- Email: [email protected]
Measurements of nanosecond lifetimes using a transient method based on pulse delay
Journal of Luminescence 33 (1985) 53—61 North-Holland, Amsterdam
53
MEASUREMENTS OF NANOSECOND LIFETIMES USING A TRANSIENT METHOD BASED ON PULSE DELAY M.F. QUINN, M.S. AL-AJEEL and F. AL-BAHRANI Laser and Electronics Department, Kuwait Institutefor Scientific Research, P.O. Box 24885 Safat. Kuwait Original manuscript received 1 November 1983 Revised manuscript received 1 March 1984
The results of lifetime measurements carried Out on a number of organic laser dyes are reported. The measurements were conducted over a wide range of solution concentrations. A modified version of a previously reported experimental system was developed for these measurements giving greater reliability and incorporating a higher degree of automation. A comparison with corresponding lifetimes determined by a standard fluorescence tail fitting technique was made to test the method. The reliability of the method is discussed.
1. Infroduction The emission characteristics of organic laser dyes have been studied for possible applications to a planar solar collector device [1]. The important parameters required for an in-depth analysis of such a collector are the absorption and emission spectra, quantum yields and the magnitude of the self-absorption and subsequent radiation trapping processes in the device [2,3]. Quantum yields and the phenomena of radiation trapping strongly influence the observed radiative lifetimes of emission from organic dyes [4]. Conversely, lifetime measurements may be used to estimate the approximate magnitudes of the quantum yields and the degree of radiation trapping, particularly at high solution concentrations. For these reasons, lifetime measurements were considered to be an important part of our investigation into the luminescence characteristics of the dyes for planar solar collector applications. In a previous publication [5], a pulse delay technique was described for measuring the lifetimes of organic dyes and scintillators. The technique was based on the fact that the fluorescence signal induced by a short excitation light pulse is distorted because of the finite lifetime of the excited state. It was shown that the difference in risetime of the excitation and fluorescence pulses (i.e., difference in times from onset to peak heights) was related to the fluorescence lifetime of the dye. The fluorescence lifetime was shown to be 0022-2313/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
54
M.F. Quin et al.
/
Nanosecond lifetime measurements.
related to the measured delay by eqs. (1) and (2) for triangular pump pulses and linearly rising pulses with exponentially falling edges respectively, i.e., —~exp
T~ ——
T~
Tf(
—~exp
T~
T~
‘rp’
T1
8
~
( 6 ~exp
—
~
——
—exp
Tf +——
T~
T
1,’
Tf
1+—
exp
6 ——
=0,
(1)
Tf
6
~
+——1~exp T~ )
——
T~’l~
+
T~
+——1~exp T~ )
——
8
—
T~.
(2) and are the base line to peak and peak to base line rise and fall times of the triangulated pumping pulses. T~is the fluorescence lifetime and 6 is the measured delay between the risetime of the fluorescence pulse and the pumping pulse. In this paper we report on the results of lifetime measurements carried out on a number of organic laser dyes. The measurements were conducted over a wide range of solution concentrations. A modified version of the previously reported experimental system was developed for these pulse delay measurements giving greater accuracy and incorporating a higher degree of automation. To test the reliability of the method, the results were compared to corresponding lifetimes determined by standard fluorescence tail fitting techniques [6]. Lifetimes determined by tail fitting procedures are generally considered to be less reliable than those involving deconvolution methods [7,8]. However, owing to the difficulties in setting up these latter procedures and their time consuming nature, tail fitting was considered adequate for our comparative purposes. One important consideration which was not mentioned in previous publications relates to the degree of accuracy which may be achieved using the pulse delay technique. This aspect of the measurements was considered and is discussed in detail later.
2. Experimental system The experimental system is shown schematically in fig. 1. The N2 laser was an M2000 Lambda Physik device with a pulse energy of approximately 5 mJ and a peak power of approximately 1 M Watt. The laser was used to pump a dye laser oscillator and amplifier system which was designed in this laboratory. The dye laser oscillator was of the Hansch design [9] and featured a short cavity (approximately 5 cm) incorporating a grating as the tunable element and
M.F. Quin et al.
/
Nanosecond lifetime measurements.
55
100% reflector. The output coupler had a reflectivity of approximately 20%. The dye laser oscillator was operated just above threshold to suppress relaxation oscillations and gave short output pulses with a typical duration of 2.0 ns (FWHM) when R.6G or pilot 386 dyes in ethanol (5 X 103M) were used as active media. The dye laser beam was focussed into a small localized volume of sample solution (approximately 1 mm3) contained in a 1 mm path length quartz fluorometer cell with an optically polished base. The focal spot was located as close to the polished base as possible. Fluorescence emission was collected from the base of the cell (i.e., at 900 to the direction of excitation). A mask with a 1 mm wide slit was used to define an aperture through which fluorescence emission could be collected by the detection system. Luminescence emission transmitted through the slit was collected by a quartz lens (focal length 100 mm, aperture 100 mm) and focused into an optical light guide (quartz rod, diameter 6 mm) coupled to the photocathode of a fast risetime photomultiplier (Hammamatsu R406). The photomultiplier was operated in a 5 stage configuration and incorporated a specially designed circuit (10) for short pulse operation (risetime approximately 1.0 ns). Part of the dye laser excitation pulse was reflected off a pellicle beam splitter and focused into a short length of optical fibre coupled to the same photomultiplier detecting the luminescence. The optical path difference between the two pulses arriving at the photomultiplier was adjusted to 6 ns. The dye laser
1
oscillator stage
N2 laser
~..
amplifier stage sample cell
.
~
filter
fibre optic
~
cell thick~ess
t1aflS~flt
[~~25A1
input lens (escitationl L
~
width slit
~~outpu~
, ,‘
lens
mask
flourescence
TEK7912AD
TEK 607
video
monitor
Fig. 1. Schematic diagram showing the experimental system used for measuring fluoresence lifetimes.
56
M. F. Quin et al.
/
Nanosecond lifetime measurements.
output of the photomultiplier was led to the input amplifier (Tektronix 7A19) of a transient digitizer (Tektronix 7912 AD). Graded neutral density filters were used to adjust the peak heights of the luminescence and excitation pulse to approximately 80 mV across the 50 ~ input impedance of the digitizer. Narrow band filters were used to select the luminescence wavelength range and eliminate unwanted scatter due to the excitation pulse. The transient digitizer was interfaced to a desk-top calculator (HP 9825A). The calculator was programmed to control and read the digitizer, to detect the onset and peaks of both pulses and to process the data. The onset of the pulse was considered to occur when the rising edges exceeded 2.5 % of the peak heights. Part of the data processing procedure involved estimating the delay 6, calculating the fluorescence lifetimes from eq. (1) and tail fitting the luminescence pulse profile to a first-order decay. The rising and falling edges of the dye laser pulse were triangulated and the magnitudes of and ‘rn. estimated for inclusion in eq. (1). A correction (approximately 300—400 ps) was included in the determination of and ‘rn. to allow for the finite risetime and falltime of the detection system (risetime and falitime approximately 1.3 ns). Since the excitation and luminescence pulses were measured using the same detection system, the error introduced by system reponse, in the determination of the delay 6, was minimized but not totally eliminated. A small correction was required to account for this source of error. This will be discussed later. The procedure described above was basically a real time technique and single shots could be used with an acceptable degree of accuracy. To improve accuracy in this work, 50 excitation and fluorescence pulses were averaged before data processing commenced. When this procedure was repeated a number of times (usually 5) for the same sample, the calculated delay 6 remained constant to ±40 ps. The average value of 6 was determined and used in all calculations.
3. Results and discussion The fluorescence lifetimes of seven organic laser dyes were measured using the techniques described above. The concentration of each dye (in ethanol) varied from 2 x 10—6 M to 5 x iO~M. Lifetimes were measured as a function of concentration and the results are presented in table 1. The behaviour of lifetime as a function of solution concentration indicated in table 1, is consistent with the radiation trapping phenomenon [11]. This effect tends to increase the measured lifetimes with increasing concentration. The effect is especially significant in dye solutions with high quantum yields and narrow Stokes shifts. To illustrate this behaviour, results pertinent to RB and R560 are plotted in fig. 2. The quantum yield of RB was measured to be 0.6 times that of R560. In fig. 2 it is seen that the measured lifetimes of R560
M.F. Quiet et al.
/
Nanosecond lifetime measurements.
57
increase much more rapidly with concentrations than those of RB. This trend is reflected in results for all dyes evaluated. The R560, R6G and R640 have quantum yields > 0.85. The oxazine dyes and cresyl violet have quantum yields <0.6 times that of R560. Relative quantum yields were measured using standard techniques [12] and agree quite closely with values reported by Kubin and Fletcher [4]. The data in fig. 2 also gives a comparison of the lifetimes measured by the pulse delay and tail fitting techniques. As shown in fig. 2, the two methods give good agreement at low solution concentrations when the effects of radiation trapping are negligible. The large deviations between the two sets of data at high solution concentrations may be partly attributed to the fact that the radiation trapping does not influence both measurement techniques to the same extent. Modelling the excitation pulse (e.g., by triangulation) is a basic feature of the pulse delay techniques and to maintain simplicity, errors introduced by this procedure must be assumed acceptable. If a well-defined excitation pulse is used, these errors are minor providing the modelling procedure is carefully applied. However, other possible sources of error should be corrected for when using the method in order to achieve lifetimes with acceptable accuracy. The major source of error identified in this work related to the following: (1) The determination of the onset of the excitation and fluorescence pulses from the detector output profiles.
10.0
Cresyl violet
‘::
___________________________________
6(
In (Cone x10
Fig. 2. Lifetime as a function of concentration for 2 dyes Cresyl violet (dashed) and Rhodamine 560 (sohd) measured by pulse delay and tail fitting techniques. The delay method is denoted (X) (+) and the tail fit (0) (s). Ordinate: lifetime (ns); Abscissa: In (cone)< 1O~).
A Excit. (nm) 386 nm 580 nm 386 nm
580 nm
580 nm
580 nm
386 nm
580 nm
Dye
methanol solution
R.6G
R.B.
R560
Oxazine 720
Ozazine 725
Cresyl Violet
Coumarin 481
RH. 640
600—: 630 Total fluor. band Total fluor. band Total fluor. band Total fluor. band Total fluor. band Total fluor. band
590— 620
Fluor. (nm)
A
—
1.7
—
2.2 2.4
—
—
1.7 2.2
—
—
0.37
—
2.5 2.9
—
2.2 2.5
0.45 0.3
—
—
0.45 0.3
—
1.0 0.6
—
—
—
0.37
2.6 2.8
0.8 0.8
1.9 2.2
3.3 4.5
0.5 0.5
1.8 3.3
—
0.44 —
2.6 4.5
1.4 2.2 0.8 3.4
2.0 4.1
2X104 M
2.5 3.6
1.6 2.15 0.45 0.95
0.95 3.0
Measured fluorescence lifetime T 6M 8x10—5 M Concentration 5 2x1O~ 4x10~5M
Table I Measured fluorescence lifetimes of 8 organic laser dyes as a function of solution concentration
5.0 5.9
0.6 0.7
2.1 3.0
—
0.41
3.3 4.3
2.0 2.25 2.7 4.9
3.25 5.5
i0~ M
4.5 5.3
0.6 0.6
2.5 4.0
—
0.43
3.5 4.2
2.5 2.5 8.20 5.8
—
—
5x103
M
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit Pulse delay Tail fit
Pulse delay Tail fit
Technique
it
C
M. F. Quiet et al.
/
Nanosecond lifetime measurements.
59
(2) The error in measuring the delay 6 introduced by the finite risetime of the detection system. (3) Errors introduced by neglecting the finite risetime of the detection system when determining the rise and fall times T~ and i-~ of the pumping pulse. In fig. 3, the dependence of fluorescence lifetime ~ is presented as a function of delay 6 for three sets of pulse rise and fall times T~and rn.. The data points indicated as circles (i.e., broken line) were calculated by evaluating ‘r~ and r~ (2.2 ns and 2.5 ns respectively) from the excitation pulse profile measured using the 5 stage photomultiplier (1.3 ns risetime). The data points indicated by the dotted line were calculated in a similar manner except that T~ and ‘ri,. (2.0 ns and 2.2 ns) were evaluated from high speed vacuum photodiode (Instrument Technology Incorporation HSD 50) pulse profiles (0.9 ns risetime). The solid curve represents the set of data obtained when a correction was applied for the finite risetime of the diode or photomultiplier detection system. According to fig. 3, if the data calculated from photomultiplier profiles are used to estimate fluorescence lifetimes from measured pulse delays without correcting for system response, errors ranging from 5% to 20% will be encountered for lifetimes ranging from 1 ns to 4 ns respectively. If similar estimates
7 S—S
Corrected
£è 6
=
2ns,
curve
for finite
T~’
2.2ns.
=
risetime
Measured by
high speed vacuum photodiode 0-0
T
p
2.2ns, T
p
‘
4
2.Sns
5
4~
...
/
3 .~
,e
2
U
0.5
1.0
1.5
2.0
Delay I (ns( Fig. 3. Calculated dependence of fluoresence lifetime i-i, on the delay for 3 sets of pulse rise and fall times and r 1,.. Ordinate: lifetimes i~(ns);Abscissa: delay 8 (ns).
60
M. F. Quin el a!.
/ Nanosecond lifetime measurements.
are made from photodiode data, the data errors will range from <1% to 7%, over the same lifetime range. The error introduced in the calculation of the delay, by the finite risetime of the detection system, is partially compensated if the same detection system is used to measure fluorescence and excitation pulses. However, this error can still be significant. Dnce the response of the detection system is known a correction may be applied, using eq. (3), to the excitation and fluorescence pulse risetimes to eliminate this source of error, F 2 2 ]1/2 [ T~+ Tsysien, risetirne 3 For example, if the measured pulse delay is 1.0 ns, according to the corrected curve in fig. 3 the fluorescence lifetime is 1.8 ns. Because of the system response however, the actual delay is approximately 145 ps larger. Applying this correction yields a lifetime of 2.4 ns. From eq. (3), the correction required for eliminating the systematic error in measuring the delay 6 ranges from 75 ps to 185 ps for pulse delays ranging from 400 ps to 1.5 ns. The random error introduced in locating the onset of the pulse was 100 ps. By averaging 50 excitation and luminescence pulses, the measured delay was found to be accurate to ±40 ps. A random error of this nature gave rise to errors of ±100 ps to ±300 ps in estimated fluorescence lifetimes ranging from 1 ns to 4 ns respectively. Finally a systematic error was introduced in the procedure for locating the onset of the pulses. By arbitrarily choosing the 2.5% of peak height as the onset point of each pulse a possible error <50 ps. was introduced. T~measured =
4. Conclusion The pulse delay technique outlined above is a relatively simple and rapid method for determining fluorescence lifetimes. The method only gives a good approximation if a well defined excitation pulse is used and if the modelling procedure is accurately applied. In addition, the method allows accurate lifetime determinations for the case of systems with single exponential decay and in the absence of secondary emission. Consequently, the lifetime data determined in this work for high solution concentrations may be somewhat distorted. Currently, attempts are being made to extend the method to multicomponent systems and to allow correction for secondary emission. We have emphasized in this paper some of the important basic sources of error which require elimination, before satisfactory lifetime determinations are possible using the pulse delay method. Finally the lifetime data reported above should prove potentially valuable to researchers using laser dyes and in the area of dye concentrators for photovoltaic applications. A thorough literature survey has revealed that much of this data has not been previously reported.
M.F. Quiet et a!.
/
Nanosecond lifetime measurements.
61
Acknowledgement The authors wish to thank the Kuwait Insitute for Scientific Research for funding this work. We also wish to thank the members of our project team who were actively involved in the project; Mr. Joseph D’Souza and Mr. Ahmad Al-Mutawa.
References [I] M.F. Quinn and MS. Al-Ajeel, Kuwait Institute for Scientific Research Final Report, ‘Planar Solar Collector Based on Light Pipe Trapping of Molecular Fluorescence for Photovoltaic Applications’, EL-4 (1982). [2] J.A. Levitt and W.H. Weber, App. Opt. 16 (1977) 2684. [3} R.W. Olsen, R.F. Loring and M.D. Fayer, App. Opt. 20 (1981) 2934. [4] R.F. Kubin and A.N. Fletcher, J. Lumin. 27 (1982) 455—462.
[5] [6] [7] [8] [9] [I0J [11] [12]
M.F. Quinn and R. O’Dowd, J. of Sci. Instrum. 13 (1980). A.J. Pesce, Fluorescence Spectroscopy (M. Dekker, New York, 1971) 186—195. DV., O’Connor and W.R. Ware, Phys. Chem. 83, No. 10 (1979) 1333—1343. A.E. Mc Kinnan, AG. Szabo and D.R. Miller, Phys. Chem. 81, No. 16 (1979) 3884—3894. T.W. Hansch, App. Opt. 11(1972) 895—898. R.W. Yip, Rev. Sci. Inst. 40 (1969) 1035. P.R. Hammond, J. Chem. Phys. 70. No. 8 (1979) 3884—3894. J.B. Birks, Florescence Quantum Yields Measurements, J. Res. National Bureau of Standards, 80A(3) (1976) 389—399.
53
MEASUREMENTS OF NANOSECOND LIFETIMES USING A TRANSIENT METHOD BASED ON PULSE DELAY M.F. QUINN, M.S. AL-AJEEL and F. AL-BAHRANI Laser and Electronics Department, Kuwait Institutefor Scientific Research, P.O. Box 24885 Safat. Kuwait Original manuscript received 1 November 1983 Revised manuscript received 1 March 1984
The results of lifetime measurements carried Out on a number of organic laser dyes are reported. The measurements were conducted over a wide range of solution concentrations. A modified version of a previously reported experimental system was developed for these measurements giving greater reliability and incorporating a higher degree of automation. A comparison with corresponding lifetimes determined by a standard fluorescence tail fitting technique was made to test the method. The reliability of the method is discussed.
1. Infroduction The emission characteristics of organic laser dyes have been studied for possible applications to a planar solar collector device [1]. The important parameters required for an in-depth analysis of such a collector are the absorption and emission spectra, quantum yields and the magnitude of the self-absorption and subsequent radiation trapping processes in the device [2,3]. Quantum yields and the phenomena of radiation trapping strongly influence the observed radiative lifetimes of emission from organic dyes [4]. Conversely, lifetime measurements may be used to estimate the approximate magnitudes of the quantum yields and the degree of radiation trapping, particularly at high solution concentrations. For these reasons, lifetime measurements were considered to be an important part of our investigation into the luminescence characteristics of the dyes for planar solar collector applications. In a previous publication [5], a pulse delay technique was described for measuring the lifetimes of organic dyes and scintillators. The technique was based on the fact that the fluorescence signal induced by a short excitation light pulse is distorted because of the finite lifetime of the excited state. It was shown that the difference in risetime of the excitation and fluorescence pulses (i.e., difference in times from onset to peak heights) was related to the fluorescence lifetime of the dye. The fluorescence lifetime was shown to be 0022-2313/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
54
M.F. Quin et al.
/
Nanosecond lifetime measurements.
related to the measured delay by eqs. (1) and (2) for triangular pump pulses and linearly rising pulses with exponentially falling edges respectively, i.e., —~exp
T~ ——
T~
Tf(
—~exp
T~
T~
‘rp’
T1
8
~
( 6 ~exp
—
~
——
—exp
Tf +——
T~
T
1,’
Tf
1+—
exp
6 ——
=0,
(1)
Tf
6
~
+——1~exp T~ )
——
T~’l~
+
T~
+——1~exp T~ )
——
8
—
T~.
(2) and are the base line to peak and peak to base line rise and fall times of the triangulated pumping pulses. T~is the fluorescence lifetime and 6 is the measured delay between the risetime of the fluorescence pulse and the pumping pulse. In this paper we report on the results of lifetime measurements carried out on a number of organic laser dyes. The measurements were conducted over a wide range of solution concentrations. A modified version of the previously reported experimental system was developed for these pulse delay measurements giving greater accuracy and incorporating a higher degree of automation. To test the reliability of the method, the results were compared to corresponding lifetimes determined by standard fluorescence tail fitting techniques [6]. Lifetimes determined by tail fitting procedures are generally considered to be less reliable than those involving deconvolution methods [7,8]. However, owing to the difficulties in setting up these latter procedures and their time consuming nature, tail fitting was considered adequate for our comparative purposes. One important consideration which was not mentioned in previous publications relates to the degree of accuracy which may be achieved using the pulse delay technique. This aspect of the measurements was considered and is discussed in detail later.
2. Experimental system The experimental system is shown schematically in fig. 1. The N2 laser was an M2000 Lambda Physik device with a pulse energy of approximately 5 mJ and a peak power of approximately 1 M Watt. The laser was used to pump a dye laser oscillator and amplifier system which was designed in this laboratory. The dye laser oscillator was of the Hansch design [9] and featured a short cavity (approximately 5 cm) incorporating a grating as the tunable element and
M.F. Quin et al.
/
Nanosecond lifetime measurements.
55
100% reflector. The output coupler had a reflectivity of approximately 20%. The dye laser oscillator was operated just above threshold to suppress relaxation oscillations and gave short output pulses with a typical duration of 2.0 ns (FWHM) when R.6G or pilot 386 dyes in ethanol (5 X 103M) were used as active media. The dye laser beam was focussed into a small localized volume of sample solution (approximately 1 mm3) contained in a 1 mm path length quartz fluorometer cell with an optically polished base. The focal spot was located as close to the polished base as possible. Fluorescence emission was collected from the base of the cell (i.e., at 900 to the direction of excitation). A mask with a 1 mm wide slit was used to define an aperture through which fluorescence emission could be collected by the detection system. Luminescence emission transmitted through the slit was collected by a quartz lens (focal length 100 mm, aperture 100 mm) and focused into an optical light guide (quartz rod, diameter 6 mm) coupled to the photocathode of a fast risetime photomultiplier (Hammamatsu R406). The photomultiplier was operated in a 5 stage configuration and incorporated a specially designed circuit (10) for short pulse operation (risetime approximately 1.0 ns). Part of the dye laser excitation pulse was reflected off a pellicle beam splitter and focused into a short length of optical fibre coupled to the same photomultiplier detecting the luminescence. The optical path difference between the two pulses arriving at the photomultiplier was adjusted to 6 ns. The dye laser
1
oscillator stage
N2 laser
~..
amplifier stage sample cell
.
~
filter
fibre optic
~
cell thick~ess
t1aflS~flt
[~~25A1
input lens (escitationl L
~
width slit
~~outpu~
, ,‘
lens
mask
flourescence
TEK7912AD
TEK 607
video
monitor
Fig. 1. Schematic diagram showing the experimental system used for measuring fluoresence lifetimes.
56
M. F. Quin et al.
/
Nanosecond lifetime measurements.
output of the photomultiplier was led to the input amplifier (Tektronix 7A19) of a transient digitizer (Tektronix 7912 AD). Graded neutral density filters were used to adjust the peak heights of the luminescence and excitation pulse to approximately 80 mV across the 50 ~ input impedance of the digitizer. Narrow band filters were used to select the luminescence wavelength range and eliminate unwanted scatter due to the excitation pulse. The transient digitizer was interfaced to a desk-top calculator (HP 9825A). The calculator was programmed to control and read the digitizer, to detect the onset and peaks of both pulses and to process the data. The onset of the pulse was considered to occur when the rising edges exceeded 2.5 % of the peak heights. Part of the data processing procedure involved estimating the delay 6, calculating the fluorescence lifetimes from eq. (1) and tail fitting the luminescence pulse profile to a first-order decay. The rising and falling edges of the dye laser pulse were triangulated and the magnitudes of and ‘rn. estimated for inclusion in eq. (1). A correction (approximately 300—400 ps) was included in the determination of and ‘rn. to allow for the finite risetime and falltime of the detection system (risetime and falitime approximately 1.3 ns). Since the excitation and luminescence pulses were measured using the same detection system, the error introduced by system reponse, in the determination of the delay 6, was minimized but not totally eliminated. A small correction was required to account for this source of error. This will be discussed later. The procedure described above was basically a real time technique and single shots could be used with an acceptable degree of accuracy. To improve accuracy in this work, 50 excitation and fluorescence pulses were averaged before data processing commenced. When this procedure was repeated a number of times (usually 5) for the same sample, the calculated delay 6 remained constant to ±40 ps. The average value of 6 was determined and used in all calculations.
3. Results and discussion The fluorescence lifetimes of seven organic laser dyes were measured using the techniques described above. The concentration of each dye (in ethanol) varied from 2 x 10—6 M to 5 x iO~M. Lifetimes were measured as a function of concentration and the results are presented in table 1. The behaviour of lifetime as a function of solution concentration indicated in table 1, is consistent with the radiation trapping phenomenon [11]. This effect tends to increase the measured lifetimes with increasing concentration. The effect is especially significant in dye solutions with high quantum yields and narrow Stokes shifts. To illustrate this behaviour, results pertinent to RB and R560 are plotted in fig. 2. The quantum yield of RB was measured to be 0.6 times that of R560. In fig. 2 it is seen that the measured lifetimes of R560
M.F. Quiet et al.
/
Nanosecond lifetime measurements.
57
increase much more rapidly with concentrations than those of RB. This trend is reflected in results for all dyes evaluated. The R560, R6G and R640 have quantum yields > 0.85. The oxazine dyes and cresyl violet have quantum yields <0.6 times that of R560. Relative quantum yields were measured using standard techniques [12] and agree quite closely with values reported by Kubin and Fletcher [4]. The data in fig. 2 also gives a comparison of the lifetimes measured by the pulse delay and tail fitting techniques. As shown in fig. 2, the two methods give good agreement at low solution concentrations when the effects of radiation trapping are negligible. The large deviations between the two sets of data at high solution concentrations may be partly attributed to the fact that the radiation trapping does not influence both measurement techniques to the same extent. Modelling the excitation pulse (e.g., by triangulation) is a basic feature of the pulse delay techniques and to maintain simplicity, errors introduced by this procedure must be assumed acceptable. If a well-defined excitation pulse is used, these errors are minor providing the modelling procedure is carefully applied. However, other possible sources of error should be corrected for when using the method in order to achieve lifetimes with acceptable accuracy. The major source of error identified in this work related to the following: (1) The determination of the onset of the excitation and fluorescence pulses from the detector output profiles.
10.0
Cresyl violet
‘::
___________________________________
6(
In (Cone x10
Fig. 2. Lifetime as a function of concentration for 2 dyes Cresyl violet (dashed) and Rhodamine 560 (sohd) measured by pulse delay and tail fitting techniques. The delay method is denoted (X) (+) and the tail fit (0) (s). Ordinate: lifetime (ns); Abscissa: In (cone)< 1O~).
A Excit. (nm) 386 nm 580 nm 386 nm
580 nm
580 nm
580 nm
386 nm
580 nm
Dye
methanol solution
R.6G
R.B.
R560
Oxazine 720
Ozazine 725
Cresyl Violet
Coumarin 481
RH. 640
600—: 630 Total fluor. band Total fluor. band Total fluor. band Total fluor. band Total fluor. band Total fluor. band
590— 620
Fluor. (nm)
A
—
1.7
—
2.2 2.4
—
—
1.7 2.2
—
—
0.37
—
2.5 2.9
—
2.2 2.5
0.45 0.3
—
—
0.45 0.3
—
1.0 0.6
—
—
—
0.37
2.6 2.8
0.8 0.8
1.9 2.2
3.3 4.5
0.5 0.5
1.8 3.3
—
0.44 —
2.6 4.5
1.4 2.2 0.8 3.4
2.0 4.1
2X104 M
2.5 3.6
1.6 2.15 0.45 0.95
0.95 3.0
Measured fluorescence lifetime T 6M 8x10—5 M Concentration 5 2x1O~ 4x10~5M
Table I Measured fluorescence lifetimes of 8 organic laser dyes as a function of solution concentration
5.0 5.9
0.6 0.7
2.1 3.0
—
0.41
3.3 4.3
2.0 2.25 2.7 4.9
3.25 5.5
i0~ M
4.5 5.3
0.6 0.6
2.5 4.0
—
0.43
3.5 4.2
2.5 2.5 8.20 5.8
—
—
5x103
M
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit
Pulse delay Tail fit Pulse delay Tail fit
Pulse delay Tail fit
Technique
it
C
M. F. Quiet et al.
/
Nanosecond lifetime measurements.
59
(2) The error in measuring the delay 6 introduced by the finite risetime of the detection system. (3) Errors introduced by neglecting the finite risetime of the detection system when determining the rise and fall times T~ and i-~ of the pumping pulse. In fig. 3, the dependence of fluorescence lifetime ~ is presented as a function of delay 6 for three sets of pulse rise and fall times T~and rn.. The data points indicated as circles (i.e., broken line) were calculated by evaluating ‘r~ and r~ (2.2 ns and 2.5 ns respectively) from the excitation pulse profile measured using the 5 stage photomultiplier (1.3 ns risetime). The data points indicated by the dotted line were calculated in a similar manner except that T~ and ‘ri,. (2.0 ns and 2.2 ns) were evaluated from high speed vacuum photodiode (Instrument Technology Incorporation HSD 50) pulse profiles (0.9 ns risetime). The solid curve represents the set of data obtained when a correction was applied for the finite risetime of the diode or photomultiplier detection system. According to fig. 3, if the data calculated from photomultiplier profiles are used to estimate fluorescence lifetimes from measured pulse delays without correcting for system response, errors ranging from 5% to 20% will be encountered for lifetimes ranging from 1 ns to 4 ns respectively. If similar estimates
7 S—S
Corrected
£è 6
=
2ns,
curve
for finite
T~’
2.2ns.
=
risetime
Measured by
high speed vacuum photodiode 0-0
T
p
2.2ns, T
p
‘
4
2.Sns
5
4~
...
/
3 .~
,e
2
U
0.5
1.0
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2.0
Delay I (ns( Fig. 3. Calculated dependence of fluoresence lifetime i-i, on the delay for 3 sets of pulse rise and fall times and r 1,.. Ordinate: lifetimes i~(ns);Abscissa: delay 8 (ns).
60
M. F. Quin el a!.
/ Nanosecond lifetime measurements.
are made from photodiode data, the data errors will range from <1% to 7%, over the same lifetime range. The error introduced in the calculation of the delay, by the finite risetime of the detection system, is partially compensated if the same detection system is used to measure fluorescence and excitation pulses. However, this error can still be significant. Dnce the response of the detection system is known a correction may be applied, using eq. (3), to the excitation and fluorescence pulse risetimes to eliminate this source of error, F 2 2 ]1/2 [ T~+ Tsysien, risetirne 3 For example, if the measured pulse delay is 1.0 ns, according to the corrected curve in fig. 3 the fluorescence lifetime is 1.8 ns. Because of the system response however, the actual delay is approximately 145 ps larger. Applying this correction yields a lifetime of 2.4 ns. From eq. (3), the correction required for eliminating the systematic error in measuring the delay 6 ranges from 75 ps to 185 ps for pulse delays ranging from 400 ps to 1.5 ns. The random error introduced in locating the onset of the pulse was 100 ps. By averaging 50 excitation and luminescence pulses, the measured delay was found to be accurate to ±40 ps. A random error of this nature gave rise to errors of ±100 ps to ±300 ps in estimated fluorescence lifetimes ranging from 1 ns to 4 ns respectively. Finally a systematic error was introduced in the procedure for locating the onset of the pulses. By arbitrarily choosing the 2.5% of peak height as the onset point of each pulse a possible error <50 ps. was introduced. T~measured =
4. Conclusion The pulse delay technique outlined above is a relatively simple and rapid method for determining fluorescence lifetimes. The method only gives a good approximation if a well defined excitation pulse is used and if the modelling procedure is accurately applied. In addition, the method allows accurate lifetime determinations for the case of systems with single exponential decay and in the absence of secondary emission. Consequently, the lifetime data determined in this work for high solution concentrations may be somewhat distorted. Currently, attempts are being made to extend the method to multicomponent systems and to allow correction for secondary emission. We have emphasized in this paper some of the important basic sources of error which require elimination, before satisfactory lifetime determinations are possible using the pulse delay method. Finally the lifetime data reported above should prove potentially valuable to researchers using laser dyes and in the area of dye concentrators for photovoltaic applications. A thorough literature survey has revealed that much of this data has not been previously reported.
M.F. Quiet et a!.
/
Nanosecond lifetime measurements.
61
Acknowledgement The authors wish to thank the Kuwait Insitute for Scientific Research for funding this work. We also wish to thank the members of our project team who were actively involved in the project; Mr. Joseph D’Souza and Mr. Ahmad Al-Mutawa.
References [I] M.F. Quinn and MS. Al-Ajeel, Kuwait Institute for Scientific Research Final Report, ‘Planar Solar Collector Based on Light Pipe Trapping of Molecular Fluorescence for Photovoltaic Applications’, EL-4 (1982). [2] J.A. Levitt and W.H. Weber, App. Opt. 16 (1977) 2684. [3} R.W. Olsen, R.F. Loring and M.D. Fayer, App. Opt. 20 (1981) 2934. [4] R.F. Kubin and A.N. Fletcher, J. Lumin. 27 (1982) 455—462.
[5] [6] [7] [8] [9] [I0J [11] [12]
M.F. Quinn and R. O’Dowd, J. of Sci. Instrum. 13 (1980). A.J. Pesce, Fluorescence Spectroscopy (M. Dekker, New York, 1971) 186—195. DV., O’Connor and W.R. Ware, Phys. Chem. 83, No. 10 (1979) 1333—1343. A.E. Mc Kinnan, AG. Szabo and D.R. Miller, Phys. Chem. 81, No. 16 (1979) 3884—3894. T.W. Hansch, App. Opt. 11(1972) 895—898. R.W. Yip, Rev. Sci. Inst. 40 (1969) 1035. P.R. Hammond, J. Chem. Phys. 70. No. 8 (1979) 3884—3894. J.B. Birks, Florescence Quantum Yields Measurements, J. Res. National Bureau of Standards, 80A(3) (1976) 389—399.