竹内 佐年

We report a femtosecond time-resolved fluorescence study of cis-stilbene, a prototypical molecule showing ultrafast olefinic photoisomerization and photocyclization. The time-resolved fluorescence signals were measured in a nonpolar solvent over a wide ultraviolet-visible region with excitation at 270 nm. The time-resolved fluorescence traces exhibit non-single exponential decays which are well fit with bi-exponential functions with time constants of tau(A) = 0.23 ps and tau(B) = 1.2 ps, and they are associated with the fluorescence emitted from different regions of the S-1 potential energy surface (PES) in the course of the structural change. Quantitative analysis revealed that the two fluorescent components exhibit similar intrinsic time-resolved spectra extending from 320 nm to 700 nm with the (fluorescence) oscillator strength of f(A) = 0.32 and f(B) = 0.21, respectively. It was concluded that the first component is assignable to the fluorescence from the untwisted S-1 PES region where the molecule reaches immediately after the initial elongation of the central C=C bond, while the second component is the fluorescence from the substantially twisted region around a shallow S-1 potential minimum. The quantitative analysis of the femtosecond fluorescence data clearly showed that the whole isomerization process proceeds in the one-photon allowed S-1 state, thereby resolving a recent controversy in quantum chemical calculations about the reactive S-1 state. In addition, the evaluated oscillator strengths suggest that the population branching into the isomerization/cyclization pathways occurs in a very early stage when the S-1 molecule still retains a planar Ph-C-C-Ph skeletal structure. On the basis of the results obtained, we discuss the dynamics and mechanism of the isomerization/cyclization reactions of cis-stilbene, as well as the electronic structure of the reaction precursor.

Understanding ultrafast reactions, which proceed on a time scale of nuclear motions, requires a quantitative characterization of the structural dynamics. To track such structural changes with time, we studied a nuclear wavepacket motion in photoisomerization of a prototype cyanine dye, 1,1'-diethy1-4,4'-cyanine, by ultrafast pump dump probe measurements in solution. The temporal evolution of wavepacket motion was examined by monitoring the efficiency of stimulated emission dumping, which was obtained from the recovery of a ground-state bleaching signal. The dump efficiency versus pump dump delay exhibited a finite rise time, and it became longer (97 fs -> 330 fs -> 390 fs) as the dump pulse was tuned to longer wavelengths (690 nm -> 950 nm -> 1200 nm). This result demonstrates a continuous migration of the leading edge of the wavepacket on the excited-state potential from the Franck-Condon region toward the potential minimum. A slowly decaying feature of the dump efficiency indicated a considerable broadening of the wavepacket over a wide range of the potential, which results in the spread of a population distribution on the flat S(1) potential energy surface. The rapid migration as well as broadening of the wavepacket manifests a continuous nature of the structural dynamics and provides an intuitive visualization of this ultrafast reaction. We also discussed experimental strategies to evaluate reliable dump efficiencies separately from other ultrafast processes and showed a high capability and possibility of the pump dump probe method for spectroscopic investigation of unexplored potential regions such as conical intersections.

The photoinduced structural change of a prototype metal complex, [Cu(dmphen)(2)](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), was studied by ultrafast spectroscopy with time resolution as high as 30 fs. Time-resolved absorption measured with direct Si excitation clearly showed spectral changes attributable to the D(2d) (Perpendicular) -> D(2) (flattened) structural change occurring in the metal-to-ligand charge transfer singlet excited state ((1)MLCT) and the subsequent S(1) -> T(1) intersystem crossing. It was confirmed that the two processes occur with time constants of similar to 0.8 ps (structural change) and ps (intersystem crossing), and their time scales are clearly well-separated. A distinct oscillation of the transient absorption signal was observed in the femtosecond region, which arises from the coherent nuclear motion of the perpendicular S(1) state that was directly generated by photoexcitation. This demonstrated that the perpendicular Si state has a well-defined vibrational structure and can vibrate within its subpicosecond lifetime. In other words, the S(1) state stays undistorted in a short period, and the coherent nuclear motion is maintained in this state. Time-dependent density functional theory (TDDFT) calculations gave consistent results, indicating a very flat feature and even a local minimum at the perpendicular structure on the S(1) potential energy surface. The vibrational assignments of the S(1) nuclear wavepacket motion were made on the basis of the TDDFT calculation. It was concluded that photoexcitation induces a(1) vibrations containing the Cu-ligand bond length change and a b(1) vibration attributed to the ligand-twisting motion that has the same symmetry as the flattening distortion. Ultrafast spectroscopy and complementary quantum chemical calculation provided an overall picture and new understanding of the photoinduced structural change of the prototypical metal complex.

We have studied IR-induced low-frequency coherent vibration of an intramolecularly hydrogen-bonded molecule, quinizarin, by an ultrashort IR-pump-visible-probe spectroscopy with similar to 60 fs time resolution. In this experiment, the IR excitation of the symmetric OH-stretching mode induced a low-frequency vibrational coherence, which was then detected as an oscillation of the visible absorption intensity. The observed oscillation was assigned to a "hydrogen-bond modulating" vibration by the vibrational analysis based on the density functional theory (DFT). Because the vibrational coherence formation by IR excitation requires a substantial anharmonic coupling, we carried out a DFT-based numerical analysis of the anharmonic coupling between the OH-stretching and the low-frequency mode, by evaluating the transition moment of the combination band. We took account of two types of anharmonicities, i.e., the mechanical anharmonicity and the electrical anharmonicity. Although the electrical anharmonicity is often neglected, it was found that the electrical anharmonicity had a comparable contribution to the mechanical anharmonicity, in generation of vibrational coherence of the low-frequency mode in this system. This result indicates general importance of the electrical anharmonicity in strongly hydrogen-bonded systems.

Halorhodopsin (HR) is a light-driven chloride pump. Cl- is bound in the Schiff base region of the retinal chromophore, and unidirectional Cl- transport is probably enforced by the specific hydrogen-bonding interaction with the protonated Schiff base and internal water molecules. It is known that HR from Natronobacterium pharaonis (pHR) also pumps NO3- with similar efficiency, suggesting that NO3- binds to the Cl--binding site. In the present study, we investigated the properties of the anion-binding site by means of ultrafast pump-probe spectroscopy and low-temperature FTIR spectroscopy. The obtained data were surprisingly similar between pHR-NO3- and pHR-Cl-, even though the shapes and sizes of the two anions are quite different. Femtosecond pump-probe spectroscopy showed very similar excited-state dynamics between pHR-NO3- and pHR-Cl-. Low-temperature FTIR spectroscopy of unlabeled and [zeta-N-15]Lys-labeled pHR revealed almost identical hydrogen-bonding strengths of the protonated retinal Schiff base between pHR-NO3- and pHR-Cl-, which is similarly strengthened after retinal isomerization. There were spectral variations for water stretching vibrations between pHR-NO3- and pHR-Cl-, suggesting that the water molecules hydrate each anion. Nevertheless, the overall spectral features were similar for the two species. These observations strongly suggest that the anion-binding site has a flexible structure and that the interaction between retinal and the anions is weak, despite the presence of an electrostatic interaction. Such a flexible hydrogen-bonding network in the Schiff base region in HR appears to be in remarkable contrast to that in light-driven proton-pumping proteins.

Coherent hydrogen-bond stretching vibration was observed by probing ultrafast visible absorption change after impulsive mid-infrared excitation of the OH-stretching mode of an intramolecularly hydrogen-bonded molecule. The underlying mechanism of the wavepacket formation was discussed on a theoretical basis considering mechanical and electrical anharmonicities.

We studied the vibrational structure of reactive S-1, cis-stilbene through wavepacket motions generated impulsively at various delay-times. They showed gradual frequency downshift, demonstrating highly anharmonic nature of the excited-state potential and structural evolution with photoisomerization.

Ultrafast photo-induced structural change of [Cu(dmphen)(2)](+) was studied by pump-probe spectroscopy with 25-fs time-resolution. The observed nuclear wavepacket motion unveiled a new mechanism of photo-induced Jahn-Teller distortion that is a key of inorganic molecular switches.

We measured steady-state fluorescence of cis-stilbene in solution, by carefully separating the contribution of coexisting trace amount of trans isomer. The obtained 'pure' fluorescence spectrum of cis-stilbene shows a broad structureless band (peaked around 420 nm) extending to the wavelength region as long as 700 nm. Compared to the fluorescence intensity from the coexisting trans isomer, the oscillator strength of the cis fluorescence was evaluated as 0.17, which indicated that the S(1) state of cis-stilbene is an optically allowed state. (C) 2008 Elsevier B.V. All rights reserved.

Understanding a chemical reaction ultimately requires the knowledge of how each atom in the reactants moves during product formation. Such knowledge is seldom complete and is often limited to an oversimplified reaction coordinate that neglects global motions across the molecular framework. To overcome this limit, we recorded transient impulsive Raman spectra during ultrafast photoisomerization of cis- stilbene in solution. The results demonstrate a gradual frequency shift of a low- frequency spectator vibration, reflecting changes in the restoring force along this coordinate throughout the isomerization. A high- level quantum- chemical calculation reproduces this feature and associates it with a continuous structural change leading to the twisted configuration. This combined spectroscopic and computational approach should be amenable to detailed reaction visualization in other photoisomerizing systems as well.

Halorhodopsin is a retinal protein that acts as a light-driven chloride pump in the Haloarchaeal cell membrane. A chloride ion is bound near the retinal chromophore, and light-induced all-trans -> 13-cis isomerization triggers the unidirectional chloride ion pump. We investigated the primary ultrafast dynamics of Natronomonas pharaonis halorhodopsin that contains Cl-, Br-, or I- (pHR-Cl-, pHR-Br-, or pHR-I-) using ultrafast pump-probe spectroscopy with similar to 30 fs time resolution. All of the temporal behaviors of the S-n <- S, absorption, ground-state bleaching, K intermediate (13-cis form) absorption, and stimulated emission were observed. In agreement with previous reports, the primary process exhibited three dynamics. The first dynamics corresponds to the population branching process from the Franck-Condon (FC) region to the reactive (S-1(r)) and non-reactive (S-1(nr)) S, states. With the improved time resolution, it was revealed that the time constant of this branching process (tau(1)) is as short as 50 fs. The second dynamics was the isomerization process of the S-1(r) state to generate the ground-state 13-cis form, and the time constant (tau(2)) exhibited significant halide ion dependence (1.4, 1.6, and 2.2 ps for pHR-Cl-, pHR-Br-, and pHR-I-, respectively). The relative quantum yield of the isomerization, which was evaluated from the pump-probe signal after 20 ps, also showed halide ion dependence (1.00, 1. 14, and 1.35 for pHR-Cl-, pHR-Br-, and pHR-I-, respectively). It was revealed that the halide ion that accelerates isomerization dynamics provides the lower isomerization yield. This finding suggests that there is an activation barrier along the isomerization coordinate on the S, potential energy surface, meaning that the three-state model, which is now accepted for bacteriorhodopsin, is more relevant than the two-state model for the isomerization process of halorhodopsin. We concluded that, with the three-state model, the isomerization rate is controlled by the height of the activation barrier on the S, potential energy surface while the overall isomerization yield is determined by the branching ratios at the FC region and the conical intersection. The third dynamics attributable to the internal conversion of the S-1(nr) state also showed notable halide ion dependence (tau(3) = 4.5, 4.6, and 6.3 ps for pHR-Cl-, pHR-Br-, and pHR-I-). This suggests that some geometrical change may be involved in the relaxation process of the S-1(nr) state.

We carried out wavelength-dispersed time-resolved absorption measurements of cis-stilbene to investigate the mechanism of the appearance of the similar to 220 cm(-1) oscillation, which has been assigned to the nuclear wavepacket motion of the S, state. The observed oscillatory pattern showed almost same amplitude and phase across the absorption peak at 645 nm, which indicates that the modulation of the transition intensity gives rise to the quantum beat. We also carried out a semiquantitative numerical simulation of the time-resolved absorption spectra based on the effective linear response theory, in which we newly incorporated the Herzberg-Teller coupling model by introducing a coordinate-dependence of the transition moment. The results of these experiments and simulation clearly showed that the intensity of the quantum beat arises from a significant coordinate dependence of the S-n <- S-1 transition moment, i.e., non-Condon effect. It was concluded that the vibronic coupling of the S-n state with other electronic, states is so large that the Herzberg-Teller coupling predominantly contributes to the intensity of the quantum beat of the totally symmetric similar to 220 cm(-1) vibration. The present work suggests a general importance of the non-Condon effect in spectroscopy involving highly excited electronic states.

In copper(I) complex [Cu(dmphen)(2)](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), a "flattening" structural change is induced with (MLCT)-M-1 excitation, which is a prototype of the structural change accompanied with Cu(I)/Cu(II) conversion in copper complexes. Femtosecond and picosecond emission dynamics of this complex were investigated in solution at room temperature with optically allowed S-2 <- S-0 photoexcitation. Time-resolved emission was measured in the whole visible region, and the lifetimes, intrinsic emission spectra, and radiative lifetimes of the transients were obtained by quantitative analysis. It was concluded that the initially populated S-2 state is relaxed with a time constant of 45 fs to generate the S-1 state retaining the perpendicular structure, and the D-2d -> D-2 structural change (the change of the dihedral angle between the two ligand planes) occurs in the S-1 state with a time constant of 660 fs. The intersystem crossing from the S-1 state to the T-1 state takes place after this structural distortion with a time constant of 7.4 ps. Importantly, the temporal spectral evolution relevant to the structural change clearly exhibited an isoemissive point around 675 nm. This manifests that there exists a shallow potential minimum at the perpendicular geometry on the S-1 surface, and the S-1 state stays undistorted for a finite period as long as 660 fs before the structural distortion. This situation is not expected for the structural change induced by the ordinary (pseudo-)Jahn-Teller effect, because the distortion should be induced by the spontaneous structural instability at the perpendicular structure. This result sheds new light on the present understanding on the structural change occurring in the metal complexes.

The dynamics and mechanism of the double proton transfer reaction of the 7-azaindole dimer was investigated in solution by excitation wavelength dependence in steady-state and femtosecond time-resolved fluorescence spectroscopy. Femtosecond measurements in the UV region revealed that the dynamics of the dimer fluorescence exhibits remarkable change as the excitation wavelength was scanned from 280 to 313 nm. The fluorescence showed a biexponential decay (0.2 and 1.1 ps) with 280-nm excitation, whereas it exhibited a single exponential decay (1.1 ps) with 313-nm excitation (the red-edge of the dimer absorption). This observation clearly indicates that the 0.2-ps component is irrelevant to the proton transfer. In the visible region, we found that the tautomer fluorescence rises in accordance with the decay of the dimer fluorescence with a common time constant of 1.1 ps. This finding unambiguously denies the appearance of any intermediate species in between the dimer and tautomer excited states, indicating that the double proton transfer reaction is essentially a single-step process. We conclude that the double proton transfer of the 7-azaindole dimer in solution proceeds in the concerted manner from the lowest excited state with the 1.1-ps time constant. On the basis of the experimental data obtained, we discuss the long-lasting concerted versus step-wise controversy for the double proton transfer mechanism in solution.

Ultrafast dynamics of photo-induced structural change of [Cu(dmphen)(2)](+) was studied by femtosecond emission spectroscopy and 25-fs time-resolved pump-probe spectroscopy. It was found that the S-1 state remains undistorted for 660 fs before structural change, being trapped in a shallow potential minimum at the perpendicular configuration on the S-1 potential.

Vibrational coherence was generated in S-1 cis-stilbene at an optional delay time by the Raman process and the nuclear wavepacket motion was observed with 11-fs optical pulses. The frequency of the wavepacket motion exhibited gradual downshift with delaying timing of the creation, indicating highly anharmonic nature of the multi-dimensional S, potential and a structural change proceeding in a picosecond time-scale.

Ultrafast relaxation processes in the excited singlet states of all-trans retinal in cyclohexane solution was investigated by time-resolved absorption spectroscopy with 35-fs time resolution. The observed transient absorption signal consists of three components, including a fully time-resolved ultrafast component having a lifetime of 56 20 fs. The obtained result is consistent with a cascading electronic relaxation (S-3(pi pi*,B-u) -> S-2(pi pi*,A(g)) -> S-1(n pi*)) occurring after photoexcitation. The time-resolved absorption anisotropy was also measured, and it showed that the transition moments of the transient absorption from these three excited states are nearly parallel to the long axis of the molecule. (c) 2005 Elsevier B.V. All rights reserved.

Three fundamental ultrafast photochemical reactions in solution, i.e., photoisomerization of cis-stil-bene, photodissociation of diphenylcyclopropenone and excited-state intramolecular proton transfer of 10-hydroxybenzoquinoline, were studied by two color pump-probe spectroscopy with time-resolution as high as 30 - 70 fs. The coherent nuclear wavepacket motions of the excited-states that undergo ultrafast reactions were observed, and their relevance to the reaction was discussed. It was concluded that these reactions are classified to three different types for the relation between the initial nuclear wavepacket motion and the reaction coordinate.

The dynamics of the excited-state intramolecular proton transfer of 10-hydroxybenzo[h]quinotine (10-HBQ) and the associated coherent nuclear motion were investigated in solution by femtosecond absorption spectroscopy. Sub-picosecond transient absorption measurements revealed spectral features of the stimulated emission and absorption of the keto excited state (the product of the reaction). The stimulated emission band appeared in the 600-800-nm region, corresponding to the wavelength region of the steady-state keto fluorescence. It showed successive temporal changes with time constants of 350 fs and 8.3 ps and then disappeared with the lifetime of the keto excited state (260 ps). The spectral feature of the stimulated emission changed in the 350-fs dynamics, which was likely assignable to the intramolecular vibrational energy redistribution in the keto excited state. The 8.3-ps change caused a spectral blue shift and was attributed to the vibrational cooling process. The excited-state absorption was observed in the 400-600-nm region, and it also showed temporal changes characterized by the 350-fs and 8.3-ps components. To examine the coherent nuclear dynamics (nuclear wavepacket motion) in excited-state 10-HBQ, we carried out pump-probe measurements of the stimulated emission and absorption signals with time resolution as good as 27 fs. The obtained data showed substantially modulated signals due to the excited-state vibrational coherence up to a delay time of several picoseconds after photoexcitation. This means that the vibrational coherence created by photoexcitation in the enol excited state is transferred to the product. Fourier transform analysis indicated that four frequency components in the 200-700-cm(-1) region contribute to the oscillatory signal, corresponding to the coherent nuclear motions in excited-state 10-HBQ. Especially, the lowest-frequency mode at 242 cm(-1) dephased significantly faster than the other three modes. This observation was regarded as a manifestation that the nuclear motion of the 242-cm(-1) mode is correlated with the structural change of the molecule associated with the reaction (the reaction coordinate). The 242-cm(-1) mode observed in excited-state 10-HBQ was assigned to a vibration corresponding to the ground-state vibration at 243 cm-1 by referring to the results of resonance Raman measurements and density functional calculations. It was found that the nuclear motion of this lowest-frequency mode involves a large displacement of the OH group toward the nitrogen site as well as in-plane skeletal deformation that assists the oxygen and nitrogen atoms to come closer to each other. We discuss the importance of the nuclear wavepacket motion on a multidimensional potential-energy surface including the vibrational coordinate of the low-frequency modes.

Time-resolved stimulated emission of 10-hydroxybenzoquinoline was measured in solution with 27-fs resolution. It clearly showed beating feature due to the excited-state wavepacket motion during the proton transfer reaction. We discuss the coherent nuclear motion in the reactive excited state and its relation to the reaction coordinate.

Ultrafast photo isomerization reaction of cis-stilbene in solution was studied by time-resolved absorption spectroscopy with 40-fs resolution. Rapidly damping coherence of a similar to 220 cm(-1) vibrational mode was observed in the reactive excited state. The obtained data showed that the initial wavepacket motion rapidly dephases owing to the significant anhannonicity of the S-1 potential surface, and the isomerization proceeds after intramolecular vibrational energy redistribution.

Time-resolved absorption spectroscopy with similar to40 fs time resolution was applied to the S-1 state of cis-stilbene in solution. An oscillatory signal caused by similar to220 cm(-1) wavepacket motion was clearly observed. Its damping rate was much faster than the isomerization rate and insensitive to the change of the solvent. The obtained ultrafast pump-probe data showed that the dephasing of the initial coherent wavepacket motion first takes place rapidly, followed by the similar to1 ps isomerization reaction. The significance of the similar to220 cm(-1) mode, intramolecular vibrational energy redistribution (IVR) process, and high anharmonicity of the S-1 potential surface in the initial stage of cis-stilbene photochemistry is discussed. (C) 2004 Elsevier B.V. All rights reserved.

The conformational relaxation of excited-state 1, 1'-binaphthyl in solution was investigated by using femtosecond time-resolved absorption and time-resolved time-domain Raman spectroscopy. Femtosecond time-resolved absorption measurements confirmed that the first transient (the "unrelaxed" S, state) is generated in a subpicosecond time scale after photoexcitation and that it undergoes a relaxation with a time constant of 16 ps (in cyclohexane) to form the second transient (the "relaxed" S-1 state). Time-domain Raman experiments provided low-frequency Raman data of the relaxed S, state for the frequency region of 0-400 cm(-1). The obtained Raman data of the relaxed S, state were significantly different from the low-frequency So spectrum in solution. The relaxed S, spectrum resembles the spectrum of the crystalline form that has a dihedral angle of 69degrees, whereas the So Raman spectrum in solution coincides well with that of the crystalline form having a dihedral angle of 103degrees. The time-domain Raman measurements provided vibrational data that support a conformational change occurring in 1,1'-binaphthyl in the S-1 state.

Probing of the nuclear wavepacket motion generated by sufficiently short optical pulses enables one to observe the motion of nuclei in real time. This time-domain measurement of the nuclear motion has been receiving a great deal of attention in femtosecond spectroscopy. Especially, such measurements for ultrafast reactive systems give an opportunity to observe the coherent nuclear dynamics in the early stage of the reaction. It is very interesting to know the initial motion of nuclei and its relevance to the reaction coordinate, when the reactions are accompanied with a large structural change. Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give a clue to reveal its significance in chemical reactions. This chapter employs two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigates the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state.

Reaction dynamics and coherent nuclear motions in the photodissociation of diphenylcyclopropenone (DPCP) were studied in solution by time-resolved absorption spectroscopy. Subpicosecond transient absorption spectra were measured in the visible region with excitation at the second absorption band of DPCP. The obtained spectra showed a new short-lived band around 480 nm immediately after photoexcitation, which is assignable to the initially populated S-2 state of DPCP before the dissociation. The dissociation takes place from this excited state (the precursor of the reaction) with a time constant of 0.2 ps, and the excited state of diphenylacetylene (DPA) is generated as the reaction product. The transient absorption after the dissociation decayed with a time constant of 8 ps that is very close to the S-2-state lifetime of DPA, but the spectrum of this 8-ps component was different from the S-2 absorption observed with direct photoexcitation of DPA. We conclude that the dissociation of DPCP generates the S-2 state of DPA that probably has a cis-bent structure. At later delay times (>30 ps), the transient absorption signals are very similar to those obtained by direct photoexcitation of DPA. This confirmed that the electronic relaxation from the S-2 state of the product DPA occurs in a similar manner to that of DPA itself, i.e., the internal conversion to the S-1 state and subsequent intersystem crossing to the T-1 state. In order to examine the coherent nuclear dynamics in this dissociation reaction, we carried out time-resolved absorption measurements for the 480-nm band with 70 fs resolution. It was found that an underdamped oscillatory modulation with a 0.1-ps period is superposed on the decay of the precursor absorption. This indicates that DPCP exhibits a coherent nuclear motion having a similar to330-cm(-1) frequency in the dissociative excited state. Based on a comparison with the measured and calculated Raman spectra of ground-state DPCP, we discuss the assignment of the "330-cm(-1) vibration" and attribute it to a vibration involving the displacement of the CO group as well as the deformation of the Ph-C=C-Ph skeleton. We consider that this motion is closely related to the reaction coordinate of the photodissociation of DPCP. (C) 2004 American Institute of Physics.

We report a new femtosecond time-resolved fluorescence spectrometer that enables us to observe fluorescence intensity as a time-wavelength two-dimensional image in a single measurement. This method utilizes a time-to-space conversion technique and fluorescence sum-frequency mixing with a femtosecond gate pulse. It provides a fluorescence image covering temporal and spectral spans of similar to2 ps and similar to60 nm, respectively. Calibration of the time and intensity axes of the image is made by use of a long-lived dye fluorescence. The two-dimensional fluorescence image of beta-carotene obtained demonstrates the high potential of this method for quantitative studies of ultrafast excited-state dynamics. (C) 2004 Optical Society of America

Time-domain Raman measurement of the excited state of a polyatomic molecule was demonstrated for the first time. Time-resolved impulsive stimulated Raman scattering TR-ISRS) measurements were carried out for trans-stilbene in solution, and resonantly enhanced signals due to the S-1 state were observed under the resonance condition with the S-n <-- S-1 absorption. The observed signal consisted of a spike-like feature around the time origin, an oscillatory component with a period of similar to0.12 ps, and a slowly decaying traditional transient-grating (TG) signal. A Fourier transform analysis clarified that the oscillatory ISRS component was attributed to an in-plane bending vibration of S-i trans-stilbene (nu(24), 285 cm(-1)). The origin of the TG signals was examined by three-pulse absorption measurements, and it was concluded that the transient grating was created reflecting two relaxation processes following the S-n <-- S-1 excitation: the vibrational cooling process of S-1 trans-stilbene and the loss of the S-1 population. The present study demonstrated that time-resolved time-domain Raman spectroscopy can provide spectral information about low-frequency tetrahertz motions of the excited-state, which cannot be accessed by ordinary time-resolved frequency-domain Raman spectroscopy.

Vibrational coherence of the condensed-phase polyatomic molecules was studied by several types of femtosecond time-resolved spectroscopy. The Raman-active vibrational coherence in the ground-state was observed through optically-heterodyned impulsive stimulated Raman scattering (ISRS) spectroscopy. Transient ISRS experiment was also undertaken for the measurement of the low frequency vibrations of the excited state. In transient absorption spectroscopy carried out with a 40-fs time resolution, vibrational coherence was created by ultrashort pump pulses and the wavepacket motion in the excited state was observed for trans-stilbene and diphenylcyclopropenone.

Excitation-wavelength dependence of the ultraviolet fluorescence dynamics of 7-azaindole dimer was examined in solution by femtosecond up-conversion method. It was found that the fluorescence decay of the dimer excited state showed a significant wavelength dependence. It changes from a bi-exponential decay to a single-exponential decay, when we scanned the excitation wavelength from 280 toward 313 nm (the red-edge of the dimer absorption). This result demonstrates that the proton-transfer dynamics itself exhibits a single exponential behavior. The obtained fluorescence data deny the appearance of the intermediate species and strongly support the concerted mechanism of the double proton transfer reaction. (C) 2001 Elsevier Science BN. All rights reserved.

Ultrafast bimolecular reaction kinetics between photoexcited biphenyl and carbon tetrachloride, observed by femtosecond time-resolved fluorescence spectroscopy, is successfully interpreted by theories of diffusion-controlled reactions. The obtained decay kinetics is well explained by Smoluchowski's theory of diffusion-controlled reactions when the parameter R, distance between the reactants, is 0.39 nm. A modified kinetic theory by Collins and Kimball also explains the results satisfactorily, when R is 0.40 nm and the bimolecular reaction rate constant k(act) is 3.4 x 10(11) dm(3) mol(-1) s(-1). It is suggested that molecular motions in solution for a time period of a few picoseconds is described by diffusion. (C) 2001 Elsevier Science B.V. All rights reserved.

Femtosecond time-resolved fluorescence intensities of 1,8-dihydroxyanthraquinone (chrysazin) in hexane have been measured at room temperature for a wide visible wavelength region (470-670 nm) by using the up-conversion method. Time-resolved fluorescence spectra were reconstructed after deconvolution taking account of the finite instrumental response. It was found that both the 'normal-form type' fluorescence and the 'tautomeric-form type' fluorescence appear almost instantaneously, which indicates that a barrierless excited-state proton transfer occurs within 50 fs reflecting delocalization of the excited-state wave function. A fluorescence spectral change was also observed in a few picosecond time scale, which was assigned to an additional proton translocation induced by the intramolecular vibrational relaxation. (C) 2000 Elsevier Science B.V. All rights reserved.

Temporal behavior of the S-n <-- S-1 absorption of trans-stilbene was measured in solution with a = 40-fs time resolution. An underdumped oscillation due to the S-1 vibrational coherence was observed in the early times. The Fourier-transform analysis showed that the S-1 in-plane deformation mode (v(25), 200 cm(-1)) gives rise to the oscillatory signal. This mode-specificity results from significant displacements among the S-0, S-1, and S-n potentials along the corresponding vibrational coordinate. It is shown that the amplitude of the vibrational coherence signal is semi-quantitatively related to the intensities in the two frequency-domain spectroscopies, the S-1 <-- S-0 absorption and the S-1 resonance Raman. (C) 2000 Published by Elsevier Science B.V.

Femtosecond time-resolved absorption spectroscopy was employed to investigate the relaxation dynamics of photoexcited copper(II) tetrakis (4-N-methylpyridyl)porphyrin (Cu(II)(TMpy-pd)) in neat water. The transient absorption spectra and their temporal profiles show a strong pump-power dependence reflecting multiphoton ionization of water, which probably caused the discrepancies in the previous reports. Time-resolved transient absorption spectra were recorded under low pump-power condition to avoid the multiphoton effect. It was confirmed that the relaxation process finishes within 100 ps, exhibiting the double exponential recovery of Cu(II)(TMpy-P4) ground state. The relevant mechanism for the relaxation dynamics of photoexcited Cu(II)(TMpy-P4) in water was proposed. (C) 1999 Elsevier Science B.V. All rights reserved.

The electronic and vibrational relaxation of tetracene have been studied in solution by femtosecond time-resolved fluorescence spectroscopy. Tetracene was initially photoexcited to the highly excited singlet (S-n) state, B-1(b), and the dynamics of the fluorescence from the B-1(b) State and the L-1(a) state (S-1) were investigated by fluorescence up-conversion. The fluorescence from the 1Bb State was observed in the ultraviolet region, and its lifetime was determined as similar to 120 fs. The anisotropy of the B-1(b) fluorescence was close to 0.4, which assured that the fluorescence is emitted from the excited state that was prepared by photoexcitation. The visible fluorescence from the L-1(a) state showed a finite rise that agreed well with the decay of the B-1(b) fluorescence. Negative anisotropy was observed for the L-1(a) fluorescence, reflecting that the L-1(a) transition moment is parallel to the short axis of the molecule and hence perpendicular to the B-1(b) transition moment. The anisotropy of the L-1(a) fluorescence, however, showed a very characteristic temporal behavior in the femtosecond time region: it exhibited a very rapid change and reached a certain value that is deviated from -0.2. The anisotropy data indicate that the L-1(a) fluorescence contains not only a short-axis polarized component but also a long-axis polarized component and that the ratio between the two components depends on both time and wavelength. The long-axis polarized component in the L-1(a) fluorescence was assigned to the B-1(b)-type fluorescence that appears as the result of the vibronic coupling between the L-1(a) state and the B-1(b) state. The observed initial rapid change of the anisotropy suggests that the highly excited vibrational states in the L-1(a) state which are strongly coupled with the B-1(b) state are first populated preferentially when the molecule is relaxed from the B-1(b) State to the L-1(a) state. The visible fluorescence anisotropy vanishes gradually because of the rotational diffusion in a few tens of picoseconds. In the picosecond region, we also observed additional dynamics in the fluorescence intensity whose time constant was about 12 ps. This dynamics was assigned to the vibrational relaxation (cooling) in the L-1(a) state. A series of relaxation processes taking place after photoexcitation of the molecule in solution are discussed.

The dynamics of the excited-state proton-transfer reaction of 7-azaindole dimer has been investigated in hexane with use of the femtosecond fluorescence up-conversion method. Time-resolved measurements were performed in a wide fluorescence wavelength region from near-ultraviolet to visible (320-620 nm). Three fluorescence components due to the dimer were observed in addition to the fluorescence from coexisting monomer. Time-resolved fluorescence anisotropy measurements were also carried out, and the result indicated that the first (a = 0.2 ps) and the second (tau = 1.1 ps) fluorescence components due to the dimer arise from two different dimeric excited states having different transition moment directions. The decay of the second component agrees with the rise of the third component, which is attributable to the fluorescence from the tautomeric excited state (tau = 3.2 ns) formed by the proton-transfer reaction. The fluorescence spectra of these three excited states were reconstructed from time-resolved fluorescence traces taken at 27 wavelengths, and they show intensity maxima around 330, 350, and 490 nm, respectively. This sequential red shift reflects the cascaded population relaxation after the photoexcitation. By combining the spectral data with fluorescence quantum yield data, the oscillator strengths of the three excited states were evaluated as 0.13, 0.048, and 0.023. We assigned the higher- and the lower-energy dimeric excited states to the "L-1(b)" and "L-1(a)" states of the dimer on the basis of the obtained photochemical information. The deuterium substitution effects were also examined for two isotopic analogues. It was concluded that the proton transfer proceeds exclusively from the lowest "L-1(a)" excited state with a time constant of 1.1 ps, after the electronic relaxation takes place from the initially populated ("L-1(b)") state to the "L-1(a)" state. The excited-state reaction pathway as well as quantitative characterization of each excited state is discussed.

Femtosecond transient absorption of a poly(3-dodecylthiophene) film is measured in a wide spectral range from visible to near-infrared. Our results show that the transient absorption consists of four decay components. The two faster components are assigned to self-trapping and the subsequent relaxation process of photogenerated excitons. The relaxation time constant of the self-trapped exciton is evaluated as 680 +/- 50 fs. The excited species corresponding to the third slow component is identified as a pair of intrachain, oppositely-charged polarons on the basis of its power-law decay and its spectral peak around 1.4 eV, A long-lived transient absorption due to interchain polarons is also observed. (C) 1998 Elsevier Science B.V. All rights reserved.

Femtosecond time-resolved fluorescence of 7-azaindole dimer in hexane was measured at room temperature by using the up-conversion method. The measurements covering a wide visible wavelength region revealed that the fluorescence consists of two rapid decay components (tau(1) = 0.2 +/- 0.1 ps, tau(2) = 1.1 +/- 0.1 ps) and a long-lived component attributable to the tautomeric fluorescence (tau(3) = 3.2 ns and lambda(max) = 490 nm). The two decay components were assigned to the fluorescences from two dimeric excited states which are successively populated in the relaxation process prior to the double proton transfer reaction. The dynamics of the excited-state proton transfer as well as assignments of the dimeric excited states is discussed. (C) 1997 Elsevier Science B.V.

The femtosecond time-resolved fluorescence of all-trans-retinal in hexane was measured by using the fluorescence up-conversion method. It was found that the fluorescence consists of ultrafast (tau = 30 +/- 15 fs and lambda(max) approximate to m 430 nm), fast (tau = 370 +/- 20 fs and lambda(max) approximate to 440 nm), and slow (tau = 33.5 ps and lambda(max) approximate to 560 nm) components. These three components were assigned to fluorescences from the B-1(u), (1)A(g), and (1)n pi* states, respectively, which are successively populated during the relaxation process in the singlet: manifold after photoexcitation. The oscillator strengths of the three excited singlet states were determined to be 1.0 (B-1(u)), 0.024 ((1)A(g)), and 0.0018 ((1)n pi*) on the basis of the obtained lime-resolved fluorescence data. The state ordering in the singlet manifold, as well as quantitative characterization of the three low-lying excited singlet states, is discussed.

Visible to near-infrared transient spectra were measured for thin films of three substituted polyacetylenes with a time-resolution less than or equal to 300 fs. A hot self-trapped exciton (STE) and an oppositely charged, spatially confined soliton-antisoliton pair were temporally and spectrally resolved in detail, which reveals a formation process of the localized excitations with geometrical relaxation taking place-within a subpicosecond time scale. The hot STE showing an exponential decay (tau=115-135 fs) has a spectral peak in the energy region of 1.4-1.5 eV. The transition energies from the hot STE both to continuum state and to a biexciton state are discussed with referring to strength of the Coulomb interactions between the conjugated pi electrons. The soliton-antisoliton pair which decays with a power-law behavior has a dual-peaks spectrum below the band gap energy and the two peak energies vary depending on the polymers. A pi-conjugation length (lambda(c)), a soliton size (xi), and a distance (d) between the soliton and antisoliton were evaluated based on the experimental results for each polymer. The distance was found to be nearly proportional to the conjugation length with a ratio of d/lambda(c)=0.4, indicating that an overall size of the soliton-antisoliton pair approximatedly given by 2 xi+d is limited just within a segmented conjugation chain. The decay kinetics of photoexcitations in both degenerate and nondegenerate systems has been discussed together using an adiabatic potential surface in a configuration space. (C) 1996 American Institute of Physics.

Near-infrared (0.52-1.37 eV) transient spectra of a substituted polyacetylene, poly[[(o-trimethylsilyl)phenylene]acetylene], were measured with a time-resolution better than 300 fs. Whole spectra of hot self-trapped excitons and relaxed, overall neutral pairs of oppositely charged, spatially confined soliton-antisoliton were measured deep in the band gap. The confinement parameter gamma = 0.23 +/- 0.07 and on-site Coulomb repulsion U-o = 2.8 +/- 0.4 eV were determined from the results.

A slope efficiency of as high as 30% has been attained in a pulse-pumped Ti:sapphire regenerative amplifier up to a 3-kHz repetition rate. Pulse energy of 0.46 mJ was typically obtained at 800 nm when pumped by 50-ns green pulses with 2-mJ energy. Self-focusing (lensing) effect and gain saturation act cooperatively in the final stage of amplification, limiting the amplification efficiency. Numerical calculation including the two effects has revealed the optimal cavity-design for a regenerative amplifier.

A broadband near-infrared pulse covering 700 nm around 2.4 mum has been generated by a dispersion-free parametric mixing in a KTiOPO4 crystal between the fundamental pulse (0.8 mum) from a Ti:sapphire regenerative amplifier and the white continuum pulse. It is tunable within the wavelength region of 1.7-2.9 mum. The comparison with a theoretical bandwidth indicates that the noncollinear phase matching plays a part for the spectral broadening.

Formation time of a soliton-antisoliton pair has been determined as 98 +/- 8 fs experimentally by femtosecond transient spectroscopy performed on a thin film of poly[(o-trimethylsilyl)phenylacetylene], a substituted polyacetylene. It agrees favourably with results of the numerical simulation based on the Su-Schrieffer-Heeger model. A temporal behavior of photoinduced absorption was fitted successfully by a sum of an exponentially and a power-law decaying components. This allowed one to determine separately the spectrum of the soliton-antisoliton pair and that of an exciton-polaron created by an intrachain photoexcitation.