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Anti-Stokes luminescence of europium(III)-doped lanthanum oxychloride
Voiume 112. number 3
ANT&STOKES
CHEMICAL PHYSICS LETTERS
LUMINESCENCE OF EUROPI~~iII)-DOPED
7 December 1984
L~T~NUM
OXYCHLORIDE
Jorma HdLSk Departrnmt of Clrernist~, Helsinki Urn3ersity of Teclmology. SF-02150 Espoo IS. Firdatrd and ER 210 du CNRS. 1, PI. A. Briand. F-92190 Mceudon, Frame Received 1 September
1984
The anti-stokes luminescence from the siD,,z,s 1eveIs of Eu3*-doped LaOCI was observed at 300 and 77 I( under dycIaxr escitation to the 5Do level. A two-photon absorption from the 7Fo ground level to charge-transfer states via the sD0 level was concluded to be the mechanism involved. The absorption of the first photon through the forbidden sDod7Fo transition determines the upconversion efficiency. The two-photon absorption seems to occurasan intra-ion processwithout interionic energy transfer.
1. Introduction The anti-Stokes Iurninescence of rare-earth(RE-) doped materi& has been studied extensivefy since the late 1960s [I]. The emphasis of these studies has been in developing IR-to-visible converting phosphors for detection of IR radiation. Most studies have dealt with RE compounds co-doped with two RE ions, usually a combination of Yb3+ with Er3+, Ho3+ or Tm3+_ The upconversion in these materials requires cooperative
energy
processes
between
like and unlike
RE ions. A power conversion efficiency of one percent has been achieved with yttrium oxychloride doped with Yb3+ and Er3+ [2]_ In a few cases anti-Stokes luminescence has been observed in systems co~l~ining only one doping RE ion. The materials studied recently include RE batides doped with Pr3+ [3] and Gd3+ [4] as well as Sm2+doped BaFCl [5] _The upconversion with these dopants has been achieved with an absorption of two photons followed by emission from higher 4f levels. The absorption processes may occur in one RE center or may involve cooperative phenomena between several ions. The present work is a continuation of the study of the luminescence properties ofeuropium (III)-doped RE osyhalides [6,7] _The emphasis in this work has so far been placed upon the evaluation of crystal field effects on the energy-levci scheme and line-to&e tran316
sition intensities. The aim of this particular paper is to describe, for the first time, the anti-Stokes iuminescence of Eu3’doped crystalline material, LaOCl: Eu3+ under dye-laser excitation. The possible mechanisms involved in the two-photon absorption are discussed.
2. Experimental The europium ~I~I)~oped lanthanum oxychloride was prepared by a solid-state reaction between the corresponding oxide mixture and ammonium chloride_ A detailed description of this method can be found elsewhere IS] _The concentration of the dopant ion was one mole per cent of the total RE content. A continuous-~vave Spectra Physics 375 rliodamine 6G dye laser (pumped by a Spectra Physics 164 arson ion laser) was used to excite the SD0 level (578.8 run) of the Eu3+ ion in LaOCI. Optical measurements were made both at ambient and liquid-nitrogen temperatures. The luminescence emitted was dispersed with a 1 m Jarrell-Ash nlo~ocl~omator and detected with a Hamamatsu R374 photomultiplier in the whole wavelength range from 400 to 750 nm.
0 009-2614/S4/S 03.00 0 EIsevier Science Pubhhers (North-Holland Physics Publishing Division)
B.V.
Volume 112, number 3
CHEMICAL
PHYSICS
7 December
LETTERS
1984
The excitation of the 5Do level of Eu3+ ion at 17276 cm-l resulted in strong emission between 585
earlier [S] for the BaFCl system, this contribution becomes non-negligible when the sample temperature is increased_ The main contribution to the anti-Stokes lumines-
and 702 nm. These transitions originate from the relaxation of the excited 5Do level to 7F,2J,4 levels
cence in LaOCl: Eu3+ can thus be assumed to arise from two-photon absorption processes_ An efficient
of the ground multiplet. In addition to the strong5Do emission, transitions from higher 5D levels, 5D1,2,3, were observed (cf. fig. 1). The intensities of these transitions were at least one order of magnitude weaker, decreasing with increasing J-value of the emitting level. Consequently no emission from the sD4 level was observed owing to efficient multiphonon deexcitation processes. Two explanations can be found for the origin of the anti-Stokes luminescence from 5D levels: (i) thermal population of higher 5D levels accompanied by direct radiative relaxation, and (ii) excitation of higher levels by an absorption of a second photon on the SD, level. The thermal excitation of the higher 5D levels requires an inevitable decrease in intensity of anti-Stokes emission with decreasing sample temperature_ Since no such variation in transition intensities was observed between the spectra obtained in 77 and 300 K, thermal excitation cannot give any major contribution to the intensities observed. However, as was pointed out
excitation of the sDo level depends on the absorption strength of the forbidden 7FodD0 transition_ Despite the high intensity of the 5Do+7Fo transition in the LaOCl matrix, the absorption of the first photon remains the rate-determining step in the two-photon absorption process_ On the other hand, owing to the large energy gap between the 5Do level and the ground multiplet (12000 cm-l) (fig. 2) and the forbidden nature of intraconfigurational4f transitions, the lifetime of the 5Do level is very long - of the order of several milliseconds - ensuring sufficient population of this level. The absorption of the second photon presents no difficulties since the transition from the 5Do level to the charge-transfer statg (CTS) is strongly allowed as a transition between levels of different parity_ Moreover, the CTS absorption band has in LaOCl an optimum position with an enera of twice the “Do one (== 35000 cm-l) [6] _The absorption of the second photon into the CTS band is followed by a rapid multiphonon de-excitation to the 5D levels
3. Results and discussion
‘0-j” 'F.
I
’ Dz-
7F,
25
F& L Parts of the anti-Stokes luminescencespectrumof LaOCI:Eu3+at 77
K
!L
575
“Ill
-I
under dye-laserescitation on the ‘Do 1eveL 247
Volume 112, number 3
CHEhfICAL PHYSICS LE’XTERS
7 Reccmbcr
1984
4. Conclusions
The presence of anti-Stokes Iuminescence from 5D1W levels ofLaOC1: Eu3+ has been observed under dyell%er excitation to the lowest 5D level, sDo_ The largest cont~bntion to the upconversion arises from the two-photon absorption via the SD0 level to chargetransfer states. The thermal population of the higher SD levels is, at least at lower temperatures, negligible, The two-photon absorption seems to involve only one center with no energy transfer between Eu3+ ions. The high intensity of the 5Do+7Fo transition and the optimum energy position of the charge-t~sfer states renders the LaOCI host an excellent candidate for t~orou~ studies of the upc~~~e~s~onpI~e~omena ir. tile w+ Fig. 2. Encp,y-level scheme of the europium(Ili)
via the closeIy spaced energy levels. This cascade process terminates eventua)ly at the SD0 level resulting in the do~nation of red iuminescence. The two-photon absorption in binary doped RE materials requires energy transfer between both like and unlike ions. With only one doping ion the energy transfer is, however, not always necessary for observation 0fantiStokes luminescence. It has been shown that, e.g. in Cd3*-doped LaC13, rite energy transfer between gadolinium ions is not probable [4], whereas in Pr3+-doped LaF3, cooperative energy transfer betwcen three praseody~um ions is a prereq~si~e [J]. In the case of LaWI: Et?, the low concentration (one moie per cent) of the dopant renders the distances between adjacent europium ions Iarge enough to decrase the probabitity ofenergy transfer_ The verification of the presence of energy-trzmsfer processes requirs a complete analysis of the decay time measurements which is beyond the scope of this communication_
24s
ion.
ion.
The author expresses hisgratitude to Dr. P. Percher for giving the initiative to this work.
References 111 F. Auzef, Compt. Rend. Acad. Sci. (Paris] 3263, (5966) 1016. [2] J-E_ Gcusic, F.W. Ostermayer, Ii&%. hfarcos, L.G. ~itn Uitert and J.P. van der Zicf, J. Appl. Phys. 42 (19711 1958. [31 L.-S. Lee, SC. Rand and A.L. Sc~a~~lo~v, Phys. Reu. I329 (1984) 6901. [4 ] C. Liuares, P. Jung, G. GouIon and F, Gaume, J. LessCommon hiet. 93 (1983) 89. [5] J.C. Gacon, N.F. Joubert, hl. Bianchard and 3. Jacquier,
PJlys. Rev. B29 (3984) 1155. [6] J. Ii&5 and P. Porcher, J. Chem. Phys. 75 {198ff 210&_ [7] J. H61d and P. Porcher, J. Chem. Phys. 76 (1982) 2790. [8] J, HSfd. M Leskelg and L. X&iM, Mater. Res. BuK I4 (1979) 1403.
ANT&STOKES
CHEMICAL PHYSICS LETTERS
LUMINESCENCE OF EUROPI~~iII)-DOPED
7 December 1984
L~T~NUM
OXYCHLORIDE
Jorma HdLSk Departrnmt of Clrernist~, Helsinki Urn3ersity of Teclmology. SF-02150 Espoo IS. Firdatrd and ER 210 du CNRS. 1, PI. A. Briand. F-92190 Mceudon, Frame Received 1 September
1984
The anti-stokes luminescence from the siD,,z,s 1eveIs of Eu3*-doped LaOCI was observed at 300 and 77 I( under dycIaxr escitation to the 5Do level. A two-photon absorption from the 7Fo ground level to charge-transfer states via the sD0 level was concluded to be the mechanism involved. The absorption of the first photon through the forbidden sDod7Fo transition determines the upconversion efficiency. The two-photon absorption seems to occurasan intra-ion processwithout interionic energy transfer.
1. Introduction The anti-Stokes Iurninescence of rare-earth(RE-) doped materi& has been studied extensivefy since the late 1960s [I]. The emphasis of these studies has been in developing IR-to-visible converting phosphors for detection of IR radiation. Most studies have dealt with RE compounds co-doped with two RE ions, usually a combination of Yb3+ with Er3+, Ho3+ or Tm3+_ The upconversion in these materials requires cooperative
energy
processes
between
like and unlike
RE ions. A power conversion efficiency of one percent has been achieved with yttrium oxychloride doped with Yb3+ and Er3+ [2]_ In a few cases anti-Stokes luminescence has been observed in systems co~l~ining only one doping RE ion. The materials studied recently include RE batides doped with Pr3+ [3] and Gd3+ [4] as well as Sm2+doped BaFCl [5] _The upconversion with these dopants has been achieved with an absorption of two photons followed by emission from higher 4f levels. The absorption processes may occur in one RE center or may involve cooperative phenomena between several ions. The present work is a continuation of the study of the luminescence properties ofeuropium (III)-doped RE osyhalides [6,7] _The emphasis in this work has so far been placed upon the evaluation of crystal field effects on the energy-levci scheme and line-to&e tran316
sition intensities. The aim of this particular paper is to describe, for the first time, the anti-Stokes iuminescence of Eu3’doped crystalline material, LaOCl: Eu3+ under dye-laser excitation. The possible mechanisms involved in the two-photon absorption are discussed.
2. Experimental The europium ~I~I)~oped lanthanum oxychloride was prepared by a solid-state reaction between the corresponding oxide mixture and ammonium chloride_ A detailed description of this method can be found elsewhere IS] _The concentration of the dopant ion was one mole per cent of the total RE content. A continuous-~vave Spectra Physics 375 rliodamine 6G dye laser (pumped by a Spectra Physics 164 arson ion laser) was used to excite the SD0 level (578.8 run) of the Eu3+ ion in LaOCI. Optical measurements were made both at ambient and liquid-nitrogen temperatures. The luminescence emitted was dispersed with a 1 m Jarrell-Ash nlo~ocl~omator and detected with a Hamamatsu R374 photomultiplier in the whole wavelength range from 400 to 750 nm.
0 009-2614/S4/S 03.00 0 EIsevier Science Pubhhers (North-Holland Physics Publishing Division)
B.V.
Volume 112, number 3
CHEMICAL
PHYSICS
7 December
LETTERS
1984
The excitation of the 5Do level of Eu3+ ion at 17276 cm-l resulted in strong emission between 585
earlier [S] for the BaFCl system, this contribution becomes non-negligible when the sample temperature is increased_ The main contribution to the anti-Stokes lumines-
and 702 nm. These transitions originate from the relaxation of the excited 5Do level to 7F,2J,4 levels
cence in LaOCl: Eu3+ can thus be assumed to arise from two-photon absorption processes_ An efficient
of the ground multiplet. In addition to the strong5Do emission, transitions from higher 5D levels, 5D1,2,3, were observed (cf. fig. 1). The intensities of these transitions were at least one order of magnitude weaker, decreasing with increasing J-value of the emitting level. Consequently no emission from the sD4 level was observed owing to efficient multiphonon deexcitation processes. Two explanations can be found for the origin of the anti-Stokes luminescence from 5D levels: (i) thermal population of higher 5D levels accompanied by direct radiative relaxation, and (ii) excitation of higher levels by an absorption of a second photon on the SD, level. The thermal excitation of the higher 5D levels requires an inevitable decrease in intensity of anti-Stokes emission with decreasing sample temperature_ Since no such variation in transition intensities was observed between the spectra obtained in 77 and 300 K, thermal excitation cannot give any major contribution to the intensities observed. However, as was pointed out
excitation of the sDo level depends on the absorption strength of the forbidden 7FodD0 transition_ Despite the high intensity of the 5Do+7Fo transition in the LaOCl matrix, the absorption of the first photon remains the rate-determining step in the two-photon absorption process_ On the other hand, owing to the large energy gap between the 5Do level and the ground multiplet (12000 cm-l) (fig. 2) and the forbidden nature of intraconfigurational4f transitions, the lifetime of the 5Do level is very long - of the order of several milliseconds - ensuring sufficient population of this level. The absorption of the second photon presents no difficulties since the transition from the 5Do level to the charge-transfer statg (CTS) is strongly allowed as a transition between levels of different parity_ Moreover, the CTS absorption band has in LaOCl an optimum position with an enera of twice the “Do one (== 35000 cm-l) [6] _The absorption of the second photon into the CTS band is followed by a rapid multiphonon de-excitation to the 5D levels
3. Results and discussion
‘0-j” 'F.
I
’ Dz-
7F,
25
F& L Parts of the anti-Stokes luminescencespectrumof LaOCI:Eu3+at 77
K
!L
575
“Ill
-I
under dye-laserescitation on the ‘Do 1eveL 247
Volume 112, number 3
CHEhfICAL PHYSICS LE’XTERS
7 Reccmbcr
1984
4. Conclusions
The presence of anti-Stokes Iuminescence from 5D1W levels ofLaOC1: Eu3+ has been observed under dyell%er excitation to the lowest 5D level, sDo_ The largest cont~bntion to the upconversion arises from the two-photon absorption via the SD0 level to chargetransfer states. The thermal population of the higher SD levels is, at least at lower temperatures, negligible, The two-photon absorption seems to involve only one center with no energy transfer between Eu3+ ions. The high intensity of the 5Do+7Fo transition and the optimum energy position of the charge-t~sfer states renders the LaOCI host an excellent candidate for t~orou~ studies of the upc~~~e~s~onpI~e~omena ir. tile w+ Fig. 2. Encp,y-level scheme of the europium(Ili)
via the closeIy spaced energy levels. This cascade process terminates eventua)ly at the SD0 level resulting in the do~nation of red iuminescence. The two-photon absorption in binary doped RE materials requires energy transfer between both like and unlike ions. With only one doping ion the energy transfer is, however, not always necessary for observation 0fantiStokes luminescence. It has been shown that, e.g. in Cd3*-doped LaC13, rite energy transfer between gadolinium ions is not probable [4], whereas in Pr3+-doped LaF3, cooperative energy transfer betwcen three praseody~um ions is a prereq~si~e [J]. In the case of LaWI: Et?, the low concentration (one moie per cent) of the dopant renders the distances between adjacent europium ions Iarge enough to decrase the probabitity ofenergy transfer_ The verification of the presence of energy-trzmsfer processes requirs a complete analysis of the decay time measurements which is beyond the scope of this communication_
24s
ion.
ion.
The author expresses hisgratitude to Dr. P. Percher for giving the initiative to this work.
References 111 F. Auzef, Compt. Rend. Acad. Sci. (Paris] 3263, (5966) 1016. [2] J-E_ Gcusic, F.W. Ostermayer, Ii&%. hfarcos, L.G. ~itn Uitert and J.P. van der Zicf, J. Appl. Phys. 42 (19711 1958. [31 L.-S. Lee, SC. Rand and A.L. Sc~a~~lo~v, Phys. Reu. I329 (1984) 6901. [4 ] C. Liuares, P. Jung, G. GouIon and F, Gaume, J. LessCommon hiet. 93 (1983) 89. [5] J.C. Gacon, N.F. Joubert, hl. Bianchard and 3. Jacquier,
PJlys. Rev. B29 (3984) 1155. [6] J. Ii&5 and P. Porcher, J. Chem. Phys. 75 {198ff 210&_ [7] J. H61d and P. Porcher, J. Chem. Phys. 76 (1982) 2790. [8] J, HSfd. M Leskelg and L. X&iM, Mater. Res. BuK I4 (1979) 1403.