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Gain measurements in binary and ternary dye mixture solutions under nitrogen laser excitation
SPECTROCHIMICA ACTA PART A
SpectrochimicaActa Part A 53 (1997) 713-720
ELSEVIER
Gain measurements in binary and ternary dye mixture solutions under nitrogen laser excitation Sujata Sanghi, D. Mohan *, R.D. Singh Department
of Physics, Received
Mahnrshi
16 March
Dayanand 1996: received
Utmersity, in revised
Rohtakform
124 001, Haryana. 1 October
India
1996
Abstract Energy transfer studies in the case of binary [Coumarin 485 (C485) + Rhodamine 610 perchlorate (Rh 610)] and ternary [C440 + C485 + Rh610 perchlorate] dye mixture solutions in ethanol have been made and discussed through optical gain characteristics at various acceptor concentrations and pump powers of the N,-laser. In case of binary mixtures, we observe that the optical gain of the acceptor dye (Rh610) improves and the threshold power required for lasing action decreases when donor dye (C485) is added. The concentration dependence of peak gain and peak lasing wavelength of the energy transfer dye laser (ETDL) has been studied. The results have also been compared with the optical gain characteristics of these dyes in single component solutions. The energy transfer rate constants (&. the FiSrster type non-radiative and KR, the radiative rate constants) and critical transfer radius (R,) have been calculated using a Stern-Volmer analysis of the pump power dependence of the gain. From the experimental results, we find that the dominant mechanism responsible for efficient excitation transfer in this bimixture is of a radiative nature. Further, the gain equation for a ternary dye mixture has been derived using the transfer rate constants for two different possible bimixtures; C440 + C485 and C485 + Rh610, which is helpful in obtaining the optimum concentration to be used in the ternary mixture for higher gains. 8 1997 Elsevier Science B.V.
1. Introduction
Intermolecular energy transfer between dye molecules has been used to couple the absorption of a lasing dye to the emission spectrum of an optically pumped dye in a mixture. This technique of energy transfer excitation between the dyes is a very effective and simple method of improving the dye laser efficiency and its spectral region of operation. A given dye may lase poorly or not at all because its absorption is too weak at the pump * Corresponding 1386-1425/97/$17.00 PIIS1386-1425(96)01829-6
author. G 1997 Elsevier
Science
B.V. All
rights
reserved
wavelength. The problem arises particularly when the excitation source is a N,-laser having a single UV line at 337.1 nm. By adding a second dye (donor) which strongly absorbs at the pump wavelength and subsequently transfers its energy to lasing dye (acceptor), lasing action can be enhanced. The greater the overlap of absorption spectrum of acceptor and emission spectrum of donor, the faster will be the transfer rate [l]. In addition to this overlap, another physical property which plays an important role in improving the dye laser performance is the optical gain. Therefore, various groups of research workers
have studied the role of concentration, pump power and different types of combinations on the optical gain of the different dyes in mixture solutions [2-81. From the various proposed mechanisms of energy transfer [3], in the mixtures studied here, the transfer of energy takes place mainly due to radiative and non radiative long range dipole-dipole interaction mechanisms. For the coumarinxanthene system of ETDL [C485 +Rh610 (ClO;) dye mixture] under investigation, the energy transfer by exchange interaction has been neglected because (a) the absorption spectrum of any substituted coumarin was found to remain unchanged when adding a xanthene dye, (b) no new fluorescence peaks were detected in the mixture to indicate any fluorescent exciplex formation, (c) the concentration of coumarin and xanthene was always taken less than or equal to 5 x 10 -- ’ M and hence collisional encounters due to short range would be very rare. The two transfer rates in the case of the bimixture under reference have been estimated. The value of the non-radiative (long range dipole-dipole type) rate constant, K,, is obtained from the Stern-Volmer plot. Further, the radiative transfer rate constant, KR, is estimated from the pump power dependence of the gain per molecule. The gain per acceptor acceptor molecule is given by the equation [5,9],
=
gc
i
A ‘A
- + (KF + K&J: %I>
XII - 0-r
Fil
~ l-FbKFNA+l/rb
N*
(1)
where xi, = a,DW(t)z, and CIA = a?W(t)r,; a,D, gt, 0: and crt are the emission and absorption cross-sections of donor and acceptor; W(r) is the pump rate; zD and zA are the florescence life times of donor and acceptor respectively; F,, = ND/ (N, + N,,) is the fractional donor population, N, and N* being the state population densities of donor and acceptor respectively. This equation shows that the gain of acceptor per acceptor molecule in the case of bimixture is increased by a factor X
The experimental setup for the measurement of gain has been described elsewhere [lo]. Dyes used were obtained from Exciton company (USA). The solutions of dyes C485 (donor) and Rh610 (acceptor) were prepared in analytical grade ethanol at a fixed donor concentration of 5 x 10 ’ M and varying acceptor concentration from 5 x IO ’ M to 1 x lo--’ M. The excitation of the solution contained in a quartz dye cell (internal area of cross-section, 8 mm x 8 mm) was carried out using a nitrogen laser (Molectron UV-24, peak power 900 kW, repetition rate 10 pps, pulse duration 10 ns. beam area 0.6 x 3.2 cm’). The output was detected using a monochromator, photomultiplier tube, and an oscilloscope. In order to make the intensity variation studies, the pump intensity was attenuated using a number of glass filters in between the dye cell and the cylindrical lens.
2. Results and discussion ETDL chmucteristics: The gain spectra of C485 + Rh610 dye mixture at fixed donor concentration of 5 x 10 - ’ M and varying acceptor concentrations in the range from 5 x 10 ~~’ M to 1 x 10 ~ ’ M have been investigated. The donor concentration is chosen in such a way that its gain is reported to be maximum [l 11, and the acceptor concentration is kept lower than or equal to that of donor for energy transfer studies. The use of higher concentrations would increase the energy loss due to collisional encounters followed by self-absorption, and therefore is neglected. 2.1. Concentration
dependerm
qf optical guin
Fig. 1 depicts the typical gain spectra of the bimixture at 2 x 10 ~’ M concentration of the acceptor. All other curves, not shown in the figure, exhibit similar behaviour except for the variation of peak wavelength and peak gain values (Table 1). It has been reported earlier that the single solution of C485 at 5 x lo--’ M concentration exhibits gain from 520 to 560 nm, with peak gain value of 0.7 cm -~ ’ and the corresponding
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Fig. 2. Concentration dependence of peak lasing wavelength; (a) C485: (b) C485 (5 x lo-” M)+Rh610 (I x IO- ’ 5 x IO‘-‘M); (c) Rh610.
Fig. 1. Gain spectra for a C485 + Rh610 dye mixture in ethanol at a donor concentration of 5 x lo-’ M and an acceptor concentration of 2 x IO-’ M.
of the gain spectra of bimixture solutions with acceptor concentration as well as of donor and acceptor alone. It is clear that there is a red shift in all cases, due to emission-reabsorption effect [12]. Also, the figure shows that there is a blue shift in the emission from mixtures as compared to acceptor dye alone at all concentrations, which may be understood to be caused by the reduction in aggregate formation in the acceptor molecule because of the formation of new donor-acceptor aggregates [13]. Again, the red shift in the mixture is much larger than that in donor and acceptor alone, because of the emission-reabsorption effect being two-fold effective in case of mixture under study.
peak wavelength occurring at 548 nm [I 11. When an acceptor dye (Rh610) is added, the peak wavelength of gain spectra show a red shift and the shift increases with increasing concentration of acceptor. An increase in peak gain value is also observed with acceptor concentration. The results thus obtained confirm the energy transfer between dyes due to reabsorption effects [4]. Also, the single solution of C-485 shows two peaks in the gain spectra at all concentrations, which was attributed to emission from twisted intramolecular charge transfer (TICT) state formation in the excited states [1 I]. However, this type of dual fluorescence behaviour is not observed in mixture solutions, where coumarin acts as a donor dye, because the energy from the donor dye is used to excite the acceptor dye, and the acceptor dye in the present case, Rh610, being rigid in structure, has no tendency of TICT state formation.
2.1.2. Concentration dependenceof’peuk gctbr
Fig. 3 shows the acceptor concentration dependence of the peak gain in bimixtures along with that of acceptor and donor alone. Although the peak gain value of the donor is smaller at higher concentrations, the use of 5 x 10 -- ’ M concentration of donor dye in the mixture does not affect the peak gain value in the mixture system. This is
2.1.1. Concentration dependenceof peak lasing b~~avelengtll
Fig. 2 depicts the variation Table 1 -____ Donor concentration (M) --__ 5x10-’ 5 x IO ~-3 5 x 10--j 5x lo-’ 5x IO -?
in peak wavelength Acceptor concentration (M) 5 x 10-j 2X 10-’ 1x1o-q 5 x low I x IOF4
- _^_--__
-___ Peak gain (cm ’ ) 2.60 I .4O 0.82 0.77 0.50
Peak wavelength (nm) --~.-----610 608 596 594 584
-~-
716
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et al. /‘Spec.trochimic.u
understood because of the larger gain value of the acceptor alone at higher concentration and relatively higher concentration had to be chosen for efficient energy transfer from donor to acceptor by varying the acceptor concentration in relatively wider range of concentration. The decreasing value of peak gain in the case of donor dye alone is understood to be caused by effects of concentration quenching which decreases the lifetime [l 1,141; whereas in acceptor dye alone, it is caused by increase in lifetime, which in fact depends upon its molecular structure. However, in bimixtures the increased value of peak gain with increasing acceptor concentration is caused by the interaction between excited donor molecules and ground state acceptor molecules, which enhance the effective lifetime of the acceptor. 2.2. Energy trunsfkr
mte constants
In order to calculate the energy transfer rate constants in the bimixture of C485 + Rh610, firstly, the variations of the ratio of the fluorescence intensity of donor in the presence and absence of acceptor with the latter concentration ranging from 1 x lo-” M to 5 x 10-j M was studied (Fig. 4). The results obey Stern-Volmer equation. I,,/l, = 1 + &.z&!]. where ILo and IL are the fluorescence intensities of donor in the
Fig. 3. Concentration (b) C485 (5 x IOW’ Rh610.
dependence M)+Rh610
of the peak gain; (a) C485: (1 x lo-‘-5 x lOPA M); (c)
Acta
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Fig. 4. The Stern--Volmer in ethanol.
plot for C485 + Rh610
dye mixture
absence and presence of acceptor respectively and [A] is the acceptor concentration. The slope of this line, KFrfo = 0.09 x 10” M - ‘, gives the value of I&, the resonance transfer rate due to long range dipole-dipole interaction (also known as Fiirster type transfer rate). which is equal to 0.26 x 10” M ~’ s- ’ where zfc, is the lifetime of the excited donor in absence of acceptor ( - 3.5 ns). Fig. 4 also gives the value of R,, the critical transfer radius, using the relation R,, = 7.35 x ([A],,z)P “’ A [15]. Here [A],,? is the half quenching concentration of the acceptor at which IL = $ILo and is equal to 3.34 x 10 -~’ M. Thus, the calculated value of R, is 49.16 A, which corresponds to an average of one molecule of the acceptor in a sphere of radius 49 A with excited donor as the centre. The value agrees well with that obtained in resonance transfer of the Fiirster type for other donor-acceptor pairs using different techniques [161. To evaluate K,, the radiative transfer rate constant, the ETDL output at peak wavelength (2,) was measured for various pump powers. The variation of the gain per acceptor molecule and pump power for a 2 x lo- 3 M solution of Rh610 with and without donor (i.e. ND = 2 x lo-? M and N, = 0 respectively) is shown in Fig. 5. It is remarkable that at this wavelength, donor absorption as well as emission are negligible. The peak ETDL outputs in terms of gain/acceptor molecule do not show any saturation tendency as can be seen from Fig. 5. Also, the lasing threshold for Rh610 in the mixture is lower than that for Rh610 alone, thereby showing that ETDL lowers the required threshold pump power. Again, the gain/ acceptor molecule at 0.20 MW is negative ( 1.2 x 10 .~” cm’) due to singlet state reabsorption
S. Sanghi
et al. !Spectrochimica
by acceptor molecules. When the donor is present, the gain/acceptor molecule at 0.20 MW increases by 4.5 x 10 ~ I9 cm?. The slopes of the gain versus pump power curves for the mixture and for the acceptor alone are 3.2 x 10 - ” and 2.1 x lo-” cm’ kW - ’ respectively, with the donor-to-acceptor concentration ratio being equal to one. The corrected slope calculated by subtracting the contribution due to the acceptor alone from that of the mixture (2 1.1 x 10 ” cm’ kW -- ‘) is equated with the relation given by Eq. (1). Putting the value of all the constants in this equation, the value of K, comes out to be 0.10 x lOI M ~ ’ s ~ ’ which is quite large as that for KF. Thus, in the present mixture, the radiative energy transfer is the most dominant mechanism in the energy transfer process. 17.3. The gain equution jbr u ternmy dye nzixture solutim From the measurements of energy transfer rate parameters (Table 2) in bimixtures of C440 + C485 [17] and C485 + Rh610, it is predicted that a dye mixture C440 + C485 + Rh610 is also quite effective. Fig. 6 shows the kinetic scheme for such a ternary dye mixture system. To find out the gain of the ETDL, the three dyes are denoted as dye 1, dye 2, dye 3 where the main donor dye is dye 1; dye 2 is intermediator as it acts as donor w.r.t.
Acta Part Table
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111
2
Donor
dye
Acceptor
dye
Energy stant
transfer
17, (M-’
Coumarin 440 Coumarin 485
Coumarin 485 Rhodamine 610
s-
rate con-
1)
K, 1)
(M-’
s
1.054 x IO”
2.55 n IO”
0.26 x IO”
1.0x
IO”
dye 3 and as acceptor w.r.t. dye 1; and dye 3 is the main acceptor. The energy transfer rates in cascade processes are denoted by K,, and K,,; and K,, is the transfer rate for parallel process; No, and NIi are the state population densities of ground and first singlet state of dye i (i = 1, 2, 3) respectively. The singlet states are pumped with a short pulse from a laser source of intensity I,, at the rate cr,,(p)Z,. Lasing occurs in the acceptor molecules at the rate aLI,, where ZL is the generated dye laser intensity. Absorption losses at the dye laser frequency are represented by the rate of gb,( IV,. The population rate equations are written as dN, -= 1 o;, W(t)N,, dt
- (K,2N02 + K,,N,,)N,,
- ?lL ;I
(2)
dN,, -- N,, r,
(3)
CC.“O,,dl.l, CUO
Fig. 5. Pump power dependence acceptor molecule at 580 nm with
of gain of acceptor No/N, = 1.
Inlcrm~dlotor (dw2)C&BS
Rh 610
per Fig. 6. Kinetic
scheme for the ternary
dye mixture
system.
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N=N,+N2+N3=zN,
-_ N,,
(4)
r3
where W(t) is the rate of excitation, oh, and ri are the absorption cross-section at pump wavelength and decay time of singlet states of dye i respectively in absence of transfer. The effects of triplet states have been omitted because of the simple reason that these states are not effective due to short pump pulse duration [18]. The gain of the dye mixture system at wavelength i. in terms of stimulated and absorption cross-section is given by [14]
I
and xi = crb,W(i)z,
18)
For unsaturated gain, the ground state populations are negligibly perturbed. So No3= N;. i.e. n()j = -N3 = 1 N3 NO2 n 02 = --=F,
=$
N3
(9)
v(2) = o,iN,, - (T;:N,,~ + “,?N,, - o-;Nn2 + a;N,, - g:rJ,,
(5)
F,, F, being the fractional populations of dye 1 and 2, respectively. Now, under steady state approximation
neglecting the absorption and emission of dye 1 and dye 2 in the fluorescence spectrum of dye 3, the net gain coefficient is given by
dn,, dn,, -=---.--=-=
v(A) = o,3N,, - CT:N,,,
Simplifying Eq. (7)
The rate equations less by multiplying
(6)
(2-4) can be made dimensionby r3/N3 to give:
dn,, = x,n,,, - (K,,N,n,,n,,z, __
ds
dx
dn,,
d.u
0
dx
(Fn+ F,VGzN, + K, n’3 = “’ + (K,,N, + K,)(K,,F,N,
+ K,)
a4 K,,N, (10)
+ fLN,nn,r~,,d
where K, = l/r, is the natural decay rate of dye i and x = x, = x,. So Eq. (6) becomes
=
g3
5
+
{(Fn
=CI
V&N,
+
F,KzN, +
+
K,
lF&Ni
KJW,,F,N,
+
(T3 e K,)
xN, 1
The gain per acceptor molecule is written as v(i)
v(2) = N 3
where =
& 1 +
N,, n,i = - , N,
No, n,,j = - 3 N3
Nj= N,,+N,,
z i(Fn + FJKzN3
+ K,lF&N
(K,,N, + K,)(KnF,N,
- (T?‘L
+ K)
g3 cI
’1 (11)
S. Sanghi et al. /ISpectrochiniica Acta Part A 5.1 (19971 713-720
wVELENGlHlnm1
Fig. 7. Absorption and fluorescence spectra of C440, C485 and Rh610 dye molecules.
This equation shows that the gain per acceptor molecule in the ternary mixture is increased by the factor (F,, + F,X,,N, + K, (Kz3N3+ K,)(K,,F,N, + K,)
F’K2’Nycta,3
(12)
which is much greater than that for the binary mixture (Eq. (I)). Also the gain per acceptor molecule is proportional to the pump power and a plot between ~(2) and pump power is a straight line with the slope given by Eq. (12). Thus one can easily calculate F, and F,, the concentration of the dyes to be used in the ternary dye mixture, by equating the experimental and theoretical values.
‘19
Improvement of efficiency of the dye laser is anticipated using cascade energy transfer as indicated by the above equation, where S, and S, denote the ground state and excited singlet state of each dye respectively. The gain spectra of C440 at 5 x 10 ’ M concentration give the peak wavelength at 460 nm; while for the mixtures of C440 + C485 and C485 + Rh610 the peak wavelength is 552 nm and 610 nm respectively for fixed donor and acceptor concentration at 5 x 10 .- ’ M. The gain spectra of C440 dye alone give a red shift of peak wavelength of the order = 14 nm ranging from 446-~ 460 nm; while the mixture of C440 + C485 increases the tuning range by 20 nm (532- 552 nm) and that in C485 + Rh610 dye mixture the tuning is increased by 26 nm (5844610 nm). Thus. the cascade energy transfer in ternary dye mixture would further increase the tuning range which clearly confirms that by adding the second and third dye of proper concentration in the first dye. the tuning can be increased to cover the lasing range from 460-610 nm ( = 150 nm) without disturbing the alignment of the experimental system.
24. Cuscade ETDL operatiorl
3. Conclusions
The fluorescence and absorption spectra of the three dyes are shown in Fig. 7. From the figure it is clear that there is no overlap between the C440 dye fluorescence spectrum and the Rh610 dye absorption spectrum, thereby neglecting the possibility of energy transfer in C440 + Rh610 bimixture. When an intermediate dye (C485) is used, the Rh610 dye can be made to lase. Also, as shown in Table 2, the energy transfer rate constants between C440 + C485 dye mixture are large as compared to those of C485 + Rh610 which shows that the energy transfer between excited singlet levels in the ternary dye mixture is produced with high efficiency. The energy transfer sequence in the ternary dye mixture is understood in the following way [19].
1. The study of TICT states become important when non-rigid dyes of coumarin class are chosen individually or in mixture form. 2. The non-radiative type of transfer rate is dominant in case of sister molecules (coumarincoumarin) because of the closely spaced vibrational levels, while in coumarin-xanthene bimixture system, the radiative type transfer rate becomes dominant. 3. The efficiency of the nitrogen laser pumped dye laser is increased through energy transfer excitation in the ternary dye mixture system.
S,(C440) + $,(C485) +S,(C440) + S,(C485) S,(C485) + S,,(Rh610) + S,(C485) + S,(Rh610)
Acknowledgements
The first author is grateful to the Council of Scientific and Industrial Research for providing financial assistance.
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References [lo] [l] [2] [3] [4] [5] [6] [7] [8] [9]
LB. Berlman, M. Rokni and C.R. Goldschmidt. Chem. Phys. Lett., 22 (1973) 458. S. Spieser, Opt. Commun.. 29 (1979) 213. M.I. Savadatti, S.R. Inamdar, N.N. Math and A.D. Mulla, J. Chem. Sot. Trans. 2, 82 (1986) 2417. A.J. Cou and B.K. Matise. Chem. Phys. Lett.. 76 (1980) 125. T. Govindanunny, B.M. Sivaram and M. Srinivasan, Ind. J. Phys., 54B (1980) 32. E. Wiess and S. Spieser. Chem. Phys. Lett., 40 (1976) 220. S. Muto. C. Ito and H. Inaba, Jap. J. Appl. Phys.. 21 (1982) L535. V. Yu, Kurstak, A.N., Rubinov and T. Sn. Ffendiev, J. Appl. Spectrosc., 54 (1991) 568. T. Govindanunny and B.M. Sivaram, J. Lumin.. 21
[ll] [l2] [13] [14] [15] [16] [l7] [18] [l9]
A 53 (1997)
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(1980) 397. L. Taneja. A.K. Sharma, R.D. Singh, Opt. Commun.. 106 (1994) 79. D. Mohan. L. Taneja, A. Gaur, A.K. Sharma and R.D. Singh, J. Lumin.. 50 (1991) 127. M. Tarada and J. Muto, Opt. Commun., 59 (1986) 199. P.J. Sebastian and K. Sathianandan. Opt. Commun.. 35 (1980) 113. O.G. Peterson, J.R. Webb and W.C. McGolgin. Appl. Phys.. 42 (1971) 917. R.D. Singh. A.K. Sharmd, N.V. Unnikrishanan and D. Mohan. J. Mod. Opt., 37 (1990) 419. C. Lin and A. Dienes, J. Appl. Phys.. 44 (1973) 5050. I. Taneja. A.K. Sharma and R.D. Singh. Opt. Commun.. 111 (1994) 463. T. Urisa and K. Kajiyama. J. Appl. Phys., 47 (1976) 3559. S. Muto, C. Ito and H. Inabe. Electron. Commun. Jpn.. 67-C (1984) 104.
SpectrochimicaActa Part A 53 (1997) 713-720
ELSEVIER
Gain measurements in binary and ternary dye mixture solutions under nitrogen laser excitation Sujata Sanghi, D. Mohan *, R.D. Singh Department
of Physics, Received
Mahnrshi
16 March
Dayanand 1996: received
Utmersity, in revised
Rohtakform
124 001, Haryana. 1 October
India
1996
Abstract Energy transfer studies in the case of binary [Coumarin 485 (C485) + Rhodamine 610 perchlorate (Rh 610)] and ternary [C440 + C485 + Rh610 perchlorate] dye mixture solutions in ethanol have been made and discussed through optical gain characteristics at various acceptor concentrations and pump powers of the N,-laser. In case of binary mixtures, we observe that the optical gain of the acceptor dye (Rh610) improves and the threshold power required for lasing action decreases when donor dye (C485) is added. The concentration dependence of peak gain and peak lasing wavelength of the energy transfer dye laser (ETDL) has been studied. The results have also been compared with the optical gain characteristics of these dyes in single component solutions. The energy transfer rate constants (&. the FiSrster type non-radiative and KR, the radiative rate constants) and critical transfer radius (R,) have been calculated using a Stern-Volmer analysis of the pump power dependence of the gain. From the experimental results, we find that the dominant mechanism responsible for efficient excitation transfer in this bimixture is of a radiative nature. Further, the gain equation for a ternary dye mixture has been derived using the transfer rate constants for two different possible bimixtures; C440 + C485 and C485 + Rh610, which is helpful in obtaining the optimum concentration to be used in the ternary mixture for higher gains. 8 1997 Elsevier Science B.V.
1. Introduction
Intermolecular energy transfer between dye molecules has been used to couple the absorption of a lasing dye to the emission spectrum of an optically pumped dye in a mixture. This technique of energy transfer excitation between the dyes is a very effective and simple method of improving the dye laser efficiency and its spectral region of operation. A given dye may lase poorly or not at all because its absorption is too weak at the pump * Corresponding 1386-1425/97/$17.00 PIIS1386-1425(96)01829-6
author. G 1997 Elsevier
Science
B.V. All
rights
reserved
wavelength. The problem arises particularly when the excitation source is a N,-laser having a single UV line at 337.1 nm. By adding a second dye (donor) which strongly absorbs at the pump wavelength and subsequently transfers its energy to lasing dye (acceptor), lasing action can be enhanced. The greater the overlap of absorption spectrum of acceptor and emission spectrum of donor, the faster will be the transfer rate [l]. In addition to this overlap, another physical property which plays an important role in improving the dye laser performance is the optical gain. Therefore, various groups of research workers
have studied the role of concentration, pump power and different types of combinations on the optical gain of the different dyes in mixture solutions [2-81. From the various proposed mechanisms of energy transfer [3], in the mixtures studied here, the transfer of energy takes place mainly due to radiative and non radiative long range dipole-dipole interaction mechanisms. For the coumarinxanthene system of ETDL [C485 +Rh610 (ClO;) dye mixture] under investigation, the energy transfer by exchange interaction has been neglected because (a) the absorption spectrum of any substituted coumarin was found to remain unchanged when adding a xanthene dye, (b) no new fluorescence peaks were detected in the mixture to indicate any fluorescent exciplex formation, (c) the concentration of coumarin and xanthene was always taken less than or equal to 5 x 10 -- ’ M and hence collisional encounters due to short range would be very rare. The two transfer rates in the case of the bimixture under reference have been estimated. The value of the non-radiative (long range dipole-dipole type) rate constant, K,, is obtained from the Stern-Volmer plot. Further, the radiative transfer rate constant, KR, is estimated from the pump power dependence of the gain per molecule. The gain per acceptor acceptor molecule is given by the equation [5,9],
=
gc
i
A ‘A
- + (KF + K&J: %I>
XII - 0-r
Fil
~ l-FbKFNA+l/rb
N*
(1)
where xi, = a,DW(t)z, and CIA = a?W(t)r,; a,D, gt, 0: and crt are the emission and absorption cross-sections of donor and acceptor; W(r) is the pump rate; zD and zA are the florescence life times of donor and acceptor respectively; F,, = ND/ (N, + N,,) is the fractional donor population, N, and N* being the state population densities of donor and acceptor respectively. This equation shows that the gain of acceptor per acceptor molecule in the case of bimixture is increased by a factor X
The experimental setup for the measurement of gain has been described elsewhere [lo]. Dyes used were obtained from Exciton company (USA). The solutions of dyes C485 (donor) and Rh610 (acceptor) were prepared in analytical grade ethanol at a fixed donor concentration of 5 x 10 ’ M and varying acceptor concentration from 5 x IO ’ M to 1 x lo--’ M. The excitation of the solution contained in a quartz dye cell (internal area of cross-section, 8 mm x 8 mm) was carried out using a nitrogen laser (Molectron UV-24, peak power 900 kW, repetition rate 10 pps, pulse duration 10 ns. beam area 0.6 x 3.2 cm’). The output was detected using a monochromator, photomultiplier tube, and an oscilloscope. In order to make the intensity variation studies, the pump intensity was attenuated using a number of glass filters in between the dye cell and the cylindrical lens.
2. Results and discussion ETDL chmucteristics: The gain spectra of C485 + Rh610 dye mixture at fixed donor concentration of 5 x 10 - ’ M and varying acceptor concentrations in the range from 5 x 10 ~~’ M to 1 x 10 ~ ’ M have been investigated. The donor concentration is chosen in such a way that its gain is reported to be maximum [l 11, and the acceptor concentration is kept lower than or equal to that of donor for energy transfer studies. The use of higher concentrations would increase the energy loss due to collisional encounters followed by self-absorption, and therefore is neglected. 2.1. Concentration
dependerm
qf optical guin
Fig. 1 depicts the typical gain spectra of the bimixture at 2 x 10 ~’ M concentration of the acceptor. All other curves, not shown in the figure, exhibit similar behaviour except for the variation of peak wavelength and peak gain values (Table 1). It has been reported earlier that the single solution of C485 at 5 x lo--’ M concentration exhibits gain from 520 to 560 nm, with peak gain value of 0.7 cm -~ ’ and the corresponding
S. Sanghi
et al. /Spectrochimica
Acla
Part
A 5.3 (1997)
713-
715
7-70
1.‘ .
12 I.0 0.8
YE ” 06 5 ”-: 0.0
.P
02
Fig. 2. Concentration dependence of peak lasing wavelength; (a) C485: (b) C485 (5 x lo-” M)+Rh610 (I x IO- ’ 5 x IO‘-‘M); (c) Rh610.
Fig. 1. Gain spectra for a C485 + Rh610 dye mixture in ethanol at a donor concentration of 5 x lo-’ M and an acceptor concentration of 2 x IO-’ M.
of the gain spectra of bimixture solutions with acceptor concentration as well as of donor and acceptor alone. It is clear that there is a red shift in all cases, due to emission-reabsorption effect [12]. Also, the figure shows that there is a blue shift in the emission from mixtures as compared to acceptor dye alone at all concentrations, which may be understood to be caused by the reduction in aggregate formation in the acceptor molecule because of the formation of new donor-acceptor aggregates [13]. Again, the red shift in the mixture is much larger than that in donor and acceptor alone, because of the emission-reabsorption effect being two-fold effective in case of mixture under study.
peak wavelength occurring at 548 nm [I 11. When an acceptor dye (Rh610) is added, the peak wavelength of gain spectra show a red shift and the shift increases with increasing concentration of acceptor. An increase in peak gain value is also observed with acceptor concentration. The results thus obtained confirm the energy transfer between dyes due to reabsorption effects [4]. Also, the single solution of C-485 shows two peaks in the gain spectra at all concentrations, which was attributed to emission from twisted intramolecular charge transfer (TICT) state formation in the excited states [1 I]. However, this type of dual fluorescence behaviour is not observed in mixture solutions, where coumarin acts as a donor dye, because the energy from the donor dye is used to excite the acceptor dye, and the acceptor dye in the present case, Rh610, being rigid in structure, has no tendency of TICT state formation.
2.1.2. Concentration dependenceof’peuk gctbr
Fig. 3 shows the acceptor concentration dependence of the peak gain in bimixtures along with that of acceptor and donor alone. Although the peak gain value of the donor is smaller at higher concentrations, the use of 5 x 10 -- ’ M concentration of donor dye in the mixture does not affect the peak gain value in the mixture system. This is
2.1.1. Concentration dependenceof peak lasing b~~avelengtll
Fig. 2 depicts the variation Table 1 -____ Donor concentration (M) --__ 5x10-’ 5 x IO ~-3 5 x 10--j 5x lo-’ 5x IO -?
in peak wavelength Acceptor concentration (M) 5 x 10-j 2X 10-’ 1x1o-q 5 x low I x IOF4
- _^_--__
-___ Peak gain (cm ’ ) 2.60 I .4O 0.82 0.77 0.50
Peak wavelength (nm) --~.-----610 608 596 594 584
-~-
716
S. Sanghi
et al. /‘Spec.trochimic.u
understood because of the larger gain value of the acceptor alone at higher concentration and relatively higher concentration had to be chosen for efficient energy transfer from donor to acceptor by varying the acceptor concentration in relatively wider range of concentration. The decreasing value of peak gain in the case of donor dye alone is understood to be caused by effects of concentration quenching which decreases the lifetime [l 1,141; whereas in acceptor dye alone, it is caused by increase in lifetime, which in fact depends upon its molecular structure. However, in bimixtures the increased value of peak gain with increasing acceptor concentration is caused by the interaction between excited donor molecules and ground state acceptor molecules, which enhance the effective lifetime of the acceptor. 2.2. Energy trunsfkr
mte constants
In order to calculate the energy transfer rate constants in the bimixture of C485 + Rh610, firstly, the variations of the ratio of the fluorescence intensity of donor in the presence and absence of acceptor with the latter concentration ranging from 1 x lo-” M to 5 x 10-j M was studied (Fig. 4). The results obey Stern-Volmer equation. I,,/l, = 1 + &.z&!]. where ILo and IL are the fluorescence intensities of donor in the
Fig. 3. Concentration (b) C485 (5 x IOW’ Rh610.
dependence M)+Rh610
of the peak gain; (a) C485: (1 x lo-‘-5 x lOPA M); (c)
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Fig. 4. The Stern--Volmer in ethanol.
plot for C485 + Rh610
dye mixture
absence and presence of acceptor respectively and [A] is the acceptor concentration. The slope of this line, KFrfo = 0.09 x 10” M - ‘, gives the value of I&, the resonance transfer rate due to long range dipole-dipole interaction (also known as Fiirster type transfer rate). which is equal to 0.26 x 10” M ~’ s- ’ where zfc, is the lifetime of the excited donor in absence of acceptor ( - 3.5 ns). Fig. 4 also gives the value of R,, the critical transfer radius, using the relation R,, = 7.35 x ([A],,z)P “’ A [15]. Here [A],,? is the half quenching concentration of the acceptor at which IL = $ILo and is equal to 3.34 x 10 -~’ M. Thus, the calculated value of R, is 49.16 A, which corresponds to an average of one molecule of the acceptor in a sphere of radius 49 A with excited donor as the centre. The value agrees well with that obtained in resonance transfer of the Fiirster type for other donor-acceptor pairs using different techniques [161. To evaluate K,, the radiative transfer rate constant, the ETDL output at peak wavelength (2,) was measured for various pump powers. The variation of the gain per acceptor molecule and pump power for a 2 x lo- 3 M solution of Rh610 with and without donor (i.e. ND = 2 x lo-? M and N, = 0 respectively) is shown in Fig. 5. It is remarkable that at this wavelength, donor absorption as well as emission are negligible. The peak ETDL outputs in terms of gain/acceptor molecule do not show any saturation tendency as can be seen from Fig. 5. Also, the lasing threshold for Rh610 in the mixture is lower than that for Rh610 alone, thereby showing that ETDL lowers the required threshold pump power. Again, the gain/ acceptor molecule at 0.20 MW is negative ( 1.2 x 10 .~” cm’) due to singlet state reabsorption
S. Sanghi
et al. !Spectrochimica
by acceptor molecules. When the donor is present, the gain/acceptor molecule at 0.20 MW increases by 4.5 x 10 ~ I9 cm?. The slopes of the gain versus pump power curves for the mixture and for the acceptor alone are 3.2 x 10 - ” and 2.1 x lo-” cm’ kW - ’ respectively, with the donor-to-acceptor concentration ratio being equal to one. The corrected slope calculated by subtracting the contribution due to the acceptor alone from that of the mixture (2 1.1 x 10 ” cm’ kW -- ‘) is equated with the relation given by Eq. (1). Putting the value of all the constants in this equation, the value of K, comes out to be 0.10 x lOI M ~ ’ s ~ ’ which is quite large as that for KF. Thus, in the present mixture, the radiative energy transfer is the most dominant mechanism in the energy transfer process. 17.3. The gain equution jbr u ternmy dye nzixture solutim From the measurements of energy transfer rate parameters (Table 2) in bimixtures of C440 + C485 [17] and C485 + Rh610, it is predicted that a dye mixture C440 + C485 + Rh610 is also quite effective. Fig. 6 shows the kinetic scheme for such a ternary dye mixture system. To find out the gain of the ETDL, the three dyes are denoted as dye 1, dye 2, dye 3 where the main donor dye is dye 1; dye 2 is intermediator as it acts as donor w.r.t.
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111
2
Donor
dye
Acceptor
dye
Energy stant
transfer
17, (M-’
Coumarin 440 Coumarin 485
Coumarin 485 Rhodamine 610
s-
rate con-
1)
K, 1)
(M-’
s
1.054 x IO”
2.55 n IO”
0.26 x IO”
1.0x
IO”
dye 3 and as acceptor w.r.t. dye 1; and dye 3 is the main acceptor. The energy transfer rates in cascade processes are denoted by K,, and K,,; and K,, is the transfer rate for parallel process; No, and NIi are the state population densities of ground and first singlet state of dye i (i = 1, 2, 3) respectively. The singlet states are pumped with a short pulse from a laser source of intensity I,, at the rate cr,,(p)Z,. Lasing occurs in the acceptor molecules at the rate aLI,, where ZL is the generated dye laser intensity. Absorption losses at the dye laser frequency are represented by the rate of gb,( IV,. The population rate equations are written as dN, -= 1 o;, W(t)N,, dt
- (K,2N02 + K,,N,,)N,,
- ?lL ;I
(2)
dN,, -- N,, r,
(3)
CC.“O,,dl.l, CUO
Fig. 5. Pump power dependence acceptor molecule at 580 nm with
of gain of acceptor No/N, = 1.
Inlcrm~dlotor (dw2)C&BS
Rh 610
per Fig. 6. Kinetic
scheme for the ternary
dye mixture
system.
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N=N,+N2+N3=zN,
-_ N,,
(4)
r3
where W(t) is the rate of excitation, oh, and ri are the absorption cross-section at pump wavelength and decay time of singlet states of dye i respectively in absence of transfer. The effects of triplet states have been omitted because of the simple reason that these states are not effective due to short pump pulse duration [18]. The gain of the dye mixture system at wavelength i. in terms of stimulated and absorption cross-section is given by [14]
I
and xi = crb,W(i)z,
18)
For unsaturated gain, the ground state populations are negligibly perturbed. So No3= N;. i.e. n()j = -N3 = 1 N3 NO2 n 02 = --=F,
=$
N3
(9)
v(2) = o,iN,, - (T;:N,,~ + “,?N,, - o-;Nn2 + a;N,, - g:rJ,,
(5)
F,, F, being the fractional populations of dye 1 and 2, respectively. Now, under steady state approximation
neglecting the absorption and emission of dye 1 and dye 2 in the fluorescence spectrum of dye 3, the net gain coefficient is given by
dn,, dn,, -=---.--=-=
v(A) = o,3N,, - CT:N,,,
Simplifying Eq. (7)
The rate equations less by multiplying
(6)
(2-4) can be made dimensionby r3/N3 to give:
dn,, = x,n,,, - (K,,N,n,,n,,z, __
ds
dx
dn,,
d.u
0
dx
(Fn+ F,VGzN, + K, n’3 = “’ + (K,,N, + K,)(K,,F,N,
+ K,)
a4 K,,N, (10)
+ fLN,nn,r~,,d
where K, = l/r, is the natural decay rate of dye i and x = x, = x,. So Eq. (6) becomes
=
g3
5
+
{(Fn
=CI
V&N,
+
F,KzN, +
+
K,
lF&Ni
KJW,,F,N,
+
(T3 e K,)
xN, 1
The gain per acceptor molecule is written as v(i)
v(2) = N 3
where =
& 1 +
N,, n,i = - , N,
No, n,,j = - 3 N3
Nj= N,,+N,,
z i(Fn + FJKzN3
+ K,lF&N
(K,,N, + K,)(KnF,N,
- (T?‘L
+ K)
g3 cI
’1 (11)
S. Sanghi et al. /ISpectrochiniica Acta Part A 5.1 (19971 713-720
wVELENGlHlnm1
Fig. 7. Absorption and fluorescence spectra of C440, C485 and Rh610 dye molecules.
This equation shows that the gain per acceptor molecule in the ternary mixture is increased by the factor (F,, + F,X,,N, + K, (Kz3N3+ K,)(K,,F,N, + K,)
F’K2’Nycta,3
(12)
which is much greater than that for the binary mixture (Eq. (I)). Also the gain per acceptor molecule is proportional to the pump power and a plot between ~(2) and pump power is a straight line with the slope given by Eq. (12). Thus one can easily calculate F, and F,, the concentration of the dyes to be used in the ternary dye mixture, by equating the experimental and theoretical values.
‘19
Improvement of efficiency of the dye laser is anticipated using cascade energy transfer as indicated by the above equation, where S, and S, denote the ground state and excited singlet state of each dye respectively. The gain spectra of C440 at 5 x 10 ’ M concentration give the peak wavelength at 460 nm; while for the mixtures of C440 + C485 and C485 + Rh610 the peak wavelength is 552 nm and 610 nm respectively for fixed donor and acceptor concentration at 5 x 10 .- ’ M. The gain spectra of C440 dye alone give a red shift of peak wavelength of the order = 14 nm ranging from 446-~ 460 nm; while the mixture of C440 + C485 increases the tuning range by 20 nm (532- 552 nm) and that in C485 + Rh610 dye mixture the tuning is increased by 26 nm (5844610 nm). Thus. the cascade energy transfer in ternary dye mixture would further increase the tuning range which clearly confirms that by adding the second and third dye of proper concentration in the first dye. the tuning can be increased to cover the lasing range from 460-610 nm ( = 150 nm) without disturbing the alignment of the experimental system.
24. Cuscade ETDL operatiorl
3. Conclusions
The fluorescence and absorption spectra of the three dyes are shown in Fig. 7. From the figure it is clear that there is no overlap between the C440 dye fluorescence spectrum and the Rh610 dye absorption spectrum, thereby neglecting the possibility of energy transfer in C440 + Rh610 bimixture. When an intermediate dye (C485) is used, the Rh610 dye can be made to lase. Also, as shown in Table 2, the energy transfer rate constants between C440 + C485 dye mixture are large as compared to those of C485 + Rh610 which shows that the energy transfer between excited singlet levels in the ternary dye mixture is produced with high efficiency. The energy transfer sequence in the ternary dye mixture is understood in the following way [19].
1. The study of TICT states become important when non-rigid dyes of coumarin class are chosen individually or in mixture form. 2. The non-radiative type of transfer rate is dominant in case of sister molecules (coumarincoumarin) because of the closely spaced vibrational levels, while in coumarin-xanthene bimixture system, the radiative type transfer rate becomes dominant. 3. The efficiency of the nitrogen laser pumped dye laser is increased through energy transfer excitation in the ternary dye mixture system.
S,(C440) + $,(C485) +S,(C440) + S,(C485) S,(C485) + S,,(Rh610) + S,(C485) + S,(Rh610)
Acknowledgements
The first author is grateful to the Council of Scientific and Industrial Research for providing financial assistance.
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et al. /Spectrochtnica
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Part
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