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Vibrational relaxation of dye molecules in solution studied by femtosecond time-resolved stimulated emission pumping fluorescence depletion
12 January 1996
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 248 (1996) 277-282
Vibrational relaxation of dye molecules in solution studied by femtosecond time-resolved stimulated emission pumping fluorescence depletion Qinghua Zhong, Zhaohui Wang, Ya Sun, Qihe Zhu, Fanao Kong
*
Laboratory of Molecular Reaction Dynamics. Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100080, People's Republic of China
Received 9 August 1995; in final form 29 September 1995
Abstract
A new method, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), has been developed to study the vibrational relaxation of electronic excited states of molecules. Two relaxation rates of dye molecules in different solvents have been observed: (i) the intramolecular redistribution of energy, with a short time constant of less than 500 fs; (ii) the subsequent cooling of the vibrationally hot molecules on the picosecond time scale. The rates of these processes strongly depend on the solvent. This may result from a solvent-induced structural modification of the dye molecules.
I. I n t r o d u c t i o n
Vibrational relaxation of dye molecules in solution has been investigated extensively with ultrashort lasers. A variety of experimental techniques, such as transient grating and four wave mixing [1,2], fluorescence upconversion [3-6], nonlinear absorption spectroscopy [7-12], femtosecond photon echo [ 13], transient Raman spectroscopy [14], and two-step excitation [15], have been developed to study the vibrational relaxation pathways and the related time constants of dye molecules. Two types of processes have been identified: (i) The intramolecular energy redistribution occurs mostly on a subpicosecond time scale. Energy
* Corresponding author.
redistribution between vibrational modes in the S I state from 25 to 500 fs have been observed for several dyes, such as nile blue, cresyl violet, oxazine, and rhodamine compounds [7-12]. Energy redistribution in the high-lying S n states, excited by ultraviolet (UV) light and known to be quickly depopulated by internal conversion to $1 state (Kasha's rule), is on a similar time scale [11,12]. (ii) The vibrationally hot S~ state is cooled by collisional energy transfer to the solvent molecules on a time scale of 5 - 5 0 ps depending on the specific solvent and the amount of excess energy [10]. In this Letter, we report a new technique, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), to study the vibrational relaxation of electronically excited dye molecules. The stimulated emission pumping method was pioneered by Field and has been widely used for
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01328-8
278
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
obtaining rovibronic spectra of highly excited species [16]. We now intend to apply this technique to study ultrafast temporal behaviors of molecules. The principles of the femtosecond time-resolved stimulated emission pump-probe method has been described by Stock and Damcke [17] theoretically. To our knowledge, we firstly realize the femtosecond time-resolved SEP fluorescence depletion process and employ this method to study the ultrafast relaxation in solution. Two types of relaxation for four dye molecules have been observed. Solvent effects are also investigated.
S~
So
2. Experiment The principle of FS TR SEP FD method is the following: (i) a first ultrashort laser pulse was used to excite the molecule to the electronically excited state, leading to a fluorescent emission with a lifetime of a few nanoseconds; (ii) with a specific delay time, a second ultrashort laser was introduced to perform stimulated emission pumping (SEP) from the upper state to the electronic ground state (Fig. 1). The intensity of the fluorescence should then be reduced. The variation of the decrease on the fluorescence intensity with the delay time between the two laser beams reflects the vibrational relaxation behavior in the electronically excited state. A schematic diagram of the experimental setup is shown in Fig. 2. The laser used for these experiments was a home-made self-mode-locked Ti:sapphire femtosecond laser oscillator pumped by an argon ion laser (Beamlok 2060-10, Spectra Physics). The average power is about 500 mW at an 84 MHz pulse repetition rate near the wavelength of 796 nm. With four quartz lenses used as a dispersion device to compress the laser pulse [ 18], the laser pulse width was 52 fs (fwhm), with a sech 2 pulse shape. The spectral width was 280 c m - I at 796 nm. A BBO crystal (1 mm 13-BaB204, Fujian Castech Crystals Inc.) was used for frequency-doubling to generate an UV laser pulse. The UV pulse width (fwhm) was about 100 fs. The conversion efficiency was about 20%. A beamsplitter was used to separate the 796 nm laser as a probe beam, and the frequency doubled 398 nm laser as a pump beam. The dye molecules were excited by 398 nm laser to emit fluorescence.
Fig. 1. One-dimensional potential energy curve for the explanation of the vibrational relaxation process observed by TR SEP FD.
To generate sufficiently efficient stimulated emission pumping, the intensity of the probe beam was about three times higher than that of the pump beam. The probe beam was delayed using a scanning translation stage with a dc servo motor under computer control, providing 2 X 1.25 I~m path difference increments, that is, 8.3 fs resolution. The probe beam was collinear with the pump beam and they were focused with a f = 10 cm quartz lens to a spot with a diameter of 100 p,m, just behind the entrance window of the fused quartz sample cell. Fluorescence was collected by a lens system, and detected by a PMT (RCAC31034, semiconductor cooling, Products for Research) after a monochromator (WDG30, spectral resolution A A = 0.2 nm, Beijing Optical Instrument Factory). A mechanical light chopper was placed in the probe beam for phase sensitive detection and the PMT signal was processed by a lock-in analyzer ( E G & G 5208). The translation stage and lock-in analyzer were controlled by the computer to synchronize the scanning and the data collection. The time resolution of our system was evaluated by detecting the fluorescence of a 1 + 1' two-photon process in PPO/1,4-dioxane solution. The dye molecule could absorb one photon each from the 796
Q.
Zhong
et
a l . / Chemical Physics
Letters 248
279
(1996) 277-282
Photodiode
~
Prism Mirror
Prism
~~-sapphire
Opt,cat Delay
Lens
Lens
"~,.'~"?
'~ I
MirrorD
~,~
,~ Mirror
BBO Ar Ion Laser Beam
Choper
~
BS2
Oscilloscope monochromater
Lens
7 Cell ens Computer~
Lock-in Amplifier .
Lens
Lens
~.~ PMT
Fig. 2. Schematic of femtosecond time-resolved stimulated emission pumping fluorescence depletion (TR SEP FD). and 398 nm lasers synchronously, and the resultant emitted fluorescence was detected at 370 nm. This method yields an intensity cross-correlation function for the overlap of the pump and the probe laser beams. The instrument response function R(t) was about 188 fs fwhm as shown in Fig. 4 (below). Assuming that each laser pulse had a sech 2 profile, the time resolution was estimated to be about 120 fs in our experiment. This method is very sensitive. The signal to noise ratio (SNR) for LDS751 in ethanol, at a concentration of 10 -s M, was about 2: 1. The SNR of this method at 10 -5 M is comparable to that of up-conversion detection at 10 -4 M. The commercial dyes LDS698, LDS751, LDS765 and LDS821 (Exciton) (without additional purification) dissolved in methanol (CH3OH), ethanol (C2HsOH), acetone (CH3COCH3), chloroform (CHC13) and PC:EG (mixture of C4H603 1,2-propanediol carbonate and CH3CHOHCH2OH ethylene glycol, volume ratio 1 : 4) at concentrations less than 10- 5 M have been studied.
in Fig. 3. From the spectra, we know that the LDS698 and LDS765 molecules can be excited to the first electronic excited state, S1, and the LDS751 and 100
*I
80
I i
,o
b }a !
3. Results The absorption spectra of LDS698, LDS751, LDS765 and LDS821 dissolved in PC : EG are shown
200
I •
|
•
|
300
400
500
...
-"
600
700
800
Wavelength(rim) Fig. 3. Absorption spectra of LDS698 (short dash), LDS751 (dot dash), LDS765 (dots) and LDS821 (line) in PC: EG.
Q. Zhong et al. / ChemicalPhysics Letters 248 (1996) 277-282
280
dyes are plotted versus the delay time o f the probe pulse relative to the p u m p pulse. T w o decay processes can be seen clearly: a rapid decay within a few hundred fs and a s l o w e r d e c a y with p i c o s e c o n d time scale. In solution, the fast decay process must i n v o l v e an i n t r a m o l e c u l a r e n e r g y redistribution o f the excited m o l e c u l e s . T h e f l u o r e s c e n c e depletion signal A l ( z ) is g i v e n by
°m co e-
AI(-r) = f _ ~ s ( t ) R ( t -
r) dt,
(1)
.~_ w h e r e r is the d e l a y time b e t w e e n the p u m p and probe laser pulses, R ( t - ~') is the instrument response function, and S(t) is the relative a c c u m u l a t e d probability o f the stimulated e m i s s i o n p u m p i n g at time t,
-2000
0
2000
4000
6000
8000
10000 12000
Delay Time (fs)
S( t) • E pij( t)PifPfj • i,j.f
(2)
Fig. 4. The fluorescence depletion signal of I,a) LDS698, (b) LDS751, (c) LDS765, (d) LDS821 in PC:EG (the solid line is simulation), (e) apparatus response function.
pij represents the n u m b e r density (for i = j ) or c o h e r e n c e (for i v~j) in the excited state S 1, Pif and Pfj are the electronic transition matrix elements, w h i c h include electronic and F r a n c k - C o n d o n factors a m o n g initial states i and j in S 1 and final states f in S 0. So S(t) can be separated into t w o parts,
L D S 8 2 1 can be excited to the second electronic excited state, S 2, by the p u m p laser o f 398 nm. In Fig. 4, the f l u o r e s c e n c e depletion signals for these
S(t) a E p i i ( t ) lPiel2+ i,f
E
pii(t)PifPf)
i , j , f ( i q, j )
Table 1 Time constants of dyes in PC : EG Dyes
Spo
LDS698 LDS751 LDS765 LDS821
0.50 0.72 0.78 0.36
5:0.02 + 0.05 + 0.06 + 0.02
rp (fs)
Seo
50 5:20 84 + 10 443 5:33 38 5:18
0.50 0.28 0.22 0.64
~'e (ps) + + + +
0.02 0.05 0.06 0.02
10.7 5:0.6 4.7 5:1.0 4.9 + 1.0 2.8 + 0.6
Table 2 Time constants of LDS765 in different solvents Spo PC : EG C2H5OH CHCI 3 CH3OH CH3COCH 3
0.78 0.78 0.44 0.39 0.26
+ 0.06 + 0.03 + 0.02 + 0.05 5:0.10
~-p (fs)
Seo
443 + 33 102 + 10 121 ± 60 73 ___27 287 5:100
0.22 0.22 0.56 0.61 0.74
~'e (ps) + 0.06 + 0.03 + 0.02 -I- 0.05 5:0.10
4.9 + 1.0 4.1 + 0.7 1.3 + 0.1 0.8 + 0.1 0.8 5:0.1
(3)
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
281
and can be described approximately as a biexponential function,
S(t) cxS¢(t) S(t) = 0
+Sp(t )
(t~>0),
(/<0),
(4) (5)
where
S¢(t)=S~o[1-exp(-t/r¢) ] (t>~O), Sp(t)=Spo[1-exp(-t/~p) ] (t>~O), where the S~(t) and Sp(t) are the slow
(6)
e-
(7)
¢D r-
and fast depletion components with time constants % and rp, respectively, and the Se0, Sp0 are the depletion as t>> r e, %. S(t) is normalized, Seo + Spo = 1.
,m
~3
n~
(8)
With Eqs. (1) and (4)-(8), the normalized FD signal AI(~-) can be expressed as cc
Al(-r) = f
R(t-'r) dt
-2000
2000
4000
6000
8000
Delay Time(fs)
- SeoL~exp(--t/'re)R(t-7") dt
-SpoL==exp(-t/'rp)R(t-,)dt.
0
~~_
(b)
(9)
The experimental results were fitted by the deconvolution of Eq. (9). Table 1 shows the time constants of LDS698, LDS751, LDS765 and LDS821 in P C : E G . The time constants of the slow decay vary from 3 to 10 ps, and, except for LDS765, most time constants of the rapid decays are less than 100 fs. The slow decay may be considered as an intermolecular process. The FS TR SEP FD signals of LDS765 and LDS821 have been measured in different solvents. Fig. 5 shows the fluorescence depletion spectra of LDS765 and LDS821 in different solvents. Tables 2 and 3 show the time constants of LDS765 and LDS821 in different solvents. It can be seen that the slow decay is strongly affected by the solvents, so it must involve an energy transfer process between the dye and solvent molecules. The fast decay process has been attributed to either intramolecular energy redistribution between the vibrational modes in the electronic excited state [7,16], or internal conversion between electronic ex-
•~- ~
~
PC:EG
=o
-2000
0
2000
4000
6000
6000
Delay Time(fs) Fig. 5. The fluorescence depletion signal of (a) LDS765 and (b) LDS821 in different solvents.
282
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
Table 3 Time constants of LDS821 in different solvents S~, PC : EG C2HsOH CH3OH CH3COCH 3
0.36 0.31 0.31 0.56
~'p (fs) + ± ± ±
0.02 0.01 0.02 0.06
38 45 69 248
± ± ± +
S~0 18 20 16 29
cited states [11,12]. Usually, solute molecules are weakly coupled to the solvent, so the fast decay should be insensitive to choice of solvent. It is surprising that, for LDS765 and LDS821, the constants rp strongly depend on the solvent, changing from tens of fs to hundreds of fs. This cannot be explained by differences between the relaxation rates for initially excited vibrational levels with different excess energy, because no relationship between the band shifts in absorption spectra and the decay time constant Zp was found here. The large change in Zp may be resulted from a solvent-induced structural modification of the dye molecule. It is interesting to point out that dye molecules in alcohol solutions have smaller decay zp time constants relative to other solvents. We therefore suggest that hydrogen bonds occur between the dye and the alcohol molecules. The vibrational energy of the excited dye molecules can then be transferred to the adjacent alcohol molecules at extraordinarily fast rates.
References [1] S. Schneider, R. Bierl and M. Seischab, Chem. Phys. Letters 230 (1994) 343. [2] A.M. Weiner, S. De Silvestfi and E.P. Ippen, J. Opt. Soc. Am. B 2 (1985) 654.
0.64 0.69 0.69 0.44
r e (ps) + ± ± ±
0.02 0.01 0.02 0.06
2.8 2.7 2.0 4.6
+ ± + ±
0.6 0.1 0.1 0.7
[3] A. Mokhtari and J. Chesnoy, Chem. Phys. Letters 155 (1989) 593. [4] G.C. Walker, W. Jarzeba, T.J. Kang, A.E. Johnson and P.F. Barbara, J. Opt. Soc. Am. B 7 (1990) 1521. [5] A. Mokhtari, A. Chebira and J. Chesnoy, J. Opt. Soc. Am. B 7 (1990) 1551. [6] K. Yoshihara, A. Yartsev, Y. Nagasawa, H. Kandori, A. Douhal and K. Kemnitz, Pure Appl. Chem. 65 (1993) 1671. [7] A.M. Wemer and E.P. Ippen, Chem. Phys. Letters 114 (1985) 456. [8] C.H. Brito Cruz, R.L. Fork, W.H. Knox and C.V. Shank, Chem. Phys. Letters 132 (1986) 341. [9] A.J. Taylor, D.J. Erskine and C.L. Tang, Chem. Phys. Letters 103 (1984) 430. [10] W. Wild, A. Seilmeier, N.H. Gottfried and W. Kaiser, Chem. Phys. Letters 119 (1985) 259. [11] F. Laermer, W. Israel and T. Elsaesser, J. Opt. Soc. Am. B 7 (1990) 1604. [12] F. Laermer, T. Elsaesser and W. Kaiser, Chem. Phys. Letters 156 (1989) 381. [13] C.J. Bardeen and C.V. Shank, Chem. Phys. Letters 203 (1993) 535. [14] K. Lenz, M. Pfeiffer, A. Lau and T. Elsaesser, Chem. Phys. Letters 229 (1994) 340. [15] A. Laubereau, A. Seilmeier and W. Kaiser, Chem. Phys. Letters 36 (1975) 232. [16] Hai-lung Dai and R.W. Field, co-eds., Molecular dynamics and spectroscopy by stimulated emission pumping (Word Scientific, Singapore, 1995). [17] G. Stock and W. Domcke, J. Opt. Soc. Am. B. 7 (1990) 1970. [18] B. Proctor and F. Wise, Opt. Letters 17 (1992) 1295.
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 248 (1996) 277-282
Vibrational relaxation of dye molecules in solution studied by femtosecond time-resolved stimulated emission pumping fluorescence depletion Qinghua Zhong, Zhaohui Wang, Ya Sun, Qihe Zhu, Fanao Kong
*
Laboratory of Molecular Reaction Dynamics. Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100080, People's Republic of China
Received 9 August 1995; in final form 29 September 1995
Abstract
A new method, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), has been developed to study the vibrational relaxation of electronic excited states of molecules. Two relaxation rates of dye molecules in different solvents have been observed: (i) the intramolecular redistribution of energy, with a short time constant of less than 500 fs; (ii) the subsequent cooling of the vibrationally hot molecules on the picosecond time scale. The rates of these processes strongly depend on the solvent. This may result from a solvent-induced structural modification of the dye molecules.
I. I n t r o d u c t i o n
Vibrational relaxation of dye molecules in solution has been investigated extensively with ultrashort lasers. A variety of experimental techniques, such as transient grating and four wave mixing [1,2], fluorescence upconversion [3-6], nonlinear absorption spectroscopy [7-12], femtosecond photon echo [ 13], transient Raman spectroscopy [14], and two-step excitation [15], have been developed to study the vibrational relaxation pathways and the related time constants of dye molecules. Two types of processes have been identified: (i) The intramolecular energy redistribution occurs mostly on a subpicosecond time scale. Energy
* Corresponding author.
redistribution between vibrational modes in the S I state from 25 to 500 fs have been observed for several dyes, such as nile blue, cresyl violet, oxazine, and rhodamine compounds [7-12]. Energy redistribution in the high-lying S n states, excited by ultraviolet (UV) light and known to be quickly depopulated by internal conversion to $1 state (Kasha's rule), is on a similar time scale [11,12]. (ii) The vibrationally hot S~ state is cooled by collisional energy transfer to the solvent molecules on a time scale of 5 - 5 0 ps depending on the specific solvent and the amount of excess energy [10]. In this Letter, we report a new technique, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), to study the vibrational relaxation of electronically excited dye molecules. The stimulated emission pumping method was pioneered by Field and has been widely used for
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01328-8
278
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
obtaining rovibronic spectra of highly excited species [16]. We now intend to apply this technique to study ultrafast temporal behaviors of molecules. The principles of the femtosecond time-resolved stimulated emission pump-probe method has been described by Stock and Damcke [17] theoretically. To our knowledge, we firstly realize the femtosecond time-resolved SEP fluorescence depletion process and employ this method to study the ultrafast relaxation in solution. Two types of relaxation for four dye molecules have been observed. Solvent effects are also investigated.
S~
So
2. Experiment The principle of FS TR SEP FD method is the following: (i) a first ultrashort laser pulse was used to excite the molecule to the electronically excited state, leading to a fluorescent emission with a lifetime of a few nanoseconds; (ii) with a specific delay time, a second ultrashort laser was introduced to perform stimulated emission pumping (SEP) from the upper state to the electronic ground state (Fig. 1). The intensity of the fluorescence should then be reduced. The variation of the decrease on the fluorescence intensity with the delay time between the two laser beams reflects the vibrational relaxation behavior in the electronically excited state. A schematic diagram of the experimental setup is shown in Fig. 2. The laser used for these experiments was a home-made self-mode-locked Ti:sapphire femtosecond laser oscillator pumped by an argon ion laser (Beamlok 2060-10, Spectra Physics). The average power is about 500 mW at an 84 MHz pulse repetition rate near the wavelength of 796 nm. With four quartz lenses used as a dispersion device to compress the laser pulse [ 18], the laser pulse width was 52 fs (fwhm), with a sech 2 pulse shape. The spectral width was 280 c m - I at 796 nm. A BBO crystal (1 mm 13-BaB204, Fujian Castech Crystals Inc.) was used for frequency-doubling to generate an UV laser pulse. The UV pulse width (fwhm) was about 100 fs. The conversion efficiency was about 20%. A beamsplitter was used to separate the 796 nm laser as a probe beam, and the frequency doubled 398 nm laser as a pump beam. The dye molecules were excited by 398 nm laser to emit fluorescence.
Fig. 1. One-dimensional potential energy curve for the explanation of the vibrational relaxation process observed by TR SEP FD.
To generate sufficiently efficient stimulated emission pumping, the intensity of the probe beam was about three times higher than that of the pump beam. The probe beam was delayed using a scanning translation stage with a dc servo motor under computer control, providing 2 X 1.25 I~m path difference increments, that is, 8.3 fs resolution. The probe beam was collinear with the pump beam and they were focused with a f = 10 cm quartz lens to a spot with a diameter of 100 p,m, just behind the entrance window of the fused quartz sample cell. Fluorescence was collected by a lens system, and detected by a PMT (RCAC31034, semiconductor cooling, Products for Research) after a monochromator (WDG30, spectral resolution A A = 0.2 nm, Beijing Optical Instrument Factory). A mechanical light chopper was placed in the probe beam for phase sensitive detection and the PMT signal was processed by a lock-in analyzer ( E G & G 5208). The translation stage and lock-in analyzer were controlled by the computer to synchronize the scanning and the data collection. The time resolution of our system was evaluated by detecting the fluorescence of a 1 + 1' two-photon process in PPO/1,4-dioxane solution. The dye molecule could absorb one photon each from the 796
Q.
Zhong
et
a l . / Chemical Physics
Letters 248
279
(1996) 277-282
Photodiode
~
Prism Mirror
Prism
~~-sapphire
Opt,cat Delay
Lens
Lens
"~,.'~"?
'~ I
MirrorD
~,~
,~ Mirror
BBO Ar Ion Laser Beam
Choper
~
BS2
Oscilloscope monochromater
Lens
7 Cell ens Computer~
Lock-in Amplifier .
Lens
Lens
~.~ PMT
Fig. 2. Schematic of femtosecond time-resolved stimulated emission pumping fluorescence depletion (TR SEP FD). and 398 nm lasers synchronously, and the resultant emitted fluorescence was detected at 370 nm. This method yields an intensity cross-correlation function for the overlap of the pump and the probe laser beams. The instrument response function R(t) was about 188 fs fwhm as shown in Fig. 4 (below). Assuming that each laser pulse had a sech 2 profile, the time resolution was estimated to be about 120 fs in our experiment. This method is very sensitive. The signal to noise ratio (SNR) for LDS751 in ethanol, at a concentration of 10 -s M, was about 2: 1. The SNR of this method at 10 -5 M is comparable to that of up-conversion detection at 10 -4 M. The commercial dyes LDS698, LDS751, LDS765 and LDS821 (Exciton) (without additional purification) dissolved in methanol (CH3OH), ethanol (C2HsOH), acetone (CH3COCH3), chloroform (CHC13) and PC:EG (mixture of C4H603 1,2-propanediol carbonate and CH3CHOHCH2OH ethylene glycol, volume ratio 1 : 4) at concentrations less than 10- 5 M have been studied.
in Fig. 3. From the spectra, we know that the LDS698 and LDS765 molecules can be excited to the first electronic excited state, S1, and the LDS751 and 100
*I
80
I i
,o
b }a !
3. Results The absorption spectra of LDS698, LDS751, LDS765 and LDS821 dissolved in PC : EG are shown
200
I •
|
•
|
300
400
500
...
-"
600
700
800
Wavelength(rim) Fig. 3. Absorption spectra of LDS698 (short dash), LDS751 (dot dash), LDS765 (dots) and LDS821 (line) in PC: EG.
Q. Zhong et al. / ChemicalPhysics Letters 248 (1996) 277-282
280
dyes are plotted versus the delay time o f the probe pulse relative to the p u m p pulse. T w o decay processes can be seen clearly: a rapid decay within a few hundred fs and a s l o w e r d e c a y with p i c o s e c o n d time scale. In solution, the fast decay process must i n v o l v e an i n t r a m o l e c u l a r e n e r g y redistribution o f the excited m o l e c u l e s . T h e f l u o r e s c e n c e depletion signal A l ( z ) is g i v e n by
°m co e-
AI(-r) = f _ ~ s ( t ) R ( t -
r) dt,
(1)
.~_ w h e r e r is the d e l a y time b e t w e e n the p u m p and probe laser pulses, R ( t - ~') is the instrument response function, and S(t) is the relative a c c u m u l a t e d probability o f the stimulated e m i s s i o n p u m p i n g at time t,
-2000
0
2000
4000
6000
8000
10000 12000
Delay Time (fs)
S( t) • E pij( t)PifPfj • i,j.f
(2)
Fig. 4. The fluorescence depletion signal of I,a) LDS698, (b) LDS751, (c) LDS765, (d) LDS821 in PC:EG (the solid line is simulation), (e) apparatus response function.
pij represents the n u m b e r density (for i = j ) or c o h e r e n c e (for i v~j) in the excited state S 1, Pif and Pfj are the electronic transition matrix elements, w h i c h include electronic and F r a n c k - C o n d o n factors a m o n g initial states i and j in S 1 and final states f in S 0. So S(t) can be separated into t w o parts,
L D S 8 2 1 can be excited to the second electronic excited state, S 2, by the p u m p laser o f 398 nm. In Fig. 4, the f l u o r e s c e n c e depletion signals for these
S(t) a E p i i ( t ) lPiel2+ i,f
E
pii(t)PifPf)
i , j , f ( i q, j )
Table 1 Time constants of dyes in PC : EG Dyes
Spo
LDS698 LDS751 LDS765 LDS821
0.50 0.72 0.78 0.36
5:0.02 + 0.05 + 0.06 + 0.02
rp (fs)
Seo
50 5:20 84 + 10 443 5:33 38 5:18
0.50 0.28 0.22 0.64
~'e (ps) + + + +
0.02 0.05 0.06 0.02
10.7 5:0.6 4.7 5:1.0 4.9 + 1.0 2.8 + 0.6
Table 2 Time constants of LDS765 in different solvents Spo PC : EG C2H5OH CHCI 3 CH3OH CH3COCH 3
0.78 0.78 0.44 0.39 0.26
+ 0.06 + 0.03 + 0.02 + 0.05 5:0.10
~-p (fs)
Seo
443 + 33 102 + 10 121 ± 60 73 ___27 287 5:100
0.22 0.22 0.56 0.61 0.74
~'e (ps) + 0.06 + 0.03 + 0.02 -I- 0.05 5:0.10
4.9 + 1.0 4.1 + 0.7 1.3 + 0.1 0.8 + 0.1 0.8 5:0.1
(3)
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
281
and can be described approximately as a biexponential function,
S(t) cxS¢(t) S(t) = 0
+Sp(t )
(t~>0),
(/<0),
(4) (5)
where
S¢(t)=S~o[1-exp(-t/r¢) ] (t>~O), Sp(t)=Spo[1-exp(-t/~p) ] (t>~O), where the S~(t) and Sp(t) are the slow
(6)
e-
(7)
¢D r-
and fast depletion components with time constants % and rp, respectively, and the Se0, Sp0 are the depletion as t>> r e, %. S(t) is normalized, Seo + Spo = 1.
,m
~3
n~
(8)
With Eqs. (1) and (4)-(8), the normalized FD signal AI(~-) can be expressed as cc
Al(-r) = f
R(t-'r) dt
-2000
2000
4000
6000
8000
Delay Time(fs)
- SeoL~exp(--t/'re)R(t-7") dt
-SpoL==exp(-t/'rp)R(t-,)dt.
0
~~_
(b)
(9)
The experimental results were fitted by the deconvolution of Eq. (9). Table 1 shows the time constants of LDS698, LDS751, LDS765 and LDS821 in P C : E G . The time constants of the slow decay vary from 3 to 10 ps, and, except for LDS765, most time constants of the rapid decays are less than 100 fs. The slow decay may be considered as an intermolecular process. The FS TR SEP FD signals of LDS765 and LDS821 have been measured in different solvents. Fig. 5 shows the fluorescence depletion spectra of LDS765 and LDS821 in different solvents. Tables 2 and 3 show the time constants of LDS765 and LDS821 in different solvents. It can be seen that the slow decay is strongly affected by the solvents, so it must involve an energy transfer process between the dye and solvent molecules. The fast decay process has been attributed to either intramolecular energy redistribution between the vibrational modes in the electronic excited state [7,16], or internal conversion between electronic ex-
•~- ~
~
PC:EG
=o
-2000
0
2000
4000
6000
6000
Delay Time(fs) Fig. 5. The fluorescence depletion signal of (a) LDS765 and (b) LDS821 in different solvents.
282
Q. Zhong et al. / Chemical Physics Letters 248 (1996) 277-282
Table 3 Time constants of LDS821 in different solvents S~, PC : EG C2HsOH CH3OH CH3COCH 3
0.36 0.31 0.31 0.56
~'p (fs) + ± ± ±
0.02 0.01 0.02 0.06
38 45 69 248
± ± ± +
S~0 18 20 16 29
cited states [11,12]. Usually, solute molecules are weakly coupled to the solvent, so the fast decay should be insensitive to choice of solvent. It is surprising that, for LDS765 and LDS821, the constants rp strongly depend on the solvent, changing from tens of fs to hundreds of fs. This cannot be explained by differences between the relaxation rates for initially excited vibrational levels with different excess energy, because no relationship between the band shifts in absorption spectra and the decay time constant Zp was found here. The large change in Zp may be resulted from a solvent-induced structural modification of the dye molecule. It is interesting to point out that dye molecules in alcohol solutions have smaller decay zp time constants relative to other solvents. We therefore suggest that hydrogen bonds occur between the dye and the alcohol molecules. The vibrational energy of the excited dye molecules can then be transferred to the adjacent alcohol molecules at extraordinarily fast rates.
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0.64 0.69 0.69 0.44
r e (ps) + ± ± ±
0.02 0.01 0.02 0.06
2.8 2.7 2.0 4.6
+ ± + ±
0.6 0.1 0.1 0.7
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