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Magnetic effect on the emission of bismuth-doped calcium oxide
Journal of Luminescence 12/13 (1976) 413—417 © North-Holland Publishing Company
MAGNETIC EFFECT ON THE EMISSION OF BISMUTH-DOPED CALCIUM OXIDE W.A. RUNCIMAN, N.B. MANSUN and M. MARSHALL Department of Solid State Physics, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2600, Australia
A study of the CaO Bi violet emission in fields up to 5 T has shown that the transi1 (383.75 nm)and some of its vibrational sidebands tion intensities of a linewith at 26051 cm increase quadratically field strength although no Zeeman splitting is observed. In consequence, a revised energy level scheme is proposed. The magnetic behaviour is consistent with a fluorescent 6s6p(3P 2(’ S 3~ion. 0) -. 6s 0) transition in a Bi
1. Introduction Calcium oxide doped with bismuth gives absorption [11 and emission [1—3] with well-defined vibrational structure. In the absorption spectrum a zero-phonon line has been identified at 27239 cm’ (367.02 nm) and is attributed to the 6s2(1S 3P 3~ion [1,2,4]. The 6s6p( stress 1) electric dipole experiments transition within theabsorption Bi results 0)of uniaxial and Zeeman on this line are consistent with the 1S 3P 0 1 assignment and show conclusively that the center involved 3~ion has °h symmetry [2]. In the commonly center is a Bi replacing a Ca2F ion substitutionally withaccepted non-localmodel, chargethe compensation [2]. Recently the reverse 3P 1S 1 0 transition was seen in emission with the sample at a temperature of 100—200 K [5]. At helium temperatures, however, all the emission lies ~l500 cm~lower in energy and there is no associated 3P absorption 2(1S [21. This latter emission has been attributed [1,2,4] to the 6s6p( 0) 6s 0) transition, as such a transition would be highly forbidden and would be too weak to be observed in absorption. The vibrational pattern in this emission spectrum has previously been represented by the double series [11: —~
-÷
-*
—~
v25717—493v1
(1)
—195v2,
where v is the wavenumber in cm~and v1 and v2 are the numbers of vibrational quanta for a given line. This representation however has to be considerably modified in the light of the magnetic field observations to be discussed below.
413
414
W.A. Runciman et al./Magnetic effect in the emission of CaO: Bi
2. Experimental The samples were prepared by heating a mixture of one part by weight of bismuth oxide to 2000 parts analytical reagent grade calcium carbonate at 1200°Cin air for two hours. Pressed powder pellets were placed inside a split coil superconducting solenoid which provided magnetic fields up to 5 T. The emission was stimulated by a 125 watt high pressure mercury black lamp, dispersed by a Jarrell Ash 1 meter spectrometer, and detected by an EMI 9658 photomultiplier. The emitted light was detected perpendicular to the field direction and by using a piezo-electric modulator plus a linear polarizer between the sample and spectrometer it was possible to modulate the collected light between ir and a polarization.
3. Results On the application of a field of 5 T no Zeeman splitting is observed in the low temperature fluorescence spectrum. This is expected for a 3P0 1S0 transition as —~
there is no electronic degeneracy in the electronic states involved. However a marked intensity change in the spectrum is observed (fig. 1). The most striking effect is the
u, ~
Co C’J
A
I
E U
~ IU,
t
3
-
0
H-5T
1000
2000
3000
Wave numb e r (c m5
Fig. 1. The fluorescence spectrum of CaO: Bi at 4.2 K. The crossestdenote and 25743 Hg lines cm1. and the arrows indicate the double series starting on the lines at 26051 cm
WA. Runciman etal./Magnetic effect in the emission of CaO : Bi
415
increase in intensity of a line at 26051 cm~.This line is detectable at zero field, but is so weak that it was not included in the earlier analysis [1]. The 26051 cm’ line is the highest energy line and therefore seems to be the real zero-phonon line. The revised formula for the double vibrational series is then =
26051
—
488vi
—
308u2.
(2)
All the lines with v2 = 0 increase in intensity with a magnetic field and have the same field dependence. Likewise all the lines with v2 = 1 show the same slight decrease in intensity with increasing field strength. Therefore a measure of the relative magnitudes of the two types of line givenintensity by the intensities the 26051 cm~ 1 (‘~) lines. Theistotal (‘o ~‘~) isofindependent of magnetic field value, but the relative intensity I~/l~ grows quadratically with field (‘o) and the 25743 cm strength (fig. 2).
4. Discussion and conclusions A quadratic dependence of the transition probability on field strength is expected if the magnetic field mixes in a state which makes the transition allowed. In the absence of a magnetic field the oscillator strength of the CaO : Bi fluorescence transition is of the order l0~ [2]. The zero-phonon transition at 26051 cm~and the part of the sideband which borrows intensity from it, only contribute 7% of the total intensity (the major part being vibrationally induced) and therefore is only very weakly allowed, of orderf= 7 X 10g. This small intensity is believed to be due to strain in the crystallites [5]. The magnetic effect can be calculated in the Russell—Saunders coupling scheme.
bO1~j0~5 H~(TesIaC)
Fig. 2. The ratio of intensities IØ/I(~.)as a function of (magnetic field)2 where J~denotes the intensity of the line at 26051 cm1 and I~,that of the line at 25743 cm1.
416
W.A. Runciman et al/Magnetic effect in the emission of CaO Bi
The magnetic field perturbation transforms according to Tig symmetry and therefore mixes the 3P1 state of T1~symmetry and the 3P0 state of Ai~symmetry. The former state has an allowed electric dipole transition, oscillator3Pstrength 1S 7 X l0~ [4], to the ground state. Consequently, the mixing makes the 0 0 transition allowed and it has been shown by Hughes and Pells [5] that the induced oscillator 3P strength fmag can be expressed 1~~ transition by the equation: in terms of the oscillator strengthf of the 1 to —~
fmag/f ~ (i3B/~.E)2 (3) where fl is the Bohr magneton, B is the field strength in tesla and ~E is the energy difference 3P 3P 1 0 in crn~. The difference ~E = 1188 cm~and substitution gives fm~ 2 X l08 for B = 5 T. This is three times the zero field value which is in rough agreement with observation (fig. I). The magnetic effect is useful in analysing the vibrational structure in the sideband. The features which do not increase with the magnetic field comprise the vibrationally induced sideband of the Aiu Aig transition in which the active vibration will be of Tig symmetry. This sideband includes the strong line at 25743 cm~ but there is also a shoulder at 330 cm~and a feature at 650 cm~.These lines may all be repeated at a frequency of 488 cm~but can only be followed for the strongest component. The lines which do increase with field (other than the zero-phonon line) arise from the coupling of Aig symmetry vibrations onto the allowed electronic transition. The dominant Aig feature is at 488 cm~but there is also a weaker feature at 200 cm~ Since the magnetic field of the Tig irreducible 3P transforms as only one component 3P representation, only the 1 z state is mixed into the 0 state and hence the magnetic field-induced transition will be entirely polarized parallel to the field direction. The zero-phonon line at 26051 cm~ and the sideband which borrows its intensity from it should be ir polarized, and the magnetically induced linearly polarized emission (MLE) spectrum shown in fig. 3 confirms this prediction. —
—~
,
-
,
0
1000
2000
300J
Wave number Fig. 3. Magnetically induced linear dichroism. Spectrum measured at 4.2 K with a field of 5 T. The resolution used is indicated.
W.A. Runciman etal./Magnetic effect in the emission of CaO:Bi
417
Acknowledgement We wish to thank Dr. J.T. Gourley for running some of the Zeeman spectra and Drs. A.E. Hughes and E.R. Vance for useful discussions.
References [11 W.A.
Runciman, Proc. Phys. Soc. (London) A68 (1955) 647. [2] A.E. Hughes and W.A. Runciman, J. Phys. C2 (1969) 37. [3] J. Ewles, Proc. Roy. Soc. A167 (1938) 34. [4] A.E. Hughes and G.P. Pells, Phys. Stat. Sol. 25A (1974) 437. [5] A.E. Hughes and G.P. Pells, Phys. Stat. Sol. 71B (1975) 707.
Discussion G. Boulon: Recently, we have published a paper on bismuth photoluminescence at low 3P shown that, for T < 60 K, the 0 metastable level has a high emission probability for several oxides. 3~)have We youhave measured also calculated the trap depth the trap-depths and the emission (J. Phys.probabilities 36 (1975) 265). from the temperature dependence on decay times in order to compare these parameters with those In the case of CaO (Bi obtained in other host lattices? W.A. Runciman: The oscillator strengths are given in the paper. The lifetime is 4ms at 4K falling rapidly to less than 0.1 ms between 130 and 170 K. The trap depth has been calculated from the temperature dependence of the decay times, and is found to be 1320 ±100 cm1 from measurements over the temperature range 120—180 K [51.This is reasonably consistent with the value of 1188 cm~ obtained from our spectroscopic measurements. The spectroscopic method is preferred for greater accuracy when zero-phonon lines are observable for absorption and fluorescence. G. Blasse: Have you an explanation for the fact that Bi3~in CaO shows such a pronounced fine structure, whereas usually Bi3+ shows no fine structure in oxides? W.A. Runciman: I think the ionic radii are such that there is no major misfit. The high symmetry may also help as the low g-value of the absorption line indicates covalency. Regular octahedral coordination might well favour the formation of a bismuthate complex.
temperature and we have
MAGNETIC EFFECT ON THE EMISSION OF BISMUTH-DOPED CALCIUM OXIDE W.A. RUNCIMAN, N.B. MANSUN and M. MARSHALL Department of Solid State Physics, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2600, Australia
A study of the CaO Bi violet emission in fields up to 5 T has shown that the transi1 (383.75 nm)and some of its vibrational sidebands tion intensities of a linewith at 26051 cm increase quadratically field strength although no Zeeman splitting is observed. In consequence, a revised energy level scheme is proposed. The magnetic behaviour is consistent with a fluorescent 6s6p(3P 2(’ S 3~ion. 0) -. 6s 0) transition in a Bi
1. Introduction Calcium oxide doped with bismuth gives absorption [11 and emission [1—3] with well-defined vibrational structure. In the absorption spectrum a zero-phonon line has been identified at 27239 cm’ (367.02 nm) and is attributed to the 6s2(1S 3P 3~ion [1,2,4]. The 6s6p( stress 1) electric dipole experiments transition within theabsorption Bi results 0)of uniaxial and Zeeman on this line are consistent with the 1S 3P 0 1 assignment and show conclusively that the center involved 3~ion has °h symmetry [2]. In the commonly center is a Bi replacing a Ca2F ion substitutionally withaccepted non-localmodel, chargethe compensation [2]. Recently the reverse 3P 1S 1 0 transition was seen in emission with the sample at a temperature of 100—200 K [5]. At helium temperatures, however, all the emission lies ~l500 cm~lower in energy and there is no associated 3P absorption 2(1S [21. This latter emission has been attributed [1,2,4] to the 6s6p( 0) 6s 0) transition, as such a transition would be highly forbidden and would be too weak to be observed in absorption. The vibrational pattern in this emission spectrum has previously been represented by the double series [11: —~
-÷
-*
—~
v25717—493v1
(1)
—195v2,
where v is the wavenumber in cm~and v1 and v2 are the numbers of vibrational quanta for a given line. This representation however has to be considerably modified in the light of the magnetic field observations to be discussed below.
413
414
W.A. Runciman et al./Magnetic effect in the emission of CaO: Bi
2. Experimental The samples were prepared by heating a mixture of one part by weight of bismuth oxide to 2000 parts analytical reagent grade calcium carbonate at 1200°Cin air for two hours. Pressed powder pellets were placed inside a split coil superconducting solenoid which provided magnetic fields up to 5 T. The emission was stimulated by a 125 watt high pressure mercury black lamp, dispersed by a Jarrell Ash 1 meter spectrometer, and detected by an EMI 9658 photomultiplier. The emitted light was detected perpendicular to the field direction and by using a piezo-electric modulator plus a linear polarizer between the sample and spectrometer it was possible to modulate the collected light between ir and a polarization.
3. Results On the application of a field of 5 T no Zeeman splitting is observed in the low temperature fluorescence spectrum. This is expected for a 3P0 1S0 transition as —~
there is no electronic degeneracy in the electronic states involved. However a marked intensity change in the spectrum is observed (fig. 1). The most striking effect is the
u, ~
Co C’J
A
I
E U
~ IU,
t
3
-
0
H-5T
1000
2000
3000
Wave numb e r (c m5
Fig. 1. The fluorescence spectrum of CaO: Bi at 4.2 K. The crossestdenote and 25743 Hg lines cm1. and the arrows indicate the double series starting on the lines at 26051 cm
WA. Runciman etal./Magnetic effect in the emission of CaO : Bi
415
increase in intensity of a line at 26051 cm~.This line is detectable at zero field, but is so weak that it was not included in the earlier analysis [1]. The 26051 cm’ line is the highest energy line and therefore seems to be the real zero-phonon line. The revised formula for the double vibrational series is then =
26051
—
488vi
—
308u2.
(2)
All the lines with v2 = 0 increase in intensity with a magnetic field and have the same field dependence. Likewise all the lines with v2 = 1 show the same slight decrease in intensity with increasing field strength. Therefore a measure of the relative magnitudes of the two types of line givenintensity by the intensities the 26051 cm~ 1 (‘~) lines. Theistotal (‘o ~‘~) isofindependent of magnetic field value, but the relative intensity I~/l~ grows quadratically with field (‘o) and the 25743 cm strength (fig. 2).
4. Discussion and conclusions A quadratic dependence of the transition probability on field strength is expected if the magnetic field mixes in a state which makes the transition allowed. In the absence of a magnetic field the oscillator strength of the CaO : Bi fluorescence transition is of the order l0~ [2]. The zero-phonon transition at 26051 cm~and the part of the sideband which borrows intensity from it, only contribute 7% of the total intensity (the major part being vibrationally induced) and therefore is only very weakly allowed, of orderf= 7 X 10g. This small intensity is believed to be due to strain in the crystallites [5]. The magnetic effect can be calculated in the Russell—Saunders coupling scheme.
bO1~j0~5 H~(TesIaC)
Fig. 2. The ratio of intensities IØ/I(~.)as a function of (magnetic field)2 where J~denotes the intensity of the line at 26051 cm1 and I~,that of the line at 25743 cm1.
416
W.A. Runciman et al/Magnetic effect in the emission of CaO Bi
The magnetic field perturbation transforms according to Tig symmetry and therefore mixes the 3P1 state of T1~symmetry and the 3P0 state of Ai~symmetry. The former state has an allowed electric dipole transition, oscillator3Pstrength 1S 7 X l0~ [4], to the ground state. Consequently, the mixing makes the 0 0 transition allowed and it has been shown by Hughes and Pells [5] that the induced oscillator 3P strength fmag can be expressed 1~~ transition by the equation: in terms of the oscillator strengthf of the 1 to —~
fmag/f ~ (i3B/~.E)2 (3) where fl is the Bohr magneton, B is the field strength in tesla and ~E is the energy difference 3P 3P 1 0 in crn~. The difference ~E = 1188 cm~and substitution gives fm~ 2 X l08 for B = 5 T. This is three times the zero field value which is in rough agreement with observation (fig. I). The magnetic effect is useful in analysing the vibrational structure in the sideband. The features which do not increase with the magnetic field comprise the vibrationally induced sideband of the Aiu Aig transition in which the active vibration will be of Tig symmetry. This sideband includes the strong line at 25743 cm~ but there is also a shoulder at 330 cm~and a feature at 650 cm~.These lines may all be repeated at a frequency of 488 cm~but can only be followed for the strongest component. The lines which do increase with field (other than the zero-phonon line) arise from the coupling of Aig symmetry vibrations onto the allowed electronic transition. The dominant Aig feature is at 488 cm~but there is also a weaker feature at 200 cm~ Since the magnetic field of the Tig irreducible 3P transforms as only one component 3P representation, only the 1 z state is mixed into the 0 state and hence the magnetic field-induced transition will be entirely polarized parallel to the field direction. The zero-phonon line at 26051 cm~ and the sideband which borrows its intensity from it should be ir polarized, and the magnetically induced linearly polarized emission (MLE) spectrum shown in fig. 3 confirms this prediction. —
—~
,
-
,
0
1000
2000
300J
Wave number Fig. 3. Magnetically induced linear dichroism. Spectrum measured at 4.2 K with a field of 5 T. The resolution used is indicated.
W.A. Runciman etal./Magnetic effect in the emission of CaO:Bi
417
Acknowledgement We wish to thank Dr. J.T. Gourley for running some of the Zeeman spectra and Drs. A.E. Hughes and E.R. Vance for useful discussions.
References [11 W.A.
Runciman, Proc. Phys. Soc. (London) A68 (1955) 647. [2] A.E. Hughes and W.A. Runciman, J. Phys. C2 (1969) 37. [3] J. Ewles, Proc. Roy. Soc. A167 (1938) 34. [4] A.E. Hughes and G.P. Pells, Phys. Stat. Sol. 25A (1974) 437. [5] A.E. Hughes and G.P. Pells, Phys. Stat. Sol. 71B (1975) 707.
Discussion G. Boulon: Recently, we have published a paper on bismuth photoluminescence at low 3P shown that, for T < 60 K, the 0 metastable level has a high emission probability for several oxides. 3~)have We youhave measured also calculated the trap depth the trap-depths and the emission (J. Phys.probabilities 36 (1975) 265). from the temperature dependence on decay times in order to compare these parameters with those In the case of CaO (Bi obtained in other host lattices? W.A. Runciman: The oscillator strengths are given in the paper. The lifetime is 4ms at 4K falling rapidly to less than 0.1 ms between 130 and 170 K. The trap depth has been calculated from the temperature dependence of the decay times, and is found to be 1320 ±100 cm1 from measurements over the temperature range 120—180 K [51.This is reasonably consistent with the value of 1188 cm~ obtained from our spectroscopic measurements. The spectroscopic method is preferred for greater accuracy when zero-phonon lines are observable for absorption and fluorescence. G. Blasse: Have you an explanation for the fact that Bi3~in CaO shows such a pronounced fine structure, whereas usually Bi3+ shows no fine structure in oxides? W.A. Runciman: I think the ionic radii are such that there is no major misfit. The high symmetry may also help as the low g-value of the absorption line indicates covalency. Regular octahedral coordination might well favour the formation of a bismuthate complex.
temperature and we have