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Intramolecular energy transfer in ytterbium organic chelates
lntramofecular energy tmnsfer in ytterbium orgmic cheIatest G. A, CLWSBY~and M. KASHA Department of Chemistry, Plarida State University, Tallaha8sae, Florids
energy transfer, between the lowest r, V* excited singlet states of rare-earth chelates and the elecltronic states of the rare-earth ion, was first observed by ~~~~~~~~~ [l] and investigated later by SEVCIEEENKO et al. [23. Chelates of trivalent europiumt samarium, and terbium were studied, using various ligands, such as dihe~~~ylm~thane and be~oy~a~eto~e, ~~~~~a~~~~ of the broad, intense near-~travio~e~ adsorption band of such chelates with 3850 A light yields a strong line-like l~rni~e~~e~~eemission, which is ~~a~~~~~~3~~~of &he rare-earth ion. The discrete luminescence lines bear a close resemblance to the ver,y weak sharp absorption bands observed for the trivalent ion in solution. These latter lines have been recognized generally as originating in the 4;f orbital8 of the lanthanide ion. Since there is considerable interest in the mechanism of electronic energy transfer, and the previous observations had indicated a rather high efficiency for the intramolecular process in the rare-earth chelates, wa hav;e undertaken a general investigation of this problem. In this paper we report the intramolecular energy transfer for the rather exee~tion~l ease of trivalent ytterbium chelates, which had eluded de&&on ~re~o~s~~ f1J For a des~~ption of the s~~~t~us~o~i~behavior of or~~~i~ chelates of the trivalent fanthanide rare-earth ions, it is convenient to divide them into three gmerai classes. Into CEass I we pfece the ions @a+++ and &u+++) with the closed &orbital configurations: 4f” and c4f1*,for which there are no electronic transitions in the visible and infrared regions, The ion Gd+++, with an f-orbital configuration 4f7, also falls into this class. If dibenzoylmethane is used as a lfgand in the chelate formation, the lowest excited singlet and triplet r, 7~*states of the chelates lie at INTRAMOLECULAR
G. A. CROSBY and M. KASITA
about 23500 and 20500 cm-l above the ground state [3], respectively. Consequently, in the tris-dibenzoylmethane chelates of the ions of Class I, the intramolecular energy transfer is energetically impossible (see Fig. 1). Therefore, the main effect of the rare-earth ions on the spectroscopic properties of the chelates of this class is to enhance the singlet-triplet mixing in the rr-electronic states of the chelate, as a spin-orbital, or magnetic, perturbing agent [3, 41. Into Class II we place the remaining rare-earth trivalent ions which include a 4f” to 4f’” configuration. In these cases, as indicated schematically in Fig. 1, energy transfer is feasible from the excited singlet or triplet 7~, rr* states of the chelate to the lower electronic 7 :*5
E
c 20 +? $ l5 z <5 &” 15 11 ClassI Fig. 1. Classification of rare-earth chelates. Electronic levels of the rare-earth ion on right of diagrams.
states of the chelated ion. However, as will be described later in another place [5], the efficiency of the energy transfer apparently depends on the electronic configuration of the rare-earth ion, efficient transfer correlating with the proximity of the Into Class III we place the exceptional f-orbital configuration to 4f”, 4f’ or 4f’“. lanthanide ions with 4f1 and 4fx3 f-orbital configurations, namely Ce+++ and Yb+++. There is a single electronic transition which can take place within the f-orbital in these cases (see Discussion), and is observed in the infrared region. The corresponding energy diagram for Class III chelates (Yb+++ case) is given in Fig. 1. Particular interest is attached to the efficiency of the energy transfer in this unusual situation. Moreover, the uniqueness of the rare-earth level in this case simplifies the energy transfer analysis. Although as we shall see, the Ce+++ chelate case was unamenable for study, the tris-dibenzoylmethane-Yb+++ chelate exhibited a remarkably efficient intramolecular energy transfer.
Experimental The dibenzoylmethane used was Eastman, White Label grade, and was twice sublimed in a high vacuum. Piperidine used as a precipitating agent was a Matheson, Coleman, and Bell product, and was purified by distillations to give a colorless liquid. The YbsO, was* first converted to the chloride, then the chloride dissolved in alcohol and added to an equimolar solution of dibenzoylmethane The precipitate was washed twice with absolute ethanol and piperidine in alcohol. and dried in a vacuum desiccator. The samples were dissolved in absolute ethanol For the low temperatures (77’K) for the absorption curve determinations. * Obtained from the Institute for Atomic Research, Iowa State College, Ames, Iowa.
378
I~ine~~eH~ ~t~~~~ a 1 : f : 1 frolnme ~~~) mixtureof ethyl ether, ~~o~nta~e~ abf&&e ethanol was used 88 8 so1vent. The dilute ~OIutjo~ of the chefate
were frozen in a liquid-~trogen bath to form transparent rigid glass soIutio~* The luminescence spectra were recorded on Eaatman I-M spectroscopio plates, hypersensitized in ammonia solution. The spectrograph used was a 3-p&m Steinheil Universal GH, equipped with (approx. f3.5) glass optics. A 1 kW X00 atmosphere AH-B mercury arc was used for excitation, filtered by a C&Q, solution (5’cm, 100 g/l. of the pentahydrate) and a Corning Glass 5840 standard filter, yielding mainly 3650 A light. The absorption spectra were determined on a Beckman DB-1 recording apectrophotometer.
Rare-earth ions having f’ and $1” inner orbital oonfigurations may have a single intra-f-orbital tranaition, corresponding to the unique recoupling of L and S angular momenta (in the language of Russell-Saundera coupling). This leads to a single relatively sharp and characteristically weak line-like absorption in the spectrum of the aqueous ion. The fact that the two levels involved in the transition are doublet components, split by spin-orbital interaction, results in the ocourrenee of these lines relatively far in the infrared. ~obably it is for this latter reason that in the literature [6, 7, 81 it is generally not recognized that the Ce+f+ and Yb+++ ions have a sharp line absorption in addition to their relatively well~k~o~ nearly ~ontiuuo~~ ultra~olet absorption. ~~~rbiurn as a trivalent positive ion has a 2P71a ground state, and a eF,la upper state for the intra-multiplet transition. The energy of this transition for the gaseous ion, which could be deduced from appropriate atomic term values, is unknown. In aqueous solution the corresponding sharp absorption band for Yb+++ was first observed by FREYMAN and TAKV~RIAN [9] at 10270cm-l (9735 A), and corroborated by G~BRECHT [IO, 111, who assigned it as the SF,,% --w SF&,* transition. The absorption band has been observed subaequently by other investigators 1:12, 131 and its intensity measured by HWWCXA~EX fl43. The m&r absorption coeflioient defined by Q = fl/ccQ log,, ~~~~~~ with c in mole@. a;nd bi in cm, is 2-08 st the band maximum (see Fig. 2)” The ~o~~n~ng ~~~~ator 379
a. A. CBOSBY
and
M. HA~HA
strength defined by f = 4.32 x 1O-8 fe 4% is 3‘86 x IO-* [15]. The weakness and characteristic sharpness of this transition makes GOBRECHT'S assignment secure. Although the corresponding luminescence emission has not been reported for solutions or crystals of ytterbium salts, BI~AUEBand BRAVER [XS] have observed a lins emission at about 9700 A for trivalent ytterbium oxide as an impurity in strontium and aluminium oxide phosphors, excited by 20 kV cathode rays. For the intramolecular energy transfer study the ahelate used was tris-dibenzoylmethane-Yb+++ (see Fig, 3). The lowest n, 7~* allowed absorption band (see Fig. 2) of the chelate was excited by 3650 A light with the sample in dilute rigid glass solution at 77OK (see Experimental). An intense luminescence emission
with &,, at approximately 9710 Ifi PO 6 was observed {see Fig. 2) for the Yb+f+ chelate, which must be the W,,2 -+ 2F7/2emission for the chelated ion. The low dispersion of our spectrograph in this region prevented aur obtaining a greater precision. The observed luminesoence is remarkable for its absence of subsidiary structure and for its sharpness. The quantum yield of the chelate luminescence was not determined absolutely, but a rough estimate from slit-width and exposure times and comparison with other efficient luminescence oases in this region leads to a probable value of quantum yield within the range 0*2-l.In other words, the intramol~ular energy transfer is certainly e&ient in this case. The only other emission from the Yb+++ chelate was a feeble blue fluoresoence and a fong-lived green lu~nes~en~e~ both of which are due probably to an organic impurity in the sample. The observed efiiciency is all the more remarkable because the radiationless transition leading to the Yb+++ luminescence in the chelate requires an energy loss of 17,000 cm-r, which must be dissipated thermally. The differences in energy between the Yb+++ excited 2E’s12level and the zeropoint V, r* levels are of course somewhat less than the value quoted above, based If the energy transfer is from the 7r97~* singlet level of on the exciting frequency. the ohelate, an apparent energy difference of about 13,500 cm-l would be involved; for the P, 7~* triplet of the chelate, the corresponding difference would be 10,500 An efBcient energy transfer between electronic states which are this far cm-l. 380
Intramolecular energytramferin ytterbiumorganicchelates
apart would not seem to be plausible on a qu&ntum mechanical basis, since levels so widely separated do not interact very strongly. Consequently, it seems necessary to assume that the mechanism of energy transfer involves interaction between (a) high vibrational levels of the chelate coupled with the 2Fs,z excited state of the ytterbium ion and (b) the pure ?r, n * electronic states of the chelate. The requirement of vibrational interaction, for the energy transfer mechanism, is not ruled out by the absence of vibrational fine structure in the luminescence emission of the Yb+++ chelate. Optical transitions of the type 2.F5,2 ---t 2F,,2 in the chelate undoubtedly are accompanied by insignificant molecular distortion, and as a consequence of the Franck-Condon principle, no vibrational fine structure should appear. , It is of interest to point out that the lowest m, 7~* absorption band of dibenzoylmethane is significantly altered upon formation of the Yb+++ chelate (see Fig. 2). A shift of about 400 cm-l in the band maximum is observed, accompanied by a considerable diminution in intensity (the absorption coefficient for the chelate being calculated per mole of dibenzoylmethane). It is probable that the 4f orbitals of Yb+++ are not directly involved in the chelate binding (since the chelate emission is so little displaced from the frequency of the analogous transition in the ion in solution). Nevertheless the possibility of strong vibrational coupling is suggested by the effect of chelation on the 7~,r* absorption noted above. The intra-multiplet transition should be observable in other rare earth ions, and the observations of BRAUER and BRAUER [16] give three examples in addition to the trivalent ytterbium. The owe of Ce+++ is an interesting one to consider, even though direct observations are lacking for it. The ground state of Ce+++ is 2F 5,2, and for the gaseous ion the 2Fs,2 - 2F,,2 separation has been deduced by LANQ [l?] to be 2253-O cm-r; this assignment has been confirmed by a Zeeman pattern analysis [18]. Unfortunately, the study of electronic transitions near 5 p is difficult, and we have not yet succeeded in observing this transition either in absorption (ion) or in emission (chelate). In the case of the a&&de rare earths, with incompletely filled 5f orbit&, the hexavalent neptunium ion and the normally unstable pentavalent uranium ion should exhibit s, unique intra-multiplet transition [ 193. A~k~o~~~dge~~nt~- The authors are indebted to Prof. ROBEIST E. CONNICK of the University of California, and Dr. C. J. RODDEN, now of the New Brunswick AEC Laboratory, for valuable communications on the spectra of rare earth ions. It is a pleasure to acknowledge the co-operation of Prof. F. H. SPEDDING in helping ua procure the ytterbium oxide sample.
References WEISSMAN S. I. J. Chern. Phys. 1942 10 214. [2] SEVCEENHO A. N. and TROFIMOV A. K. J. Ezp. Theor. Phyrr. 1951 21 220; SEVCHENKO A. N. and MORACHEVSEY A. G. Izv. Akad. Nauk SSSR (ser. j&z.) 1951 15 628. [3] YUSTER P. and WEISSMAN S. I. J. Chem. Phys. 1949 17 1182. [4] %SHA M. and MCGLYNN S. P. AWL Rev. Phye. Chem. 1956 7 403. [Fi]t?ROSBY G. A. To be published. [6] VAN VLECKJ. H. J. Phys. Chem. 1937 41 67. [l]
381
G. A. CROSBY and M.
KASHA
177YOST D. AI., RUSSELL H., JR. and GARNER C. S. The Rare Earth ~~~~8 an& Their C~~~~8. John Wiley, New York 1947. [S] E0JzMAXN W. Q~~~u~ ~~~~~~y p. 671. Academic Press, New York 1957. [9] FREYMANR. and TAKVORIANS. G. R. Acad. Sci., Pa& 1932194963. [IO]GOBRECRTH. 2. Cea. Nature. 19378 351. [ll] GOBRECHTH. Ann. Physirc 1938 31 600 755. [12] ROSENTHALG. Phyeik. 2. 1939 40 508. [13] RODDEN C. J. J. Res. Nat. BUY. Stand. Wmh. 1942 28 265. [14] HOOCXZHAUEN J. Phytica 1946 11 513. [15] HOO*SCHAUENJ. and GORTERC. J. Phyeica 194814 197. [16] BRAVI~IR E. and BRAUER P. Naturwissen&qftichaften 194784 120. [17] LANU R. J. Canud. J. Res. 193518A 1; 193614A 127; Phys. Rev. 1936 49 552. [18] VAX DE VLIET H. J. Dissertation, Univ. Amsterdam, 1939; Chew. A&&-. 1939 38 2033. [lQ] CONNICKR. E., KASEA M., MCVEY W. H. and SHELINEG. E. The TTa~~ra~~u~ E~~~t~ (Edited by SEABORG,KATZ and ~~I~~) Chap. 4.20. Nudge NucZea$Energy Series. ~~cGraw-Hip, New York 1949.
382
energy transfer, between the lowest r, V* excited singlet states of rare-earth chelates and the elecltronic states of the rare-earth ion, was first observed by ~~~~~~~~~ [l] and investigated later by SEVCIEEENKO et al. [23. Chelates of trivalent europiumt samarium, and terbium were studied, using various ligands, such as dihe~~~ylm~thane and be~oy~a~eto~e, ~~~~~a~~~~ of the broad, intense near-~travio~e~ adsorption band of such chelates with 3850 A light yields a strong line-like l~rni~e~~e~~eemission, which is ~~a~~~~~~3~~~of &he rare-earth ion. The discrete luminescence lines bear a close resemblance to the ver,y weak sharp absorption bands observed for the trivalent ion in solution. These latter lines have been recognized generally as originating in the 4;f orbital8 of the lanthanide ion. Since there is considerable interest in the mechanism of electronic energy transfer, and the previous observations had indicated a rather high efficiency for the intramolecular process in the rare-earth chelates, wa hav;e undertaken a general investigation of this problem. In this paper we report the intramolecular energy transfer for the rather exee~tion~l ease of trivalent ytterbium chelates, which had eluded de&&on ~re~o~s~~ f1J For a des~~ption of the s~~~t~us~o~i~behavior of or~~~i~ chelates of the trivalent fanthanide rare-earth ions, it is convenient to divide them into three gmerai classes. Into CEass I we pfece the ions @a+++ and &u+++) with the closed &orbital configurations: 4f” and c4f1*,for which there are no electronic transitions in the visible and infrared regions, The ion Gd+++, with an f-orbital configuration 4f7, also falls into this class. If dibenzoylmethane is used as a lfgand in the chelate formation, the lowest excited singlet and triplet r, 7~*states of the chelates lie at INTRAMOLECULAR
G. A. CROSBY and M. KASITA
about 23500 and 20500 cm-l above the ground state [3], respectively. Consequently, in the tris-dibenzoylmethane chelates of the ions of Class I, the intramolecular energy transfer is energetically impossible (see Fig. 1). Therefore, the main effect of the rare-earth ions on the spectroscopic properties of the chelates of this class is to enhance the singlet-triplet mixing in the rr-electronic states of the chelate, as a spin-orbital, or magnetic, perturbing agent [3, 41. Into Class II we place the remaining rare-earth trivalent ions which include a 4f” to 4f’” configuration. In these cases, as indicated schematically in Fig. 1, energy transfer is feasible from the excited singlet or triplet 7~, rr* states of the chelate to the lower electronic 7 :*5
E
c 20 +? $ l5 z <5 &” 15 11 ClassI Fig. 1. Classification of rare-earth chelates. Electronic levels of the rare-earth ion on right of diagrams.
states of the chelated ion. However, as will be described later in another place [5], the efficiency of the energy transfer apparently depends on the electronic configuration of the rare-earth ion, efficient transfer correlating with the proximity of the Into Class III we place the exceptional f-orbital configuration to 4f”, 4f’ or 4f’“. lanthanide ions with 4f1 and 4fx3 f-orbital configurations, namely Ce+++ and Yb+++. There is a single electronic transition which can take place within the f-orbital in these cases (see Discussion), and is observed in the infrared region. The corresponding energy diagram for Class III chelates (Yb+++ case) is given in Fig. 1. Particular interest is attached to the efficiency of the energy transfer in this unusual situation. Moreover, the uniqueness of the rare-earth level in this case simplifies the energy transfer analysis. Although as we shall see, the Ce+++ chelate case was unamenable for study, the tris-dibenzoylmethane-Yb+++ chelate exhibited a remarkably efficient intramolecular energy transfer.
Experimental The dibenzoylmethane used was Eastman, White Label grade, and was twice sublimed in a high vacuum. Piperidine used as a precipitating agent was a Matheson, Coleman, and Bell product, and was purified by distillations to give a colorless liquid. The YbsO, was* first converted to the chloride, then the chloride dissolved in alcohol and added to an equimolar solution of dibenzoylmethane The precipitate was washed twice with absolute ethanol and piperidine in alcohol. and dried in a vacuum desiccator. The samples were dissolved in absolute ethanol For the low temperatures (77’K) for the absorption curve determinations. * Obtained from the Institute for Atomic Research, Iowa State College, Ames, Iowa.
378
I~ine~~eH~ ~t~~~~ a 1 : f : 1 frolnme ~~~) mixtureof ethyl ether, ~~o~nta~e~ abf&&e ethanol was used 88 8 so1vent. The dilute ~OIutjo~ of the chefate
were frozen in a liquid-~trogen bath to form transparent rigid glass soIutio~* The luminescence spectra were recorded on Eaatman I-M spectroscopio plates, hypersensitized in ammonia solution. The spectrograph used was a 3-p&m Steinheil Universal GH, equipped with (approx. f3.5) glass optics. A 1 kW X00 atmosphere AH-B mercury arc was used for excitation, filtered by a C&Q, solution (5’cm, 100 g/l. of the pentahydrate) and a Corning Glass 5840 standard filter, yielding mainly 3650 A light. The absorption spectra were determined on a Beckman DB-1 recording apectrophotometer.
Rare-earth ions having f’ and $1” inner orbital oonfigurations may have a single intra-f-orbital tranaition, corresponding to the unique recoupling of L and S angular momenta (in the language of Russell-Saundera coupling). This leads to a single relatively sharp and characteristically weak line-like absorption in the spectrum of the aqueous ion. The fact that the two levels involved in the transition are doublet components, split by spin-orbital interaction, results in the ocourrenee of these lines relatively far in the infrared. ~obably it is for this latter reason that in the literature [6, 7, 81 it is generally not recognized that the Ce+f+ and Yb+++ ions have a sharp line absorption in addition to their relatively well~k~o~ nearly ~ontiuuo~~ ultra~olet absorption. ~~~rbiurn as a trivalent positive ion has a 2P71a ground state, and a eF,la upper state for the intra-multiplet transition. The energy of this transition for the gaseous ion, which could be deduced from appropriate atomic term values, is unknown. In aqueous solution the corresponding sharp absorption band for Yb+++ was first observed by FREYMAN and TAKV~RIAN [9] at 10270cm-l (9735 A), and corroborated by G~BRECHT [IO, 111, who assigned it as the SF,,% --w SF&,* transition. The absorption band has been observed subaequently by other investigators 1:12, 131 and its intensity measured by HWWCXA~EX fl43. The m&r absorption coeflioient defined by Q = fl/ccQ log,, ~~~~~~ with c in mole@. a;nd bi in cm, is 2-08 st the band maximum (see Fig. 2)” The ~o~~n~ng ~~~~ator 379
a. A. CBOSBY
and
M. HA~HA
strength defined by f = 4.32 x 1O-8 fe 4% is 3‘86 x IO-* [15]. The weakness and characteristic sharpness of this transition makes GOBRECHT'S assignment secure. Although the corresponding luminescence emission has not been reported for solutions or crystals of ytterbium salts, BI~AUEBand BRAVER [XS] have observed a lins emission at about 9700 A for trivalent ytterbium oxide as an impurity in strontium and aluminium oxide phosphors, excited by 20 kV cathode rays. For the intramolecular energy transfer study the ahelate used was tris-dibenzoylmethane-Yb+++ (see Fig, 3). The lowest n, 7~* allowed absorption band (see Fig. 2) of the chelate was excited by 3650 A light with the sample in dilute rigid glass solution at 77OK (see Experimental). An intense luminescence emission
with &,, at approximately 9710 Ifi PO 6 was observed {see Fig. 2) for the Yb+f+ chelate, which must be the W,,2 -+ 2F7/2emission for the chelated ion. The low dispersion of our spectrograph in this region prevented aur obtaining a greater precision. The observed luminesoence is remarkable for its absence of subsidiary structure and for its sharpness. The quantum yield of the chelate luminescence was not determined absolutely, but a rough estimate from slit-width and exposure times and comparison with other efficient luminescence oases in this region leads to a probable value of quantum yield within the range 0*2-l.In other words, the intramol~ular energy transfer is certainly e&ient in this case. The only other emission from the Yb+++ chelate was a feeble blue fluoresoence and a fong-lived green lu~nes~en~e~ both of which are due probably to an organic impurity in the sample. The observed efiiciency is all the more remarkable because the radiationless transition leading to the Yb+++ luminescence in the chelate requires an energy loss of 17,000 cm-r, which must be dissipated thermally. The differences in energy between the Yb+++ excited 2E’s12level and the zeropoint V, r* levels are of course somewhat less than the value quoted above, based If the energy transfer is from the 7r97~* singlet level of on the exciting frequency. the ohelate, an apparent energy difference of about 13,500 cm-l would be involved; for the P, 7~* triplet of the chelate, the corresponding difference would be 10,500 An efBcient energy transfer between electronic states which are this far cm-l. 380
Intramolecular energytramferin ytterbiumorganicchelates
apart would not seem to be plausible on a qu&ntum mechanical basis, since levels so widely separated do not interact very strongly. Consequently, it seems necessary to assume that the mechanism of energy transfer involves interaction between (a) high vibrational levels of the chelate coupled with the 2Fs,z excited state of the ytterbium ion and (b) the pure ?r, n * electronic states of the chelate. The requirement of vibrational interaction, for the energy transfer mechanism, is not ruled out by the absence of vibrational fine structure in the luminescence emission of the Yb+++ chelate. Optical transitions of the type 2.F5,2 ---t 2F,,2 in the chelate undoubtedly are accompanied by insignificant molecular distortion, and as a consequence of the Franck-Condon principle, no vibrational fine structure should appear. , It is of interest to point out that the lowest m, 7~* absorption band of dibenzoylmethane is significantly altered upon formation of the Yb+++ chelate (see Fig. 2). A shift of about 400 cm-l in the band maximum is observed, accompanied by a considerable diminution in intensity (the absorption coefficient for the chelate being calculated per mole of dibenzoylmethane). It is probable that the 4f orbitals of Yb+++ are not directly involved in the chelate binding (since the chelate emission is so little displaced from the frequency of the analogous transition in the ion in solution). Nevertheless the possibility of strong vibrational coupling is suggested by the effect of chelation on the 7~,r* absorption noted above. The intra-multiplet transition should be observable in other rare earth ions, and the observations of BRAUER and BRAUER [16] give three examples in addition to the trivalent ytterbium. The owe of Ce+++ is an interesting one to consider, even though direct observations are lacking for it. The ground state of Ce+++ is 2F 5,2, and for the gaseous ion the 2Fs,2 - 2F,,2 separation has been deduced by LANQ [l?] to be 2253-O cm-r; this assignment has been confirmed by a Zeeman pattern analysis [18]. Unfortunately, the study of electronic transitions near 5 p is difficult, and we have not yet succeeded in observing this transition either in absorption (ion) or in emission (chelate). In the case of the a&&de rare earths, with incompletely filled 5f orbit&, the hexavalent neptunium ion and the normally unstable pentavalent uranium ion should exhibit s, unique intra-multiplet transition [ 193. A~k~o~~~dge~~nt~- The authors are indebted to Prof. ROBEIST E. CONNICK of the University of California, and Dr. C. J. RODDEN, now of the New Brunswick AEC Laboratory, for valuable communications on the spectra of rare earth ions. It is a pleasure to acknowledge the co-operation of Prof. F. H. SPEDDING in helping ua procure the ytterbium oxide sample.
References WEISSMAN S. I. J. Chern. Phys. 1942 10 214. [2] SEVCEENHO A. N. and TROFIMOV A. K. J. Ezp. Theor. Phyrr. 1951 21 220; SEVCHENKO A. N. and MORACHEVSEY A. G. Izv. Akad. Nauk SSSR (ser. j&z.) 1951 15 628. [3] YUSTER P. and WEISSMAN S. I. J. Chem. Phys. 1949 17 1182. [4] %SHA M. and MCGLYNN S. P. AWL Rev. Phye. Chem. 1956 7 403. [Fi]t?ROSBY G. A. To be published. [6] VAN VLECKJ. H. J. Phys. Chem. 1937 41 67. [l]
381
G. A. CROSBY and M.
KASHA
177YOST D. AI., RUSSELL H., JR. and GARNER C. S. The Rare Earth ~~~~8 an& Their C~~~~8. John Wiley, New York 1947. [S] E0JzMAXN W. Q~~~u~ ~~~~~~y p. 671. Academic Press, New York 1957. [9] FREYMANR. and TAKVORIANS. G. R. Acad. Sci., Pa& 1932194963. [IO]GOBRECRTH. 2. Cea. Nature. 19378 351. [ll] GOBRECHTH. Ann. Physirc 1938 31 600 755. [12] ROSENTHALG. Phyeik. 2. 1939 40 508. [13] RODDEN C. J. J. Res. Nat. BUY. Stand. Wmh. 1942 28 265. [14] HOOCXZHAUEN J. Phytica 1946 11 513. [15] HOO*SCHAUENJ. and GORTERC. J. Phyeica 194814 197. [16] BRAVI~IR E. and BRAUER P. Naturwissen&qftichaften 194784 120. [17] LANU R. J. Canud. J. Res. 193518A 1; 193614A 127; Phys. Rev. 1936 49 552. [18] VAX DE VLIET H. J. Dissertation, Univ. Amsterdam, 1939; Chew. A&&-. 1939 38 2033. [lQ] CONNICKR. E., KASEA M., MCVEY W. H. and SHELINEG. E. The TTa~~ra~~u~ E~~~t~ (Edited by SEABORG,KATZ and ~~I~~) Chap. 4.20. Nudge NucZea$Energy Series. ~~cGraw-Hip, New York 1949.
382