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Nonradiative decay from 5d states of rare earths in crystals
Solid State Communications,
Vol.12, pp. 741—744, 1973.
Pergamon Press.
Printed in Great Britain
NONRADIATIVE DECAY FROM 5d STATES OF RARE EARTHS IN CRYSTALS* M.J. Weber Raytheon Research Division, Waltham, Massachusetts 02154, U.S.A. (Received 10 January 1973 byR.H. Silsbee)
Nonradiative decay from 4f~~ Sd states was investigated for trivalent rare earths in Y3A15012. The rates of both 5d—’4f and 5d—~5dtransitions were determined from3~ measurements of the and ion-lattice intensities coupling, of 5d and Pr3~.Because of lifetimes the stronger nonradiative rates for transitions involving Sd states are much faster fluorescence decay from Ce than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the Sd fluorescence lifetimes from 77 to 700 K are reported.
NONRADIATIVE transitions between 4f” states of rare-earth ions in crystals involving the simultaneous emission of several phonons have been studied extensively.1 The dependences of the rate of multiphonon emission on energy gap to the next-lower level, tern-
investigate radiative and nonradiative transitions from Sd states. Relaxation of 5d states was studied for trivalent rare earths in Y 3A15012 (YAG). In this host the crystalfield splitting of the 5d states is large which results in relatively low-lying Sd3levels (~ l0~cm’ lowerappear than At 300°K,the 5d levels those found in CaF2). as moderately broad, structureless bands in the absorption spectra. Since the fundamental absorption edge of Y 3A15012 begins at ~ 52,000 cm~ many 4f—Sd absorptions are unobserved. The locations of the Sd bands relative to the 4f levels for several rare earths in YAG are shown in Fig. 1. The extent of the shaded regions denotes the full width of the bands at onehalf maximum; the solid half-circles indicate levels from which fluorescence was observed at 300 or 77°K. The lowest Sd levels of Nd3~and Th3~from Fig. I are in the proximity of the 4f levels and very fast nonradiative 5d—~4frelaxation occurs. This was established from the appearance of the Sd bands in the excitation spectra of the Nd3~2P 3,~fluorescence and 3~5D 5D the Tb 3 and 4 fluorescences. The efficiency of the transfer can be determined by comparing the relative intensities of the Sd bands in absorption and excitation spectra. The measured peak absorption cross sections of the 4f—~5d transitions Y3A15O12Sdwere4f 2 therefore theinpredicted 1—5 X 10-18 cm
perature, and phonon spectrum of the host have also been established. Here we investigate similar nonradiative transitions states of thethe excited 4f~~ 5d configuration of rarefrom earths. Because ion-lattice coupling is greater for configurations containing outer d electrons, nonradiative decay is faster and transitions
,~
bridging large energy gaps are possible. Radiative decay from Sd to 4f levels is also faster since electric-dipole transitions are allowed. The 4f~ Sd levels of trivalent rare earths are generally located 2atFor energies> 50,000 cm’ many lanthanide ions,above levelsthe of 4f’~ground level. the 4f’~’Sd configuration overlap those of the 4f’~ configuration. In such cases ions excited into 5d levels rapidly decay nonradiatively to nearby 4f levels. For several ions, however, the energy separation from the lowest Sd level to levels of 4f is very large (e.g. trivalent Ce, Tm, Yb and divalent Eu). Nonradiative decay is therefore less probable and 5d—~4femission may be anticipated. This emission can be used to *
Research supported in part by the Army Night Vision Laboratory, Ft. Belvoir, Va.
,~
741
—~
742
NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS Vol. 12, No. 7 soc .
50
~so 40
—
—
4f25
-
ce3+
40 60
—
20 —
30
_____
S S
Pr3~
.5 0
~D3
3
= — u
t
—.
4f’5d
—
•
‘
20
—.~
04
— —
0
=
~
6
S U C 0
4
.
—
Lifetime
2
° —
Intert SIty
•I~y2~~~ 7F 0
= 0
—
~ — Ce~
Pr~
—
4h,
2-———-’F~
Nd~
Tb~
FIG. 1. Energy level diagrams for several trivalent rare earths in Y3A15012 showing the locations of levels of the 4f’~~round configuration and the lower bands of the 4f’~ Sd excited configuration. Levels or bands from which fluorescence has been observed are marked with solid half circles. radiative decay probabilities are > 3~and 101 seC’. Since300 no Tb~at Sd emission was observed from Nd or 77°K,the nonradiative decay rates are very much faster. For Ce3~and Pr3~there are large energy Separations from the lowest Sd levels to levels of 4f. Although the relative positions and shapes of the configuration coordinate curves for the ground and excited states are unknown, the estimated minimum Sd-to-4f energy gaps are approximately 16,500 and l0,000cm’, respectively, Because of the large energy gaps, nonradiative decay is less probable and Sd fluorescence was observed. For cerium the Sd fluorescence is in the form of a broad band peaking at 550 nm corresponding to unresolved transitions terminating on levels of 2F 2F 5,2 and 7,2 for praseodymium the strongest extends from approximately 3005d tofluorescence 450 nm with peaks corresponding to transitions terminating on the 3H~ and 3F~manifolds.6 The lifetimes of the Sd fluorescence from Ce3~ and Pr3~were measured following excitation with a
60
00
200 Temperature
400 600
000
(OK)
FIG. 2. Temperature dependence 3~and of the Pr3~ Sdinfluorescence Y lifetimes and intensities for Ce 3A15012. deuterium lamp having a 5 nsec pulse width. The fluorescence decays were fitted to a simple exponential time dependence; lifetimes less than the pulse width were derived from a convolution with the system response function. The 2.results plotted as a function of temperature in Fig. At lowaretemperatures the lifetimes were relatively constant. These values were compared to the radiative lifetimes calculated using measured integrated absorption cross sections and the Einstein A—B relationship modified7 to account for the Stokes shift. Because the required fluorescence branching ratio from Sd to levels of 4f was difficult to determine accurately and the matrix elements for absorption and emission were assumed to be equal, the resulting radiative lifetimes were only approximate. For both ions, however, the values were in reasonable agreement with the measured low-temperature lifetimes, thus indicating that the lowest 5d levels fluoresce with near-unity quantum efficiency. 3~and Pr~lifeelevatedrapidly. temperatures, theenergy Ce gap for cerium timesAtdecrease Since the is larger, more phonons are needed to conserve energy and a higher temperature is required before stimulated phonon processes compete with radiative decay. That the additional decay at high temperature is due to nonradiative decay was established from measurements
Vol. 12, No.7 NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS
Pr3~it ~s po~ibleto determine the nonradiative rate over a more extended range including low temperatures where radiative decay predominates. This was accomplished by monitoring the fluorescence
For I I
j
0’
I toe
P~’ YAG
~ i0~
/ /
i~0’
/ /
/ //
. ‘0’
intensity from 3F
“
/ /
/
.
I
0’
—
.!
/
/
/
‘O~
/
/
/ .
/ /
1
/
/400cm’
I0~
/ 6,0cm~
.
,~
/
‘ —
60
00
—
...~.
200
400
600
1Q00
llsiplrotur• (1()
FIG. 3. Temperature dependence of the 5d—.’P 0,1 ,2~ decay rate for Pr’~in Y,Al5012 The dashed curves are the temperature dependencies of stimulated multiphonon emission rates calculated using a single. frequency model. 16
743
-
of the integrated fluorescence intensity versus ternperature. These results are included in Fig. 2 and are in good agreement with the lifetime data. The ternperature dependences of the 5d nonradiative decay rates were obtained by subtracting the radiative decay probability from the total decay probability. These were compared with the temperature dependence for a process involving the emission of n-phonons of frequency v given by [1 exp (—hv/kT)J~.Phonon energies in Y 3AI5O1, range up to 860 cm’~ In the harmonic approximation, the highest energy phonons conserve however, a given energy in the lowestdependence order process possible; the temperature predicted using these phonons does not account for the behavior in Fig. 2. For Ce”~,although data was available over only a small temperature range, a better fit is obtained using phonons of 500600 cm~. This may be due in part to the fact that the high energy phonons in Y,Al 5012 arise from vibrations 4 which are probably less strongly of the AIO4 groups coupled to rare earths at Y3” sites.8 The temperature dependences of the 4fmultiphonon emission rates in YAG also indicate that phonons of less than maximum energies make the dominant contribution.9 —
.~
0 for selective excitation into the lowest Sd level. For weak pumping 3P into Sd at a rate F, the steady-state population in 0 is given by r and r’ are total decay times of the from ~ WP 3F, rr’~N, where W the is the probability for decay Sd Sd to and 3F states, and ~3Nis the Boltzmann population in 3P 0, assuming a rapid thermal equilibration among the stark levels of ‘Po,1 ,2~ ‘6. The temperature dependence of W was found3P by measuring the intensity of a fluorescence line from 0, and correcting for 3 and the the temperature variationsand of emission r, r’, and lines. j~ widths of the absorption The results, normalized to the high temperature decay rates derived from Fig. 2, are shown in Fig. 3. The decay rate appears to approach a temperature independent limit below 100°K. Since Wincludes both radiative and nonradiative processes, this could correspond to the probability for radiative decay and/or for spontaneous phonon emission. Although the Sd fluorescence branching ratio was not measured, a radiative probability to 2’ of l0~sec’ is not unreasonable. ~
‘
3’~relax. temperature Pr ationThe in Fig. 3 cannot dependence be explainedofbythe phonon emission using a single-frequency model or by an activation energy in an exponential law. No detailed analysis of the temperature dependence was undertaken because of uncertainties in the rates at low temperatures, however, as examples, the temperature dependences predicted for phonons of 400 and 600 cm~are included in Fig. 3. It is apparent that relaxation via 600 cm~phonons is insufficient to account for the behavior at the higher temperatures. The singlefrequency model may, of course, be too great an 10 have oversimplification. As Kisliuk and Moore pointed out, processes involving lower-frequency phonons, being of higher order, have steeper temperature dependences and hence should ultimately govern the relaxation at high temperatures. In addition, anharmonic effects” may be important for relaxation of Sd states. Nonradiative decay betweenspectra, Sd levelsthewas also examined. From the absorption energy separations between the ground and first excited Sd levels of Ce~and Pr3” are approximately 7500 and 7200cm~ respectively. By selectively exciting ions into the first excited state of the Sd configuration of ,
744
NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS Vol.12, No.7
Ce~at 340nm and observing the risetime of the
emission band in the ultraviolet from Ce~”:Y3 Al5
emission from the ground 5d state, a limiting value of > 5 X 108 sec’ was found for the nonradiative decay rate at 77°K.For comparison, the extrapolated phonon emission rate at 77°Kbetween similarlymultiseparated 4f’ levels in Y 3sec~.~ Al5 012 isnonradiative 10 Whereas intra—4fand3intra—5d transitions occur between initial and final electronic states having the same parity, interconfiguration 5d—4f nonradiative transitions require that at least one odd-parity vibrational mode be active in the orbit-lattice interaction. The AS = 0 spinselection rule should not be important since the spin—orbit interaction causes significant spin state admixmg in both configurations.
at 300°Kfor u.v. and cathode-ray excitation. We have not detected emissionhigh-pressure from excitedxenon Sd states 3”’ orany Pr~using and of either Ce arc lamps for excitation and samples containmercury ing 0.05 per cent Ce and ~ 0.1 per cent Pr.
If emission can be observed from several higher Sd levels, an energy gap dependence for nonradiative decay similar to those for 4f’~levels’ might be obtained. Generally, however, the Sd levels are sufficiently close that, based upon the above rates, a rapid nonradiative cascade to the lowest 2Sd would be expected to adomi. have reported observing nate. Blasse and Bril’
012
In conclusion, it should be noted that 5d—”4f nonradiative decay is also evident in the excitation spectra of 4f emission from divalent rare earths. The Sd bands of divalent rare earths are located at lower energies than for the trivalent ion.2 For Tm2”’ in CaF,, Sd bands beginning at 15 000cm1 appear in the excitation spectrum of the 2F~—p 2F 7p~tofluor13 thus indicating decay from Sd 2F escence, 5,2 across an energy gap of S000—6000cm~at 77°K. Acknowledgements I wishwith to thank Thomas Varitimos for his assistance the measurements andthese Andrew Morrison for growing the crystals used in studies. —
REFERENCES 1. 2. 3.
MOOS H.W.,J. Luminesc. 1,2, 106 (1970) and references therein. DIEKE G.H., Spectra and Energy Levels ofRare Earth Ions in Crystals, (Edited by CROSSWHITE H.M. and CROSSWI-IITE H. lnterscience, New York, (1968). LOHE.,Phys. Rev. 147, 332 (1966); ib&1. 175. 533 (1968).
4.
SLACK G.A., OLIVER D.W., CHRENKO R.M. and ROBERTS S.,Pkvs. Rev. 177, 1308 (1969).
S.
Absorption mea~urementswere made using crystals containing <0.1% rare earths substituted at Y3”’ sites and pathlengths of< 0.5mm. Whereas in YAG the lowest Sd level of Pr3~is below the ‘So level of 4J’2, in other crystals it coincides with or is above 1S 1S 0 in the latter case emission may be observed from 0 (YEN W. private communication).
6.
—
7. 8. 9.
FOWLER W.B. and DEXTER D.L., Phys. Rev. 128, 2154 (1962); J. Chem. Phys. 43, 1768 (1965). Observations of the vibronic structure associated with 4f—5d transitions at liquid helium temperatures would be useful in identifying the relative coupling for various phonons. ZVEREV G.M.,KOLODNY1 G.Ya. and ONISHCHENKO A.M., Soviet Phys. JETP 33,497(1971).
10.
KISLIUKP. and MOORE C.A.,Phys. Rev. 160,307(1967).
11. 12. 13.
RISEBERG L.A. and WEBER M.J., Solid State Commun. 9, 791 (1971). BLASSE G. and BRIL A.,Appl. Phys. Letr. 11, 53 (1967);J. chem. Phys. 47, 5139 (1967). McCLURE D.S. and KISS Z., J. chem. Phys. 39, 3251 (1963). Nous avons examine Ia dissociation non-radiative a partir d’Ctats 4ff” 5d de terres rares trivalentes dans l’Y, Al5 O~.Nous avons determine les taux 34’ etdedudurCes Pr34’. de Vuvie Ia nature de transitions Sd—’~4fet5d-~5dau moyen de mesures et d’intensités du couplage de ion-reseau, Ia fluorescence les tauxSddeprovenant dissociation du Ce non-radiative de transitions impliquant des Ctats Sd sont beaucoup plus ClevCs que dans Ic cas d’Ctats 4f. Les taux de dissociation dependent de lat temperature et de la separation energetique du niveau adjacent plus bas. On dCcrit les variations en fonction de la temperature des durées de vie de Ia fluorescence Sd entre 77°et 700°K.
Vol.12, pp. 741—744, 1973.
Pergamon Press.
Printed in Great Britain
NONRADIATIVE DECAY FROM 5d STATES OF RARE EARTHS IN CRYSTALS* M.J. Weber Raytheon Research Division, Waltham, Massachusetts 02154, U.S.A. (Received 10 January 1973 byR.H. Silsbee)
Nonradiative decay from 4f~~ Sd states was investigated for trivalent rare earths in Y3A15012. The rates of both 5d—’4f and 5d—~5dtransitions were determined from3~ measurements of the and ion-lattice intensities coupling, of 5d and Pr3~.Because of lifetimes the stronger nonradiative rates for transitions involving Sd states are much faster fluorescence decay from Ce than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the Sd fluorescence lifetimes from 77 to 700 K are reported.
NONRADIATIVE transitions between 4f” states of rare-earth ions in crystals involving the simultaneous emission of several phonons have been studied extensively.1 The dependences of the rate of multiphonon emission on energy gap to the next-lower level, tern-
investigate radiative and nonradiative transitions from Sd states. Relaxation of 5d states was studied for trivalent rare earths in Y 3A15012 (YAG). In this host the crystalfield splitting of the 5d states is large which results in relatively low-lying Sd3levels (~ l0~cm’ lowerappear than At 300°K,the 5d levels those found in CaF2). as moderately broad, structureless bands in the absorption spectra. Since the fundamental absorption edge of Y 3A15012 begins at ~ 52,000 cm~ many 4f—Sd absorptions are unobserved. The locations of the Sd bands relative to the 4f levels for several rare earths in YAG are shown in Fig. 1. The extent of the shaded regions denotes the full width of the bands at onehalf maximum; the solid half-circles indicate levels from which fluorescence was observed at 300 or 77°K. The lowest Sd levels of Nd3~and Th3~from Fig. I are in the proximity of the 4f levels and very fast nonradiative 5d—~4frelaxation occurs. This was established from the appearance of the Sd bands in the excitation spectra of the Nd3~2P 3,~fluorescence and 3~5D 5D the Tb 3 and 4 fluorescences. The efficiency of the transfer can be determined by comparing the relative intensities of the Sd bands in absorption and excitation spectra. The measured peak absorption cross sections of the 4f—~5d transitions Y3A15O12Sdwere4f 2 therefore theinpredicted 1—5 X 10-18 cm
perature, and phonon spectrum of the host have also been established. Here we investigate similar nonradiative transitions states of thethe excited 4f~~ 5d configuration of rarefrom earths. Because ion-lattice coupling is greater for configurations containing outer d electrons, nonradiative decay is faster and transitions
,~
bridging large energy gaps are possible. Radiative decay from Sd to 4f levels is also faster since electric-dipole transitions are allowed. The 4f~ Sd levels of trivalent rare earths are generally located 2atFor energies> 50,000 cm’ many lanthanide ions,above levelsthe of 4f’~ground level. the 4f’~’Sd configuration overlap those of the 4f’~ configuration. In such cases ions excited into 5d levels rapidly decay nonradiatively to nearby 4f levels. For several ions, however, the energy separation from the lowest Sd level to levels of 4f is very large (e.g. trivalent Ce, Tm, Yb and divalent Eu). Nonradiative decay is therefore less probable and 5d—~4femission may be anticipated. This emission can be used to *
Research supported in part by the Army Night Vision Laboratory, Ft. Belvoir, Va.
,~
741
—~
742
NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS Vol. 12, No. 7 soc .
50
~so 40
—
—
4f25
-
ce3+
40 60
—
20 —
30
_____
S S
Pr3~
.5 0
~D3
3
= — u
t
—.
4f’5d
—
•
‘
20
—.~
04
— —
0
=
~
6
S U C 0
4
.
—
Lifetime
2
° —
Intert SIty
•I~y2~~~ 7F 0
= 0
—
~ — Ce~
Pr~
—
4h,
2-———-’F~
Nd~
Tb~
FIG. 1. Energy level diagrams for several trivalent rare earths in Y3A15012 showing the locations of levels of the 4f’~~round configuration and the lower bands of the 4f’~ Sd excited configuration. Levels or bands from which fluorescence has been observed are marked with solid half circles. radiative decay probabilities are > 3~and 101 seC’. Since300 no Tb~at Sd emission was observed from Nd or 77°K,the nonradiative decay rates are very much faster. For Ce3~and Pr3~there are large energy Separations from the lowest Sd levels to levels of 4f. Although the relative positions and shapes of the configuration coordinate curves for the ground and excited states are unknown, the estimated minimum Sd-to-4f energy gaps are approximately 16,500 and l0,000cm’, respectively, Because of the large energy gaps, nonradiative decay is less probable and Sd fluorescence was observed. For cerium the Sd fluorescence is in the form of a broad band peaking at 550 nm corresponding to unresolved transitions terminating on levels of 2F 2F 5,2 and 7,2 for praseodymium the strongest extends from approximately 3005d tofluorescence 450 nm with peaks corresponding to transitions terminating on the 3H~ and 3F~manifolds.6 The lifetimes of the Sd fluorescence from Ce3~ and Pr3~were measured following excitation with a
60
00
200 Temperature
400 600
000
(OK)
FIG. 2. Temperature dependence 3~and of the Pr3~ Sdinfluorescence Y lifetimes and intensities for Ce 3A15012. deuterium lamp having a 5 nsec pulse width. The fluorescence decays were fitted to a simple exponential time dependence; lifetimes less than the pulse width were derived from a convolution with the system response function. The 2.results plotted as a function of temperature in Fig. At lowaretemperatures the lifetimes were relatively constant. These values were compared to the radiative lifetimes calculated using measured integrated absorption cross sections and the Einstein A—B relationship modified7 to account for the Stokes shift. Because the required fluorescence branching ratio from Sd to levels of 4f was difficult to determine accurately and the matrix elements for absorption and emission were assumed to be equal, the resulting radiative lifetimes were only approximate. For both ions, however, the values were in reasonable agreement with the measured low-temperature lifetimes, thus indicating that the lowest 5d levels fluoresce with near-unity quantum efficiency. 3~and Pr~lifeelevatedrapidly. temperatures, theenergy Ce gap for cerium timesAtdecrease Since the is larger, more phonons are needed to conserve energy and a higher temperature is required before stimulated phonon processes compete with radiative decay. That the additional decay at high temperature is due to nonradiative decay was established from measurements
Vol. 12, No.7 NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS
Pr3~it ~s po~ibleto determine the nonradiative rate over a more extended range including low temperatures where radiative decay predominates. This was accomplished by monitoring the fluorescence
For I I
j
0’
I toe
P~’ YAG
~ i0~
/ /
i~0’
/ /
/ //
. ‘0’
intensity from 3F
“
/ /
/
.
I
0’
—
.!
/
/
/
‘O~
/
/
/ .
/ /
1
/
/400cm’
I0~
/ 6,0cm~
.
,~
/
‘ —
60
00
—
...~.
200
400
600
1Q00
llsiplrotur• (1()
FIG. 3. Temperature dependence of the 5d—.’P 0,1 ,2~ decay rate for Pr’~in Y,Al5012 The dashed curves are the temperature dependencies of stimulated multiphonon emission rates calculated using a single. frequency model. 16
743
-
of the integrated fluorescence intensity versus ternperature. These results are included in Fig. 2 and are in good agreement with the lifetime data. The ternperature dependences of the 5d nonradiative decay rates were obtained by subtracting the radiative decay probability from the total decay probability. These were compared with the temperature dependence for a process involving the emission of n-phonons of frequency v given by [1 exp (—hv/kT)J~.Phonon energies in Y 3AI5O1, range up to 860 cm’~ In the harmonic approximation, the highest energy phonons conserve however, a given energy in the lowestdependence order process possible; the temperature predicted using these phonons does not account for the behavior in Fig. 2. For Ce”~,although data was available over only a small temperature range, a better fit is obtained using phonons of 500600 cm~. This may be due in part to the fact that the high energy phonons in Y,Al 5012 arise from vibrations 4 which are probably less strongly of the AIO4 groups coupled to rare earths at Y3” sites.8 The temperature dependences of the 4fmultiphonon emission rates in YAG also indicate that phonons of less than maximum energies make the dominant contribution.9 —
.~
0 for selective excitation into the lowest Sd level. For weak pumping 3P into Sd at a rate F, the steady-state population in 0 is given by r and r’ are total decay times of the from ~ WP 3F, rr’~N, where W the is the probability for decay Sd Sd to and 3F states, and ~3Nis the Boltzmann population in 3P 0, assuming a rapid thermal equilibration among the stark levels of ‘Po,1 ,2~ ‘6. The temperature dependence of W was found3P by measuring the intensity of a fluorescence line from 0, and correcting for 3 and the the temperature variationsand of emission r, r’, and lines. j~ widths of the absorption The results, normalized to the high temperature decay rates derived from Fig. 2, are shown in Fig. 3. The decay rate appears to approach a temperature independent limit below 100°K. Since Wincludes both radiative and nonradiative processes, this could correspond to the probability for radiative decay and/or for spontaneous phonon emission. Although the Sd fluorescence branching ratio was not measured, a radiative probability to 2’ of l0~sec’ is not unreasonable. ~
‘
3’~relax. temperature Pr ationThe in Fig. 3 cannot dependence be explainedofbythe phonon emission using a single-frequency model or by an activation energy in an exponential law. No detailed analysis of the temperature dependence was undertaken because of uncertainties in the rates at low temperatures, however, as examples, the temperature dependences predicted for phonons of 400 and 600 cm~are included in Fig. 3. It is apparent that relaxation via 600 cm~phonons is insufficient to account for the behavior at the higher temperatures. The singlefrequency model may, of course, be too great an 10 have oversimplification. As Kisliuk and Moore pointed out, processes involving lower-frequency phonons, being of higher order, have steeper temperature dependences and hence should ultimately govern the relaxation at high temperatures. In addition, anharmonic effects” may be important for relaxation of Sd states. Nonradiative decay betweenspectra, Sd levelsthewas also examined. From the absorption energy separations between the ground and first excited Sd levels of Ce~and Pr3” are approximately 7500 and 7200cm~ respectively. By selectively exciting ions into the first excited state of the Sd configuration of ,
744
NONRADIATIVE DECAY FROM Sd STATES OF RARE EARTHS IN CRYSTALS Vol.12, No.7
Ce~at 340nm and observing the risetime of the
emission band in the ultraviolet from Ce~”:Y3 Al5
emission from the ground 5d state, a limiting value of > 5 X 108 sec’ was found for the nonradiative decay rate at 77°K.For comparison, the extrapolated phonon emission rate at 77°Kbetween similarlymultiseparated 4f’ levels in Y 3sec~.~ Al5 012 isnonradiative 10 Whereas intra—4fand3intra—5d transitions occur between initial and final electronic states having the same parity, interconfiguration 5d—4f nonradiative transitions require that at least one odd-parity vibrational mode be active in the orbit-lattice interaction. The AS = 0 spinselection rule should not be important since the spin—orbit interaction causes significant spin state admixmg in both configurations.
at 300°Kfor u.v. and cathode-ray excitation. We have not detected emissionhigh-pressure from excitedxenon Sd states 3”’ orany Pr~using and of either Ce arc lamps for excitation and samples containmercury ing 0.05 per cent Ce and ~ 0.1 per cent Pr.
If emission can be observed from several higher Sd levels, an energy gap dependence for nonradiative decay similar to those for 4f’~levels’ might be obtained. Generally, however, the Sd levels are sufficiently close that, based upon the above rates, a rapid nonradiative cascade to the lowest 2Sd would be expected to adomi. have reported observing nate. Blasse and Bril’
012
In conclusion, it should be noted that 5d—”4f nonradiative decay is also evident in the excitation spectra of 4f emission from divalent rare earths. The Sd bands of divalent rare earths are located at lower energies than for the trivalent ion.2 For Tm2”’ in CaF,, Sd bands beginning at 15 000cm1 appear in the excitation spectrum of the 2F~—p 2F 7p~tofluor13 thus indicating decay from Sd 2F escence, 5,2 across an energy gap of S000—6000cm~at 77°K. Acknowledgements I wishwith to thank Thomas Varitimos for his assistance the measurements andthese Andrew Morrison for growing the crystals used in studies. —
REFERENCES 1. 2. 3.
MOOS H.W.,J. Luminesc. 1,2, 106 (1970) and references therein. DIEKE G.H., Spectra and Energy Levels ofRare Earth Ions in Crystals, (Edited by CROSSWHITE H.M. and CROSSWI-IITE H. lnterscience, New York, (1968). LOHE.,Phys. Rev. 147, 332 (1966); ib&1. 175. 533 (1968).
4.
SLACK G.A., OLIVER D.W., CHRENKO R.M. and ROBERTS S.,Pkvs. Rev. 177, 1308 (1969).
S.
Absorption mea~urementswere made using crystals containing <0.1% rare earths substituted at Y3”’ sites and pathlengths of< 0.5mm. Whereas in YAG the lowest Sd level of Pr3~is below the ‘So level of 4J’2, in other crystals it coincides with or is above 1S 1S 0 in the latter case emission may be observed from 0 (YEN W. private communication).
6.
—
7. 8. 9.
FOWLER W.B. and DEXTER D.L., Phys. Rev. 128, 2154 (1962); J. Chem. Phys. 43, 1768 (1965). Observations of the vibronic structure associated with 4f—5d transitions at liquid helium temperatures would be useful in identifying the relative coupling for various phonons. ZVEREV G.M.,KOLODNY1 G.Ya. and ONISHCHENKO A.M., Soviet Phys. JETP 33,497(1971).
10.
KISLIUKP. and MOORE C.A.,Phys. Rev. 160,307(1967).
11. 12. 13.
RISEBERG L.A. and WEBER M.J., Solid State Commun. 9, 791 (1971). BLASSE G. and BRIL A.,Appl. Phys. Letr. 11, 53 (1967);J. chem. Phys. 47, 5139 (1967). McCLURE D.S. and KISS Z., J. chem. Phys. 39, 3251 (1963). Nous avons examine Ia dissociation non-radiative a partir d’Ctats 4ff” 5d de terres rares trivalentes dans l’Y, Al5 O~.Nous avons determine les taux 34’ etdedudurCes Pr34’. de Vuvie Ia nature de transitions Sd—’~4fet5d-~5dau moyen de mesures et d’intensités du couplage de ion-reseau, Ia fluorescence les tauxSddeprovenant dissociation du Ce non-radiative de transitions impliquant des Ctats Sd sont beaucoup plus ClevCs que dans Ic cas d’Ctats 4f. Les taux de dissociation dependent de lat temperature et de la separation energetique du niveau adjacent plus bas. On dCcrit les variations en fonction de la temperature des durées de vie de Ia fluorescence Sd entre 77°et 700°K.