminescence under continuous and pulsed excitations, and thermal effects on some sharp lines. The experimental results were used to characterize this system. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Praseodymium; Barium yttrium fluoride
1. Introduction This work deals with trivalent praseodymium (Pr3+) in barium yttrium fluoride (BaY2F8), a monoclinic crystal. This system has shown laser emission [1] and has motivated us to characterize it spectroscopically. The electronic configuration of the ion Pr3+ is [(closed shells)+4f2]: it corresponds to 91 quantum states which group in the LS terms 3H, 3F, 1G, 1D, 1 3 I, P, and 1S. These terms are further split by the spin–orbit interaction and by the crystalline field.
*Corresponding author. Tel.: +1-6175523601; fax: +16175528478. E-mail address: [email protected], [email protected] (B. Di Bartolo). 1 Now at the Department of Cognitive and Neural Systems, Boston University, Boston, MA.
An energy level scheme of the Pr3+ ion in solids is presented in Fig. 1.
2. Experimental The absorption spectra were obtained using a Perkin–Elmer Lambda 9 spectrophotometer. For continuous luminescence measurements, the source used was an Omnichrome model 532 argon-ion laser powered by an Omnichrome model 150 power supply. Responses to pulsed excitation and time-resolved spectra were obtained by using the same excitation source: a Molectron model DL-II Tunable Dye Laser, which was pumped with a Molectron model UV12 Pulsed Nitrogen Laser. The nitrogen laser provided pulse widths of 10 ns with a repetition rate of up to 30 Hz. Dyes used
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00584-7
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
482
Energy
6000
4000
5000
3000
3
P2
22
3
20 18
1
P
3
1 P & I 1 6
0
D2
16
1
10
G
4
3
6 3
4
F3
3000
1000
2000
0
3
←77K
-1000
F
2
3
0
3
H6
2
2000
Fig. 2. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 465 to 500 nm. lexc ¼ 457:9 nm (argon laser).
F
3
300K→
-2000 0 465 470 475 480 485 490 495 500 Wavelength (nm)
12
4
4000
1000
14
8
Intensity (A.U.)
3
(cm -1 x 10 )
H5 H4
Fig. 1. Energy level scheme of Pr3+ in BaY2F8.
included Coumarin 480 and Coumarin 600. These dyes lase in the ranges 430–500 and 560–640 nm, with maximum output at 480 and 600 nm, respectively.
3. Results and discussion The absorption spectra of BaY2F8:Pr3+ were obtained at two temperatures: 300 and 77 K. The spectral region covered was 430–2400 nm. All the J levels were identified. There was no difference between the spectra of the two samples used, apart from the fact that the spectra of the sample with the higher concentration of Pr3+ were more intense. The absorption spectra that we have obtained contain a lot of detailed information that we have reported in another paper that we have prepared
on the applications of the Judd–Ofelt theory to the praseodymium ion in solids [2]. The emission spectra of BaY2F8:Pr3+ were found to originate mainly from the 3P0, 1D2, and 3 F3 levels. The excitation of the 3P0 level did not pose any problem because several of the argon laser lines matched the absorption peaks of the manifolds 3 P2, 1I6, and 3P1. We chose to excite the 3P0 level by using the argon laser output filtered to allow only the 457.9 nm emission to reach the sample, resulting in the excitation of a sublevel of 3P1. The emission spectra originating from the 3P0 level can be associated with transitions from 3P0 to the multiplet levels 3H4, 3H5, 3H6, 3F2, 3F3, and 3 F4. The emission related to the 3P0-3H4 transition is reported in Fig. 2. Emission from the 1D2 level was more difficult to obtain. Because of the very low radiative or nonradiative 3P0-1D2 transition rates, excitation by the argon laser was not suitable. Other sources were tried, including a Sylvania sungun and a xenon lamp. However, we found that the best excitation for observing the emission from the 1D2 level was provided by the dye laser using the dye Coumarin 600. The reference signal from the (pulsed) dye laser was sent to the boxcar integrator that provided the relevant electronic output.
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
200
1000
400
200
600
0
400 ← 77K 200
100
150 Intensity (A.U.)
300K →
50
300K →
100
0
50
← 77K
-50
-200
600 605 Wavelength (nm)
0
-400 610
Fig. 3. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 595 to 610 nm. lexc ¼ 589 nm (dye laser).
The emission spectra originating from the 1D2 level can be associated with transitions from 1D2 to 3H4, 3H5, 3H6, 3F2, 3F3, and 3F4. The emission related to the 1D2-3H4 transition is reported in Fig. 3. The only other manifold that displayed luminescence was 3F3 and obtaining the emission spectra was relatively easy. Because of the radiative and nonradiative pathways available, it was possible to excite the 3F3 manifold by using the argon laser. This time, however, the unfiltered output of the laser was used to excite the sample. As the upper levels relaxed, the 3F3 level was excited and then, in turn, relaxed to the lower levels in a cascade type of sequential transitions. The emission spectra originating from the 3F3 level can be associated with transitions from 3F3 to 3H4 (see Fig. 4). The lifetime versus temperature graph of the 3P0 level for both samples is given in Fig. 5. The samples were excited with the dye laser using Coumorin 480. The lasing wavelength was 480 nm and the emission was monitored at 609 nm (the 3 P0-3H6 transition). All decay patterns were exponential. The lifetime of the 3P0 level appears to be temperature independent in the temperature range measured, and to have a value of approximately 55 ms for both samples. The fact that the
1560
1580 1600 1620 Wavelength (nm)
1640
-100
Fig. 4. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 1550 to 1650 nm. lexc =unfiltered argon laser output.
70
Lifetime (microseconds)
Intensity (A.U.)
800
0 595
483
1% .3%
65
60
55
50
0
100
200
300
400
500
600
Temperature (K) Fig. 5. Lifetime of the 3P0 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 480 nm (dye laser), lem ¼ 609 nm.
lifetime is independent of the temperature in this regime is not unexpected. The energy difference between the 3P0 and the next lower lying level, the 1 D2, is over 4000 cm1. Since the phonon spectrum extends to only about 415 cm1, at least 10 phonons would be needed to bridge this energy
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
2
This possibility was suggested to one of the authors (BDB) by Dr. Marvin Weber during the International Conference on Luminescence at Budapest.
270 1% .3% Lifetime (microseconds)
gap, a very unlikely event. Lifetime measurements were also performed on emissions from the 3P1 level (at 465.9 nm), but the lifetime of this level was found to be identical to that of the 3P0 level, leading us to believe that thermalization is occurring between these two manifolds. Moving on to the response of the 1D2 manifold to pulsed excitation we found that, similar to the 3 P0 level, the lifetime of the 1D2 manifold appears to be essentially temperature independent for both samples. The measured lifetime was approximately 250 ms. The lifetime versus temperature graph for both samples is shown in Fig. 6. The samples were once again excited with the dye laser, this time using Coumarin 600 and a lasing wavelength of 589 nm. Emission was monitored at 597 nm. Again, the fact that the lifetime of the 1D2 level is essentially independent of temperature is not surprising. Here, the energy difference between this level (the 1D2) and the next lower level (the 1 G4) is 6000 cm1, even greater than the previous case. Again, multiphonon processes bridging this gap are very improbable. The slight temperature dependence of the 1D2 lifetime may be due to a rearrangement of the population of excited ions in the 1D2 sublevels, which may have intrinsically different lifetimes.2 The other level to decay radiatively is the 3F3 level. Emission was seen originating from this level and terminating on the 3H4 ground state. Unlike the previous cases, the lifetime of the 3F3 level shows a strong temperature dependence. Excitation in this case was again at 480 nm using the dye laser and Coumarin 480. This excited the 3P0 level, which then decayed through various channels to the 3F3 level. Emission was monitored at 1580 nm. The lifetime versus temperature graph for both samples is given in Fig. 7. We see that the lifetime varies from 23 ms at 77 K to about 5 ms at 500 K. We investigated the temperature dependence of the line width and position of several radiative transitions. For these studies, we chose the most intense lines in the 3P0-3H4, 3H6, 3F4, and 1 D2-3H4 transitions.
260
250
240
230 0
100
200
300
400
500
600
Temperature (K) Fig. 6. Lifetime of the 1D2 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 589 nm (dye laser), lem ¼ 597 nm.
30 25 Lifetime (milliseconds)
484
1% .3%
20 15 10 5 0 0
100
200
300
400
500
600
Temperature (K) Fig. 7. Lifetime of the 3F3 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 480 nm (dye laser), lem ¼ 1580 nm.
We fitted the experimental data for all the lines by assuming that the thermal width is mainly determined by the Raman scattering of phonons [3]. In such a model, two adjustable parameters are used: a; a coefficient that gives a measure of the coupling of the ion with the phonon spectrum, and
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486 Table 1 Line width parameters Transition
E0 (cm1)
a (cm1)
TD (K)
P0- H4 P0-3H6 3 P0-3F4 1 D2-3H4
13.5 5.9 10.5 10
195 261 150 350
500 500 500 500
3
3
3
Table 2 Line shift parameters Transition
a% (cm1)
TD (K)
P0- H4 P0-3H6 3 P0-3F4 1 D2-3H4
45 40 11 16, 20
500 500 415 415
3 3
3
TD ; the Debye temperature. These parameters for all the lines examined are reported in Table 1. We also fitted the thermal change of the position of each line by assuming a mechanism by which the ion emits and re-absorbs virtual phonons [3]. In such a model, again two adjustable parameters are used: a% ; different from a above, a coefficient that depends on the coupling of the ion with the phonon system, and TD ; the Debye temperature. We should note that, while a is intrinsically positive, a% may be positive or negative, as shown in Table 2 where we report on the thermal shifts of all the lines examined. Examining first the line width versus temperature data, we found that a Debye temperature of 500 K characterizes all four transitions. This is close to the reported cutoff of the phonon spectra of barium yttrium fluoride (415 cm1 or about 600 K) [4]. Values for a ranged from 150 to 350. All transitions had a residual line width of about 10 cm1 (5.9–13.5 cm1). Looking next at the position versus temperature data, we found that there is a thermal ‘‘blue shift’’ for the transition 3P0-3H4 in both samples. This phenomenon has been reported previously for praseodymium in other hosts [5]. The emissions from the 3P0 level to the 3H6 and 3F4 levels, on the other hand, show a thermal red shift. Similarly, the 1 D2-3H4 emission line shows a red shift with
485
increasing temperatures, with the 1% sample showing a somewhat greater shift. Numerically, we find that a Debye temperature of 500 K gives the best fit for the 3P0-3H4 line position versus temperature data. Here, as noted previously, we have a ‘‘blue shift’’ with increasing temperature and a value of 45 for a% : Likewise, the line position versus temperature data for the 3 P0-3H6 transition is best represented by a Debye temperature of 500 K, but with an a% value of –40. The line position versus temperature data for the other two transitions, 3P0-3F4 and 1D2-3H4, are best characterized by a Debye temperature of 415 K, much less than the 415 cm1 (approx. 600 K) given as the phonon cutoff energy. The amount of shift, affected by a% ; is also smaller for these transitions, with values ranging from 11 to 20. Also, the line position of the 1D2-3H4 transition shows a difference with concentration of about 3 cm1 at all temperatures with the 1% sample having the higher energy.
4. Summary and conclusions We have conducted a detailed investigation of the absorption and emission properties of the Pr3+ ion in BaY2F8. We now list our conclusions: (1) We have identified the levels responsible for the spectroscopic properties of this system with the three levels 3P0, 1D2, and 3F3 mainly responsible for the emission properties. (2) The lifetimes of the levels 3P0 and 1D2 were found to be essentially independent of temperature and not affected by the variation of Pr3+ concentration (0.3% or 1%). (3) In accordance with point 2 above, nonradiative decay processes do not affect the lifetimes of 3P0 and 1D2, and their values should be considered due entirely to radiative processes. (4) The lifetime of the 3F3 level was found to be dependent on temperature, on account of the smaller energy gap between the 3F3 level and the next lower level, 3F2. (5) In order to observe the luminescence originating in the 3P0 level, the system had to be
486
(6)
(7)
(8)
(9)
(10)
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
excited in the spectral region above 3P0 where the 3P1, 1I6, and 3P2 levels reside. From these levels the excitation quickly reached 3P0. Under these conditions of excitation, no luminescence originating from the 1D2 level is observed. Luminescence originating from the 3P1 level was observed at 300 K, but not at 77 K, giving additional proof of the thermalization of the levels above 3P0 with the 3P0 level. In order to observe the luminescence originating in the 1D2 level, the system has to be excited in the same level. Under these conditions of excitation, no luminescence originating in the 3F3 level is observed. For the excitation of the 3F3 level, we have to rely on the radiative transitions 3P0-3F4 and 3 P0-3F3, which populates the 3F3 level, following the laser excitation of level 3P0. The thermal dependence of the widths and positions of the most prominent spectral lines were adequately explained by using a simple model which postulates nonlinear interaction between the optically active ion and the phonons distributed according to a Debye law. The Debye temperature used to fit the thermal dependence of the spectral line parameters was 500 K, corresponding to a phonon cutoff at B330 cm1.
(11) The investigation of BaY2F8:Pr3+ has brought to our attention the interesting spectral features of the Pr3+ ion in this system, and may contribute to validate the renewed interest in this laser ion. Acknowledgements The authors would like to acknowledge the benefit of discussions about the spectral characteristics of the praseodymium ion with Dr. Norman Barnes of NASA Langley Research Center and Dr. Brian Walsh and Mr. Yueli Chen of Boston College, and the sponsorship of this research by the National Aeronautics and Space Administration under Research Grant NAG-01-019. The authors would also like to acknowledge Prof. A. Kaminskii for providing them with the two samples examined in this investigation. References [1] A.A. Kaminskii, Crystalline Solids, CRC Press, Boca Raton, FL, 1996, p. 438. [2] B.E. Bowlby, B. Di Bartolo, J. Lumin., to appear. [3] B. Di Bartolo, Optical Interactions in Solids, Wiley, New York, 1968, Ch. 15. [4] F. Auzel, A. Kaminskii, D. Meichenin, Phys. Stat. Sol. A 131 (1992) K63. [5] N. Raspa, Ph.D. Thesis, Concordia University, Montreal, Que., Canada, 1992.
Spectroscopic properties of trivalent praseodymium in barium yttrium fluoride B. Di Bartolo*, B.E. Bowlby1 Department of Physics, Boston College, Chestnut Hill, MA 02467, USA
Abstract We have conducted a spectroscopic investigation of Pr3+ in barium yttrium fluoride (BaY2O8). Two doping concentrations were used: BaY2F8:Pr3+ (0.3%) and BaY2F8:Pr3+ (1%). The measurements included absorption, luminescence under continuous and pulsed excitations, and thermal effects on some sharp lines. The experimental results were used to characterize this system. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Praseodymium; Barium yttrium fluoride
1. Introduction This work deals with trivalent praseodymium (Pr3+) in barium yttrium fluoride (BaY2F8), a monoclinic crystal. This system has shown laser emission [1] and has motivated us to characterize it spectroscopically. The electronic configuration of the ion Pr3+ is [(closed shells)+4f2]: it corresponds to 91 quantum states which group in the LS terms 3H, 3F, 1G, 1D, 1 3 I, P, and 1S. These terms are further split by the spin–orbit interaction and by the crystalline field.
*Corresponding author. Tel.: +1-6175523601; fax: +16175528478. E-mail address: [email protected], [email protected] (B. Di Bartolo). 1 Now at the Department of Cognitive and Neural Systems, Boston University, Boston, MA.
An energy level scheme of the Pr3+ ion in solids is presented in Fig. 1.
2. Experimental The absorption spectra were obtained using a Perkin–Elmer Lambda 9 spectrophotometer. For continuous luminescence measurements, the source used was an Omnichrome model 532 argon-ion laser powered by an Omnichrome model 150 power supply. Responses to pulsed excitation and time-resolved spectra were obtained by using the same excitation source: a Molectron model DL-II Tunable Dye Laser, which was pumped with a Molectron model UV12 Pulsed Nitrogen Laser. The nitrogen laser provided pulse widths of 10 ns with a repetition rate of up to 30 Hz. Dyes used
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00584-7
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
482
Energy
6000
4000
5000
3000
3
P2
22
3
20 18
1
P
3
1 P & I 1 6
0
D2
16
1
10
G
4
3
6 3
4
F3
3000
1000
2000
0
3
←77K
-1000
F
2
3
0
3
H6
2
2000
Fig. 2. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 465 to 500 nm. lexc ¼ 457:9 nm (argon laser).
F
3
300K→
-2000 0 465 470 475 480 485 490 495 500 Wavelength (nm)
12
4
4000
1000
14
8
Intensity (A.U.)
3
(cm -1 x 10 )
H5 H4
Fig. 1. Energy level scheme of Pr3+ in BaY2F8.
included Coumarin 480 and Coumarin 600. These dyes lase in the ranges 430–500 and 560–640 nm, with maximum output at 480 and 600 nm, respectively.
3. Results and discussion The absorption spectra of BaY2F8:Pr3+ were obtained at two temperatures: 300 and 77 K. The spectral region covered was 430–2400 nm. All the J levels were identified. There was no difference between the spectra of the two samples used, apart from the fact that the spectra of the sample with the higher concentration of Pr3+ were more intense. The absorption spectra that we have obtained contain a lot of detailed information that we have reported in another paper that we have prepared
on the applications of the Judd–Ofelt theory to the praseodymium ion in solids [2]. The emission spectra of BaY2F8:Pr3+ were found to originate mainly from the 3P0, 1D2, and 3 F3 levels. The excitation of the 3P0 level did not pose any problem because several of the argon laser lines matched the absorption peaks of the manifolds 3 P2, 1I6, and 3P1. We chose to excite the 3P0 level by using the argon laser output filtered to allow only the 457.9 nm emission to reach the sample, resulting in the excitation of a sublevel of 3P1. The emission spectra originating from the 3P0 level can be associated with transitions from 3P0 to the multiplet levels 3H4, 3H5, 3H6, 3F2, 3F3, and 3 F4. The emission related to the 3P0-3H4 transition is reported in Fig. 2. Emission from the 1D2 level was more difficult to obtain. Because of the very low radiative or nonradiative 3P0-1D2 transition rates, excitation by the argon laser was not suitable. Other sources were tried, including a Sylvania sungun and a xenon lamp. However, we found that the best excitation for observing the emission from the 1D2 level was provided by the dye laser using the dye Coumarin 600. The reference signal from the (pulsed) dye laser was sent to the boxcar integrator that provided the relevant electronic output.
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
200
1000
400
200
600
0
400 ← 77K 200
100
150 Intensity (A.U.)
300K →
50
300K →
100
0
50
← 77K
-50
-200
600 605 Wavelength (nm)
0
-400 610
Fig. 3. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 595 to 610 nm. lexc ¼ 589 nm (dye laser).
The emission spectra originating from the 1D2 level can be associated with transitions from 1D2 to 3H4, 3H5, 3H6, 3F2, 3F3, and 3F4. The emission related to the 1D2-3H4 transition is reported in Fig. 3. The only other manifold that displayed luminescence was 3F3 and obtaining the emission spectra was relatively easy. Because of the radiative and nonradiative pathways available, it was possible to excite the 3F3 manifold by using the argon laser. This time, however, the unfiltered output of the laser was used to excite the sample. As the upper levels relaxed, the 3F3 level was excited and then, in turn, relaxed to the lower levels in a cascade type of sequential transitions. The emission spectra originating from the 3F3 level can be associated with transitions from 3F3 to 3H4 (see Fig. 4). The lifetime versus temperature graph of the 3P0 level for both samples is given in Fig. 5. The samples were excited with the dye laser using Coumorin 480. The lasing wavelength was 480 nm and the emission was monitored at 609 nm (the 3 P0-3H6 transition). All decay patterns were exponential. The lifetime of the 3P0 level appears to be temperature independent in the temperature range measured, and to have a value of approximately 55 ms for both samples. The fact that the
1560
1580 1600 1620 Wavelength (nm)
1640
-100
Fig. 4. Emission spectra of BaY2F8:Pr3+(1%) at 300 K and 77 K in the spectral region 1550 to 1650 nm. lexc =unfiltered argon laser output.
70
Lifetime (microseconds)
Intensity (A.U.)
800
0 595
483
1% .3%
65
60
55
50
0
100
200
300
400
500
600
Temperature (K) Fig. 5. Lifetime of the 3P0 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 480 nm (dye laser), lem ¼ 609 nm.
lifetime is independent of the temperature in this regime is not unexpected. The energy difference between the 3P0 and the next lower lying level, the 1 D2, is over 4000 cm1. Since the phonon spectrum extends to only about 415 cm1, at least 10 phonons would be needed to bridge this energy
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
2
This possibility was suggested to one of the authors (BDB) by Dr. Marvin Weber during the International Conference on Luminescence at Budapest.
270 1% .3% Lifetime (microseconds)
gap, a very unlikely event. Lifetime measurements were also performed on emissions from the 3P1 level (at 465.9 nm), but the lifetime of this level was found to be identical to that of the 3P0 level, leading us to believe that thermalization is occurring between these two manifolds. Moving on to the response of the 1D2 manifold to pulsed excitation we found that, similar to the 3 P0 level, the lifetime of the 1D2 manifold appears to be essentially temperature independent for both samples. The measured lifetime was approximately 250 ms. The lifetime versus temperature graph for both samples is shown in Fig. 6. The samples were once again excited with the dye laser, this time using Coumarin 600 and a lasing wavelength of 589 nm. Emission was monitored at 597 nm. Again, the fact that the lifetime of the 1D2 level is essentially independent of temperature is not surprising. Here, the energy difference between this level (the 1D2) and the next lower level (the 1 G4) is 6000 cm1, even greater than the previous case. Again, multiphonon processes bridging this gap are very improbable. The slight temperature dependence of the 1D2 lifetime may be due to a rearrangement of the population of excited ions in the 1D2 sublevels, which may have intrinsically different lifetimes.2 The other level to decay radiatively is the 3F3 level. Emission was seen originating from this level and terminating on the 3H4 ground state. Unlike the previous cases, the lifetime of the 3F3 level shows a strong temperature dependence. Excitation in this case was again at 480 nm using the dye laser and Coumarin 480. This excited the 3P0 level, which then decayed through various channels to the 3F3 level. Emission was monitored at 1580 nm. The lifetime versus temperature graph for both samples is given in Fig. 7. We see that the lifetime varies from 23 ms at 77 K to about 5 ms at 500 K. We investigated the temperature dependence of the line width and position of several radiative transitions. For these studies, we chose the most intense lines in the 3P0-3H4, 3H6, 3F4, and 1 D2-3H4 transitions.
260
250
240
230 0
100
200
300
400
500
600
Temperature (K) Fig. 6. Lifetime of the 1D2 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 589 nm (dye laser), lem ¼ 597 nm.
30 25 Lifetime (milliseconds)
484
1% .3%
20 15 10 5 0 0
100
200
300
400
500
600
Temperature (K) Fig. 7. Lifetime of the 3F3 level of BaY2F8:Pr3+ versus temperature. lexc ¼ 480 nm (dye laser), lem ¼ 1580 nm.
We fitted the experimental data for all the lines by assuming that the thermal width is mainly determined by the Raman scattering of phonons [3]. In such a model, two adjustable parameters are used: a; a coefficient that gives a measure of the coupling of the ion with the phonon spectrum, and
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486 Table 1 Line width parameters Transition
E0 (cm1)
a (cm1)
TD (K)
P0- H4 P0-3H6 3 P0-3F4 1 D2-3H4
13.5 5.9 10.5 10
195 261 150 350
500 500 500 500
3
3
3
Table 2 Line shift parameters Transition
a% (cm1)
TD (K)
P0- H4 P0-3H6 3 P0-3F4 1 D2-3H4
45 40 11 16, 20
500 500 415 415
3 3
3
TD ; the Debye temperature. These parameters for all the lines examined are reported in Table 1. We also fitted the thermal change of the position of each line by assuming a mechanism by which the ion emits and re-absorbs virtual phonons [3]. In such a model, again two adjustable parameters are used: a% ; different from a above, a coefficient that depends on the coupling of the ion with the phonon system, and TD ; the Debye temperature. We should note that, while a is intrinsically positive, a% may be positive or negative, as shown in Table 2 where we report on the thermal shifts of all the lines examined. Examining first the line width versus temperature data, we found that a Debye temperature of 500 K characterizes all four transitions. This is close to the reported cutoff of the phonon spectra of barium yttrium fluoride (415 cm1 or about 600 K) [4]. Values for a ranged from 150 to 350. All transitions had a residual line width of about 10 cm1 (5.9–13.5 cm1). Looking next at the position versus temperature data, we found that there is a thermal ‘‘blue shift’’ for the transition 3P0-3H4 in both samples. This phenomenon has been reported previously for praseodymium in other hosts [5]. The emissions from the 3P0 level to the 3H6 and 3F4 levels, on the other hand, show a thermal red shift. Similarly, the 1 D2-3H4 emission line shows a red shift with
485
increasing temperatures, with the 1% sample showing a somewhat greater shift. Numerically, we find that a Debye temperature of 500 K gives the best fit for the 3P0-3H4 line position versus temperature data. Here, as noted previously, we have a ‘‘blue shift’’ with increasing temperature and a value of 45 for a% : Likewise, the line position versus temperature data for the 3 P0-3H6 transition is best represented by a Debye temperature of 500 K, but with an a% value of –40. The line position versus temperature data for the other two transitions, 3P0-3F4 and 1D2-3H4, are best characterized by a Debye temperature of 415 K, much less than the 415 cm1 (approx. 600 K) given as the phonon cutoff energy. The amount of shift, affected by a% ; is also smaller for these transitions, with values ranging from 11 to 20. Also, the line position of the 1D2-3H4 transition shows a difference with concentration of about 3 cm1 at all temperatures with the 1% sample having the higher energy.
4. Summary and conclusions We have conducted a detailed investigation of the absorption and emission properties of the Pr3+ ion in BaY2F8. We now list our conclusions: (1) We have identified the levels responsible for the spectroscopic properties of this system with the three levels 3P0, 1D2, and 3F3 mainly responsible for the emission properties. (2) The lifetimes of the levels 3P0 and 1D2 were found to be essentially independent of temperature and not affected by the variation of Pr3+ concentration (0.3% or 1%). (3) In accordance with point 2 above, nonradiative decay processes do not affect the lifetimes of 3P0 and 1D2, and their values should be considered due entirely to radiative processes. (4) The lifetime of the 3F3 level was found to be dependent on temperature, on account of the smaller energy gap between the 3F3 level and the next lower level, 3F2. (5) In order to observe the luminescence originating in the 3P0 level, the system had to be
486
(6)
(7)
(8)
(9)
(10)
B. Di Bartolo, B.E. Bowlby / Journal of Luminescence 102–103 (2003) 481–486
excited in the spectral region above 3P0 where the 3P1, 1I6, and 3P2 levels reside. From these levels the excitation quickly reached 3P0. Under these conditions of excitation, no luminescence originating from the 1D2 level is observed. Luminescence originating from the 3P1 level was observed at 300 K, but not at 77 K, giving additional proof of the thermalization of the levels above 3P0 with the 3P0 level. In order to observe the luminescence originating in the 1D2 level, the system has to be excited in the same level. Under these conditions of excitation, no luminescence originating in the 3F3 level is observed. For the excitation of the 3F3 level, we have to rely on the radiative transitions 3P0-3F4 and 3 P0-3F3, which populates the 3F3 level, following the laser excitation of level 3P0. The thermal dependence of the widths and positions of the most prominent spectral lines were adequately explained by using a simple model which postulates nonlinear interaction between the optically active ion and the phonons distributed according to a Debye law. The Debye temperature used to fit the thermal dependence of the spectral line parameters was 500 K, corresponding to a phonon cutoff at B330 cm1.
(11) The investigation of BaY2F8:Pr3+ has brought to our attention the interesting spectral features of the Pr3+ ion in this system, and may contribute to validate the renewed interest in this laser ion. Acknowledgements The authors would like to acknowledge the benefit of discussions about the spectral characteristics of the praseodymium ion with Dr. Norman Barnes of NASA Langley Research Center and Dr. Brian Walsh and Mr. Yueli Chen of Boston College, and the sponsorship of this research by the National Aeronautics and Space Administration under Research Grant NAG-01-019. The authors would also like to acknowledge Prof. A. Kaminskii for providing them with the two samples examined in this investigation. References [1] A.A. Kaminskii, Crystalline Solids, CRC Press, Boca Raton, FL, 1996, p. 438. [2] B.E. Bowlby, B. Di Bartolo, J. Lumin., to appear. [3] B. Di Bartolo, Optical Interactions in Solids, Wiley, New York, 1968, Ch. 15. [4] F. Auzel, A. Kaminskii, D. Meichenin, Phys. Stat. Sol. A 131 (1992) K63. [5] N. Raspa, Ph.D. Thesis, Concordia University, Montreal, Que., Canada, 1992.