- Email: [email protected]
Luminescence properties of Tb3+-doped strontium pyromellitate
Spectrochimica Acta Part A 55 (1999) 1193 – 1196
Luminescence properties of Tb3 + -doped strontium pyromellitate Liangjie Yuan, Jutang Sun *, Keli Zhang College of Chemistry, Wuhan Uni6ersity, Wuhan 430072, People’s Republic of China Received 27 April 1998; received in revised form 29 August 1998; accepted 29 August 1998
Abstract Tb3 + -doped strontium pyromellitate was prepared and its powder X-ray diffraction, infrared, emission, and excitation spectra were investigated. The energy transfer mechanism from C6H2(CO2)44 − to Tb3 + ion and the relationship between luminescence intensity and structure are discussed. The crystal structure of Tb3 + -doped strontium pyromellitate is monoclinic with C 12 − P2 (ITC No. 3) space group; the calculated lattice parameters are a =1.1476 90.0001, b=1.773790.0001, c=0.667690.0001 nm, b=95.2090.02°, Z= 4, V =1.35339 0.0004 nm3, and dcalc =2.087890.0006 g cm − 3. The energy of the p, p* and n, p* excited states of C6H2(CO2)44 − can be efficiently transferred to Tb3 + ion, which results in very strong emissions from the 5D4 7Fj transitions of Tb3 + . © 1999 Elsevier Science B.V. All rights reserved. Keywords: Strontium; Pyromellitate; Terbium; Luminescence
1. Introduction Many aromatic carboxylate complexes of Tb3 + and Eu3 + possess good luminescence properties [1–3]. The Tb3 + and Eu3 + -doped lanthanum, zinc and strontium phthalates could effectively transfer ultraviolet light to the Tb3 + and Eu3 + ions, which results in the characteristic emission from Tb3 + and Eu3 + ions. These lanthanide doped phthalates, which have been used as luminescent materials and transducer materials in X-ray microphotographic cameras, have better luminescence properties than pure lanthanide com* Corresponding author. Tel.: + 86-27-87682455; Fax: + 86-27-87647617.
pounds [4–6]. In order to gain further information about luminescence of rare earth ions in aromatic carboxylates, Tb3 + -doped strontium pyromellitate (Sr2PMT:Tb) was prepared by the solution reaction method. The crystal structure, infrared spectrum, and luminescence properties were investigated. The energy transfer mechanism and relationship between luminescence intensity and structure are discussed.
2. Experimental Terbium oxide and strontium carbonate were converted to their chloride solutions by treatment with hydrochloric acid, respectively. The Na4PMT
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 2 8 1 - 9
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
1194
Table 1 The powder X-ray diffraction data from Sr2PMT: Tb0.01 dobs(nm)
I/I1
dcalc(nm)
h
k
l
dobs (nm)
I/I1
dcalc (nm)
h
k
l
0.8864 0.7003 0.5913 0.5263 0.4674 0.4442 0.3810 0.3611 0.3505 0.3433 0.3223 0.3010 0.2952 – 0.2890 0.2740 0.2622 0.2510 0.2407 – 0.2339 –
100 4 2 B1 7 4 3 5 15 3 5 2 3 – 13 10 8 6 2 – 13 –
0.8868 0.7006 0.5912 0.5251 0.4694 0.4434 0.3810 0.3602 0.3503 0.3443 0.3218 0.3014 0.2956 0.2952 0.2890 0.2734 0.2625 0.2506 0.2408 0.2405 0.2342 0.2336
0 1 0 1 1 0 3 2 2 3 1 2 0 2 3 2 3 2 2 1 1 4
2 2 3 3 2 4 0 3 4 0 1 5 6 1 4 1 0 3 6 5 5 3
0 0 0 0 1 0 0 1( 0 1( 2( 0 0 2( 0 2 2( 2 1 2( 2 1
0.2240 0.2067 – 0.2015 0.1915 – 0.1866 0.1832 0.1798 – 0.1752 0.1717 0.1687 0.1637 – – 0.1601 – 0.1582 0.1561 – –
2 5 – 1 3 – 2 1 1 – 2 5 3 1 – – 1 – 1 B1 – –
0.2243 0.2067 0.2067 0.2015 0.1916 0.1913 0.1866 0.1832 0.1799 0.1799 0.1752 0.1717 0.1688 0.1638 0.1636 0.1636 0.1601 0.1600 0.1584 0.1652 0.1560 –
3 2 4 0 3 4 6 1 5 5 4 4 1 1 3 2 6 0 6 5 1 –
6 8 5 7 8 5 1 5 4 1 8 8 9 2 5 0 6 3 6 4 4 –
1( 0 1 2 0 2( 1( 3 2( 2 0 1( 2( 4( 3 4( 0 4 1( 3( 4( –
solutions were prepared according to [7]. The Sr2PMT:0.01Tb was prepared by adding the Na4C10H2O8 solutions to mixed solutions of 2:0.01 molar amounts of SrCl2 and TbCl3. The product was filtered, washed, dried at 573 K, and identified by elemental and thermal analysis. The X-ray diffraction pattern was obtained with a Rigaku D/MAX-RA model X-ray diffractometer with a graphite monochromator and ˚ ). The infrared Cu Ka1 radiation (l= 1.5405 A spectrum of the sample in a KBr pellet was recorded with a Nicolet 170SX FT-IR spectrometer. The excitation and emission spectra were examined with a Shimadzu RF-5000 spectrofluorophotometer. The ultraviolet absorption spectrum was measured with a Shimadzu UV240 UV-visible recording spectrophotometer. All spectra have been recorded at ambient temperature.
3. Results and discussion The powder X-ray diffraction data from Sr2PMT Tb0.01 are listed in Table 1. The crystal structure is monoclinic. The unit cell parameters were calculated to be a= 1.14769 0.0001, b= 1.77379 0.0001, c= 0.667690.0001 nm, b= 95.2090.02°, Z= 4, V=1.35339 0.0004 nm3, and dcalc = 2 0878 9 0.0006 (dobs = 2.08) g cm − 3. The reflection conditions are consistent with C12 − P2 (ITC No. 3) space group. The strongest diffraction peak stems from the (020) plane, whose intensity is approximately seven times that of the second strongest peak. It was suggested that the Sr2PMT Tb0.01 crystal exhibits a layered structure, with the metal ions located in the (020) plane and the PMT groups situated between two metal ion planes.
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
1195
Fig. 1. Coordination of PMT to Sr2 + ion in Sr2PMT.
The IR spectrum shows a very strong absorption band at 1383 cm − 1, which can be assigned to the symmetric stretching vibration of the OCO group, whereas the broad and very strong bands at 1605 and 1536 cm − 1 are probably due to asymmetric vibrations of the OCO group. It is revealed that there are symmetrical, nonsymmetrical bridging bidentate, and chelating coordination between carboxylate groups and the metal ions in the Sr2PMT. The PMT could bond with Sr2 + ion through carboxylate groups to form a stable seven-membered ring [8 – 10] and metal– oxygen chain structures as shown in Fig. 1. The Sr2PMT: Tb has a very strong green emission under ultraviolet radiation excitation (lEx = 306 nm). The excitation and emission spectra of Sr2PMT: Tb0.01 are shown in Fig. 2. The 5D4 7 Fj ( j= 6, 5, 4, 3) transitions emission from the Tb3 + ion is observed at 488, 544, 582 and 618
Fig. 2. Excitation (a, lEm = 544 nm) and emission (b, lEx = 306 nm) spectra of Sr2PMT: Tb0.01.
Fig. 3. Excitation (a, lEm =437 nm; c, lEm =543 nm) and emission (b, lEx =310 nm; d, lEx =306 nm) spectra of Sr2PMT (a, b) and Sr2PMT: Tb0.01 (c, d) in aqueous solution.
nm, and the emission from the 5D4 7F5 transitions is the strongest (curve a). The excitation band of the Tb3 + ion emission is broad at 306 nm (curve b), and corresponds to the electronic transition at 292 nm in the ultraviolet absorption spectrum of an aqueous solution of pyromellitate. The excitation and emission spectra of Sr2PMT: Tb0.01, and Sr2PMT in aqueous solutions are shown in Fig. 3. For Sr2PMT, the broad bands at 310 and 437 nm (curves a and b) are the excitation and emissions of PMT, respectively. For Sr2PMT: Tb0.01, the strong characteristic emission from the Tb3 + ion is observed (curve d), and the excitation bands at 306 and 280 nm (curve c) are assigned to the S1p, p* and S1n, p* transitions in PMT, respectively. Because the emission from PMT at 437 nm (curve d) is extremely quenched, it appears that the energy of the S1p, p* and S1n, p* states can be effectively transferred to the Tb3 + ion. Additionally, in the case of either solid or aqueous solution, the 5 D3 7Fj emission from the Tb3 + ion is not observed. From the above results, it can be seen that the S1p, p* (35.7×103 cm − 1) and S1n, p* (32.7× 103 cm − 1) energy levels of PMT overlap with the 5 Fj (5F3 36.6× 103, 5F4 35.4× 103 cm − 1) and 5Hj 5 ( H6 33.0× 103, 5H7 31.6× 103 cm − 1) levels of Tb3 + ion [11]. Furthermore, the T1p, p* and T1n, p* (22.9× 103 cm − 1) levels in PMT lie in between the 5D3 (26.3× 103 cm − 1) and 5D4
1196
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
(20.5× 103 cm − 1) levels of the Tb3 + ion. Thus, the energy transfer and luminescence mechanism in the Sr2PMT:Tb is proposed as follows: Under ultraviolet radiation, ligands are excited to the singlet excited state from the ground state. The energy of the S1p, p* and S1n, p* excited states of PMT decays to the T1n, p* and T1p, p* levels in PMT and then relaxes directly to the 5D4 level in the Tb3 + ion through intramolecular transfer of energy to produce the 5D4 7Fj transition emissions from Tb3 + ion. This transfer process is in agreement with those illustrated in the literature on lanthanide organic complexes [12– 14]. In this compound with a layered structure, the energy of the S1p, p* and S1n, p* excited states can be effectively transferred very far through the metal – oxygen bonding chains (OCOMOCO). Therefore, the luminescence efficiency of Sr2PMT: Tb phosphor is very high. In Sr2PMT: Tb when the Tb3 + concentration is 2 mol%, the relative emission intensity is the highest and is twice as strong as that of pure terbium pyromellitate.
Acknowledgements This study was supported by the National Natural Science Foundation of the People’s Republic
.
of China. The referee is thanked for helpful comments and suggestion.
References [1] V.F. Zolin, N.A. Kazanskaya, A.V. Moshinskaya, Yu.I. Kheruze, V.I. Tsaryuk, Opt. Spektrosk. 33 (1972) 929. [2] H.G. Brittain, J. Inorg. Nucl. Chem. 41 (1979) 567. [3] V.E. Karasev, N.V. Petrochenkova, L.I. Lifar, Koord. Khim. 16 (1990) 1003. [4] J.T. Sun, X.Z. Du, X.Q. He, Q.Y. Wang, Proceedings of the Second International Symposium on RE Spectroscopy, Su Qiang, World Scientific, 1989, p. 227. [5] J.T. Sun, C. Chen, Z.B. Qin, J. Lumin. 40/41 (1988) 246. [6] J.T. Sun, Chin. J. Lumin. 15 (1994) 242. [7] D.C. Luehrs, B.C. Cornilsen, C.B. Gloverand, T.L. Neils, Inorg. Chim. Acta 145 (1988) 81. [8] J.T. Sun, Z.H. Peng, X.Z. Du, Q.Y. Wang, Acta Chim. Sin. 49 (1991) 1094. [9] A.V.R. Warrier, P.S. Narayanan, Spectrochim. Acta 23A (1967) 1061. [10] G.B. Deacon, F. Huber, R. Phillips, J. Inorg. Chim. Acta 104 (1985) 41. [11] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4447. [12] F. Halverson, J.S. Brinen, J.R. Leto, J. Chem. Phys. 40 (1964) 2790. [13] M.L. Bhaumik, M.A. El-Sayed, J. Chem. Phys. 42 (1965) 787. [14] L.J. Nugent, J.L. Burnett, R.D. Baybarz, G.K. Werner, S.P. Tanner, J.R. Tarrant, O.L. Keller, J. Phys. Chem. 73 (1969) 1540.
Luminescence properties of Tb3 + -doped strontium pyromellitate Liangjie Yuan, Jutang Sun *, Keli Zhang College of Chemistry, Wuhan Uni6ersity, Wuhan 430072, People’s Republic of China Received 27 April 1998; received in revised form 29 August 1998; accepted 29 August 1998
Abstract Tb3 + -doped strontium pyromellitate was prepared and its powder X-ray diffraction, infrared, emission, and excitation spectra were investigated. The energy transfer mechanism from C6H2(CO2)44 − to Tb3 + ion and the relationship between luminescence intensity and structure are discussed. The crystal structure of Tb3 + -doped strontium pyromellitate is monoclinic with C 12 − P2 (ITC No. 3) space group; the calculated lattice parameters are a =1.1476 90.0001, b=1.773790.0001, c=0.667690.0001 nm, b=95.2090.02°, Z= 4, V =1.35339 0.0004 nm3, and dcalc =2.087890.0006 g cm − 3. The energy of the p, p* and n, p* excited states of C6H2(CO2)44 − can be efficiently transferred to Tb3 + ion, which results in very strong emissions from the 5D4 7Fj transitions of Tb3 + . © 1999 Elsevier Science B.V. All rights reserved. Keywords: Strontium; Pyromellitate; Terbium; Luminescence
1. Introduction Many aromatic carboxylate complexes of Tb3 + and Eu3 + possess good luminescence properties [1–3]. The Tb3 + and Eu3 + -doped lanthanum, zinc and strontium phthalates could effectively transfer ultraviolet light to the Tb3 + and Eu3 + ions, which results in the characteristic emission from Tb3 + and Eu3 + ions. These lanthanide doped phthalates, which have been used as luminescent materials and transducer materials in X-ray microphotographic cameras, have better luminescence properties than pure lanthanide com* Corresponding author. Tel.: + 86-27-87682455; Fax: + 86-27-87647617.
pounds [4–6]. In order to gain further information about luminescence of rare earth ions in aromatic carboxylates, Tb3 + -doped strontium pyromellitate (Sr2PMT:Tb) was prepared by the solution reaction method. The crystal structure, infrared spectrum, and luminescence properties were investigated. The energy transfer mechanism and relationship between luminescence intensity and structure are discussed.
2. Experimental Terbium oxide and strontium carbonate were converted to their chloride solutions by treatment with hydrochloric acid, respectively. The Na4PMT
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 2 8 1 - 9
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
1194
Table 1 The powder X-ray diffraction data from Sr2PMT: Tb0.01 dobs(nm)
I/I1
dcalc(nm)
h
k
l
dobs (nm)
I/I1
dcalc (nm)
h
k
l
0.8864 0.7003 0.5913 0.5263 0.4674 0.4442 0.3810 0.3611 0.3505 0.3433 0.3223 0.3010 0.2952 – 0.2890 0.2740 0.2622 0.2510 0.2407 – 0.2339 –
100 4 2 B1 7 4 3 5 15 3 5 2 3 – 13 10 8 6 2 – 13 –
0.8868 0.7006 0.5912 0.5251 0.4694 0.4434 0.3810 0.3602 0.3503 0.3443 0.3218 0.3014 0.2956 0.2952 0.2890 0.2734 0.2625 0.2506 0.2408 0.2405 0.2342 0.2336
0 1 0 1 1 0 3 2 2 3 1 2 0 2 3 2 3 2 2 1 1 4
2 2 3 3 2 4 0 3 4 0 1 5 6 1 4 1 0 3 6 5 5 3
0 0 0 0 1 0 0 1( 0 1( 2( 0 0 2( 0 2 2( 2 1 2( 2 1
0.2240 0.2067 – 0.2015 0.1915 – 0.1866 0.1832 0.1798 – 0.1752 0.1717 0.1687 0.1637 – – 0.1601 – 0.1582 0.1561 – –
2 5 – 1 3 – 2 1 1 – 2 5 3 1 – – 1 – 1 B1 – –
0.2243 0.2067 0.2067 0.2015 0.1916 0.1913 0.1866 0.1832 0.1799 0.1799 0.1752 0.1717 0.1688 0.1638 0.1636 0.1636 0.1601 0.1600 0.1584 0.1652 0.1560 –
3 2 4 0 3 4 6 1 5 5 4 4 1 1 3 2 6 0 6 5 1 –
6 8 5 7 8 5 1 5 4 1 8 8 9 2 5 0 6 3 6 4 4 –
1( 0 1 2 0 2( 1( 3 2( 2 0 1( 2( 4( 3 4( 0 4 1( 3( 4( –
solutions were prepared according to [7]. The Sr2PMT:0.01Tb was prepared by adding the Na4C10H2O8 solutions to mixed solutions of 2:0.01 molar amounts of SrCl2 and TbCl3. The product was filtered, washed, dried at 573 K, and identified by elemental and thermal analysis. The X-ray diffraction pattern was obtained with a Rigaku D/MAX-RA model X-ray diffractometer with a graphite monochromator and ˚ ). The infrared Cu Ka1 radiation (l= 1.5405 A spectrum of the sample in a KBr pellet was recorded with a Nicolet 170SX FT-IR spectrometer. The excitation and emission spectra were examined with a Shimadzu RF-5000 spectrofluorophotometer. The ultraviolet absorption spectrum was measured with a Shimadzu UV240 UV-visible recording spectrophotometer. All spectra have been recorded at ambient temperature.
3. Results and discussion The powder X-ray diffraction data from Sr2PMT Tb0.01 are listed in Table 1. The crystal structure is monoclinic. The unit cell parameters were calculated to be a= 1.14769 0.0001, b= 1.77379 0.0001, c= 0.667690.0001 nm, b= 95.2090.02°, Z= 4, V=1.35339 0.0004 nm3, and dcalc = 2 0878 9 0.0006 (dobs = 2.08) g cm − 3. The reflection conditions are consistent with C12 − P2 (ITC No. 3) space group. The strongest diffraction peak stems from the (020) plane, whose intensity is approximately seven times that of the second strongest peak. It was suggested that the Sr2PMT Tb0.01 crystal exhibits a layered structure, with the metal ions located in the (020) plane and the PMT groups situated between two metal ion planes.
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
1195
Fig. 1. Coordination of PMT to Sr2 + ion in Sr2PMT.
The IR spectrum shows a very strong absorption band at 1383 cm − 1, which can be assigned to the symmetric stretching vibration of the OCO group, whereas the broad and very strong bands at 1605 and 1536 cm − 1 are probably due to asymmetric vibrations of the OCO group. It is revealed that there are symmetrical, nonsymmetrical bridging bidentate, and chelating coordination between carboxylate groups and the metal ions in the Sr2PMT. The PMT could bond with Sr2 + ion through carboxylate groups to form a stable seven-membered ring [8 – 10] and metal– oxygen chain structures as shown in Fig. 1. The Sr2PMT: Tb has a very strong green emission under ultraviolet radiation excitation (lEx = 306 nm). The excitation and emission spectra of Sr2PMT: Tb0.01 are shown in Fig. 2. The 5D4 7 Fj ( j= 6, 5, 4, 3) transitions emission from the Tb3 + ion is observed at 488, 544, 582 and 618
Fig. 2. Excitation (a, lEm = 544 nm) and emission (b, lEx = 306 nm) spectra of Sr2PMT: Tb0.01.
Fig. 3. Excitation (a, lEm =437 nm; c, lEm =543 nm) and emission (b, lEx =310 nm; d, lEx =306 nm) spectra of Sr2PMT (a, b) and Sr2PMT: Tb0.01 (c, d) in aqueous solution.
nm, and the emission from the 5D4 7F5 transitions is the strongest (curve a). The excitation band of the Tb3 + ion emission is broad at 306 nm (curve b), and corresponds to the electronic transition at 292 nm in the ultraviolet absorption spectrum of an aqueous solution of pyromellitate. The excitation and emission spectra of Sr2PMT: Tb0.01, and Sr2PMT in aqueous solutions are shown in Fig. 3. For Sr2PMT, the broad bands at 310 and 437 nm (curves a and b) are the excitation and emissions of PMT, respectively. For Sr2PMT: Tb0.01, the strong characteristic emission from the Tb3 + ion is observed (curve d), and the excitation bands at 306 and 280 nm (curve c) are assigned to the S1p, p* and S1n, p* transitions in PMT, respectively. Because the emission from PMT at 437 nm (curve d) is extremely quenched, it appears that the energy of the S1p, p* and S1n, p* states can be effectively transferred to the Tb3 + ion. Additionally, in the case of either solid or aqueous solution, the 5 D3 7Fj emission from the Tb3 + ion is not observed. From the above results, it can be seen that the S1p, p* (35.7×103 cm − 1) and S1n, p* (32.7× 103 cm − 1) energy levels of PMT overlap with the 5 Fj (5F3 36.6× 103, 5F4 35.4× 103 cm − 1) and 5Hj 5 ( H6 33.0× 103, 5H7 31.6× 103 cm − 1) levels of Tb3 + ion [11]. Furthermore, the T1p, p* and T1n, p* (22.9× 103 cm − 1) levels in PMT lie in between the 5D3 (26.3× 103 cm − 1) and 5D4
1196
L. Yuan et al. / Spectrochimica Acta Part A 55 (1999) 1193–1196
(20.5× 103 cm − 1) levels of the Tb3 + ion. Thus, the energy transfer and luminescence mechanism in the Sr2PMT:Tb is proposed as follows: Under ultraviolet radiation, ligands are excited to the singlet excited state from the ground state. The energy of the S1p, p* and S1n, p* excited states of PMT decays to the T1n, p* and T1p, p* levels in PMT and then relaxes directly to the 5D4 level in the Tb3 + ion through intramolecular transfer of energy to produce the 5D4 7Fj transition emissions from Tb3 + ion. This transfer process is in agreement with those illustrated in the literature on lanthanide organic complexes [12– 14]. In this compound with a layered structure, the energy of the S1p, p* and S1n, p* excited states can be effectively transferred very far through the metal – oxygen bonding chains (OCOMOCO). Therefore, the luminescence efficiency of Sr2PMT: Tb phosphor is very high. In Sr2PMT: Tb when the Tb3 + concentration is 2 mol%, the relative emission intensity is the highest and is twice as strong as that of pure terbium pyromellitate.
Acknowledgements This study was supported by the National Natural Science Foundation of the People’s Republic
.
of China. The referee is thanked for helpful comments and suggestion.
References [1] V.F. Zolin, N.A. Kazanskaya, A.V. Moshinskaya, Yu.I. Kheruze, V.I. Tsaryuk, Opt. Spektrosk. 33 (1972) 929. [2] H.G. Brittain, J. Inorg. Nucl. Chem. 41 (1979) 567. [3] V.E. Karasev, N.V. Petrochenkova, L.I. Lifar, Koord. Khim. 16 (1990) 1003. [4] J.T. Sun, X.Z. Du, X.Q. He, Q.Y. Wang, Proceedings of the Second International Symposium on RE Spectroscopy, Su Qiang, World Scientific, 1989, p. 227. [5] J.T. Sun, C. Chen, Z.B. Qin, J. Lumin. 40/41 (1988) 246. [6] J.T. Sun, Chin. J. Lumin. 15 (1994) 242. [7] D.C. Luehrs, B.C. Cornilsen, C.B. Gloverand, T.L. Neils, Inorg. Chim. Acta 145 (1988) 81. [8] J.T. Sun, Z.H. Peng, X.Z. Du, Q.Y. Wang, Acta Chim. Sin. 49 (1991) 1094. [9] A.V.R. Warrier, P.S. Narayanan, Spectrochim. Acta 23A (1967) 1061. [10] G.B. Deacon, F. Huber, R. Phillips, J. Inorg. Chim. Acta 104 (1985) 41. [11] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4447. [12] F. Halverson, J.S. Brinen, J.R. Leto, J. Chem. Phys. 40 (1964) 2790. [13] M.L. Bhaumik, M.A. El-Sayed, J. Chem. Phys. 42 (1965) 787. [14] L.J. Nugent, J.L. Burnett, R.D. Baybarz, G.K. Werner, S.P. Tanner, J.R. Tarrant, O.L. Keller, J. Phys. Chem. 73 (1969) 1540.