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Energy structures of absorption and emission bands measured by coherent population-grating spectroscopy in dye solutions
Prog. Crystal Growth and Charact. Vol. 33, pp. 375-378, 1996
Pergamon
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0960-8974/96 $32.00
ENERGY STRUCTURES OF ABSORPTION AND EMISSION BANDS MEASURED BY COHERENT POPULATION-GRATING SPECTROSCOPY IN DYE SOLUTIONS Hirotoshi Souma* and Hideo Tashirot * Faculty of Science and Technology, Ishinomaki Senshu University, Ishinomaki 986-80, Japan i Photodynamics Research Center, The Institute of Physical and Chemical Research, Japan
Abstract The bandwidths of the resolved nonlinear spectra in the S0-Sj electronic transitions of cresyl violet and resorufin in solutions were found to be much less than those of the linear absorption and emission spectra of both samples. The linear optical bands are caused by the congested energy structures but not dominantly by the optical dephasing of the molecules on a femtosecond time scale.
t. Introduction In the development of electronically resonant third-order nonlinear spectroscopy in the frequency domain, it was explicitly noted that for the application to condensed materials the effect of the inhomogeneous nature and the relevant spectral diffusion dynamics should be taken into account[l]. When the spectral diffusion is fast, the spectral information may be smeared out. In the cases of the effective two-level systems using dye molecules in solutions, resonant Rayleigh-type four-wave mixing[2-5] has been known to be comparatively insensitive for the fast spectral diffusion, while polarization spectroscopy[4,6] has been sensitive for that. The insensitive or sensitive characters are caused by the second-order coherence grating or the population grating, respectively, depending on the time ordering of the perturbations at incident frequencies oJ ~and 092. From these gratings a third wave can be scattered to generate a fourth wave. This argument should be also valid for the cases of multienergy-level systems. The coherent anti-Stokes Raman scattering(CARS) and coherent Stokes Raman scattering(CSRS) spectroscopies under the electronically resonant condition and Rayleigh-type mixing spectroscopy definitely belong to the former, while hole burning belongs to the later. On the other hand, polarization, Raman gain, inverse Raman, and newly developed coherent population-grating(CPG) spectroscopies[7] may belong to the both cases depending on the excitation and probe frequencies including the Raman resonances. It has been confirmed that it is possible in CPG method to single out a definite molecule in resonance with co~ by using picosecond lasers for the pumping and probing where the effect of spectral diffusion is expected to be largely reduced. The experimental arcangement[7] for the picosecond CPG spectroscopy are shown in Fig. 1. Amplified dye pulses oJ ~with a width [FWHM] of 6 ps and a spectral width [FWHM] of 5cm1 were divided into two light fields. 375
376
H. Souma and H. Tashiro
As a source of probe beam 092, we used the wide range of spectra of amplified spontaneous emissions of several dye solutions with a pulse width [FWHM] of the order of 100 ps. In order to induce a population-grating the two beams w l for a selective excitation of electronic transitions intersect at a small angle 0. The probe beam with a different frequency 602 participating in a CPG resonance propagates in the opposite direction to one of the o9~ beams. The signal beam W s governed by the third-order nonlinear susceptibility X(3~(- o92;o9t,-w~,o92)emits at the angle 0 with respect to the direction of the probe beam, yielding spatial filtering. I,~ "~............... 0................... There are two types of CPG resonances which were named 2_ Fig. 1. Schematic of the type 1 and type 2. The type 1 gives rise to detailed information experimental system for CPG of energy structures of the ground state, while the type 2 offers spectroscopy, information of the energy structures of the excited state. In the previous paper[7] it was noted that the results of the fs coherent transient expe~nents reported by Bigot et al.[8] and Nibbering et al.[9] were quite different from the observations with the CPG method. It is required definitely to perform more refined CPG expe~nents in order to understand the physics of these frequency- and time-domain spectroscopies. The purpose of the present paper is to clear up the fine energy compositions hidden in both broadened absorption and emission spectra of large molecules such as cresyl violet and resorufin in solutions, which have not achieved sufficiently in the preliminary experiment[7]. There involve the determination of the spectral resolution of the CPG spectra, the coherent and incoherent contributions and the excitation frequency dependence of the CPG spectra in order to consider the selective excitation mechanisms and the origin of the free energy structures. 2. Results and Discussions In the present experiment the CPG signals are generated under the partially transient condition in which the 6 ps pulses and 100 ps pulses with the wide range spectra were used for the pumping and probing, respectively. In order to analyze the CPG spectra we need to determine the spectral resolution, by means of the measurement of the third-order frequency correlation between pump and probe pulses. For this purpose we measured the signals from well-known 992 cm-1 Rarnan mode with the linewidth of the order of 0.1 cm j of benzene under the electronically non-resonant conditions in the cases of type 1 and type 2, in addition to the CARS, as shown in Figs. 2(A) to 2(C), respectively. In the following, the terms of the type I and type 2 are used widely even for the electronically non-resonant conditions. All these spectra have the linewidth[HWHM] of the order of 3.5 cm-~, which indicates the resolution of the CPG spectra and corresponds to the correlation time of the order of 1.5 ps. It should be pointed out that when the measured linewidth of the CPG spectra is wider than 3.5 cm t the steady-state approximation for the CPG processes will be valid[10]. It is also pointed out that the third-order nonlinear processes in Figs. 2(A) and 2(B) are governed by the same
Energy Structures of Absorption and Emission Bands
377
linear spectra are considered to be composed of the transitions with such coupling modes. The other specific feature is that the regions of w~ for the clear appearance of the type 1 and type 2 signals are quite different each other. The type 1 begins at the higher frequency region than the type 2 does as indicated by the dotted and solid lines in Fig. 4, respectively. In order to specify the reason we tried to measure the time dependencies of the type 1, type 2, and CARS spectra as shown in Figs. 5(A) to 5(C), respectively. The time dependencies of the type 2 shown in Fig. 5(B) under the lower frequency excitation with cot shows almost the same behavior as those of the CARS shown in Fig. 5(C). Therefore it must be noted that there appear the coherent Raman processes especially in the lower energy side of 550 600 650 700 ENERGY SHIFF(cm -t) the absorption band due to the non-resonant excitation Fig. 4. Excitation frequency co l with co ~. It is clearly seen that the integrated incoherent dependencies of type 1 and type 2 contribution smears out the spectral information in the spectra of cresyl violet in ethanol. type 1 when w~ pulses are coincident at the reading RESYL V I O L E T ~ ,%(B) (A)I ~c) ~6 ~!- ~ edge of 092 pulses, as shown in Fig. 5(A). In conclusion, we have demonstrated the usefulness :AI of the CPG method for the spectroscopy of condensed cos matter. The detailed experimental results and the theoretical considerations will be reported elsewhere.
~
References
1.
T. Yajima and H. Souma, Phys. Rev. AI 7, 309 (1978). 2. T. Yajima, H. Souma and Y. lshida, Phys. Rev. A17, 324(1978). 3. H. Souma, T. Yajima and Y. Taira, J. Phys. Soc. Jpn. 48, 2040(1980). 1560(I 15800 158130 16000 16800 17000 WAVENUMBER(cm "11 4. Y. Taira and T. Yajima, J. Phys. Soc. Jpn. 50, Fig, 5. Time dependencies of CPG 3459(1981). and CARS spectra in the relative 5. H. Souma, E. J. Heilweil and R. M. Hochstrasser, delays of co2 against co~ pulses. J. Chem. Phys. 76, 5693(1982). 6. J.J. Song, J. H Lee and M. D. Levenson, Phys. Rev. A17, 1439(l 978). 7. H. Souma, C. Horie, M. Hoshi, T. Daiguji and H. Tashiro, Jpn. J. Appl. Phys. 33, L1218 (1994). 8. J.Y. Bigot, M. T. Portella, R. W. Schoenlein, C. J. Bardeen, A. Migus and C. V. Shank, Phys. Rev. Lett. 66, 1138(1991). 9. E.T.J. Nibbering, D. A. Wiersma and K. Duppen, Phys. Rev. Lett. 66, 2464(1991). 10. H. Souma, J. Phys. Soc. Jpn. 52, 3225(1983).
378
H. Souma and H. Tashiro
Z (3)processes as in Raman gain and inverse Raman spectroscopies, respectively. The spectra shown in Figs. 3(A) to 3(D) are type 1 BENZENE and type 2 of CPG, CARS, and CSRS signals, respectively, measured from frequency 0) ~. The 0) I=16768 Cm .J i tO2 tot frequency 0) 1 is resonant with the lower energy side of --+ the absorption band of resorufin in dimethylsulfoxide as shown in the insert of the figure. We must note that the 0~) ~ot=14915 cm- / 1 o)~ energy of the strongly enhanced vibrational mode in the ground state indicated by the spectra shown in Figs. 3(A), --÷ 3(C) and 3(D) is larger in several wavenumbers than that in the excited state seen in Fig. 3(B) as expected. It is also noted that the fully resonant CPG spectra shown in i (C) o9~=16524 cm i ][. ~ i~2 col Ws _. Figs. 3(A) and 303) are accompanied with weakly enhanced fine energy structures, remarkably different 900 950 10D0 1050 1100 ENERGY SHIFT(cm -t ) from the CARS and CSRS spectra shown in Figs. 3(C) and 3(D), respectively. This fact is supposed to arise on Fig. 2. Type 1, type 2, and CARS spectra of benzene under the account of the nearly-resonant conditions in both CARS electronically non-resonant conditions. and CSRS due to the different vibrational frequencies in "REsoRUFIN ~ U 2~o~ ]] the ground and excited states. Thus these results reveal / /ILI I 670~m ~, the high-resolution capability of the CPG method. al CPG TYPE 1 +~÷;+ b~ The excitation frequency dependencies of the CPG t.. ............. I spectra in cresyl violet in ethanol under the conditions of ....... t (b CPG TYPE 2 +-:~' ~ *~ J "7"+" / type 1 and type 2 are shown in Fig. 4, in which we gave cV / ~ ..;*,L,~ attention to the frequency region of around 600 cm ~ in the difference frequency between w l and 0)2. The time dependencies of the CPG and CARS spectra in the relative delays of w2 pulses against w~ pulses are shown in Fig. 5 in order to investigate the coherent and (el CARS .......-/ J incoherent contributions to them. The energy positions of the excitation frequencies 0)~ together with the 61~1 650 700 1 750 absorption and emission spectra, and the relative energy ENERGY stnFT(cm- ) Fig. 3. Comparison between type 1, positions of 60 ~, 0) 2 and COs are indicated in the type 2, CARS, and CSRS spectra of inserts of the figures. One of the remarkable features of resorufin in dimethylsulfoxide. the CPG spectra seen in Fig. 4 is that the energy structures of the CPG spectra are changed greatly during the tuning of w~. In the cases of type 2 one can see even dips in the frequency regions shown in Fig. 4(E) and 4(F). These results suggest strongly that in the CPG method the selective excitation with o9 ~works very well. Then we can think about the origin of the fine energy structures in the CPG spectra. Since cresyl violet is one of the low-symmetry large molecules, the fine structures might be caused by the resonant electronic transitions accompanied with a strong vibrational mode coupled with numbers of low-frequency modes such as some kinds of librational modes. The ~
co
i.
-
Jl
~o
Pergamon
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0960-8974/96 $32.00
ENERGY STRUCTURES OF ABSORPTION AND EMISSION BANDS MEASURED BY COHERENT POPULATION-GRATING SPECTROSCOPY IN DYE SOLUTIONS Hirotoshi Souma* and Hideo Tashirot * Faculty of Science and Technology, Ishinomaki Senshu University, Ishinomaki 986-80, Japan i Photodynamics Research Center, The Institute of Physical and Chemical Research, Japan
Abstract The bandwidths of the resolved nonlinear spectra in the S0-Sj electronic transitions of cresyl violet and resorufin in solutions were found to be much less than those of the linear absorption and emission spectra of both samples. The linear optical bands are caused by the congested energy structures but not dominantly by the optical dephasing of the molecules on a femtosecond time scale.
t. Introduction In the development of electronically resonant third-order nonlinear spectroscopy in the frequency domain, it was explicitly noted that for the application to condensed materials the effect of the inhomogeneous nature and the relevant spectral diffusion dynamics should be taken into account[l]. When the spectral diffusion is fast, the spectral information may be smeared out. In the cases of the effective two-level systems using dye molecules in solutions, resonant Rayleigh-type four-wave mixing[2-5] has been known to be comparatively insensitive for the fast spectral diffusion, while polarization spectroscopy[4,6] has been sensitive for that. The insensitive or sensitive characters are caused by the second-order coherence grating or the population grating, respectively, depending on the time ordering of the perturbations at incident frequencies oJ ~and 092. From these gratings a third wave can be scattered to generate a fourth wave. This argument should be also valid for the cases of multienergy-level systems. The coherent anti-Stokes Raman scattering(CARS) and coherent Stokes Raman scattering(CSRS) spectroscopies under the electronically resonant condition and Rayleigh-type mixing spectroscopy definitely belong to the former, while hole burning belongs to the later. On the other hand, polarization, Raman gain, inverse Raman, and newly developed coherent population-grating(CPG) spectroscopies[7] may belong to the both cases depending on the excitation and probe frequencies including the Raman resonances. It has been confirmed that it is possible in CPG method to single out a definite molecule in resonance with co~ by using picosecond lasers for the pumping and probing where the effect of spectral diffusion is expected to be largely reduced. The experimental arcangement[7] for the picosecond CPG spectroscopy are shown in Fig. 1. Amplified dye pulses oJ ~with a width [FWHM] of 6 ps and a spectral width [FWHM] of 5cm1 were divided into two light fields. 375
376
H. Souma and H. Tashiro
As a source of probe beam 092, we used the wide range of spectra of amplified spontaneous emissions of several dye solutions with a pulse width [FWHM] of the order of 100 ps. In order to induce a population-grating the two beams w l for a selective excitation of electronic transitions intersect at a small angle 0. The probe beam with a different frequency 602 participating in a CPG resonance propagates in the opposite direction to one of the o9~ beams. The signal beam W s governed by the third-order nonlinear susceptibility X(3~(- o92;o9t,-w~,o92)emits at the angle 0 with respect to the direction of the probe beam, yielding spatial filtering. I,~ "~............... 0................... There are two types of CPG resonances which were named 2_ Fig. 1. Schematic of the type 1 and type 2. The type 1 gives rise to detailed information experimental system for CPG of energy structures of the ground state, while the type 2 offers spectroscopy, information of the energy structures of the excited state. In the previous paper[7] it was noted that the results of the fs coherent transient expe~nents reported by Bigot et al.[8] and Nibbering et al.[9] were quite different from the observations with the CPG method. It is required definitely to perform more refined CPG expe~nents in order to understand the physics of these frequency- and time-domain spectroscopies. The purpose of the present paper is to clear up the fine energy compositions hidden in both broadened absorption and emission spectra of large molecules such as cresyl violet and resorufin in solutions, which have not achieved sufficiently in the preliminary experiment[7]. There involve the determination of the spectral resolution of the CPG spectra, the coherent and incoherent contributions and the excitation frequency dependence of the CPG spectra in order to consider the selective excitation mechanisms and the origin of the free energy structures. 2. Results and Discussions In the present experiment the CPG signals are generated under the partially transient condition in which the 6 ps pulses and 100 ps pulses with the wide range spectra were used for the pumping and probing, respectively. In order to analyze the CPG spectra we need to determine the spectral resolution, by means of the measurement of the third-order frequency correlation between pump and probe pulses. For this purpose we measured the signals from well-known 992 cm-1 Rarnan mode with the linewidth of the order of 0.1 cm j of benzene under the electronically non-resonant conditions in the cases of type 1 and type 2, in addition to the CARS, as shown in Figs. 2(A) to 2(C), respectively. In the following, the terms of the type I and type 2 are used widely even for the electronically non-resonant conditions. All these spectra have the linewidth[HWHM] of the order of 3.5 cm-~, which indicates the resolution of the CPG spectra and corresponds to the correlation time of the order of 1.5 ps. It should be pointed out that when the measured linewidth of the CPG spectra is wider than 3.5 cm t the steady-state approximation for the CPG processes will be valid[10]. It is also pointed out that the third-order nonlinear processes in Figs. 2(A) and 2(B) are governed by the same
Energy Structures of Absorption and Emission Bands
377
linear spectra are considered to be composed of the transitions with such coupling modes. The other specific feature is that the regions of w~ for the clear appearance of the type 1 and type 2 signals are quite different each other. The type 1 begins at the higher frequency region than the type 2 does as indicated by the dotted and solid lines in Fig. 4, respectively. In order to specify the reason we tried to measure the time dependencies of the type 1, type 2, and CARS spectra as shown in Figs. 5(A) to 5(C), respectively. The time dependencies of the type 2 shown in Fig. 5(B) under the lower frequency excitation with cot shows almost the same behavior as those of the CARS shown in Fig. 5(C). Therefore it must be noted that there appear the coherent Raman processes especially in the lower energy side of 550 600 650 700 ENERGY SHIFF(cm -t) the absorption band due to the non-resonant excitation Fig. 4. Excitation frequency co l with co ~. It is clearly seen that the integrated incoherent dependencies of type 1 and type 2 contribution smears out the spectral information in the spectra of cresyl violet in ethanol. type 1 when w~ pulses are coincident at the reading RESYL V I O L E T ~ ,%(B) (A)I ~c) ~6 ~!- ~ edge of 092 pulses, as shown in Fig. 5(A). In conclusion, we have demonstrated the usefulness :AI of the CPG method for the spectroscopy of condensed cos matter. The detailed experimental results and the theoretical considerations will be reported elsewhere.
~
References
1.
T. Yajima and H. Souma, Phys. Rev. AI 7, 309 (1978). 2. T. Yajima, H. Souma and Y. lshida, Phys. Rev. A17, 324(1978). 3. H. Souma, T. Yajima and Y. Taira, J. Phys. Soc. Jpn. 48, 2040(1980). 1560(I 15800 158130 16000 16800 17000 WAVENUMBER(cm "11 4. Y. Taira and T. Yajima, J. Phys. Soc. Jpn. 50, Fig, 5. Time dependencies of CPG 3459(1981). and CARS spectra in the relative 5. H. Souma, E. J. Heilweil and R. M. Hochstrasser, delays of co2 against co~ pulses. J. Chem. Phys. 76, 5693(1982). 6. J.J. Song, J. H Lee and M. D. Levenson, Phys. Rev. A17, 1439(l 978). 7. H. Souma, C. Horie, M. Hoshi, T. Daiguji and H. Tashiro, Jpn. J. Appl. Phys. 33, L1218 (1994). 8. J.Y. Bigot, M. T. Portella, R. W. Schoenlein, C. J. Bardeen, A. Migus and C. V. Shank, Phys. Rev. Lett. 66, 1138(1991). 9. E.T.J. Nibbering, D. A. Wiersma and K. Duppen, Phys. Rev. Lett. 66, 2464(1991). 10. H. Souma, J. Phys. Soc. Jpn. 52, 3225(1983).
378
H. Souma and H. Tashiro
Z (3)processes as in Raman gain and inverse Raman spectroscopies, respectively. The spectra shown in Figs. 3(A) to 3(D) are type 1 BENZENE and type 2 of CPG, CARS, and CSRS signals, respectively, measured from frequency 0) ~. The 0) I=16768 Cm .J i tO2 tot frequency 0) 1 is resonant with the lower energy side of --+ the absorption band of resorufin in dimethylsulfoxide as shown in the insert of the figure. We must note that the 0~) ~ot=14915 cm- / 1 o)~ energy of the strongly enhanced vibrational mode in the ground state indicated by the spectra shown in Figs. 3(A), --÷ 3(C) and 3(D) is larger in several wavenumbers than that in the excited state seen in Fig. 3(B) as expected. It is also noted that the fully resonant CPG spectra shown in i (C) o9~=16524 cm i ][. ~ i~2 col Ws _. Figs. 3(A) and 303) are accompanied with weakly enhanced fine energy structures, remarkably different 900 950 10D0 1050 1100 ENERGY SHIFT(cm -t ) from the CARS and CSRS spectra shown in Figs. 3(C) and 3(D), respectively. This fact is supposed to arise on Fig. 2. Type 1, type 2, and CARS spectra of benzene under the account of the nearly-resonant conditions in both CARS electronically non-resonant conditions. and CSRS due to the different vibrational frequencies in "REsoRUFIN ~ U 2~o~ ]] the ground and excited states. Thus these results reveal / /ILI I 670~m ~, the high-resolution capability of the CPG method. al CPG TYPE 1 +~÷;+ b~ The excitation frequency dependencies of the CPG t.. ............. I spectra in cresyl violet in ethanol under the conditions of ....... t (b CPG TYPE 2 +-:~' ~ *~ J "7"+" / type 1 and type 2 are shown in Fig. 4, in which we gave cV / ~ ..;*,L,~ attention to the frequency region of around 600 cm ~ in the difference frequency between w l and 0)2. The time dependencies of the CPG and CARS spectra in the relative delays of w2 pulses against w~ pulses are shown in Fig. 5 in order to investigate the coherent and (el CARS .......-/ J incoherent contributions to them. The energy positions of the excitation frequencies 0)~ together with the 61~1 650 700 1 750 absorption and emission spectra, and the relative energy ENERGY stnFT(cm- ) Fig. 3. Comparison between type 1, positions of 60 ~, 0) 2 and COs are indicated in the type 2, CARS, and CSRS spectra of inserts of the figures. One of the remarkable features of resorufin in dimethylsulfoxide. the CPG spectra seen in Fig. 4 is that the energy structures of the CPG spectra are changed greatly during the tuning of w~. In the cases of type 2 one can see even dips in the frequency regions shown in Fig. 4(E) and 4(F). These results suggest strongly that in the CPG method the selective excitation with o9 ~works very well. Then we can think about the origin of the fine energy structures in the CPG spectra. Since cresyl violet is one of the low-symmetry large molecules, the fine structures might be caused by the resonant electronic transitions accompanied with a strong vibrational mode coupled with numbers of low-frequency modes such as some kinds of librational modes. The ~
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i.
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Jl
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