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4f-5d transitions of Pr3+ in CaF2
Journal of Luminescence 14(1976)115—120 © North-Holland Publishing Company
4f—5d TRANSITIONS OF Pr3~IN CaF2 V.P. BHOLA D~partementde Physique, Université de Sherbrooke, Quebec, Canada
*
Received 20 November 1975 Revised manuscript received 12 April 1976 3~ion in CaF The luminescence excitation spectrum of the Pr 2 crystals hase been investigated, and new bands at 3950, 3660, 3380, 2740 and 2620 A have been observed. These bands are found to be concentration dependent.
1. Introduction In this paper, we present resultsbands on theatluminescence excitation 3~.Interconfigurational 3950, 3660, 3380, 2740spectrum and 2620ofA have CaF2 observed, : Pr been and, among these the strongest band is centered at 3660 A. Mukerjee Li] has observed bands at 3570, 2790, and 2420 A in the case of Pr3~in aqueous solution. Employing luminescence and thermoluminescence techniques, Schlesinger and Whippey [2] have investigated the 4f Sd transitions of Ce3~in CaF 2 crystals. 3~ion in CaF At higher concentrations of Pr 2 a reduction in intensity for the strongest band has been observed. Many research workers [3,41 have reported similar results earlier in the literature. —
2. Experimental method The crystals used in the present work were obtained from the Harshaw chemical 3~was 0.05%, 0.1%, 0.5%, and Co. and from Optovac Inc. The concentration of Pr 1.0% in the samples of CaF 3~. 2 : Pr set up for recording the luminescence excitation Fig. 1 shows the experimental spectrum. An exciting light from a 1000 W xenon lamp was focused on the entrance slit of a one meter McPherson Model 225 monochromator. A crystal 12 X 8 X 2 mm was placed in the crystal-holder of a custom built Andonian vacuum cryostat fitted with a copper—constantan thermocouple for recording the temperature of the *
This work was done at the University of Windsor, Windsor, Ont., Canada.
115
3~in CaF
/
116
V.P. Bhola 4f—5d transitions of Pr
2
~E1Li~ ~89
POWer SUPptY
1
Ignitor
2
Xenon Lamp 3 Lens 48 Flap valve 5714
pump
~ 15 Roughing valve 16 Foreline valve 17 Roughing line trapl8 Mechnical pump 19 Air inlet I
12 Mainchamber CryStal Spectrograph Photomultiplier i.~ti ammeter Recorder
6 9 10 11 12 13
Fig. 1. Experimental set-up for recording the excitation spectrum.
1.G
0.5 LU I—
(a)
(b)
0.0
I
I
4200
I
3800
I
Fig. 2.AExcitation 2200 to 4200 A.spectra of CaF2 : Pr
3400 3000 2600 2200 3~ at: (a) RT from 3200 A to 4200 A; (b) LNT from WAVELENGTH
3 in CaF
V.P. Bhola / 4f—5d transitions of Pr
2
117
crystal. The light coming out from the exit slit of the monochromator was focussed on the large face of the crystal facing the monochromator. The other face 12 X 8 mm of the crystal was facing the entrance slit of the home-made grating spectrograph with a low f number of about 2.5 and, of medium resolution better than 0.5 A in the first order. The home-made spectrograph was pre-adjusted for the blue-green spectral region. The entrance slit of the spectrograph was opened to 0.3 mm. A photomultiplier tube, type 95 14S, supplied by the McPherson Company, was fixed on the exit slit of the spectrograph. The photocurrent from the photomultiplier was fed to a Keithley 410 micro-microammeter, the output of which was recorded by a strip chart recorder model 3~ 71(0.1%) 28A supplied by LNT Hewlett—Packard at RT and are shown in Co. fig. 2. excitation spectra ofofCaF2: Pr Fig.The 3 shows the recording concentration dependent spectra at LNT.
I I~Ill
4200
4000
3~00 3600 WAVE LEI’~GTH
Fig. 3. Concentration dependent bands of CaF
3400
3200
3~:(a) 0.05%; (b) 0.5%; (c) 1.0%. 2 : Pr
3~in CaF
V.P. Bhola / 4f—5d transitions of Pr
118
2
1.0 Hg
Hg
Hg
~0
19
16
14
Hg
05
0.0
I
I
I
4550
4650 4750 4850 4950 W&VELENGT 3~at LNT from 4550H A to 4950 A ex~itedwith 3660 A. Fig. 4. Emission spectrum of CaF2 : Pr
The emission spectrum of CaF 3~at LNT was also photographed and the 2 inPr microdensitometer traces are shown fig. 4.
3. Discussion of results When the Pr3~ion in the matrix of CaF 2 is irradiated with monochromatic radiation, the ion undergoes a transition from the ground state Eg to an excited state Ee. Subsequently, the ion will lose a part of its energy in a radiationless process or by cascade process, thus coming to an intermediate state E1. From intermediate state E~,it will decay to the ground state Eg, thus emitting electromagnetic radiations, which can be measured experimentally. By changing the wavelength of exciting light, one gets an excitation spectrum. It3~ has earlier ionbeen in thereported matrix of CaF [5], that there exist cubic and tetragonal sites around Pr 2, which means that the crystal field will be stronger due to the cubic field and weaker due to the tetragonal. Therefore, the effect on the low lying levels of the 4fSd configuration mixed configuration 4f5d isfield very[7—9].In close to the 1Swill be2a and alsoone it is[6]. veryThe sensitive to the crystal the enercubic gy level 0 of 4flevel of the 4f5d configuration will lie below the 1S field, the lowest 2. 0 level of the 4f The position of is 0 is reported to be 47.2 X ~ cm~[10,11], while the lowest excited level of 5d is situated at 45.6 X l0~cm~[7,12]. On other hand,1Sunder C4v symmetry the splitting of 1S 5d is small and its lowest level will lie above 0 [6]. For both sites the position of 0 will be the same, due to the screening character of 4f electron [13].
3 ~in CaF
V.P. Bhola / 4f—5d transitions of Pr
2
119
In the present experiment, fluorescence is observed from 2000 A to 42001), A and the are 2740 seen at 3950 A(25316 cm~),3660 A(27322 cm— the 3380important A (29585peaks cm—1), A (36500 cm—1), and 2620 A (38168 cm~).For 4f2 configuration, there is no energy level of Pr3+ in this region. The fluorescence observed in our experiment is ascribed to the 4f—5d transition. The large number of these bands is due to the presence of different sites around Pr3+ [5]. The strongest band has been observed at 3660 A (27322 cm 1). The presence of this band is further confirmed from the emission spectrum of CaF 3+ (see fig. 4). Lines due to mixed 2: Pr sites are also present in it. In fig. 4 peak numbers are the same as in table 1 in ref. [5]. In aqueous solution Mukherjee [1] has observed it at 3570 A (2800 cm—1). The band at 2620 A is also attributed to 4f—5d transition. From emission spectrum its position has been found to be 2550 A in case of LiYF 3~[141. The difference in the po4 : 3000 Pr and 4500 A, Weber [15]has sitions is due to different host lattices. Between also observed the fluorescence of Pr3+ due to Sd bands. Fig. 3 shows the concentration dependent bands of CaF 3~at LNT. It is clear 2 Pr that as we increase the concentration of Pr3+ in CaF 2, the intensity of the bands goes on decreasing. The effect of concentration on intensity of luminescence excitation bands at LNT can be described qualitatively by Dexter—Schulman theory [3]. According to this theory, the average distance (F) between the ions is inversely proportional to the cube root of the concentration. The intensity also depends on F where n is the multiplicity of interaction and takes the values 6, 8, and 10 for dipole—dipole, dipole-—quadrupole and quadrupole—quadrupole interactions, respectively. As we in3+ in CaF crease the concentration of Pr 2, the distance between Pr—Pr ions is decreased and the intensity will also decrease. 4. Conclusion 4f—5d transitions at 3950, 3660, 3380, 2740 A are havefound been to observed from 3+.and The2620 bands be concenluminescence excitation spectrum of CaF2 : Pr tration dependent. Acknowledgement The author is thankful to Professor Ferd Williams, Chairman, Department of Physics, University of Delaware, Delaware, for his invalued inspiration. Thanks are also due to the referee for pointing out some mistakes in the original manuscript. References [1] P.C. Mukerjee, Z. Physik 109 (1938) 573. [21 M. Schlesinger and P.W. Whippey, Phys. Rev. 171 (1968) 361.
120
3~in CaF
V.P. Bhola /4f—5d transitions of Pr
2
[3] D.L. Dexter and J.H. Schulman, J. Chem. Phys. 22 (1954) 1063. [41 N.S. Poluektov and S.A. Gava, Opt. Spectry. 31(1971)45. [5] V.P. Bhola, J. Luminescence 10 (1975) 185. [6] J.L. Sommerdijk et al., J. Luminescence 9 (1974) 288. [7] E. Loh, Phys. Rev. 147 (1966) 332. [81 J. Sugar, J. Opt. Soc. Am. 55 (1965) 1058. [9] W.T. Carnall et al., J. Chem. Phys. 49 (1968) 4424. [10] M. Schlesinger and T. Szczurek, Phys. Rev. 8 (1973) 2367. [111 E. Loh, Phys. Rev. 140 (1965) A1463. [12] M.H. Crozier, Bull. Am. Phys. Soc. 9 (1964) 631. [1 3] G.H. Dieke, Spectra and energy levels of rare-earth ions in crystals (Interscience, New York, 1968). [14] W.W. Piper et al., J. Luminescence 8 (1974) 344. [15] M.J. Weber, Solid State Commun. 12 (1973) 741.
4f—5d TRANSITIONS OF Pr3~IN CaF2 V.P. BHOLA D~partementde Physique, Université de Sherbrooke, Quebec, Canada
*
Received 20 November 1975 Revised manuscript received 12 April 1976 3~ion in CaF The luminescence excitation spectrum of the Pr 2 crystals hase been investigated, and new bands at 3950, 3660, 3380, 2740 and 2620 A have been observed. These bands are found to be concentration dependent.
1. Introduction In this paper, we present resultsbands on theatluminescence excitation 3~.Interconfigurational 3950, 3660, 3380, 2740spectrum and 2620ofA have CaF2 observed, : Pr been and, among these the strongest band is centered at 3660 A. Mukerjee Li] has observed bands at 3570, 2790, and 2420 A in the case of Pr3~in aqueous solution. Employing luminescence and thermoluminescence techniques, Schlesinger and Whippey [2] have investigated the 4f Sd transitions of Ce3~in CaF 2 crystals. 3~ion in CaF At higher concentrations of Pr 2 a reduction in intensity for the strongest band has been observed. Many research workers [3,41 have reported similar results earlier in the literature. —
2. Experimental method The crystals used in the present work were obtained from the Harshaw chemical 3~was 0.05%, 0.1%, 0.5%, and Co. and from Optovac Inc. The concentration of Pr 1.0% in the samples of CaF 3~. 2 : Pr set up for recording the luminescence excitation Fig. 1 shows the experimental spectrum. An exciting light from a 1000 W xenon lamp was focused on the entrance slit of a one meter McPherson Model 225 monochromator. A crystal 12 X 8 X 2 mm was placed in the crystal-holder of a custom built Andonian vacuum cryostat fitted with a copper—constantan thermocouple for recording the temperature of the *
This work was done at the University of Windsor, Windsor, Ont., Canada.
115
3~in CaF
/
116
V.P. Bhola 4f—5d transitions of Pr
2
~E1Li~ ~89
POWer SUPptY
1
Ignitor
2
Xenon Lamp 3 Lens 48 Flap valve 5714
pump
~ 15 Roughing valve 16 Foreline valve 17 Roughing line trapl8 Mechnical pump 19 Air inlet I
12 Mainchamber CryStal Spectrograph Photomultiplier i.~ti ammeter Recorder
6 9 10 11 12 13
Fig. 1. Experimental set-up for recording the excitation spectrum.
1.G
0.5 LU I—
(a)
(b)
0.0
I
I
4200
I
3800
I
Fig. 2.AExcitation 2200 to 4200 A.spectra of CaF2 : Pr
3400 3000 2600 2200 3~ at: (a) RT from 3200 A to 4200 A; (b) LNT from WAVELENGTH
3 in CaF
V.P. Bhola / 4f—5d transitions of Pr
2
117
crystal. The light coming out from the exit slit of the monochromator was focussed on the large face of the crystal facing the monochromator. The other face 12 X 8 mm of the crystal was facing the entrance slit of the home-made grating spectrograph with a low f number of about 2.5 and, of medium resolution better than 0.5 A in the first order. The home-made spectrograph was pre-adjusted for the blue-green spectral region. The entrance slit of the spectrograph was opened to 0.3 mm. A photomultiplier tube, type 95 14S, supplied by the McPherson Company, was fixed on the exit slit of the spectrograph. The photocurrent from the photomultiplier was fed to a Keithley 410 micro-microammeter, the output of which was recorded by a strip chart recorder model 3~ 71(0.1%) 28A supplied by LNT Hewlett—Packard at RT and are shown in Co. fig. 2. excitation spectra ofofCaF2: Pr Fig.The 3 shows the recording concentration dependent spectra at LNT.
I I~Ill
4200
4000
3~00 3600 WAVE LEI’~GTH
Fig. 3. Concentration dependent bands of CaF
3400
3200
3~:(a) 0.05%; (b) 0.5%; (c) 1.0%. 2 : Pr
3~in CaF
V.P. Bhola / 4f—5d transitions of Pr
118
2
1.0 Hg
Hg
Hg
~0
19
16
14
Hg
05
0.0
I
I
I
4550
4650 4750 4850 4950 W&VELENGT 3~at LNT from 4550H A to 4950 A ex~itedwith 3660 A. Fig. 4. Emission spectrum of CaF2 : Pr
The emission spectrum of CaF 3~at LNT was also photographed and the 2 inPr microdensitometer traces are shown fig. 4.
3. Discussion of results When the Pr3~ion in the matrix of CaF 2 is irradiated with monochromatic radiation, the ion undergoes a transition from the ground state Eg to an excited state Ee. Subsequently, the ion will lose a part of its energy in a radiationless process or by cascade process, thus coming to an intermediate state E1. From intermediate state E~,it will decay to the ground state Eg, thus emitting electromagnetic radiations, which can be measured experimentally. By changing the wavelength of exciting light, one gets an excitation spectrum. It3~ has earlier ionbeen in thereported matrix of CaF [5], that there exist cubic and tetragonal sites around Pr 2, which means that the crystal field will be stronger due to the cubic field and weaker due to the tetragonal. Therefore, the effect on the low lying levels of the 4fSd configuration mixed configuration 4f5d isfield very[7—9].In close to the 1Swill be2a and alsoone it is[6]. veryThe sensitive to the crystal the enercubic gy level 0 of 4flevel of the 4f5d configuration will lie below the 1S field, the lowest 2. 0 level of the 4f The position of is 0 is reported to be 47.2 X ~ cm~[10,11], while the lowest excited level of 5d is situated at 45.6 X l0~cm~[7,12]. On other hand,1Sunder C4v symmetry the splitting of 1S 5d is small and its lowest level will lie above 0 [6]. For both sites the position of 0 will be the same, due to the screening character of 4f electron [13].
3 ~in CaF
V.P. Bhola / 4f—5d transitions of Pr
2
119
In the present experiment, fluorescence is observed from 2000 A to 42001), A and the are 2740 seen at 3950 A(25316 cm~),3660 A(27322 cm— the 3380important A (29585peaks cm—1), A (36500 cm—1), and 2620 A (38168 cm~).For 4f2 configuration, there is no energy level of Pr3+ in this region. The fluorescence observed in our experiment is ascribed to the 4f—5d transition. The large number of these bands is due to the presence of different sites around Pr3+ [5]. The strongest band has been observed at 3660 A (27322 cm 1). The presence of this band is further confirmed from the emission spectrum of CaF 3+ (see fig. 4). Lines due to mixed 2: Pr sites are also present in it. In fig. 4 peak numbers are the same as in table 1 in ref. [5]. In aqueous solution Mukherjee [1] has observed it at 3570 A (2800 cm—1). The band at 2620 A is also attributed to 4f—5d transition. From emission spectrum its position has been found to be 2550 A in case of LiYF 3~[141. The difference in the po4 : 3000 Pr and 4500 A, Weber [15]has sitions is due to different host lattices. Between also observed the fluorescence of Pr3+ due to Sd bands. Fig. 3 shows the concentration dependent bands of CaF 3~at LNT. It is clear 2 Pr that as we increase the concentration of Pr3+ in CaF 2, the intensity of the bands goes on decreasing. The effect of concentration on intensity of luminescence excitation bands at LNT can be described qualitatively by Dexter—Schulman theory [3]. According to this theory, the average distance (F) between the ions is inversely proportional to the cube root of the concentration. The intensity also depends on F where n is the multiplicity of interaction and takes the values 6, 8, and 10 for dipole—dipole, dipole-—quadrupole and quadrupole—quadrupole interactions, respectively. As we in3+ in CaF crease the concentration of Pr 2, the distance between Pr—Pr ions is decreased and the intensity will also decrease. 4. Conclusion 4f—5d transitions at 3950, 3660, 3380, 2740 A are havefound been to observed from 3+.and The2620 bands be concenluminescence excitation spectrum of CaF2 : Pr tration dependent. Acknowledgement The author is thankful to Professor Ferd Williams, Chairman, Department of Physics, University of Delaware, Delaware, for his invalued inspiration. Thanks are also due to the referee for pointing out some mistakes in the original manuscript. References [1] P.C. Mukerjee, Z. Physik 109 (1938) 573. [21 M. Schlesinger and P.W. Whippey, Phys. Rev. 171 (1968) 361.
120
3~in CaF
V.P. Bhola /4f—5d transitions of Pr
2
[3] D.L. Dexter and J.H. Schulman, J. Chem. Phys. 22 (1954) 1063. [41 N.S. Poluektov and S.A. Gava, Opt. Spectry. 31(1971)45. [5] V.P. Bhola, J. Luminescence 10 (1975) 185. [6] J.L. Sommerdijk et al., J. Luminescence 9 (1974) 288. [7] E. Loh, Phys. Rev. 147 (1966) 332. [81 J. Sugar, J. Opt. Soc. Am. 55 (1965) 1058. [9] W.T. Carnall et al., J. Chem. Phys. 49 (1968) 4424. [10] M. Schlesinger and T. Szczurek, Phys. Rev. 8 (1973) 2367. [111 E. Loh, Phys. Rev. 140 (1965) A1463. [12] M.H. Crozier, Bull. Am. Phys. Soc. 9 (1964) 631. [1 3] G.H. Dieke, Spectra and energy levels of rare-earth ions in crystals (Interscience, New York, 1968). [14] W.W. Piper et al., J. Luminescence 8 (1974) 344. [15] M.J. Weber, Solid State Commun. 12 (1973) 741.