Charles F. Rapp and John Chrysochoos
103
Again, as with FMN, these values do not represent the true excited state pK* but must be attributed to the quenching reaction clnly. With respect to the indole derivatives one discrepancy should be noted. If our assumptions are correct then since k , = &-I* the ratio of any directly measured lifetime to that of another subrstance should equal the ratios of their quenching constants. In Table 111 the reference compound
is p-hydroxyethylindole (HEI) and we see that for substances 2, 3, and 4 the agreement is fairly good. For compounds 5 to 7 however ~ H E I /is~ too large. However it should be noted that the latter compounds all carry an ionized -NH3+ group. We feel therefore that the above discrepancy might be attributed to a lower rate constant for the reaction between H+ and the positively charged indole derivative.
ce Lifetimes of Neodymium-Doped Glasses and Glass-Ceramics Charles F. Rappl Owens-Illinois, Corporate Research Laboratories, Technical Center, Toledo, Ohio
and John Chrysochoos* De,oartment of Chemistry, 7he University of Toledo, Toledo, Ohio 43606 (Received December 11, 1972) Pualicabon costs assisted by Owens-Illinois
The fluorescence lifetimes of Nd3+ in neodymium-doped glass-ceramics are much shorter than the lifetimes in neodymium-doped glasses under comparable Nd3+ concentrations, although the absorption and emission spectra are identical. These results indicate that the neodymium ions are excluded from the crystalline phase of the glass-ceramic and they are entirely accumulated in the glassy phase. Under these conditions shorter lifetimes result due to enhanced concentration quenching. The self-quenching rate in the glasses varies linearly with the [Nd3+]2 which is proportional to 1/R6. This dependence may imply either a dipole-dipole type of energy transfer or a rapid energy migration to "ion pairs" in which energy may dissipate by exchange interactions.
Introduction It has been shown recently that lasing action can be obtained from a transparent neodymium-doped glass-ceramic.2 The glass-ceramic is a two-phase material, whereas almost all previous solid-state laser materials have been either single crystals or glasses. Since the glass-ceramic is a two-phase material, a significant question ariijes regarding the distribution of the neodymium ions between the crystalline and the residual glassy phases and the effect of such a distribution upon the emission characteristics of the Nd3+ ions. A preferential segregation of the neodymium ions into one of the two phases would be accompanied by an increase in the concentration quenching of the Nd3+ fluorescence. To gain some insight into this question, the fluorescence lifetimes of Nd3+ were measured in neodymium-doped glasses and in glass-ceramics at various extents of crystallization. Experimental Proeedures The base composition, in mole percentages, of the glasses and the glass-ceramics used is Si02 73.24%, A1203 13.73%, Liz0 8.69510, RaO 1.75%, Ti02 1.51%, and ZrO2 1.08%. Melts were prepared containing 0.097, 0.189, 0.361, 0.550, and 0.90'7% NdzO3 which replaces Si82. The glasses were melted a t 1600" and they were annealed. Small samples, 2 in. X 718 In. 0.25 in., were cut and were given addiThe Journal of Physical Chemistry, Vol. 77, No. 8,7973
tional one- and two-stage heat treatments to produce glass-ceramics with up to 70% crystallinity. An X-ray diffraction pattern was obtained for each of the heat-treated samples to identify the phases present and to determine their contribution to the composition of the glass-ceramic. The main crystalline phase present, in addition to a small quantity of cubic ZrOz, was a high-quartz solid-solution phase.2-4 The percentage of the glass remaining in each sample was estimated by comparing the intensity of the broad maximum in the diffraction pattern of the glassceramic to that for the original glass.59 The fluorescence lifetimes were measured by exciting the samples with a xenon flashlamp and by photographing the oscilloscope display of the intensity of fluorescence us. time. The duration of the xenon flash was about 20 psec.
Results and Discussion As it is commonly observed with neodymium-doped glasses,sb the fluorescence decays in the systems studied This work is based in part on a thesis submitted in partial fulfillment of the Ph.D requirement at the University of Toledo. C. F. Rapp and J, Chrysochoos, J. Mater. Sci, 7, .I090(1972). S.Ray and G. M. Muchow, J. Amer. Ceram. Soc., 51,678 (1968). R. Roy,Z. Kristallogr., 111, 165 (1959). (a) S.M. Ohlberg and D. W. Stricker, J. Amer. Ceram. Soc., 45,170 (1962);(b) E. Snitzer and C. G . Young, Lasers," Vol. 2. A. K. Levins, Ed., Marcel DeCker, New York, N. Y . . 1963,p 202.
Neodymium-Doped Gla!eses and Glass-Ceramics
1017
TABLE I: Fluorescence ILifetimes of Neodymium-Doped
Glasses and Glass-Ceramics
0.097 0.097 0.189 0.189 0.189 0.1 89 0.189 0.189 0.189 0,189 (1.189 U.189 0.361 0.361 0.361 0.361 0.550 Q.907
0.073 0.073 0.141
100 40
41 5 348 380 358 239 254 243 291 253 241 245 224 31 6 347 302 155 229 134
100
0.141
98 40 45 40 55 40 40 35 30
0.141 0.141
0.141 0.141 0.141 0.141 0.141 0.141 0.269 0.269 0.269
100 95 80
(3.269
4.0 100 100
0.41 1 0.692
0.1
2Nd
-
3+
4
(
Ii5/2)
-2Nd
-
3+
0.3
o,4
0.5
[Nd3+f~moie$/iit~1a i/ R6
Figure 1. Dependence of the reciprocal fluorescence lifetimes of Nd3+, measured at ’/2/20, upon the square of the concentration of Nd3+ in neodymium-doped glasses.
were not simple expanentials. Therefore, to obtain a decay time characteristic of the “average” ion, the fluorescence lifetimes were measured at the time a t which the fluorescence intensity had decayed to ?ne-half of 120 where 40 represents the fluorescence intensity at 20 hsec after the initiation of the flash. Results obtained in this way for various glasses and glass-ceramics are given in Table I. The fluorescence lifetimes of Nd3+ decrease as the concentration of the rare earth ion increases. This was observed in all glasses and glass-ceramics employed. This is known as concentration quenching and it is attributed6 to a nonradiative exchange of energy between a n excited Nd3+ ion in the metastable 4F3/2 state and a neighboring Nd3+ ion in the 4Xea/z ground state. The energy exchange is of the type Nd3’ (4F3jz)-5. Nd3’ (419,2)
0.2
4
I9/2)
-
It has also been suggested that the transition 4F3/2 $113’2 coupled with the transition 419/2 4I15/2 may contribute to the quenching process.7 Although there has been considerable uncertainty regarding the energy transfer rmechanism,8 i. (> , dipole-dipole, dipole-quadrupole, elc., Lt has been observed that the rate of quenching increase linearly with the square of the neodymium concentration.7.9 This would imply a dependence of the quenching rate upon 1/R6 which in turn may indicate energy transfer via a dipole-dipole interaction.lOJ1 A very strict dependence of the quenching rate upon the square of the neodymium concentration was observed in this study for all neodymium-doped glasses. ‘This dependence is shown in Figure 1 where l / r (the reciprocal of the fluorescence lifetimle) is plotted us. [Nd3+]2. Since the square of the neodymium concentration is proportional to 1/R6, where R is the distance between two Nd3+ ions, it may appear that the quenching rate varies linearly with 1/R6. Therefore, one could speculate that these results imply a correlation between the self-quenching process and ;a dipoledipole type of energy transfer. However, i t should be pointed out that there is an alternative interpreeation for the pinear dependence of the quenching rate
upon the square of the neodymium concentration. If the resonant energy transfer from one neodymium ion to a neighboring one (which will be excited to the 4F3/2 state) is sufficiently rapid, the quenching rate will be proportional to the number of neodymium ions which have another neodymium ion as a nearest neighbor in the glass (not considering the anion positions). Therefore, the excitation energy will migrate from neodymium to neodymium until it reaches a neodymium “pair.” The rate of quenching will be proportional to the concentration of such pairs which in turn will be proportional to the square of the neodymium concentration. Since i t has been reported that the resonant energy transfer is completed in nanoseconds12 for Nd3+, such an interpretation is possible. This effect has been observed experimentally13 in the quenching of Eu3+ fluorescence by Nd3+. In the case of different donor and acceptor ions, the number of pairs, and therefore the quenching rate, are proportional to the first power of the quencher concentration. In the case in which both the donor and the acceptor are the same species, the number of pairs will be proportional to the square of the concentration. The neodymium absorption spectra in the glasses and in glass-ceramics were nearly identical. No differences were found between the line widths of the neodymium absorption bands in the two materials. This would imply that the neodymium ions are excluded from the crystalline phase and are accumulated in the residual glassy phase of the glass-ceramic. Narrowing of the absorption lines should be observed if the neodymium ions were entering the crystalline phase (decreased inhomogeneous line broadening). (6) G. E. Peterson and P. M. Bridenbaugh, J . Opt. SOC.Amer., 54, 664 (1964). (7) L. G. Van Uitert, E. F. Dearborn, and J. J. Rubin, J , Chem. Phys., 47, 547 (1967). (8) J. S. Stroud, Appi. Opt., 7, 751 (1968). (9) W. W. Holloway, Jr., and M. Kestigian, J. Chem Phys., 43, 147 (1965). (10)T. Foerster, Z. Naturforsch. A , 4, 321 (1949). (11) D. L. Dexter,J. Chem. Phys.. 21, 836 (1953). (12) D. K. Duston, Thesis, Rensselaer Polytechnic Institute, 1969. (13) L. G. Van Uitert, E. F. Dearborn, and J. J. Rubin, J. Chem. Phys., 46, 420 (1967). The Journal of Physicai Chemistry, Voi. 77, No. 8,1973
Charles F. Rapp and John Chrysochoos
X
Oil97 MOLE % N%03
0 0.189 MOLE %
a
0.361 MOLE%Nd203
0
k I
03 APPARENT [Nd37 FROM
L d
0
200
400
1
600 t
803
1000
1200
1400
(ped
Figure 2. Fluorescence decay in neadymium-doped glasses and glass-ceramics, Any segregation of the neodymium ions in the glass-ceraniic should a h be accompanied by an increase in the rate of concentratior L quenching because of the decreased Nd-Nd distance. This would result in a decrease in the fluorescence lifetime. Since the neodymium absorption spectra are identical in both materials, the neodymium transition probabilities and the energy overlap of the transitions involved in the quenching process would be the same in both materials. Therefore, any differences observed in the fluorescence lifetimes of the two materials could only be the result of different neodymium corrcentrations. If it were assumed that all the neodymium ions were Pegregated into the glassy phase present in the glass~u~ could be culcuceramic, a local n e o ~ y m concentration lated from the ~ e r c ~ ~ of~ the t ~ gglass e present. Figure 2 depicts plots of log 1 us 1 for several glass and glass-ceramic samples with (different total [Nd3+] and calculated local [Nd"]. As it can e seen in the figure, the fluorescence lifetimes of the glass-ceramics are in close agreement with the lifetimes expected for their calculated local neodymium concentrations. For example, the lifetimes in a 100'7~ glass containing (3.550 mol of Ndz& and ~n a
The Jolafnai of Physical Chc?rnistry, Voi. 77,
No. 8, 7973
04
'4 DATA
0.5
Q6
Q7
(m~les/liter)
Figure 3. Correlation between the apparent neodymium concentrations obtained via t h e observed fluorescence lifetimes and Figure 1 and the calculated neodymium concentration assuming that t h e neodymium ions are excluded from the crystalline phase of the glass-ceramic. Solid line represents a theoretical correlation with slope equal to unity.
glass-ceramic with 30% glass containing 0.189 mol of Ndz& are almost the same (229 and 224 bsec) . From the fluorescence decay times of the glass-ceramics and from Figure 1, it is also possible to determine "apparent" neodymium concentrations for the glass-ceramics, that is, neodymium concentrations which should produce the observed decay times. If all of the neodymium ions were being segregated into the glassy phase of the glassceramic, these apparent neodymium concentrations should be the same as those calculated from the per cent glass in the glass-ceramics. A correlation between the values obtained via both ways is shown in Figure 3 . A fairly straight line is obtained with a slope equal to one. It should be pointed out that the percentage of glass can be determined with an accuracy of +5 to &lo%. It appears from these data on absorption and decay times that all or nearly all of the neodymium ions in the LizO-Al~03-SiOz based glass-ceramics are entering the residual glassy phase. The present data are not sufficient in distinguishing between the dipole-dipole interaction or the participation of neodymium pairs in the concentration quenching of the fluorescence of neodymium. Studies along these lines are under way.
