Browsing by Author "Wang, Zhuan"

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Chirped modulation of molecular vibration in quinoidal thiophene after sub-5 fs excitation
(2006-10-19)
Wang, Zhuan
;
Otsubo, Tetsuo
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Fourier transformation of the real-time trace of sub-5 fs pump-induced molecular vibration in quinoidal thiophene in solution revealed coupled modes to the electronic excitation at 206, 447, and 1466 cm(-1). Spectrogram analysis of 1466 cm(-1) (C=C stretching) mode showed that the amplitude and frequency of the mode are modulated with another low-frequency mode at about 220 cm(-1), which is down chirped being induced by the coupling with modes of even lower frequency, resulting in the irreversible relaxation due to high mode density in the low-frequency range. This Letter thus reports on observation of the modulation of vibrational frequency with a chirped frequency. (c) 2006 Elsevier B.V. All rights reserved.
Circa conservation of vibrational energy among three strongly coupled modes of a cyanine dye molecule studied by quantum-beat spectroscopy with a 7 fs laser
(2008-11-24)
Kobayashi, Takayoshi
;
Wang, Ying
;
Wang, Zhuan
;
Iwakura, Izumi
;
電子物理學系
Real-time energy exchange among the three vibrational modes forming a triplet with frequencies omega(1), omega(2), and omega(3) satisfying a 'circa sum-frequency relation', namely omega(1) + omega(2) congruent to omega(3) in a cyanine dye molecule has been studied with a 7-fs pulse laser. There are two triplets with average frequencies of the components of (146, 279, 432 cm (1)) and (279, 432, 738 cm (1)). Even though the circa sum-frequency relation is satisfied, circa- conservation of the vibrational energy holds among the three components only in the latter triplet. The difference between the two is ascribed to the symmetry of the molecular vibrational modes. (C) 2008 Elsevier B. V. All rights reserved.
Classification of dynamic vibronic couplings in vibrational real-time spectra of a thiophene derivative by few-cycle pulses
(2007-12-20)
Kobayashi, Takayoshi
;
Wang, Zhuan
;
Otsubo, Tetsuo
;
電子物理學系
;
Department of Electrophysics
Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.
Confined breather-type excitation in a quinoidal thiophene after sub-5 fs pulse excitation
(2006-07-28)
Kobayashi, Takayoshi
;
Wang, Haibo
;
Wang, Zhuan
;
Otsubo, Tetsuo
;
電子物理學系
A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5 fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the C = C stretching was observed for the first time. It generates two sidebands of 2238 and 700 cm(-1) with a separation of 769 cm(-1) corresponding to the modulation frequency. The latter mode is very close to the C-S-C ring deformation, which indicate that this mode mediates coupling between the breather mode and the C-beta-C-beta stretching mode. It was also found that the breather-type excitation has a longer lifetime (2-3 ps) than in all-trans-polyacetylene (50 fs) due to its confinement. (c) 2006 American Institute of Physics.
Correlations of instantaneous transition energy and intensity of absorption peaks during molecular vibration: toward potential hyper-surface
(2008-06-30)
Kobayashi, Takayoshi
;
Wang, Zhuan
;
電子物理學系
;
Department of Electrophysics
Time-resolved spectrum after ultrashort pulse excitation revealed fine structure of instantaneous vibronic absorption spectra in a thiophene derivative. The probe photon energy-dependent amplitudes of molecular vibration coupled to the induced absorption were composed of several peaks. An absorbance-change peak-tracking method revealed four vibronic transitions buried in the time-integrated spectra over several vibrational periods of typical molecular vibration. Four vibronic transitions located at 2.024, 1.921, 1.818 and 1.731 eV were found to be correlated among themselves with respect to the photon energies and intensities of the peaks in the difference absorbance change spectra. From the size and sign of the correlation strengths the mechanism of the vibronic coupling was related to non-Condon mechanism and Herzberg-Teller vibronic coupling.
Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces
(2011-05-01)
Teramoto, Takahiro
;
Du, Juan
;
Wang, Zhuan
;
Liu, Jun
;
Tokunaga, Eiji
We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1', 3'-dihydro-1', 3', 3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2'-(2H)-indole] in a photochromic state was demonstrated. (C) 2011 Optical Society of America
Electronic relaxation and vibrational dynamics in a thiophene oligomer studied under the same experimental condition with a sub-5 fs laser
(2008-12-16)
Wang, Zhuan
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Using a sub-5 fs pulse laser, the pump-probe experiment was performed on a quinoid thiophene derivative and both electronic relaxation and vibrational dynamics were clarified with the same experimental data. From the data, two population decay times of the lowest electronic excited state were determined to be 200 fs and 1.8 ps, which can be explained by the assumption that the 2(1)A(g) state lies below the exciton 1(1)B(u) state together with the very low fluorescence quantum efficiency. The relaxation process after excitation by the pump pulse was studied to be 1(1)B(u) -> 1(1)B(u)* -> 2(1)A(g) -> 1(1)A(g). According to the experimental data, the location of the energy level was also discussed. The electronic dephasing time was determined to be 64 +/- 4 fs by utilizing the data in the 'negative time' range. The vibrational dephasing time constant of the most strongly coupled mode with frequency of 1466 cm(-1) was determined to be 520 +/- 20 fs from the widths of the corresponding Fourier power spectra.
Non-Condon vibronic coupling of coherent molecular vibration in MEH-PPV induced by a visible few-cycle pulse laser
(2009-01-26)
Kobayashi, Takayoshi
;
Zhang, Jun
;
Wang, Zhuan
;
電子物理學系
;
Department of Electrophysics
Vibrational real-time spectra of poly-[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH- PPV) were measured in a 5 fs pump-probe experiment simultaneously at 128 probe wavelengths with a multi-channel detection system. The spectral dependence of the coherent vibrational amplitudes obtained from the Fourier transform (FT) on the probe wavelength detected was found to be given by the sum of the ground-state absorption spectrum and its first and second derivatives. This indicates that the change of the transition probability caused by a wave packet motion can be explained as induced by both the non-Condon effect (non-Condon (NC) mechanism) and the time-dependent Franck-Condon factor (Frank-Condon (FC) mechanism). The FC mechanism can contribute to the first and second derivatives' dependence. On the other hand, the NC mechanism is dominant in the zeroth-order derivative. This result proves that the 1(1)B(u) exciton is strongly coupled with the excited (1)A(g) state, which is known to be essential in third-order optical nonlinearity. The amounts of shift of the absorption peaks and changes in the bandwidth due to the wave packet motions were determined for the four most prominent modes in the FT power spectra. The shift due to the FC mechanism was about 1.3-1.1% of the peak transition energy and the broadening of the vibronic transition due to the NC mechanism was about 8.0-7.6% of the bandwidth for the four modes. A novel ultrafast optical switch utilizing the modulation of electronic transition probability by molecular vibration through vibronic coupling and the interference of the wave packets is proposed.
Real-time vibrational amplitude change in the ground and excited states of a quinoid thiophene induced by few-cycle pulses
(2007-10-25)
Wang, Zhuan
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Several vibrational modes impulsively excited by 6.7 fs pulses were observed in a quinoid thiophene. From the amplitude decay time and the phase of vibration, the modulation mechanism of the electronic transition by the wave packet motion is discussed and classified in terms of vibronic coupling. Modes with wavenumbers of 1343 and 1420 cm(-1) are attributed to the wave packet motions on the potential surface of the excited and ground state, respectively, and a mode with 1539 cm(-1) is due to those of both states. The apparent low-frequency modulation of these three modes is ascribed to the artificial interference effect in the calculation of the spectrogram. (C) 2007 Elsevier B.V. All rights reserved.
Simultaneous measurement of electronic and vibrational dynamics to clarify a geometrical relaxation process in a conjugated polymer
(2008-05-01)
Du, Juan
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Wang, Zhuan
;
Feng, Wei
;
Yoshino, Katsumi
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Kobayashi, Takayoshi
The real-time vibrational spectroscopy was utilized to obtain both electronic and vibrational dynamics in a conjugated polymer under the same excitation and probing condition by using the same sample and observing at the same time. Most of the conjugated polymers are in an amorphous phase, in which there is a relatively broad distribution of chain length, conjugation length, and degree of interaction strength between neighboring chains. Because of the varieties, a simultaneous measurement of electronic and vibrational dynamics in a broad spectral range is considered to be most powerful to study such systems as conjugated polymers. In the present paper, we performed this type of experiment of the simultaneous probing of electronic and vibrational relaxations at 128 wavelengths. The sample studied here is an amino-moiety-containing conjugated polymer, poly{[3-hexylthiophene-2,5-diyl]-[p-dimethylaminobenzylidenequinoidmethene]} (PHTDMABQ), whose monomer is a derivative of a thiophene oligomer. The light source is a few-cycle pulse laser with an ultimate time resolution of 0.2 fs in the visible and near IR ranges. The data containing both electronic relaxation and vibrational dynamics were analyzed to obtain a more reliable relaxation mechanism of the excitations than a combination of individual studies of electronic and vibrational relaxations. By utilizing the ultrafast pump-probe spectroscopic system, we could identify the exciton state with a pi-electron delocalized in benzilidene ring. In the polymer PHTDMABQ, the geometrical relaxation from the free exciton to the exciton polaron takes place in 60-100 fs. This time is close to that observed in several polydiacetylenes previously reported. The C-C single bond stretching frequencies of the free exciton and exciton polaron are about 1300 and 1350 cm(-1). The coherent molecular vibration after the geometrical relaxation is considered to be a kind of reaction induced coherence. This coherent vibration decays with the time constant of about 450 fs.
Spectral Oscillation in Optical Frequency-Resolved Quantum-Beat Spectroscopy With a Few-Cycle Pulse Laser
(2008-11-01)
Kobayashi, Takayoshi
;
Wang, Zhuan
;
電子物理學系
;
Department of Electrophysics
Impulsive stimulated resonant Raman scattering was induced by a 5.7 fs visible pulse from a non-collinear optical parametric amplifier. The pulse duration corresponded to 2.8 cycles of the central wavelength of the laser spectrum at 610 nm. The induced spectral change was time-resolved with a 0.8 fs delay time step and the effective absorbance change Delta A (lambda, t) was calculated from the normalized transmittance change Delta T(lambda, t)/T. The observed optical and vibrational 2-D spectrum was analyzed to separate the electronic and vibrational contributions to the transient difference absorption spectrum. The probe wavelength dependence of the vibrational amplitude was explained in terms of the coupling between the laser field, Stokes field, and the vibrational coordinates. Simultaneous measurement of both the time and wavelength dependence of the vibrational amplitudes clearly showed that the broadband spectra of the components corresponding to the laser and Stokes lights are oscillating in pi out of phase with the corresponding vibrational frequency. The probe delay time and probe wavelength dependence of the oscillation in the absorbance change induced by the femtosecond laser were fully analyzed. The results were explained by the imaginary and real part of the third-order susceptibility, respectively.
Sub-5-fs spectroscopy of a thiophene derivative with a quinoid structure
(2006-07-26)
Kobayashi, Takayoshi
;
Wang, Haibo
;
Wang, Zhuan
;
Otsubo, Tetsuo
;
電子物理學系
Sub-5-fs spectroscopy of a thiophene derivative with an electron donative and an acceptive moieties and quinoid structure provided the experimental evidence of dynamic mode coupling. It was shown that two out-of-plane bending modes mediate the dynamic mode coupling between 1469 and 1379 cm(-1) and between 1603 and 1469 cm(-1). These couplings are considered to be associated with geometrical relaxation relevant to the neutral bipolaron formation in all-s-trans polyacetylene. (c) 2006 Elsevier B.V. All rights reserved.
The observation of strong dynamic electron-phonon couplings due to a breather state in trans-polyacetylene
(2009)
Teramoto, Takahiro
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Wang, Zhuan
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Kobryanskii, Valerii M.
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Taneichi, Takashi
;
Kobayashi, Takayoshi
The dynamics preceding the separation process of a charged soliton pair after photoexcitation in trans-polyacetylene was successfully time-resolved using ultrafast spectroscopy with a few-optical-cycle laser. It was directly verified that after photoexcitation, the electron-hole pair relaxes with a breather mode as theory predicts with an electron-hole pair lifetime of 33-50 fs, which is in agreement with the electronic dephasing time. By applying the spectrogram analysis to the time trace of the absorbance change, the ultrafast amplitude and frequency modulations of C-C and C=C stretching modes lasting only shorter than 100 fs induced by breather modes can be observed simultaneously for the first time. The frequency shift of C-C and C=C stretching modes caused by the breather mode are observed in good agreement with the SSH Hamiltonian. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The relation between the symmetry of vibrational modes and the potential curve displacement associated with electronic transition studied by using real-time vibrational spectroscopy
(2008-06-30)
Kobayashi, Takayoshi
;
Wang, Zhuan
;
Iwakura, Izumi
;
電子物理學系
;
Department of Electrophysics
The relation between the vibronic coupling strength of the vibronic state combined with the optical transitions with short pulses and the symmetry of the coupled vibrational modes was studied by using multi-wavelength vibrational real-time spectroscopy. A pump-probe experiment of thiophene oligomers using 6.7 fs pulsed laser as an ultrashort pulse light source and a multi-channel lock-in amplifier as a broadband detector was performed to obtain simultaneously the real-time traces at 128 wavelengths. At all wavelengths, the absorbance changes were plotted against pump-probe delay time, which was between-200 and 2000 fs. The vibronic coupling in both the ground and excited states was classified into two types: the displaced (D) type with displaced potential surfaces and the non-displaced (ND) type with non-displaced potential surfaces between the two electronic studies. The D and ND types were found to be related to inversion anti-symmetric and symmetric molecular vibrations, respectively, with respect to the center of the pseudo-inversion symmetry of the molecule. The symmetry center is located at the middle point of the C=C bond connecting the two quinoid-type thiophene rings. It was shown that multi-wavelength measurement of the vibrational real-time spectroscopy provides information on the mechanism of vibronic coupling in the process of transition from the ground state to the excited state and that between the two excited states.
Ultrafast real-time vibronic coupling of a breather soliton in trans-polyacetylene using a laser pulse with few cycles
(2009-01-01)
Teramoto, Takahiro
;
Wang, Zhuan
;
Kobryanskii, Valerii M.
;
Taneichi, Takashi
;
Kobayashi, Takayoshi
Simultaneous modulations of both frequencies and amplitudes of C-C and C=C stretching vibration modes, due to dynamical coupling to a breather soliton, in trans-polyacetylene was successfully time resolved. The frequency shifts of both modes were in good agreement with a simulation based on the Su-Schrieffer-Heeger model. It was found that the intensities of transition dipoles changed due to the breather, whereas transition energies were dominantly modulated by C=C stretching mode as recent theoretical work predicted.
Vibrational fine structures revealed by the real-time vibrational phase and amplitude in MEH-PPV using few cycle pulses
(2008-04-01)
Zhang, Jun
;
Wang, Zhuan
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
It was found that the vibronic transition peaks hidden in a featureless spectrum of induced absorption could be clearly revealed by utilizing the spectra of phases and amplitudes of the molecular vibrational modes. Some of the peaks were also found in a second derivative of a transition spectrum integrated over a delay time from 100 to 700 fs. This is because of the localization of a wave packet in a limited region along the potential multimode hypersurfaces. The transition energy of an induced absorption or an induced emission corresponds to some localized point (space) on the potential hypersurface to which the wave packets visit with some fixed phases.