By directly measuring the donor fluorescence lifetime as a function of acceptor concentration in the laser dye mixture Rhodamine 6G-Cresyl violet, we found that the Stern-Volmer relation is obeyed, from which the rate of excitation transfer is determined. The experimental results indicate that the dominant mechanism responsible for the efficient excitation transfer is that of resonance transfer due to long range dipole-dipole interaction

Dienes, A.

Lin, C.

English

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STUDY OF EXCITATION TRANSFER IN LASER DYE MIXTURES BY
10 DIRECT MEASUREMENT OF FLUORESCENCE LIFETIME
2;i Chinlon Lin and A. Dieces
Department of Electrical Engineering and Computer Sciences
and Electronics Research Laboratory
o' University of California, Berkeley, California 94720
H
< o ABSTRACT
UOH -r By directly measuring the donor fluorescence lifetime as a function
4 4
H U of acceptor concentration in the laser dye mixture Rhodamine 6G-Cresyl
0 P4 
pmu violet, we found that the Stern-Volmer relation is obeyed, from which the
rate of excitation transfer is determined. The experimental results indi-
A * cate that the dominant mechanism responsible for the efficient excitation
,- transfer is that of resonance transfer due to long range dipole-dipole
- A "interaction.
•Visiting MacKay Lecturer, on leave from Bell Laboratories, Holmdel,
*Visiting MacKay Lecturer, on leave from Bell Laboratories, Holmdel,
New Jersey, 07733. Address after September 1973: Department of Electrical
Engineering, University of California, Davis, California.
https://ntrs.nasa.gov/search.jsp?R=19740014051 2020-03-23T09:16:56+00:00Z
1. Introduction
The concept of radiative transfer has been applied [1,2] to improve
the pumping efficiency of Neodymium lasers. By circulating fluorescent
dye solutions between the flashlamp and the laser rod the light conversion
is enhanced since the dyes absorb the visible and UV radiation strongly
and then transfer the excitation to the Nd+ 3 ions by emission and reabsorp-
tion. Similar improvement in the efficiency of flashlamp-pumped dye lasers
has also been reported [3]. Excitation transfer in laser dye mixtures has
also been utilized to achieve better dye laser performance at the desired
wavelengths. For example, Rhodamine 6G-Cresyl violet mixtures are reported
[4,5] to have enhanced the laser efficiency of Cresyl violet, and RhodamineB-
DODC mixture has enabled the generation of tunable near infrared radiation
with nitrogen laser pumping[6]. This type of excitation transfer is called
"sensitized fluorescence" and is well known in photochemistry [7]. Study
of laser dye mixtures using gain spectroscopy [8] indicated that the exci-
tation transfer in .the -case of Rhodamine 6G-Cresyl violet mixture is very
efficient, although the mechanism responsible has not been determined. In
this paper we report experimental work for determining the dominant excita-
tion transfer mechanism in such laser dye mixtures.
2. Excitation Transfer Mechanisms
The main mechanisms for the intermolecular, singlet-singlet electronic
energy transfer in dye mixtures, or donor-acceptor pairs, are [7,9,10]:
(a) radiative transfer--emission of donors absorbed by acceptors (so-
called "trivial mechanism");
-1-
(b) diffusion controlled collisional transfer;
(c) resonance transfer due to long-range dipple-dipole interaction.
Mechanisms (b) and (c) are also called nonradiative transfer because
the excitation transfer takes place before the excited donor emits, in
contrast to mechanism (a). Mechanism (c) occurs with donor-acceptor
separation much greater than collisional diameters. The quantum mechan-
ical treatment of resonance transfer is due to Fbrster [17]. The origin
of such resonance transfer is the long-range dipole-dipole Couloumb in-
teraction. The probability of energy transfer due to such interaction is
large if the emission spectrum of the donor overlaps strongly with the
absorption spectrum of the acceptor-i.e., precisely the conditions required
of mechanism (a), the purely radiative emission-reabsorption.
Although all these mechanisms contribute to the donor fluorescence
quenching and enhance the acceptor flourescence yield, they can be ex-
perimentally distinguished [11] by noting that (i) the radiative transfer
does not affect the donor fluorescence lifetime; (ii) the diffusion-
controlled collisional transfer rate should decrease with the rigidity of
the environment, or more specifically should be inversely proportional to
the solvent viscosity; (iii) theory and experiments have indicated clearly
that the long-range resonance transfer rate is at least ten times faster
than that of collisional transfer. Thus, by simply measuring the donor
fluorescence lifetime as a function of acceptor concentration distinction
can be made between the various mechanisms. For example, if the donor
lifetime is little affected by the acceptor molecules, then mechanism (a)
can be considered the dominant one.
-2-
3. Experimental
The fluorescence lifetimes of organic laser dyes are typically in
the 1-10 nsec range, which can be directly measured [12,13,14] by using
picosecond or subnanosecond optical pulse excitation and observing the
fluorescence decay with a fast electronic detection system. The conven-
tional phase-shift method and other systems for measuring nanosecond
radiative lifetimes are less accurate and not as simple to use.
In this experiment the output of a ruby laser, mode-locked with DDI
in methanol, is frequency doubled in a KDP crystal. The resultant UV
pulses are 8 psec in duration and are separated by the 9.4 nsec cavity round
trip time. These pulses are passed through a tilted dye sample cell close
to the cell wall and after proper filtering the flourescence is detected
from a near perpendicular direction. The beam size of the excitation is
about 2.5 mm. A fast vacuum photodiode and a Tektronix 7904 oscilloscope
with theamplifier plug-in unit are used. With this detection system the
combined time resolution is approximately 0.9 nsec (fall time). This is
somewhat longer than the resolution obtainable if a direct access oscillo-
scope is used [12,13,15]. However, the sensitivity (20 mv/cm) is 20 times
better so that much weaker fluorescence signals can be detected. The possi-
bility of decay time shortening with high excitation intensity [13] is thus
avoided. The lifetime data are obtained by fitting the decay curve to a
straight line on semi-log plots. Within the experimental error the decay
curves were found to be simple exponentials. Lifetimes were measured in
pure Rhodamine 6G solutions and in solutions of Rhodamine 6G-Cresyl violet
mixtures for a fixed Rhodamine 6G concentration (5x10- 3 M) and different
Cresyl violet concentrations. Both methanol and ethylene glycol were used
-3-
as solvents. The results are shown in Table 1 and Figure 1. Typical
error is ~-10% except for lifetimes shorter than 1 nsec where accurate
measurements become difficult.
4. Discussion
Table 1 shows that concentration quenching [21] in pure Rhodamine
6G solutions is quite important. Similar observation has been previously
reported [15]. The reduction in lifetime for very low concentrations as
observed here is also noted in [15]. It is possible that the effect-of
radiation trapping (self-reabsorption-reemission), which tends to lengthen
the observed lifetime [16], is present and superimposed on the effect of
concentration quenching which reduces the lifetime.
Figure 1 shows a plot of (Tf)- 1 versus Cresyl violet concentration,
where Tf is the observed decay time of the Rhodamine 6G fluorescence. The
-3
Rhodamine 6G concentration was fixed at 5x10- 3 M. The plots are straight
lines, apparently obeying the Stern-Volmer relation [7]
f--- 1 + K A, (1)
Tf et
where Tfo is the measured lifetime in the absence of acceptor molecules,
Ket is the excitation transfer rate and A is the acceptor concentration.
Equation (1) can be obtained from the rate equation
* *
dD_ P(t) D K AD , (2)
dt fo et
assuming 6-function excitation for P(t). D is the excited donor concen-
tration. Although F6rster's theory indicates a deviation from simple ex-
-4-
ponential decay, it has been observed [181 that both Forster's formulation
and Vavilov's phenomenological treatment [19], which 
uses a simple ex-
ponential decay with an effective time constant 
Tf, fit experimental
results with slightly different decay time constants.
From Figure 1, we obtain Al, the acceptor concentration 
at which
2
fo K is calculated from
f 2 et
K1 (3)
Ket Al =
et 2 fo
R , the critical separation of donor and acceptor 
molecules for which
energy transfer from D to A-and emission from D 
are equally probable,
can be calculated from [7].
3
R = 000 7.35in A (4)
o 4N AI A
where N is Avogardro's number. This corresponds to 
an average of one
molecule of A in a sphere of radius R with D as the 
center. The values
obtained are shown in Table 2.
Note that the values of Ro obtained agree well with 
those obtained
in resonance transfer studies of other donor-acceptor pairs using dif-
ferent techniques [10,20] and are considerably greater than 
the value for
the collisional excitation transfer, which is 4-6 A [7]. In 
terms of
Ket , the excitation transfer 
rate obtained here is typical of resonance
transfer [9,10,20]. This is in contrast to the values for 
the case of
collisional transfer (10 - 1010 liter mole
-1 sec- ) as observed experi-
-5-
mentally or calculated from the Debye equation [7,9]. It is also seen
that the excitation transfer rate does not decrease with a more than twenty
times increase in solvent viscosity. On the contrary, Ket is found to be
greater in ethylene glycol than in methonol, which might be due 
to the so-
called "cage effect" [21].
We conclude that although radiative and collisional processes probably
contribute to the energy transfer in the Rhodamine 6G-Cresyi violet laser
dye mixture, resonance transfer due to long-range dipole-dipole interaction
is the dominating mechanism responsible for the efficient excitation transfer.
The technique described here is simple and can be applied to studies of
other excitation transfer processes.
The authors would like to acknowledge useful conversations with
Professor Martin McClain.
-6-
References
1. W.W. Morey, IEEE J. Quantum Electronics QE-8 (1972) 818.
2. D.D. Bhawalkar and L. Pandit, IEEE J. Quantum Electronics QE-9 (1973) 43.
3. J.M. Drake, E.M. Tam, and R.I. Morse, IEEE J. Quantum Electronics QE-8
(1972) 92.
4. C.E. Moeller, C.M. Verber, and A.H. Adelman, Appl. Phys. Letters 18
(1971) 278.
5. W. Schmidt, W. Appt, and N. Wittekindt, Z. Naturforschung 27a (1972) 37.
6. F.B. Dunning and E.D. Stokes, Opt. Commun. 6 (1972) 160.
7. N.J. Turro, "Molecular Photochemistry," W.A. Benjamin, Inc., New York,
1967.
8. A. Dienes and M. Madden, to be published in J. Appl. Phys., September 1973.
9. F. Wilkinson, in "Luminescence in Chemistry," E.J. Bowen, Ed., Van
Nostrand, Princeton, 1967.
10. R.G. Bennet and R.E. Kellog, Photchem. and Photobio. 7 (1968) 571.
11. F. Wilkinson, in "Advances in Photochemistry," Vol. III, W.A. Noyes, Jr.,
G.S. Hammond, and J.N. Pitts, Jr., Ed., Interscience, New York, 1964.
12. M.E. Mack, J. Appl. Phys. 39 (1968) 2483.
13. H.E. Lessing, E. Lippert, and W. Rapp,.Chem. Phys. Letters 7 (1970) 247.
14. H. Merkelo, S.R. Hartman, T. Mar, and G.S. Singhal Govindjee, Science
164 (1969) 301.
15. R.R. Alfano, S.L. Shapiro, and W. Yu, Opt. Commun. 7(1973) 191.
16. J.B. Birks, "Photophysics of Aromatic. Molecules," Wiley-Interscience,
New York, 1970.
17.- Th. Fdrster, Ann. Physik. 2 (1948) 55; Z. Naturforschung 4a (1949) 32.
-7-
18. K.B. Eisenthal, Chem. Phys. Letters 6 (1970) 155.
19. S.I. Vavilov, Dokl. Akad. Nauk USSR 45 (1944) No. 1.
20. W. Ware, J. Am. Chem. Soc. 83 (1961) 4374.
21. P.R. Pringsheim, "Fluorescence and Phosphorescence," Interscience, 
New
York, 1949.
-8-
Table 1
Fluorescence lifetimes of pure Rhodamine 6G solutions
Concentration (M) Tf in methanol (nsec) Tf in ethylene glycol (nsec)
10 - 2  2.4 2.5
5 x 10- 3  2.7 2.9
10 - 3  3.5 4.1
5 x 10- 4  6.1 6.4
10 - 5  9.3 9.7
10- 6  6.2 6.4
-9-
Table 2
Al ("half-quenching concentration"), Ket (excitation transfer rate),
and R ("critical separation") calculated from Fig. 1.
0
Solvent A 2 (M) Ket(liter mole sec-) Ro(A)
Methanol 2.6 x 10- 3  1.4 x 1011 53
Ethylene glycol 1.5 x 10
- 3  2.2 x 1011 64
-10-
Figure Caption
Fig. 1. (Tf) - 1 versus Cresyl violet concentration 
for fixed Rhodamine
6G concentration 5x1 0
- 3 11 in two solvents: o for methanol,
A for ethylene glycol. Dotted section corresponds 
to f < 1
nsec where accurate measurement is not obtained.
-11-
I (nsec)f /
1.2-
o
0.8
0.4 -
0.0
0 10-3  2 x 3  3 x 10-3  4 x 103  5 x 10 3
Cresyl violet concentration


Study of excitation transfer in laser dye mixtures by direct measurement of fluorescence lifetime

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