Browsing by Author "Feng, Wei"

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Excited-state molecular vibration observed for a probe pulse preceding the pump pulse by real-time optical spectroscopy
(2008-07-18)
Kobayashi, Takayoshi
;
Du, Juan
;
Feng, Wei
;
Yoshino, Katsumi
;
電子物理學系
It is shown experimentally that the absorbance change observed in the "negative" time range, where probe pulse precedes pump pulse in real-time vibrational spectroscopy is induced only by the excited-state wave-packet motion as theoretically expected. Coherent molecular vibration of a polymer in the excited state was observed in the real-time trace without the effect of wave-packet motion in the ground state, which usually makes it difficult to ascribe the signal either to the ground state or to the excited state.
Observation of breather and soliton in a substituted polythiophene with a degenerate ground state
(2011)
Kobayashi, Takayoshi
;
Du, Juan
;
Feng, Wei
;
Yoshino, Katsumi
;
Tretiak, Sergei
The ultrafast dynamics of a unique polythiophene derivative that has a degenerate ground state due to a quantum mechanical resonance was investigated using a sub-5-fs pulse laser. The method allowed us to study the electronic relaxation and vibrational dynamics in completely same conditions at the same time. The dynamics of a breather composed of a dynamic bound state of solitons generated immediately after photoexcitation was time-resolved to reveal coupling between the vibrational modes and the solitons. The C-C and C=C stretching modes were found to be modulated by the breather, whose lifetime was determined to be 30-50 fs. The results of quantum-chemical excited-state molecular dynamics simulation are consistent with experimental results. Our modeling results allow to identify related vibrational normal modes strongly coupled to the electronic degrees of freedom. Moreover, analysis of calculated trajectories of excited state shows appearance of short-lived breather excitation decaying due to intramolecular vibrational energy equilibration on a timescale of hundreds of femtoseconds, which also agrees well with the experimental results. [GRAPHICS] Contour maps of the two-dimensional Fourier power of the vibrational components obtained by spectrogram calculation for real-time data covering 680-690 nm (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Observation of breather excitons and soliton in a substituted polythiophene with a degenerate ground state
(2010-02-01)
Kobayashi, Takayoshi
;
Du, Juan
;
Feng, Wei
;
Yoshino, Katsumi
;
Tretiak, Sergei
We investigated the ultrafast dynamics of a unique polythiophene derivative that has a degenerate ground state due to an alternating benzenoid-quinoid resonance. We probed 128 different wavelengths at the same time by using a sub-5-fs pulse laser and a detection system composed of a polychromator and a multichannel lock-in amplifier. The method allowed us to study the electronic relaxation and vibrational dynamics in completely same conditions at the same time. Because the polythiophene derivative has degenerate ground state, solitons are expected to be generated after photoexcitation. The dynamics of a breather composed of a dynamic bound state of solitons generated immediately after photoexcitation was time-resolved to reveal coupling between the vibrational modes and the solitons. The C-C and C=C stretching modes were found to be modulated by the breather, whose lifetime was determined to be 30-50 fs. The results of quantum-chemical excited-state molecular dynamics simulation are consistent with experimental results. Our modeling results allow to identify related vibrational normal modes strongly coupled to the electronic degrees of freedom. Moreover, analysis of calculated trajectories of excited state shows appearance of short-lived breather excitation decaying due to intramolecular vibrational energy equilibration on a time scale of hundreds of femtoseconds, which also agrees well with the experimental results.
Simultaneous measurement of electronic and vibrational dynamics to clarify a geometrical relaxation process in a conjugated polymer
(2008-05-01)
Du, Juan
;
Wang, Zhuan
;
Feng, Wei
;
Yoshino, Katsumi
;
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.