Energy & Fuels 1992,6, 545-552
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Temperature Dependence of Fluorescence of Crude Oils and Related Compounds Yifu Zhu and Oliver C. Mullins' Schlumberger-Doll Research, Ridgefield, Connecticut 06877 Received December 23, 1991. Revised Manuscript Received June 1, 1992
The temperature dependence of fluorescence has been measured for eight crude oils in the temperature range 20-200 "C and for related compounds in the temperature range -80 to 200 "C. The measurements show that crude oils fluorescestrongly at elevated temperatures but more weakly than at room temperature. Furthermore, elevated temperatures produce very little effect on the spectral distribution of fluorescence. The decrease in fluorescence intensity is explained using a simple theoretical model which consists of Arrhenius activation of thermal quenching of fluorescence coupled with a Boltzmann distribution of molecular complexes.
I. Introduction Fluorescence spectroscopy is one of the most powerful, Sensitive,and widely used methods in the chemicalanalysis of molecular structure and dynamics. Fluorescence directly probes excited (electronic) states of molecules, providing detailed structural information. Molecular dynamics such as competing decay channels of excited states, complex formation, and solvent interactions can be investigated via fluorescence spectroscopy.' Fluorescence measurements are used ubiquitously for real time analysis of flow streams such as in liquid chromatography, and in certain circumstances, even single molecules can be detected by fluorescencee2 In the oil industry, fluorescence analysis of crude oils, crude oil components, source rocks, and reservoir rocks has been carried out for a variety of purposes. It is of interest to us whether fluorescence can be used to analyze process flow streams of crude oils at elevated temperatures. Molecules in excited states can either radiativelyor nonradiatively decay back to their ground states. It is well known3 that fluorescence can be sensitive to temperature; as temperature increases,radiationlessrelaxation generallyincreases and can lead to total thermal quenching of fluorescence. Here, we first show that relative fluorescence quantum yields of crude oils decrease as the concentration of chromophores increases, suggesting that significant fluorescence quenching is occurring. Next, we experimentally determine the temperature dependence of fluorescence (and fluorescence quenching) for various crude oils and related compounds and compare the measured data with a simple theoretical model. We have found that crude oils fluoresce strongly at elevated temperatures, even as high as 200 "C; the fluorescence intensity of crude oils generally decreases by less than a factor of 2 from room temperature to 200 "C.Our measurements of fluorescence spectra for two crude oils show that crude oils possess broad fluorescence spectra which indicate the existence of many different fluorophoresin crude oils. The measured spectral line profiles are essentially independent of the temperature, indicating that these fluorophores may have (1) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/ Cummings Publishing Co., Inc.: Menlo Park, CA, 1978. (2) Orrit, M.; Bernard, J. Phys. Reu. Lett. 1990,65, 2716. ( 3 )Dorn,H.;Muller,A. Chem.Phys.Lett. 1986,130,426andreferences therein.
similar temperature characteristics. The primary temperature effect on fluorescence is Arrhenius activation of fluorescencequenching. Another effect of increasingtemperature may be to cause dissociation of molecular complexes, which acts to increase fluorescence with increasing temperature.
11. Experimental Section The experimental setup is depicted in Figure 1. The excitation source was an NRC 780 tungsten-halogen lamp. The excitation wavelengths were selected with two short-wave-pass filters (Corion LS500 and LS450). After passing through the filter pair, the light was chopped at a frequency of -80 Hz and then imaged onto the front window of the airtight sample cell. The fluorescence from the crude oil samples was collected by an optical fiber bundle aimed at the front window of the sample cell, then passed through a Corion LL500 long-wave-pass filter, and incident on a photodiode detector. The output from the photodiode was fed into an EG&G 5210 lock-in amplifier and then processed and stored in a Mac I1 computer. The excitation wavelengths after filtration were approximately in the range 390-460 nm. The fluorescence was collected at wavelengths 2500 nm. The utilization of three filters for excitation and emission ensures that the scattered light from the excitation source is minimized. The background level due to the scattered light from the front window of the cell was measured when the cell was empty and found to be -10% of the total signal registered by the photodiode when the cell was filled with crude oils; the background signal was time independent and subtracted out from the fluorescence data. Two cells were employed for these measurements a hightemperature cell and a cryogenic cell. The high-temperature, stainless-steel cell was wrapped by a heating tape; the cell temperature was controlled using the heating tape and monitored by a thermocouple placed inside the fluid sample. In the course of the experiment, the temperature was increased slowly from 19 up to about 200 "C in a period of more than 1 h to ensure uniformity of the sample temperature. Before the temperature was raised, the crude oil sample inside the cell was purged with Nz gas in order to remove oxygen. Oxygen can react with crude oil especially at elevated temperatures, and oxygen can (partially) quench fluorescence. Oxygen quenching is particularly significant for species with long fluorescent lifetimes. All crude oil samples were purged with Nz for more than 0.5 h before the temperature was raised. However,we found that the measurements for the crude oil samples with and without purging were essentially the same, indicating that oxygen quenching is negligible for the crude oils. The apparent lack of oxygen
0SS7-0624/92/2506-0545$03.00/ 0 0 1992 American Chemical Society
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646 Energy & Fuels, Vol. 6,No. 5, 1992 LSSOO
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Figure 1. Experimental setup consisting of a quartz-tungstenhalogen (QTH) lamp, short-wave-pass flters (LS450, LS500), a chopper, a fiber bundle, a sample cell, a long-wave-passfilter (LL500),and a photodiode (PD).
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Cut-off Wavelength (nm) Figure 2. Relative fluorescencequantum yields of crude oils vs their cutoff wavelength (defined to be the wavelength where the absorbance equals 3 for a 2-mm path length). This figure shows that fluorescence yields decrease as the concentration of chromophores increases.
quenching may be due to short natural fluorescent lifetimes of crude oils and/or due to the dominance of other quenching mechanisms. The low-temperature cell consisted of two concentric glass test tubes; the annulus between the tubes was evacuated. The sample was placed in the inner tube, which was fitted with a cold finger to which liquid nitrogen could be added. The fluorescence was generallymonitored during the warming cycle,which typically lasted for 30 min. The sample in the inner tube was purged with nitrogen if the measurement of fluorescence lifetime for the sample indicated that oxygen quenching was significant. The low-temperaturelimit was established by the freezing point (-94 "C) of the solvent (methanol)used for the pure compounds.Crude oils were not used in the low-temperaturecell as wax precipitated from the crudes at temperatures not much below room temperature. Fluorescence spectra of crude oils in the temperature range 20-190 O C were measured using a PTI LS-100fluorometer using the steady-statemode. The crude oils were purged with nitrogen gas before the measurements. The measurements were made in the front surface mode since all of the crude oils used here show a large absorbance at the excitation wavelengths. A high-temperature cell was used for the spectral measurements. 111. Results and Discussion
Crude Oil Fluorescence. Considerable fluorescence quenching occurs in many crude oils. Figure 2 plots the relative fluorescence quantum yield for various crude oils against the cutoff wavelength of a crude oil. We define the cutoff wavelength of a crude oil to be that wavelength
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Wavelength (nm) Figure 3. Absorption spectra for a 2-mm path length of seven crude oils used in the measurements. The curves from left to right correspond to those of Hunt, STI, Escravos, Vixburg, Acoustics, BG-5, and UG-8(the optical density of the Alaska crude is off scale at all wavelengths). The absorption peaks at about 2200,1750,1400,and 1200nm are thevibrationalovertones. which corresponds to an absorbance equal to 3 for a 2-mm path length; this is a measure of the concentration of chromophores in the crude oil. Figure 2 shows that the relative quantum yield decreases as the concentration of chromophores increases. Thus, significant quenching of fluorescence, which is probably associated with the complex formation, occurs in crude oils. It is the temperature dependence of this quenching which we wish to determine. The decrease of fluorescence with increasing optical density of the crude oil is most likely not due to selfabsorption. Crude oils always show increasing (electronic) absorption a t shorter wavelength^^*^ (see Figure 31, so that the excitation absorption length is smaller than that for fluorescence emission. Thus, front surface fluorescence spectra should not suffer significant self-absorptioneffede. High-temperature measurements were performed for eight different crude oils. As shown in the optical absorption spectra from 300 to 2600 nm in Figure 3, they range from light oil (Hunt, with appreciable electronic absorption beginning at 500 nm) to heavy oil (Alaska,with appreciable electronic absorption beginning at >3000 nm) and span a very wide spectral range. The broad absorption tails are exponential and characteristic of all crude oil^^^^ and asphaltenes? the solid component of crude oils. The measurements of the temperature dependence of fluorescence intensity for the eight crude oils are presented in Figure 4a-h; the dashed curves are the experimental data, and the solid curves are the theoretical fits which will be explained below. The measurements show that crude oils fluoresce strongly a t elevated temperature; furthermore, the fluorescence from crude oils does not strongly depend on temperature. In the temperature range between 19and 100 "C, the fluorescence intensity changes very little. For a few crude oils, the fluorescence intensity even increases with a small temperature increase. For temperatures up to 200 OC, the fluorescence intensity is typically reduced by only 15% for heavy oil and up to about 60% for the light oil. We note that the use of a 60-nm excitation bandwidth and a 600-nm long-pass filter for fluorescence emission (vs narrow band source and (4) Mullins, 0. C. Anal. Chem. 1990, 62, 508. (5) Mullins, 0. C.; Kirtley, S.M.; Zhu, Y. Appl. Spectroac., accepted for publication. (6) Mullins, 0. C.; Zhu, Y. Appl. Spectroac. 1992, 46, 345.
Energy & Fuels, Vol. 6, No. 5,1992 547
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Figure 4. Temperature dependence of fluorescence for eight crude oils: (a) Hunt; (b) STI; (c) Escravos; (d) Vixburg; (e) Acoustics; (f)BG5;(9) UG8; (h) Alaska. The dashed line is the experimental data, and the solid line is the least-squares fit to eq 8. The intensity ~
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detection) implies that our measurements are fairly robust and insensitive to the exact wavelength selections. However, these filter selections emphasize the red fluorophores to some extent. Temperature-dependent shifts in the fluorescence spectra could influence our measurements. We therefore collected fluorescence emission spectra as a function of temperature to determine whether large spectral shifts occur with elevated temperatures. Figures 5 and 6 show the emission spectra for two crude oils, respectively, for several excitation wavelengths. These figuresclearly show that elevated temperatures do not induce substantial spectral changes (aside from total intensity). In fact, the spectral distribution of fluorescence is almost unaffected by increasing temperature. The predominant effect of elevated temperatures observed in these figures is to decrease the fluorescence intensity uniformly at all wavelengths. For small temperature increases there is actually a small increase in fluorescence intensity, again uniformly
at all wavelengths. This data is in agreement with the observations usingthe discrete component assemblyshown in Figure 1 (although the bandwidths and optical design of the spectrometer and discrete component assembly differ somewhat). In order to further test the wavelength independence of our observations, we replaced the long-wave-passfilter LL500 with the LL600 and collected the fluorescence light at wavelengths 1600 nm; the measured temperature dependence of fluorescence from crude oils is essentially the same as that collected at wavelengths 1500 nm, indicating that temperature-induced wavelength shifta probably are not significant for these experiments. The measured fluorescence spectra for two crude oils from 20 to 190 "C confirmed the above conclusion. Crude oils contain a complex mixture of chromophores. Many crude oilsexhibit electronic absorption continuously from the near-IR through the visible to the near-UV, yet the electronic absorption spectra of crude oils show no
Zhu and Mullina
Energy & Fuels, Vol. 6,No. 5, 1992 Acoustics
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Figure 5. Fluorescence spectra of the crude oil, Acoustics, measured for a series of temperatures from 25 to -190 "C: (a) excitationwavelength 300 nm; (b) excitationwavelength 400 nm; (c) excitation wavelength 500 nm. Note that for long-wave excitations (b and c), the fluorescence increases as the temperature is raised above 25 "C (e100 "C)and then decreases when T > 100 "C. structural peaks a t all in this spectral range. Even the intense Soret band at -400nm of the porphyrin complexes present in crude oils is not discernible as a peak. This continuous and monotonically increasing electronic absorption4s5is in contrast to the sharp and highly structured spectra obtained from individual crude oil components. It is easily shown that crude oils contain mixtures of fluorophores; it is well known that, for most pure chemical compounds, the fluorescence spectrum in the condensed phase is independent of excitation wavelength (Kasha's rule). Independent of the electronic excitation energy, molecules in the condensed phase rapidly relax (- 10-l2 s) by collision to the first excited (singlet) electronic state. The large energy gap between the two lowest (singlet) electronicstates (in accordance with the Rydberg equation) impedes further collisional relaxation; thus, fluorescence can take place between these two lowest electronic states. In contrast, for all crude oils the fluorescence spectra are different for each different excitation wavelength, as demonstrated for a crude oil in Figure 7. From these spectra, we can conclude that crude oils consist of complicated mixtures of fluorophores. The different fluorophores span a wide range of spectral characteristics and lead to a broad spectral distribution in the observed fluorescence spectra of crude oils shown in Figures 5-7. Thermal Quenchingof Fluorescence. As indicated in Figure 2, where fluorescence intensity decreases with
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Figure 6. Fluorescence spectra of the crude oil, Escravos, measured for a series of temperatures from 25 to -190 "C: (a) excitationwavelength 300 nm; (b)excitationwavelength 400nm; (c) excitation wavelength 500 nm. For all excitation wavelengths, the fluorescence increases as the temperature is raised above 25 "C (e100 "C) and then decreases when T > 100 "C.
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Figure 7. Fluorescence spectra of the crude oil Acoustics. The spectralprofilesare different for different excitation wavelengths, indicating that the crude oil contains complex mixtures of fluorophores. increasingchromophore concentration, significantquenching of fluorescence occurs in crude oils. Fluorescence quenching is probably associated with complex formation in the crude oils. A variety of pathways exist for quenching of fluorescence, including intra- and intermolecular processes. Furthermore, intermolecular energy transfer can produce fluorescence but at longer wavelengths. The thermal characteristics of crude oil fluorescence are governed by the thermal characteristics of the different
Temperature Dependence of Crude Oil Fluorescence
fluorophores and of the different quenching and energytransfer processes. If the fluorophoreshave quite different temperature characteristics, then the fluorescence spectrum at elevated temperatures should shiftin ita line profile compared with that at room temperature. However, the measured spectral line profile of crude oils is essentially temperature independent, as shown in Figures 5 and 6 for two crude oils. From the lack of spectral shifta at elevated temperatures in the measured fluorescence spectra in Figures 6 and 7, it is reasonable to assume that various fluorophores possess approximately the same thermal quenching characteristics. For samples of such complexity as crude oils, our measurements do not provide sufficient information to identify all of the major absorbers and fluorescent species, let alone their individual fluorescence characteristics. A detailed and complete analysis is very difficult to attempt. However, our measurements do provide useful information regarding the dominant thermal behavior of the fluorescence of crude oils. To the first-order approximation, we consider all of the fluorophores as having very similar temperature characteristics. We treat the thermally activated fluorescence quenching of crude oils to be represented by a single activation energy. This activation energy includes contributions from the thermal decay of individual chromophores, but may also include contributions from the temperature dependence of energy transfer. This treatment is equivalent to a onecomponent analysis. Figures 6 and 7 lend credence to such an approach due to the observed uniform spectral variation with temperature. Of course, if one process dominates the temperature dependence of quenching, then our treatment using a single activation energy is quite appropriate. For a one-componentanalysis,we define the fluorescence intensity I as the number of excited molecules multiplied by the total radiative decay rate kf normalized by the total relaxation rate kf + kt:
Here kt is the total nonradiative decay rate. Frequently, the total radiationless relaxation rate increases with temperature. To a good approximation, the radiationless relaxation rate can be written as kt = ko + kl exp(-PE/ k T ) , where k is Boltzmann's constant, T is the absolute temperature, and PE represents a weighted auerage activation energy for thermal quenching. The temperature-dependent term corresponds to the familiar Arrhenius term which describes a process that is subject to kinetic control (as opposed to equilibrium control). After some rearrangement, the fluorescence intensity can be written 88
Here, N is the number of excited molecules in the observation region and k' = kl/(kf + ko). The resulting curves which obey eq 2 are sigmoid or "S" c ~ r v e s . ~ To verify the above one-component, theoretical model, we have measured the temperature dependence of fluorescence for five pure chemical compounds in dilute solutions in the temperature range -80 to 200 "C. After (7) Organic Molecular Photophysics; Burke, J. B., Ed.; Wiley: New York, 1973.
Energy & Fuels, Vola6, No. 5, 1992 549
normalization, the high- and low-temperature seta of data are spliced with matching slopes. The experimental data are shown in Figure 8a for the laser dye rhodamine B, Figure 8b for rhodamine 6G, Figure 8c for DCM, Figure 8d for tetracene, and Figure 8e for perylene. The dashed curves are the experimental measurements, and solid curves are the computer fittingsto eq 2. These five organic compounds show quite different temperature behaviors. For rhodamine B, DCM, and tetracene, the fluorescence intensity drops very quickly as the temperature is increased. A smooth S-shaped curve is obtained. For rhodamine 6G and perylene, the transition region of the S curve occurs at high temperatures, T 1 120 "C. The efficacy of thermally induced fluorescence quenching is related partially to the flexibilityof the fluorophoric group. DCM is a styryl dye; the prototypical styryl dye, stilbene, possesses a low fluorescence quantum yield, perhaps due to internal rotation about the vinyl-phenyl bond. In addition to this location of flexibility, the fluorophore of DCM also contains an amine function, which also can undergo internal rotation. These accessible internal modes can quench fluorescence. The substantial difference between rhodamine 6G and rhodamine B may be related to the relative ability of the amine function to undergo internal rotation. Rotation of the (secondary) phenyl amine of rhodamine 6G is sterically hindered by an a-methyl group, while the (tertiary) phenyl amine of rhodamine B possesses a more facile internal rotation because only H atoms are in the a-positions. Tetracene can readily undergo intersystem crossing as can many polycyclic aromatic hydrocarbons. Generally, the data for these pure compounds are accurately represented by eq 2. The parameters from the fitting are listed in Table I. The averagethermal activation energies are typical for thermally induced chemical processes. Particularly, the energies correspond to activation of several quanta of low-frequency vibrational modes or activation of one or two quanta of aromatic ring stretching modes. In fact, even for pure compounds, several relaxation pathways may exist, each with its own activation energy. For these compounds, the measured room-temperature quantum yields are consistent with our experimental curves (assumingb,the temperatureindependent quenching constant, equalszero). For instance, the quantum yield of rhodamine B is 0.64.8 The fluorescence intensity curve for rhodamine B, Figure 8a, shows that the room-temperature fluorescence for this dye is 0.53 of the maximum value which occurs at low temperatures. (Variations of 20% in reported quantum yields are typical.) Thus, at low temperatures the dye has a quantum yield of approximately 1. Likewise, the room-temperature fluorescence quantum yield of rhodamine 6G is 0.95: similar to the ratio of fluorescence intensity at room temperature vs low temperatures in Figure 8b. The room-temperature quantum yield of tetracene is 0.20 and of perylene is 1,' consistent with Figure 8d,e, respectively. Using the values of k' listed in Table I and given that the fluorescence lifetimes are -10-8 s (which we have confirmed), the preexponential factors or "Arrhenius frequency factors" in thermal quenching are approximately 10l2s-l. These are typical values for chemical processes induced via vibrational excitation. (8)
Kubin, R. F.; Fletcher, A. N. J . Lumin. 1982, 27, 455.
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DCM tetracence perylene
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Complex Formation. In addition to thermal quenching, fluorescence quenching resulting from molecular complexes may be important for concentrated solutions of fluorophores such as crude oils. Complex formation is consistent with the data shown in Figure 2. These complexes are generally weakly bound and can exist either in the ground electronic state and/or in the excited electronic state. Radiationless transitions dominate excited-state complexes by processes which may be related to relaxation of "floppy" molecules. Here, we employ a simple model to describe the observed concentration and temperature dependence of solutions of rhodamine B. In this model we retain the thermal quenching previously described. Recall that this decay process is considered to be rate limited or kinetically controlled. With regard to complex formation, we assume that rhodamine B forms monomers and dimers only and that only monomers can fluoresce. The monomer-dimer equilibrium (and higher aggregates) of rhodamine B has been shown to be important for the absorption and emission characteristics of rhodamine solutions."12 The decreasing fluorescence quantum yields with increasing concentration of rhodamine B solutions has been explained in terms of the rhodamine
dimer being nonfluorescent.gJ1J2 Complex formation is considered to be under equilibrium control; only that fraction of rhodamine B which exists as monomer can fluoresce. The simple equilibrium
x, + 2x is governed by the equilibrium equation
(3)
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where X is rhodamine B, K is the equilibrium constant, and the brackets indicate concentration (moles/liter).The concentration of monomer depends on the total concentration of added dye, XT (= [XI + 2[X21). By trivial substitution, one finds
The temperature dependence of the equilibrium constant is given by the standard equation K = KOexp(-AHlkT) (6) where AH is the enthalpy required to break a complex. (9) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762. (10) Levshin, L. V.; Slavnova, T. D.; Jushakov, V. I.; Zorov, N. B.; Pastchenko, V. 2. Russ. J . Phys. Chem. 1974,48,46. (11) Bojarski, C.; Zurkowska, G.;Tyrzyk, J. 2.Naturforsch. 1982,374, 74. (12) Lopez Arbeloa, F.; Ruiz Ojeda, P.; Lopez Arbeloa, I. Chem. Phys. Lett. 1988, 148, 253.
Energy & Fuels, Vol. 6, No. 5, 1992 551
Temperature Dependence of Crude Oil Fluorescence 140
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