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EXCITATION PROFILES OF RESONANCE RAMAN
SCATTERING AND FLUORESCENCE IN DYE
MOLECULES
S. Kinoshita, J. Watanabe, T. Kushida
To cite this version:
Colloque C7, supplkment au nOIO, Tome 46, octobre 1985 page C7-419
E X C I T A T I O N PROFILES OF RESONANCE RAMAN SCATTERING AND FLUORESCENCE I N DYE MOLECULES
S. Kinoshita, J. Watanabe and T. Kushida
Department o f Physics, Osaka University, Toyonaka, Osaka 560, Japan
Abstract
-
The excitation spectra for the Raman and luminescence components of 6-carotene have been measured in solutions over a wide spectral range using ion lasers and tunable dye lasers for excitation. The intensity ratio of the fluorescence to Raman scattering has been found to decrease rapidly when the exciting wavelength is tuned away from resonance. This result clearly shows the importance of the non-motional narrowing effect. The experimental data are compared with existing theories that take into account the non-motional narrowing effect.I
-
INTRODUCTIONIn considering an optical response of matter, we usually separate the object into the system which interacts with the radiation field and the reservoir which affects the system but does not couple with the light directly. The interaction between the system and the reservoir gives rise to various relaxation phenomena and also to the broadening of optical spectra. In the resonance Raman scattering and luminescence, the interaction between the intermediate state of the system and the reservoir plays an essential role, because this interaction is related to the quantum coherence be- tweenthe light absorption and emission in these second-order optical processes.
When the system-reservoir interaction is suffici.ently weak, or the correlation time of the reservoir TC is sufficiently short, the optical response can be treated well under the motional-narrowing approximation, or equivalently, by the theories based on the optical Bloch equations that take into account two relaxation parameters, TI and T2. However, when the interaction is not very weak, or
-rC
is not very short, this type of theoretical treatment is no longer applicable because of the memory effect of the reservoir. This non-motional narrowing effect is clearly observed in the difference of the excitation profiles between the Raman scattering and lumines- cence, since the motional narrowing theory predicts that the intensity ratio between these two secondary emission components is independent of the exciting photon ener- gy. Usually, the Raman lines are predominant over luminescence under off-resonant excitations, while at the resonance, luminescence is overwhelming. To our know- ledge, however, no quantitative comparison between the two excitation profiles has been performed yet.In the present paper, we report measurements of the excitation spectra from reso- nance to far off-resonance for the Raman scattering and fluorescence in @-carotene in solutions. Organic dye molecules doped in solutions are very suitable for this study, because they interact with surroundings strongly and accordingly the non- motionalnarrowing effect plays an important role. Their absorption and fluorescence
C7-420 JOURNAL
DE
PHYSIQUEare broad, while the Raman lines due to intramolecular vibrations are narrow and clearly discernible from luminescence. Further, the inhomogeneous broadening of the spectra bands can be dealt with statistically because thermal equilibrium is consi- dered 'co be well established in solutions. 6-carotene is especially convenient for the experiment, because the Raman signals are intense, while the fluorescence is rather weak even for the resonant excitation.
I1
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EXPERIMENTSThe commercially available crystal of 6-carotene was recrystallized and dissolved in two kinds of solvents, tetrachloromethane (CC14) and isopentane. We adjusted their concentration to about 2 x M for the measurement in the resonance region,
M in the near-resonance and 10-4Q-3 M in the off-resonance. To check the pres- ence of various isomers, we used the liquid chromatography and found the 96 % of the dye is in all-trans form with trace of 13- and 15-cis isomers. Further, to confirm the absence of photo-induced isomerization, we compared the absorption spectra and also made liquid chromatographic investigations before and after the laser illumi- nation. No trace of isomerization was found to be present though slight decrease in optical density was sometimes observed after the long exposure to the laser light. Exciting lasers employed were an Ar laser, dye lasers (coumarin 7 and rhodamine 6G) and a Kr laser. In using the dye lasers, the output beam was transmitted through a single monochromator to reduce the background fluorescence emitted from the laser dyes. The contribution from such background is estimated to be only loa5 at 100 cm-I from the laser frequency.
In determining the excitation wavelength dependence of the Raman and luminescence intensities, we measured the intensity ratio of the Raman scattering and the lumi- nescence to the 465-cm-I Raman line of CC14 or 1446-cmdl line of isopentane. Since the intensities of these non-resonant Raman lines of solvent molecules depend only
Fig. 1
-
Emission spectra of 6-carotene in CC14 for various excitation wavenum- bers.x
indicates a Raman line from ~ ~ 1 4 .o Raman scattering
A Luminescence
the wavelength, such as the reabsorption effect, local field correction, vivs3-de- pendence, the transmission efficiency of the monochromator and the detection effi- ciency of the photomultiplier, are automatically compensated.
I11 - RESULTS
In Fig. 1, we show the emission spectra of 8-carotene in CC14 for various excitation wavenumbers at room temperature. The three sharp lines, located at 1005, 1155, 1525 cm-I from the laser line, are assigned to the V3, V2 and V 1 Raman modes of 8-caro- tene, whereas the other sharp lines are due to the non-resonant Raman scattering from CCl4. On the other hand, the broad bands are ascribed to luminescence from @-carotene.
Several features of the luminescence of 8-carotene are noticeable from this figure: 1) The luminescence spectra are almost Gaussian in shape and have vibrational struc- tures only when the lower energy side of the absorption band is excited. 2) When the laser wavelength is varied, the luminescence bands shift in position, but the spectral shape remains almost unchanged. 3) The half-width of the luminescence band is always a little smaller than that of the absorption band of 730 cm-l.
Most of organic dyes, such as rhodamine, also have Gaussian-shaped broad spectra, but they do not show the shift of luminescence band with excitation wavelength. Furthermore, 8-carotene in solutions shows a laser-induced fluorescence line-narrow- ing effect even at room temperature, i.e., the spectral width of luminescence under laser excitation is narrower than that under broad-band excitation. The peculiar behavior of luminescence in 8-carotene is considered to arise mainly from the very
short luminescence decay time (
2
1 ps) in this material. Namely, the dye molecules with the equal transition energy are selectively excited, and the observed fluores- cence is emitted before the change in the transition energy becomes appreciable. Therefore, we must consider two types of system-reservoir interactions with differ- ent time scales. The perturbation slower than the luminescence decay time, which gives the inhomogeneous broadening of the absorption band, probably comes from the change in the configuration of the environments of the dye molecules. On the other hand, the one faster than the luminescence decay time, which gives the homogeneous broadening, may be due to the fluctuation of solvent molecules around their equi- librium positions. From this consideration, the inhomogeneous broadening parameter is obtained using the difference between the absorption and luminescence band- widths.In Fig. 1, longer the excitation wavelength, larger the intensity ratio of the three Raman lines of @-carotene to the luminescence. This is more clearly seen in Fig. 2, where the relative intensities of the v2 Raman and luminescence peaks are plotted against the detuning from the resonance. In going from resonance to off- resonance, the luminescence intensity decreases rapidly and its dependence is approximated by a Gaussian function. On the other hand, the Raman intensity de- creases rather slowly and its dependence appears to have a Lorentzlan tail. In Fig. 3, we show the intensity ratio of the luminescence to the Raman scattering for 8-carotene in CC14 and isopentane at room temperature and at 125 K. The exper- imental data for the two different solvents at room temperature are almost the same with each other, but those at 125 K are different considerably from the dependence at room temperature.
C7-422 JOURNAL DE PHYSIQUE
IV
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DISCUSSIONThe excitation profile for the resonance Raman scattering in 6-carotene has been extensively investigated /I/, and Albrecht A-term is known to be predominant for the V1 V3 Raman modes. Due to the Franck-Condon factors, a molecular vibrational transition (0,0,1), i.e., V=O + V'=0 -t VU=l, is also known to dominate over the
other vibrational transitions. Furthermore, the oscillator strength for the first optically-allowed transition 1~~ + is so large that the contribution from the other electronic transition is considered to be negligibly small. Therefore, the first-order Raman scattering and luminescence in question, are well approximated by the interaction between the opticalfieldand a three-level system, in which only the intermediate state is perturbed by the reservoir.
Several theories which treat the second-order optical processes in a three-level system have been reported. In the motional narrowing case (I), where the memory effect of the reservoir is completely ignored, it is known that the excitation pro- files for the Raman scattering and luminescence coincide with each other, and show a Lorentzian dependence on the excitation energy. The intensity ratio of lumines- cence to Raman scattering in this case is given by 2yAg/ym, where yAg is the de- phasing rate between the ground and the intermediate exclted states and ym is the population relaxation rate in the excited state /2/.
Ron and Ron (11) /3/ took into account the memory effect as the first-order pertur- bation and obtained the result that the dephasing rate becomes energy-dependent as
where @ is the detuning in the units of angular frequency. Clearly, their theory predicts a Lorentzian excitation-energy dependence for the luminescence-to-Raman intensity ratio.
Fig. 3
-
Luminescence-to-Raman intensity ratio for 6-carotene. Solid lines arecalculated curves by using the model (IV) Fig. 4
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Calculated curves for lumines- with the parameters given in the figure. cence-to-Raman intensity ratio by usina two-state jump model (III), in which it is assumed that the energy of intermediate state is adiabatically modulated and the energy modulation takes only two values
+6
randomly with the rate of y. It has been found that, in addition to broad lumines- cence, two Raman components, i-e., a sharp one and a broad one, appear inthe second- ary emission spectrum. In the case of our experiment, the broad m a n component is considered to be overlapped on the luminescence band and it is difficult to discrimi- nate it from luminescence. Therefore, we separate the total secondary emission into the broad and sharp components. Then, the excitation-energy dependence of the in- tensity ratio between the two components obtained by Takagahara et al. agrees com- pletely with that of Ron and Ron, if we put ym/2+
y = 7.
'
-A more realistic model employed by Takagahara et al. andCalso by Mukamel is to assume the random energy modulation of the intermediate state as a Gaussian- Markovian process characterized by the correlation function:
where x(t) is the energy modulation of the intermediate state as a function of time, and 6 is its amplitude. Takagahara et al. obtained a formula to calculate the sec- ondary emission spectrum numerically. The spectra calculated with this formula again have a broad Raman band besides a sharp Raman line and a broad luminescence band. Mukamel (IV), on the other hand, factorized the second-order optical process into two first-order processes and derived a method to calculate the secondary emis- sion spectra rather easily. The spectrum obtained by this method shows only a sharp Raman line and a broad luminescence band.
Now, in order to compare these theories with each other, we calculated the excita- tion-energy dependence of the intensity ratio between the broad band and the sharp line using the theoretical expressions (I)
-
(111) and the numerical calculation(IV) with the same values of 6 and .rc. Though all the theories gave very similar results for the case of 6~,<<1, the difference was clear when 67 was larger than
F:
unity. An example is shown in Fig. 4. It is understood from thls result that, in order to determine the values of the stochastic parameters, it is necessary first to choose the proper model. Since it is considered that the two-state jump model over- simplifies the real situation, and also that the theory of Ron and Ron is applicable only to the case not far from the motional narrowing limit, we attempted to fit our experimental data with the Mukamel's theory. The inhomogeneous broadening was assumed to be Gaussian with half-width of 6i. The results are shown in Figs. 2 and 3 by solid lines. The agreement of these curves with the experimental dependence is fairly good. The values of
6
and Tc obtained by this fitting indicate that, as expected, the system-reservoir interaction is rather strong (6-rC>1) in 8-carotene in solutions. It has also been found that 6 decreases with decreasing temperature, while-rc
is rather insensitive to the temperature change.According to the Gaussian-Markovian theory of Takagahara et al., the contribution of the broad Raman band is not negligible in the case of strong intermediate-state in- teraction. Therefore, it is supposed that the factorization approximation adopted by Mukamel is not very good in our case. The calculation of the excitation profiles for the three secondary emission components by the formula of Takagahara et al. is now in progress and comparisons of the result with other theories and also with our experimental data will be reported shortly.
ACKNOWLEDGEMENT
We are indebted to Prof. Y. Koyama for the measurement of liquid chromatography on 6-carotene and also to Dr. T. Takagahara for the stimulating discussions.
REFERENCES
/1/ Inagaki, F., Tasumi, M. and Miyazawa, T., J. Mol. Spectro.
50
(1974) 286. /2/ Kushida, T., Solid State Comrnun.2
(1979) 33./3/ Ron, A. and Ron, A., Chem. Phys. Lett.
