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Fast time-resolved luminescence emission spectroscopy in some feldspars
PII:
Radiation Measurements Vol. 29, No. 5, pp. 553±560, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1350-4487(98)00068-7 1350-4487/98 $19.00 + 0.00
FAST TIME-RESOLVED LUMINESCENCE EMISSION SPECTROSCOPY IN SOME FELDSPARS R. J. CLARK and I. K. BAILIFF* Luminescence Dating Laboratory, Environmental Research Centre, South Road, Durham University, Durham DH1 3LE, UK (Received 4 February 1998; revised 20 May 1998; in ®nal form 21 May 1998) AbstractÐMeasurements of fast time-resolved 850 nm stimulated luminescence (resolution 5 ns) and also CW infra-red stimulated luminescence (IRSL) emission spectroscopy have been performed with a set of six specimen feldspars comprising orthoclase, albite, sanidine and oligoclase. The IRSL was observed following the administration of ionising radiation dose. Detection of the time-resolved spectrum was de®ned by the use of bandpass (FWHM 40 nm) interference ®lters enabling low resolution emission spectroscopy to be performed between 300 and 550 nm. Multi-exponential lifetime analysis suggests the possibility of a link between luminescence emitted in the UV spectral region (0300 nm) and the visible spectral region (i.e. >400 nm). The results also support a model of lattice stabilised electron vacancies for the luminescence emission rather than intrinsic transition metal ion luminescence. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION Studies of optically stimulated luminescence mechanisms in natural dosimetric materials (e.g. quartz and feldspar) used for dating have been based mainly on explorations of charge carrier (trap) excitation (e.g. HuÈtt and Jaek, 1993) and recombination by CW emission spectroscopy (e.g. Huntley et al., 1991; Clarke and Rendell, 1997). The information yielded by such studies is limited in terms of the recombination process, the data obtainable with the latter technique being limited by the complexities and possible interactions of the emission bands. However, fast (ns) time-resolved emission spectroscopy substantially extends the study of the nature of recombination processes because it has the potential to deconvolute the dierent recombination processes and identify them by measuring the recombination lifetimes. In preliminary work, time-resolved measurements performed with samples of feldspar showed that the spectra could be resolved into one or more exponential components by the use of multi-exponential non-linear regression analysis (Clark et al., 1997). Measurements were performed using three broad spectral ranges obtained by placing band colour glass ®lters (FWHM 280±380, 350±575 and 460± 625 nm) in the detection system. The evaluated lifetimes ranged from 030 ns to 030 ms and, where present, the fastest component (030 ns) appeared to be associated with the UV emissions (detection window FWHM 280±380 nm). The results also indi-
cated the presence of a rise time (signal growth rather than decay) in one sample, orthoclase moonstone. In this paper we report on further investigations of the wavelength dependency of the lifetime components with greater spectral resolution, and the interactions between lifetime components at dierent emission wavelengths.
2. MEASUREMENTS The time-resolved luminescence (TRL) spectrometer was used in the con®guration described in Clark et al. (1997) with the exception that the detection window was de®ned by the use of bandpass interference ®lters. The ®lters comprised a set with central wavelengths spaced at 50 nm intervals between 300 and 550 nm and with FWHM of 040 nm; the transmission characteristics for the ®lters were checked using a Perkin±Elmer Lambda 19 UV/Vis/NIR spectrometer, as shown in Fig. 1. Optical stimulation was performed with a Quanta Ray MOPO 710 laser adjusted to 850 nm (linewidth of <4 nm); the pulse repetition rate is 10 Hz. The luminescence was detected using a fast EMI 9813QB photomultiplier, the pulse output of which was fed to a multi-channel scalar (MCS) adjusted to provide a dwell time of 5 ns. The start of each sweep of the MCS was triggered by an advanced Q switch pulse from the laser control unit. The PMT detection system was active during the delivery of each laser pulse.
*To whom all correspondence should be addressed. Fax: 0191-374-3619; E-mail: ian.baili@durham.ac.uk. 553
R. J. CLARK and I. K. BAILIFF
554
Fig. 1. Transmission curves for the interference ®lters used in this study.
The feldspar samples (see Table 1) were in the form of cleaved chips and further details are given in Barnett and Baili (1997). All of the aliquots of each sample were bleached at a distance of 10 cm from a 150 W tungsten-halogen lamp for 2 days prior to use. Doses of ionising radiation in the range 2±100 Gy were delivered by a Sr90/Y90 beta source, the level depending on the measurement requirements. Optical bleaching and the delivery of ionising radiation dose were performed at ambient temperatures. Following the administration of radiation dose, all samples were preheated at 508C for 1 min and stored for several hours before measurement to allow traps unstable at room temperature to be emptied of charge. A time resolution of 5 ns per channel over 8k channels (i.e. 40 ms per sweep) was employed for TRL measurements with all samples with one exception. In the case of albite, where the TRL was detected through a 550 nm ®lter, additional measurements were performed using time resolutions of 80 ns and 2 ms per channel. The intensity of the incident laser beam was adjusted (estimated Table 1. Summary of the feldspars used in this study Sample Orthoclase perthite Orthoclase moonstone Amelia albite Irkutsk orthoclase (high temperature form) Sanidine Oligoclase
Laboratory No
Ratio K:Na:Ca
Mi20232
77:8:15
Ð Al7646
72:26:2 3:95:2
Or]SP Sa8433 Ol7647
90:10:0.1 82:16:2 9:44:47
energy 01±100 nJ per pulse) until a maximum rate of 1 cps was recorded in the channel with the greatest number of recorded photon counts (referred to as the peak channel) to avoid pulse pile-up. Data were acquired until at least 1000 counts were recorded in the peak channel, except for those measurements indicated in the results where signal levels were too low (such that raising the laser power to increase the signal intensity would introduce unacceptable interference from background luminescence signals). It is worth noting here an important dierence between time-resolved luminescence discussed in this paper and the more commonly observed IRSL under continuous or pulsed stimulation. Following the delivery of each laser pulse the time of arrival of photons detected at the PMT is registered. Thus the TRL spectra obtained, as described further below, are not equivalent to decay curves recorded using CW stimulation since they represent the time-integrated distribution of the luminescence. The time-resolved spectra were analysed using multi-exponential nonlinear regression analysis (using a Marquardt± Levenberg algorithm) with the form X I ai exp ÿt=ti 1 i
where I = intensity (detected photon counts), ai =pre-exponential factor (i.e. magnitude of each exponential component), t = time and ti=lifetime of each exponential component (same units as t). CW IRSL emission spectra for each of the samples used in the TRL study were measured using a fast scanning interference ®lter spectrometer (Baili and Poolton, 1991; Baili et al., 1977)
FAST TIME RESOLVED LUMINESCENCE coupled to the output of a Ti±sapphire laser adjusted to 850 nm, employed as the stimulation source. All of the samples were in the form of cleaved chips and had received a radiation dose of 200 Gy about 2 months before the spectra were recorded at room temperature. The spectrometer comprises 16 interference ®lters and scans from 340 to 640 nm in 125 ms; the photon counts registered by the PMT during the passage of each ®lter through the light collection path is registered, enabling the spectra to be integrated over selected time intervals in the subsequent data analysis.
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3. RESULTS AND DISCUSSION The emission spectra for each of the samples are shown in Fig. 2 and the results of the regression analysis of the time-resolved spectra can be seen in Table 2. We note here additional TRL data that were obtained as a result of the comparatively long lifetimes evaluated in two cases. For the albite sample (Table 2; Fig. 3) the time-resolved spectrum in the 550 nm detection window has a minor component that persists well beyond the 40 ms sweep using a 5 ns dwell time. The lifetime evaluated using
Fig. 2. CW IRSL emission spectra for each of the minerals used in this study with a stimulation wavelength of 850 nm. The spectra were integrated over the ®rst 100±30 s of stimulation, depending on the intensity of emission, and are corrected for instrument response.
R. J. CLARK and I. K. BAILIFF
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Table 2. Summary of the lifetime components of each feldspar in each of the detection windows. The numbers in brackets are the percentage contributions of each component to the total signal as measured by the pre-exponential factors. Stimulation at 850 nm (a) Orthoclase perthite 20232 Detection wavelength/nm t1/ns 300 4292 1 (91%) 1 2822 4 (73%) 350 400 2582 4 (58%) 450 2282 2 (66%) 500 2032 2 (67%) 550 2072 1 (75%)
t2/ns Ð 954233 986233 1030230 756232 810234
(23%) (32%) (25%) (22%) (17%)
t3/ns 32002 100 99002 300 39002 200 40002 200 32002 100 36002 200
(6%) (3%) (9%) (8%) (9%) (7%)
t4/ms 18.320.6 (2%) 2222 (1%) 2422 (1%) 1821 (1%) 21.321.4 (1%)
z
(b) Amelia albite 7646 Detection wavelength/nm t1/ns 300 Ð 350 37.32 0.1 (84%) 400 1672 9 (13%) 450 Ð 500 17.02 0.4 (36%) 550 22.02 0.3 (50%) Ð 5502 3 550 Ð
t2/ns 37521 (97%) 46125 (14%) Ð 433219 (46%) 17927 (21%) 16026 (22%) 17424 (48%) Ð
t3/ns Ð Ð 6272 3 (73%) 8582 47 (48%) 7572 17 (34%) 6412 (18 (22%) 9362 22 (43%) Ð
t4/ns 19002100 (3%) 3290260 (2%) 2800210 (14%) 36002200 (6%) 31002100 (7%) 30002100 (5%) 58002200 (6%) 2740210 (95%)
(c) Orthoclase moonstone Detection wavelength/nm t1/ns 300 19.02 0.4 (39%) 350 25.52 0.4 (52%) 400 862 1 (ÿ34%) 450 1262 2 (ÿ33%) 500 Ð 550 672 2 (35%)
t2/ns 46623 (45%) Ð Ð Ð 83929 (ÿ31%) Ð
t3/ns Ð Ð 10102 10 (30%) Ð Ð 10202 30 (ÿ15%)
t4/ns 34002100 (12%) 4570230 (44%) 5450210 (32%) 50002200 (35%) Ð Ð
t5/ms 10.920.4 (4%) 17.320.7 (4%) 13.020.1 (5%) 9.520.2 (32%) 9.320.014 (69%) 9.3620.020 (50%)
(d) Irkutsk orthoclase Detection wavelength/nm t1/ns 300 32.42 0.1 (92%) 350 34.52 0.1 (98%) 400 542 1 (ÿ28%) 450 32.02 0.3 (ÿ34%) 500 282 1 (ÿ26%) 550 282 1 (ÿ28%) 5504 19.82 0.2 (ÿ39%)
t2/ns Ð Ð Ð Ð 226213 (13%) 18625 (27%) 22524 (24%)
t3/ns 4822 5 (8%) 8062 39 (2%) 6822 3 (68%) 7492 3 (62%) 8292 12 (55%) 8442 11 (41%) 9642 13 (34%)
t4/ns 44002200 59002900 33002200 32002200 29002200 31002200 41002300
t5/ms Ð Ð 1623 (<1%) 1623 (<1%) 14.421.4 (1%) 2423 (<1%) 3526 (<1%)
(e) Sanidine 8433 Detection wavelength/nm 300 350 400 450 500 550
t1/ns 42.12 0.1 (91%) 43.92 0.1 (90%) 632 1 (ÿ33%) 722 1 (ÿ36%) 1292 4 (ÿ19%) 542 1 (41%)
(f) Oligoclase 7647 Detection wavelength/nm t1/ns 3001 33.32 0.1 (96%) 350 36.82 0.1 (97%) 400 442 1 (ÿ24%) 450 312 0.5 (ÿ28%) 500 372 1 (ÿ21%) Ð 5501 5501 Ð
t2/ns Ð Ð Ð Ð Ð Ð
t3/ns 4062 6 (8%) 6422 6 (9%) 7472 3 (62%) 9632 4 (59%) 11722 10 (70%) 12642 15 (47%)
t2/ns Ð Ð Ð Ð Ð Ð Ð
t3/ns 4412 12 (4%) 5612 12 (2%) 5892 3 (70%) 6432 3 (65%) 8282 5 (75%) 9472 5 (96%) 9332 6 (95%)
(1%) (<1%) (4%) (4%) (5%) (4%) (3%)
t4/ns 4360290 (1%) 5150290 (1%) 36002200 (4%) 47002300 (4%) 50002400 (7%) 51002500 (7%) t4/ns 58002500 (<1%) 54002400 (<1%) 2040290 (6%) 2270290 (6%) 36002300 (4%) 58002800 (4%) 50002600 (5%)
t5 1122 ms (<1%) 14.520.6 ms (1%) 30215 ms (<1%) 2923 ms (<1%) 164217 ms (1%) 4.220.2 ms (2%) 2.1420.07 ms (2%) & 11.120.1 ms (3%)
t5/ms Ð Ð 1522 (<1%) 2525 (<1%) 2822 (3%) 2522 (4%) t5/ms Ð Ð 15.420.8 (1%) 16.020.9 (1%) 2525 (1%) {1022110 (<1%)}? {83248 (1%)}?
1, Low signal level, poor signal statistics; 2, scalar dwell time, 80 ns; 3, scalar dwell time, 2 ms; 4, repeated measurement.
FAST TIME RESOLVED LUMINESCENCE
557
Fig. 3. Amelia albite (7646) TRL spectra obtained with a stimulation wavelength of 850 nm, detection window 550 D 40 nm. (a) 5 ns dwell time, (b) 2 ms dwell time. R2 and RSS refer to the correlation coef®cient and the residual sum of squares respectively.
this con®guration is likely to be in error; accordingly TRL spectra were recorded using longer dwell times (80 ns and 2 ms). Although the signal levels recorded with the 550 nm detection window were low for the oligoclase sample (Table 2), repeated measurements were in reasonable agreement, yielding lifetimes of 102 and 83 ms. However, they represent weak components of a low sensitivity sample and thus the evaluations of these lifetimes should be treated with caution. As with all studies of this type we must add a further caveat to any conclusions drawn from the lifetime evaluations because of the possible eects of non-radiative competition which have yet to be investigated. Discussions of the recombination mechanism in feldspars have previously been advanced on the basis of the results of steady-state emission spectroscopy and EPR and several types of defect site may serve as recombination centres. Luminescence emission bands in the visible region of the spectrum assigned to lattice defects have been proposed by a number of authors (i.e. Prescott and Fox, 1993; Kirsh and Townsend, 1988; Speit and Lehmann, 1982a; Huntley et al., 1988). The main candidates for such defects include: (i) hole centres due to vacancies or substitution of a structural ion by an impurity ion of lower valency (Petrov et al., 1989); (ii) vacancies at K or Na positions (or Ca for plagioclase); and (iii) substitution of structural Si4+ by Al3+ or substitution of structural Si/Al by a bivalent cation such as Cu2+ or Mg2+. The hole created by charge imbalance in such defects locates around a nearby oxygen ion forming bridges (i.e. Al±Oÿ±Al, Si±Oÿ±Al, Si±Oÿ±M2+). There are
many types of paramagnetic hole centres, with nine being identi®ed by Petrov et al. (1989). Also there are 32 dierent T±O±T bridges that can be potential hole centres (e.g. Al±Oÿ±Al), eight of which are distinguishable by EPR measurements. Thus the number of potential recombination modes for released charge in feldspar is likely to be high.
4. THE ROLE OF Mn2+ Two of the most studied emission bands at 560 nm and in the range 680±730 nm in feldspar are attributed to Mn2+ and Fe3+ respectively. The 560 nm emission band detected in lunar plagioclase was assigned to a Mn2+ centre, substituting for structural Ca2+ (Geake et al., 1971, 1973, 1977) with the luminescence resulting from intrinsic d 5 electron transitions. This was supported by excitation spectroscopy (Telfer and Walker, 1975, 1978), where excitation bands were assigned to Mn2+ electronic transitions by ligand ®eld theory calculations. In microcline and albite (with few Ca sites), however, it has been suggested that the Mn2+ ions substitute for Al3+ T sites forming a lattice stabilised hole centre termed Si±Oÿ±Mn2+ (Telfer and Walker, 1978; Petrov et al., 1989; Kirsh et al., 1987). Both types of centre are expected to be dependent on the concentration of Mn present in the mineral, although it may be expected that the recombination lifetimes for each centre will be dierent. Transition metal ion luminescence typically has a long lifetime (i.e. >100 ms) since electronic transitions occur between levels of the
558
R. J. CLARK and I. K. BAILIFF
incompletely ®lled d shell and are thus parity forbidden. In Mn2+ the radiative transition is also spin forbidden (Blasse and Grabmaier, 1994) and thus is expected to have a lifetime of the order of ms or greater. Luminescence resulting from electron-hole recombination at lattice defects (e.g. Si± Oÿ±Mn2+) on the other hand do not have such constraints and may be expected to have a much shorter lifetime. Consequently the short lifetimes reported here for feldspars are of particular signi®cance in attempting to clarify the general mode(s) of recombination. Of the samples used in this study, the Amelia albite (7646) sample shows the strongest 560 nm emission band (see Fig. 2). Minor 560 nm emission bands can also be seen in the orthoclase perthite (20232), Irkutsk orthoclase, the oligoclase (7647) sample and to a lesser extent in the sanidine (8433) sample. Whereas the albite sample has luminescence lifetime components of the order of milliseconds (02 and 11 ms, Fig. 3) when employing the 550 nm detection window they, at most, constitute only 5% of the lifetime components and the emission is dominated by sub-microsecond dynamics. On the basis of these results the intrinsic Mn2+ transitions with associated millisecond lifetime dynamics do not contribute signi®cantly to IRSL emission for the potassic feldspars observed under these conditions and their role in the albite studied appears to be minor. This indicates that the dominant radiative recombination occurs at lattice hole defects. Although better spectral resolution might resolve the Mn2+ components, the 560 nm emission band
associated with the albite sample is not dominated by intrinsic Mn2+ luminescence. As stated earlier, the intrinsic role of Mn2+ was described (Geake et al. 1971, 1973, 1977) for calcic plagioclase feldspars (mainly lunar feldspars). This type of mineral was not incorporated in the current TRL study, and future examination of such minerals using TRL spectroscopy may reveal much greater ms contributions (revealing greater dependence on intrinsic Mn2+ luminescence).
5. EMISSION BAND INTERACTION An interesting feature of the TRL is the presence of initial rising components (indicated by negative pre-exponential factors) in the spectra obtained with some of the feldspars [see Fig. 4(b) for an example]. For orthoclase moonstone, Irkutsk orthoclase, sanidine and oligoclase samples the rising components are associated with emission bands located >400 nm. Three of the samples (Irkutsk orthoclase, sanidine, oligoclase) have emission bands at 400 nm, while the orthoclase moonstone sample has a 0430 nm emission band. Although part of the broad 0400 nm emission bands is likely to be transmitted by the 350 nm detection ®lter, there is no evidence of a rising component in any of the TRL spectra measured using that ®lter. Thus it seems likely that the rising component is dominated by emission in the region of 420±460 nm, rather than 400 nm. The two samples that do not exhibit rising components are albite and orthoclase perthite. The
Fig. 4. Irkutsk orthoclase TRL spectra obtained with a stimulation wavelength of 850 nm. (a) Detection window 350 D 40 nm, (b) detection window 450 D 40 nm.R2 and RSS refer to the correlation coecient and the residual sum of squares, respectively.
FAST TIME RESOLVED LUMINESCENCE orthoclase perthite sample has a 440 nm emission peak but we note that it lacks a 030 ns lifetime component in the 300/350 nm detection windows. For the TRL spectra exhibiting rise times, it can be seen that the value of the rise time is very similar to the fastest decaying component (030±40 ns) in the 300/350 nm detection windows. The complementary nature of these components suggests a connection between the dierent emission bands, and one possible mechanism for this is the self-absorption of rapidly produced UV luminescence. In the sanidine, oligoclase and Irkutsk orthoclase samples, the UV emission (300/350 nm) is dominated (90±98%) by a single component of approximately 30 ns, and in the visible luminescence (400±500/550 nm) a single rise time of approximately the same value is observed. In the orthoclase moonstone sample, the 030 ns component in the UV makes a smaller contribution to the emission and a range of rise times is associated with the visible emissions, supporting a correlation, but not as closely. The emission centres in the 400±550 nm region have been identi®ed as dierent hole centres located on an oxygen ion, stabilised by the lattice (Petrov et al., 1989; Speit and Lehmann, 1982b; Kirsh and Townsend, 1988), and the short lifetimes observed support this interpretation. If self-absorption is stimulating luminescence in the 400±550 nm region it would seem likely that the mechanism is a two-step process associated with the eviction of electrons from traps followed by electron-hole recombination at a lattice defect rather than absorption by transition metal ions. The data obtained with the orthoclase perthite opens up a further question. It is the only sample from the six to exhibit a 440 nm emission peak without yielding a 30 ns component in the 350 nm detection window or a rising component in the 400±550 nm detection windows. This sample has very low sensitivity in the 350 nm detection window and it is likely that the luminescence observed results from the tails of the emission bands on either side, therefore lacking the appropriate combination of lifetime and emission wavelength possessed by the other samples. This suggests that the rising components depend on the presence of the 30 ns decay in the 350 nm detection window, rather than simply 300±400 nm photons, indicating that some form of energy transfer other than selfabsorption is responsible. However, on the basis of the present results, there is no reason to reject the self-absorption mechanism. One ®nal observation is that in two cases (oligoclase and orthoclase perthite) the CW IRSL emission spectra contain relatively strong components in the region of 550 and 350 nm, respectively, yet the TRL is relatively weak when employing detection windows that would pass these wavelengths. This opens up the question of whether the TRL and CW emission spectra dier signi®cantly.
559
6. CONCLUSION The results have revealed important characteristics in feldspar IRSL under narrow width pulse stimulation, ®rstly demonstrating that intrinsic metal ion luminescence (especially manganese) has a very small contribution to sodic and potassic feldspar luminescence and this evidence supports lattice stabilised hole centres on oxygen ions forming T±O±T bridges as the main recombination centres. Secondly, evidence has been presented for a strong interaction between UV and visible emission bands in some of the feldspars, leading to the suggestion that self-absorption of some of the UV emission stimulates part of the visible luminescence. AcknowledgementsÐThe work described in this paper was ®nancially supported by the University of Durham and research grant GST/02/0756 from the NERC within the LOEPS core programme and as part of the Land±Ocean Interaction Study (LOIS), special topic 240, and is LOIS publication number 343. We thank Professor J. R. Prescott for supplying the orthoclase moonstone and Dr S. Petrov for the sample of Irkutsk orthoclase. We are grateful to the Department of Geology for access to their reference mineral collection and for their generous supply of the other feldspars tested in this study.
REFERENCES Baili, I. K. and Poolton, N. R. J. (1991) Studies of charge transfer mechanisms in feldspars. Nuclear Tracks and Radiation Measurements 18(1/2), 111± 118. Baili, I. K., Morris, D. A. and Aitken, M. J. (1977) A rapid-scanning interference spectrometer; application to low level thermoluminescence emission. Journal of Physics E: Scienti®c Instrumentation 10, 1156±1160. Barnett, S. M. and Baili, I. K. (1997) The temperature dependence of luminescence in some feldspars (80± 300 K). Journal of Physics D: Applied Physics 30, 683±689. Blasse, G. and Grabmaier, B. C. (1994) Luminescent Materials. Springer-Verlag, Berlin. Clark, R. J., Baili, I. K. and Tooley, M. J. (1977) A preliminary study of time resolved luminescence in some feldspars. Radiation Measurements 27, 211±220. Clarke, M. L. and Rendell, H. M. (1997) IR stimulated luminescence spectra of alkali feldspars. Radiation Measurements 27, 221±236. Geake, J. E., Walker, G., Mills, A. A. and Garlick, G. F. J. (1971) Luminescence of Apollo lunar samples. Proceedings of the 2nd Lunar Science Conference: Geochemica et Cosmochimica Acta Supplement 2 3, 2265±2275. Geake, J. E., Walker, G., Telfer, D. J. and Mills, A. A. (1977) The cause and signi®cance of luminescence in lunar plagioclase. Philosophical Transactions of the Royal Society of London, A 285, 403±408. Geake, J. E., Walker, G., Telfer, D. J., Mills, A. H. and Garlick, G. F. J. (1973) Luminescence of lunar, terrestrial and synthesized plagioclase caused by Mn2+ and Fe3+. Proceedings of the 4th Lunar Science Conference: Geochemica et Cosmochimica Acta, Supplement 4 3, 3181±3189. Huntley, D. J., Godfrey-Smith, D. I., Thewalt, M. L. W. and Berger, G. W. (1988) Thermoluminescence
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spectra of some mineral samples relevant to thermoluminescence dating. Journal of Luminescence 39, 123±136. Huntley, D. J., Godfrey-Smith, D. I. and Haskell, E. H. (1991) Light induced emission spectra from some quartz and feldspars. Nuclear Tracks and Radiation Measurements 18(2), 127±131. HuÈtt, G. and Jaek, I. (1993) Photostimulated luminescence of some materials and its dosimetry applications. Nuclear Tracks and Radiation Measurements 21(1), 95±98. Kirsh, Y. and Townsend, P. D. (1988) Speculations on the blue and red bands in the TL emission spectrum of albite and microcline. Nuclear Tracks and Radiation Measurements 14(1/2), 43±49. Kirsh, Y., Shoval, S. and Townsend, P. D. (1987) Kinetics and emission spectra of thermoluminescence in the feldspars albite and microcline. Physica Status Solidi (a) 101, 253±262.
Petrov, I., Agel, A. and Hafner, S. S. (1989) Distinct defect centres at oxygen positions in albite. American Mineralogist 74, 1130±1141. Prescott, J. R. and Fox, P. J. (1993) Three-dimensional thermoluminescence spectra of feldspars. Journal of Physics D: Applied Physics 26, 2245±2254. Speit, B. and Lehmann, G. (1982a) A comparative study of thermoluminescence and isothermal destruction of radiation defects in feldspars. Journal of Luminescence 27, 127±136. Speit, B. and Lehmann, G. (1982b) Radiation defects in feldspars. Physics and Chemistry of Minerals 8, 77± 82. Telfer, D. J. and Walker, G. (1975) Optical detection of Fe3+ in lunar plagioclase. Nature 258, 694±695. Telfer, D. J. and Walker, G. (1978) Ligand ®eld bands of Mn2+ and Fe3+ luminescence centres and their site occupancy in plagioclase feldspars. Modern Geology 6, 199±210.
Radiation Measurements Vol. 29, No. 5, pp. 553±560, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1350-4487(98)00068-7 1350-4487/98 $19.00 + 0.00
FAST TIME-RESOLVED LUMINESCENCE EMISSION SPECTROSCOPY IN SOME FELDSPARS R. J. CLARK and I. K. BAILIFF* Luminescence Dating Laboratory, Environmental Research Centre, South Road, Durham University, Durham DH1 3LE, UK (Received 4 February 1998; revised 20 May 1998; in ®nal form 21 May 1998) AbstractÐMeasurements of fast time-resolved 850 nm stimulated luminescence (resolution 5 ns) and also CW infra-red stimulated luminescence (IRSL) emission spectroscopy have been performed with a set of six specimen feldspars comprising orthoclase, albite, sanidine and oligoclase. The IRSL was observed following the administration of ionising radiation dose. Detection of the time-resolved spectrum was de®ned by the use of bandpass (FWHM 40 nm) interference ®lters enabling low resolution emission spectroscopy to be performed between 300 and 550 nm. Multi-exponential lifetime analysis suggests the possibility of a link between luminescence emitted in the UV spectral region (0300 nm) and the visible spectral region (i.e. >400 nm). The results also support a model of lattice stabilised electron vacancies for the luminescence emission rather than intrinsic transition metal ion luminescence. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION Studies of optically stimulated luminescence mechanisms in natural dosimetric materials (e.g. quartz and feldspar) used for dating have been based mainly on explorations of charge carrier (trap) excitation (e.g. HuÈtt and Jaek, 1993) and recombination by CW emission spectroscopy (e.g. Huntley et al., 1991; Clarke and Rendell, 1997). The information yielded by such studies is limited in terms of the recombination process, the data obtainable with the latter technique being limited by the complexities and possible interactions of the emission bands. However, fast (ns) time-resolved emission spectroscopy substantially extends the study of the nature of recombination processes because it has the potential to deconvolute the dierent recombination processes and identify them by measuring the recombination lifetimes. In preliminary work, time-resolved measurements performed with samples of feldspar showed that the spectra could be resolved into one or more exponential components by the use of multi-exponential non-linear regression analysis (Clark et al., 1997). Measurements were performed using three broad spectral ranges obtained by placing band colour glass ®lters (FWHM 280±380, 350±575 and 460± 625 nm) in the detection system. The evaluated lifetimes ranged from 030 ns to 030 ms and, where present, the fastest component (030 ns) appeared to be associated with the UV emissions (detection window FWHM 280±380 nm). The results also indi-
cated the presence of a rise time (signal growth rather than decay) in one sample, orthoclase moonstone. In this paper we report on further investigations of the wavelength dependency of the lifetime components with greater spectral resolution, and the interactions between lifetime components at dierent emission wavelengths.
2. MEASUREMENTS The time-resolved luminescence (TRL) spectrometer was used in the con®guration described in Clark et al. (1997) with the exception that the detection window was de®ned by the use of bandpass interference ®lters. The ®lters comprised a set with central wavelengths spaced at 50 nm intervals between 300 and 550 nm and with FWHM of 040 nm; the transmission characteristics for the ®lters were checked using a Perkin±Elmer Lambda 19 UV/Vis/NIR spectrometer, as shown in Fig. 1. Optical stimulation was performed with a Quanta Ray MOPO 710 laser adjusted to 850 nm (linewidth of <4 nm); the pulse repetition rate is 10 Hz. The luminescence was detected using a fast EMI 9813QB photomultiplier, the pulse output of which was fed to a multi-channel scalar (MCS) adjusted to provide a dwell time of 5 ns. The start of each sweep of the MCS was triggered by an advanced Q switch pulse from the laser control unit. The PMT detection system was active during the delivery of each laser pulse.
*To whom all correspondence should be addressed. Fax: 0191-374-3619; E-mail: ian.baili@durham.ac.uk. 553
R. J. CLARK and I. K. BAILIFF
554
Fig. 1. Transmission curves for the interference ®lters used in this study.
The feldspar samples (see Table 1) were in the form of cleaved chips and further details are given in Barnett and Baili (1997). All of the aliquots of each sample were bleached at a distance of 10 cm from a 150 W tungsten-halogen lamp for 2 days prior to use. Doses of ionising radiation in the range 2±100 Gy were delivered by a Sr90/Y90 beta source, the level depending on the measurement requirements. Optical bleaching and the delivery of ionising radiation dose were performed at ambient temperatures. Following the administration of radiation dose, all samples were preheated at 508C for 1 min and stored for several hours before measurement to allow traps unstable at room temperature to be emptied of charge. A time resolution of 5 ns per channel over 8k channels (i.e. 40 ms per sweep) was employed for TRL measurements with all samples with one exception. In the case of albite, where the TRL was detected through a 550 nm ®lter, additional measurements were performed using time resolutions of 80 ns and 2 ms per channel. The intensity of the incident laser beam was adjusted (estimated Table 1. Summary of the feldspars used in this study Sample Orthoclase perthite Orthoclase moonstone Amelia albite Irkutsk orthoclase (high temperature form) Sanidine Oligoclase
Laboratory No
Ratio K:Na:Ca
Mi20232
77:8:15
Ð Al7646
72:26:2 3:95:2
Or]SP Sa8433 Ol7647
90:10:0.1 82:16:2 9:44:47
energy 01±100 nJ per pulse) until a maximum rate of 1 cps was recorded in the channel with the greatest number of recorded photon counts (referred to as the peak channel) to avoid pulse pile-up. Data were acquired until at least 1000 counts were recorded in the peak channel, except for those measurements indicated in the results where signal levels were too low (such that raising the laser power to increase the signal intensity would introduce unacceptable interference from background luminescence signals). It is worth noting here an important dierence between time-resolved luminescence discussed in this paper and the more commonly observed IRSL under continuous or pulsed stimulation. Following the delivery of each laser pulse the time of arrival of photons detected at the PMT is registered. Thus the TRL spectra obtained, as described further below, are not equivalent to decay curves recorded using CW stimulation since they represent the time-integrated distribution of the luminescence. The time-resolved spectra were analysed using multi-exponential nonlinear regression analysis (using a Marquardt± Levenberg algorithm) with the form X I ai exp ÿt=ti 1 i
where I = intensity (detected photon counts), ai =pre-exponential factor (i.e. magnitude of each exponential component), t = time and ti=lifetime of each exponential component (same units as t). CW IRSL emission spectra for each of the samples used in the TRL study were measured using a fast scanning interference ®lter spectrometer (Baili and Poolton, 1991; Baili et al., 1977)
FAST TIME RESOLVED LUMINESCENCE coupled to the output of a Ti±sapphire laser adjusted to 850 nm, employed as the stimulation source. All of the samples were in the form of cleaved chips and had received a radiation dose of 200 Gy about 2 months before the spectra were recorded at room temperature. The spectrometer comprises 16 interference ®lters and scans from 340 to 640 nm in 125 ms; the photon counts registered by the PMT during the passage of each ®lter through the light collection path is registered, enabling the spectra to be integrated over selected time intervals in the subsequent data analysis.
555
3. RESULTS AND DISCUSSION The emission spectra for each of the samples are shown in Fig. 2 and the results of the regression analysis of the time-resolved spectra can be seen in Table 2. We note here additional TRL data that were obtained as a result of the comparatively long lifetimes evaluated in two cases. For the albite sample (Table 2; Fig. 3) the time-resolved spectrum in the 550 nm detection window has a minor component that persists well beyond the 40 ms sweep using a 5 ns dwell time. The lifetime evaluated using
Fig. 2. CW IRSL emission spectra for each of the minerals used in this study with a stimulation wavelength of 850 nm. The spectra were integrated over the ®rst 100±30 s of stimulation, depending on the intensity of emission, and are corrected for instrument response.
R. J. CLARK and I. K. BAILIFF
556
Table 2. Summary of the lifetime components of each feldspar in each of the detection windows. The numbers in brackets are the percentage contributions of each component to the total signal as measured by the pre-exponential factors. Stimulation at 850 nm (a) Orthoclase perthite 20232 Detection wavelength/nm t1/ns 300 4292 1 (91%) 1 2822 4 (73%) 350 400 2582 4 (58%) 450 2282 2 (66%) 500 2032 2 (67%) 550 2072 1 (75%)
t2/ns Ð 954233 986233 1030230 756232 810234
(23%) (32%) (25%) (22%) (17%)
t3/ns 32002 100 99002 300 39002 200 40002 200 32002 100 36002 200
(6%) (3%) (9%) (8%) (9%) (7%)
t4/ms 18.320.6 (2%) 2222 (1%) 2422 (1%) 1821 (1%) 21.321.4 (1%)
z
(b) Amelia albite 7646 Detection wavelength/nm t1/ns 300 Ð 350 37.32 0.1 (84%) 400 1672 9 (13%) 450 Ð 500 17.02 0.4 (36%) 550 22.02 0.3 (50%) Ð 5502 3 550 Ð
t2/ns 37521 (97%) 46125 (14%) Ð 433219 (46%) 17927 (21%) 16026 (22%) 17424 (48%) Ð
t3/ns Ð Ð 6272 3 (73%) 8582 47 (48%) 7572 17 (34%) 6412 (18 (22%) 9362 22 (43%) Ð
t4/ns 19002100 (3%) 3290260 (2%) 2800210 (14%) 36002200 (6%) 31002100 (7%) 30002100 (5%) 58002200 (6%) 2740210 (95%)
(c) Orthoclase moonstone Detection wavelength/nm t1/ns 300 19.02 0.4 (39%) 350 25.52 0.4 (52%) 400 862 1 (ÿ34%) 450 1262 2 (ÿ33%) 500 Ð 550 672 2 (35%)
t2/ns 46623 (45%) Ð Ð Ð 83929 (ÿ31%) Ð
t3/ns Ð Ð 10102 10 (30%) Ð Ð 10202 30 (ÿ15%)
t4/ns 34002100 (12%) 4570230 (44%) 5450210 (32%) 50002200 (35%) Ð Ð
t5/ms 10.920.4 (4%) 17.320.7 (4%) 13.020.1 (5%) 9.520.2 (32%) 9.320.014 (69%) 9.3620.020 (50%)
(d) Irkutsk orthoclase Detection wavelength/nm t1/ns 300 32.42 0.1 (92%) 350 34.52 0.1 (98%) 400 542 1 (ÿ28%) 450 32.02 0.3 (ÿ34%) 500 282 1 (ÿ26%) 550 282 1 (ÿ28%) 5504 19.82 0.2 (ÿ39%)
t2/ns Ð Ð Ð Ð 226213 (13%) 18625 (27%) 22524 (24%)
t3/ns 4822 5 (8%) 8062 39 (2%) 6822 3 (68%) 7492 3 (62%) 8292 12 (55%) 8442 11 (41%) 9642 13 (34%)
t4/ns 44002200 59002900 33002200 32002200 29002200 31002200 41002300
t5/ms Ð Ð 1623 (<1%) 1623 (<1%) 14.421.4 (1%) 2423 (<1%) 3526 (<1%)
(e) Sanidine 8433 Detection wavelength/nm 300 350 400 450 500 550
t1/ns 42.12 0.1 (91%) 43.92 0.1 (90%) 632 1 (ÿ33%) 722 1 (ÿ36%) 1292 4 (ÿ19%) 542 1 (41%)
(f) Oligoclase 7647 Detection wavelength/nm t1/ns 3001 33.32 0.1 (96%) 350 36.82 0.1 (97%) 400 442 1 (ÿ24%) 450 312 0.5 (ÿ28%) 500 372 1 (ÿ21%) Ð 5501 5501 Ð
t2/ns Ð Ð Ð Ð Ð Ð
t3/ns 4062 6 (8%) 6422 6 (9%) 7472 3 (62%) 9632 4 (59%) 11722 10 (70%) 12642 15 (47%)
t2/ns Ð Ð Ð Ð Ð Ð Ð
t3/ns 4412 12 (4%) 5612 12 (2%) 5892 3 (70%) 6432 3 (65%) 8282 5 (75%) 9472 5 (96%) 9332 6 (95%)
(1%) (<1%) (4%) (4%) (5%) (4%) (3%)
t4/ns 4360290 (1%) 5150290 (1%) 36002200 (4%) 47002300 (4%) 50002400 (7%) 51002500 (7%) t4/ns 58002500 (<1%) 54002400 (<1%) 2040290 (6%) 2270290 (6%) 36002300 (4%) 58002800 (4%) 50002600 (5%)
t5 1122 ms (<1%) 14.520.6 ms (1%) 30215 ms (<1%) 2923 ms (<1%) 164217 ms (1%) 4.220.2 ms (2%) 2.1420.07 ms (2%) & 11.120.1 ms (3%)
t5/ms Ð Ð 1522 (<1%) 2525 (<1%) 2822 (3%) 2522 (4%) t5/ms Ð Ð 15.420.8 (1%) 16.020.9 (1%) 2525 (1%) {1022110 (<1%)}? {83248 (1%)}?
1, Low signal level, poor signal statistics; 2, scalar dwell time, 80 ns; 3, scalar dwell time, 2 ms; 4, repeated measurement.
FAST TIME RESOLVED LUMINESCENCE
557
Fig. 3. Amelia albite (7646) TRL spectra obtained with a stimulation wavelength of 850 nm, detection window 550 D 40 nm. (a) 5 ns dwell time, (b) 2 ms dwell time. R2 and RSS refer to the correlation coef®cient and the residual sum of squares respectively.
this con®guration is likely to be in error; accordingly TRL spectra were recorded using longer dwell times (80 ns and 2 ms). Although the signal levels recorded with the 550 nm detection window were low for the oligoclase sample (Table 2), repeated measurements were in reasonable agreement, yielding lifetimes of 102 and 83 ms. However, they represent weak components of a low sensitivity sample and thus the evaluations of these lifetimes should be treated with caution. As with all studies of this type we must add a further caveat to any conclusions drawn from the lifetime evaluations because of the possible eects of non-radiative competition which have yet to be investigated. Discussions of the recombination mechanism in feldspars have previously been advanced on the basis of the results of steady-state emission spectroscopy and EPR and several types of defect site may serve as recombination centres. Luminescence emission bands in the visible region of the spectrum assigned to lattice defects have been proposed by a number of authors (i.e. Prescott and Fox, 1993; Kirsh and Townsend, 1988; Speit and Lehmann, 1982a; Huntley et al., 1988). The main candidates for such defects include: (i) hole centres due to vacancies or substitution of a structural ion by an impurity ion of lower valency (Petrov et al., 1989); (ii) vacancies at K or Na positions (or Ca for plagioclase); and (iii) substitution of structural Si4+ by Al3+ or substitution of structural Si/Al by a bivalent cation such as Cu2+ or Mg2+. The hole created by charge imbalance in such defects locates around a nearby oxygen ion forming bridges (i.e. Al±Oÿ±Al, Si±Oÿ±Al, Si±Oÿ±M2+). There are
many types of paramagnetic hole centres, with nine being identi®ed by Petrov et al. (1989). Also there are 32 dierent T±O±T bridges that can be potential hole centres (e.g. Al±Oÿ±Al), eight of which are distinguishable by EPR measurements. Thus the number of potential recombination modes for released charge in feldspar is likely to be high.
4. THE ROLE OF Mn2+ Two of the most studied emission bands at 560 nm and in the range 680±730 nm in feldspar are attributed to Mn2+ and Fe3+ respectively. The 560 nm emission band detected in lunar plagioclase was assigned to a Mn2+ centre, substituting for structural Ca2+ (Geake et al., 1971, 1973, 1977) with the luminescence resulting from intrinsic d 5 electron transitions. This was supported by excitation spectroscopy (Telfer and Walker, 1975, 1978), where excitation bands were assigned to Mn2+ electronic transitions by ligand ®eld theory calculations. In microcline and albite (with few Ca sites), however, it has been suggested that the Mn2+ ions substitute for Al3+ T sites forming a lattice stabilised hole centre termed Si±Oÿ±Mn2+ (Telfer and Walker, 1978; Petrov et al., 1989; Kirsh et al., 1987). Both types of centre are expected to be dependent on the concentration of Mn present in the mineral, although it may be expected that the recombination lifetimes for each centre will be dierent. Transition metal ion luminescence typically has a long lifetime (i.e. >100 ms) since electronic transitions occur between levels of the
558
R. J. CLARK and I. K. BAILIFF
incompletely ®lled d shell and are thus parity forbidden. In Mn2+ the radiative transition is also spin forbidden (Blasse and Grabmaier, 1994) and thus is expected to have a lifetime of the order of ms or greater. Luminescence resulting from electron-hole recombination at lattice defects (e.g. Si± Oÿ±Mn2+) on the other hand do not have such constraints and may be expected to have a much shorter lifetime. Consequently the short lifetimes reported here for feldspars are of particular signi®cance in attempting to clarify the general mode(s) of recombination. Of the samples used in this study, the Amelia albite (7646) sample shows the strongest 560 nm emission band (see Fig. 2). Minor 560 nm emission bands can also be seen in the orthoclase perthite (20232), Irkutsk orthoclase, the oligoclase (7647) sample and to a lesser extent in the sanidine (8433) sample. Whereas the albite sample has luminescence lifetime components of the order of milliseconds (02 and 11 ms, Fig. 3) when employing the 550 nm detection window they, at most, constitute only 5% of the lifetime components and the emission is dominated by sub-microsecond dynamics. On the basis of these results the intrinsic Mn2+ transitions with associated millisecond lifetime dynamics do not contribute signi®cantly to IRSL emission for the potassic feldspars observed under these conditions and their role in the albite studied appears to be minor. This indicates that the dominant radiative recombination occurs at lattice hole defects. Although better spectral resolution might resolve the Mn2+ components, the 560 nm emission band
associated with the albite sample is not dominated by intrinsic Mn2+ luminescence. As stated earlier, the intrinsic role of Mn2+ was described (Geake et al. 1971, 1973, 1977) for calcic plagioclase feldspars (mainly lunar feldspars). This type of mineral was not incorporated in the current TRL study, and future examination of such minerals using TRL spectroscopy may reveal much greater ms contributions (revealing greater dependence on intrinsic Mn2+ luminescence).
5. EMISSION BAND INTERACTION An interesting feature of the TRL is the presence of initial rising components (indicated by negative pre-exponential factors) in the spectra obtained with some of the feldspars [see Fig. 4(b) for an example]. For orthoclase moonstone, Irkutsk orthoclase, sanidine and oligoclase samples the rising components are associated with emission bands located >400 nm. Three of the samples (Irkutsk orthoclase, sanidine, oligoclase) have emission bands at 400 nm, while the orthoclase moonstone sample has a 0430 nm emission band. Although part of the broad 0400 nm emission bands is likely to be transmitted by the 350 nm detection ®lter, there is no evidence of a rising component in any of the TRL spectra measured using that ®lter. Thus it seems likely that the rising component is dominated by emission in the region of 420±460 nm, rather than 400 nm. The two samples that do not exhibit rising components are albite and orthoclase perthite. The
Fig. 4. Irkutsk orthoclase TRL spectra obtained with a stimulation wavelength of 850 nm. (a) Detection window 350 D 40 nm, (b) detection window 450 D 40 nm.R2 and RSS refer to the correlation coecient and the residual sum of squares, respectively.
FAST TIME RESOLVED LUMINESCENCE orthoclase perthite sample has a 440 nm emission peak but we note that it lacks a 030 ns lifetime component in the 300/350 nm detection windows. For the TRL spectra exhibiting rise times, it can be seen that the value of the rise time is very similar to the fastest decaying component (030±40 ns) in the 300/350 nm detection windows. The complementary nature of these components suggests a connection between the dierent emission bands, and one possible mechanism for this is the self-absorption of rapidly produced UV luminescence. In the sanidine, oligoclase and Irkutsk orthoclase samples, the UV emission (300/350 nm) is dominated (90±98%) by a single component of approximately 30 ns, and in the visible luminescence (400±500/550 nm) a single rise time of approximately the same value is observed. In the orthoclase moonstone sample, the 030 ns component in the UV makes a smaller contribution to the emission and a range of rise times is associated with the visible emissions, supporting a correlation, but not as closely. The emission centres in the 400±550 nm region have been identi®ed as dierent hole centres located on an oxygen ion, stabilised by the lattice (Petrov et al., 1989; Speit and Lehmann, 1982b; Kirsh and Townsend, 1988), and the short lifetimes observed support this interpretation. If self-absorption is stimulating luminescence in the 400±550 nm region it would seem likely that the mechanism is a two-step process associated with the eviction of electrons from traps followed by electron-hole recombination at a lattice defect rather than absorption by transition metal ions. The data obtained with the orthoclase perthite opens up a further question. It is the only sample from the six to exhibit a 440 nm emission peak without yielding a 30 ns component in the 350 nm detection window or a rising component in the 400±550 nm detection windows. This sample has very low sensitivity in the 350 nm detection window and it is likely that the luminescence observed results from the tails of the emission bands on either side, therefore lacking the appropriate combination of lifetime and emission wavelength possessed by the other samples. This suggests that the rising components depend on the presence of the 30 ns decay in the 350 nm detection window, rather than simply 300±400 nm photons, indicating that some form of energy transfer other than selfabsorption is responsible. However, on the basis of the present results, there is no reason to reject the self-absorption mechanism. One ®nal observation is that in two cases (oligoclase and orthoclase perthite) the CW IRSL emission spectra contain relatively strong components in the region of 550 and 350 nm, respectively, yet the TRL is relatively weak when employing detection windows that would pass these wavelengths. This opens up the question of whether the TRL and CW emission spectra dier signi®cantly.
559
6. CONCLUSION The results have revealed important characteristics in feldspar IRSL under narrow width pulse stimulation, ®rstly demonstrating that intrinsic metal ion luminescence (especially manganese) has a very small contribution to sodic and potassic feldspar luminescence and this evidence supports lattice stabilised hole centres on oxygen ions forming T±O±T bridges as the main recombination centres. Secondly, evidence has been presented for a strong interaction between UV and visible emission bands in some of the feldspars, leading to the suggestion that self-absorption of some of the UV emission stimulates part of the visible luminescence. AcknowledgementsÐThe work described in this paper was ®nancially supported by the University of Durham and research grant GST/02/0756 from the NERC within the LOEPS core programme and as part of the Land±Ocean Interaction Study (LOIS), special topic 240, and is LOIS publication number 343. We thank Professor J. R. Prescott for supplying the orthoclase moonstone and Dr S. Petrov for the sample of Irkutsk orthoclase. We are grateful to the Department of Geology for access to their reference mineral collection and for their generous supply of the other feldspars tested in this study.
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