Browsing by Author "Xue, Bing"

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Coherent phonon coupled with exciton in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser
(2014-08-01)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Du, Juan
;
Xue, Bing
;
電子物理學系
The vibrational wavepackets dynamics of single-walled carbon nanotubes (SWCNTs) are studied through the modulation of the transition probability in the visible spectral range of the systems. The modulations corresponding to the radial breathing mode (RBM), observed in the time traces for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed. The vibrational modes of the coherent phonon spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. The present study pointed out that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to derivative type dependence of the absorbed photon energy spectrum due to molecular-phase modulation, Raman (and Raman-like) gain and loss processes, and molecular phase modulation, respectively. (C) 2014 Elsevier B.V. All rights reserved.
Exploring the ultrafast dynamics of a diarylethene derivative using sub-10 fs laser pulses
(2019-01-07)
Jarota, Arkadiusz
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Pastorczak, Ewa
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Tawfik, Walid
;
Xue, Bing
;
Kania, Rafal
A diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DMP), is a photoswitch molecule utilizing a reversible aromatic ring-opening reaction. The quantum yield of the ring-opening reaction is however remarkably low. We investigate the origin of this behaviour by means of ultrafast transient absorption spectroscopy utilizing sub-10 fs pulses, which is an invaluable tool for simultaneously studying both the electronic and the vibrational molecular dynamics. Namely, a noncollinear optical parametric amplifier (NOPA) generating sub-10 fs pulses in the spectral range 605-750 nm is employed. The transient absorption signal is modulated by several vibrational modes, which are compared with experimental and computational Raman spectra and then assigned to the ground or excited electronic state. We observe that the most pronounced vibrational mode - the ethylenic stretching mode at a frequency of 1501 cm(-1) - exhibits instantaneous frequency and amplitude modulation. The observed modulations occur due to weak coupling with another 1431 cm(-1) stretching mode mediated by a vibrational mode of low frequency, i.e. around 60 cm(-1). Fast internal conversion S-1 S-0 originates in a relaxation through a conical intersection (found by density-functional theory computations), facilitated by the two aforementioned stretching modes.
Femtosecond electronic relaxation and real-time vibrational dynamics in 2 '-hydroxychalcone
(2019-03-14)
Yamakita, Yoshihiro
;
Yokoyama, Nanae
;
Xue, Bing
;
Shiokawa, Naoyuki
;
Harabuchi, Yu
Femtosecond ultrafast electronic relaxation and vibrational dynamics in 2'-hydroxychalcone after deep ultraviolet (DUV) excitation were observed by two types of pump-probe spectroscopy experiments, i. e., DUV-pump pulse and visible-broadband-probe pulse (DUV/Vis) experiments and DUV-pump and DUV-probe (DUV/DUV) pulse experiments. Time-dependent density functional theory (TDDFT) calculations were performed to elucidate relaxation dynamics from the third singlet electronic excited state S-3. The DUV/Vis experiments and TDDFT calculations have disclosed the ultrafast dynamics of internal conversion from the initial S-3 state (tau(1) approximate to 35 fs) to the S-1 state via a rapid process through the S-3/S-2 conical intersection and proton transfer [OH: tau(2)(H) approximate to 93 and OD: tau(2)(D) approximate to 164 fs] before deactivation through the S-1/S-0 conical intersection (tau(3) approximate to 690 fs). Thanks to the ultrashort pump and probe pulses, real-time observation of vibrational modes coupled to the electronic excitation was realized providing both amplitudes and phases. Spectrogram analyses were performed based on the real-time spectra obtained by the DUV/Vis experiments, in which instantaneous vibrational frequencies reflecting molecular structural change after the impulsive excitation were visualized. The vibrational frequency of central C=C bond stretch decreases from similar to 1600 cm(-1) to similar to 1560 cm(-1) in about 200-500 fs and recovers in similar to 550 fs. Normal mode analyses along the decay path support the observed variation of the C=C stretching frequency. The temporal weakening of the central C=C bond is connected with the angle of the two aromatic rings which flip back to the initial conformation.
Real-Time Spectroscopy of Single-Walled Carbon Nanotubes for Negative Time Delays by Using a Few-Cycle Pulse Laser
(2014-02-13)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Xue, Bing
;
Kataura, Hiromichi
;
Sakakibara, Youichi
Several investigations using the femtosecond pump-probe technique have been made for coherent phonon dynamics in single-walled carbon nanotubes (SWNTs), which are promising candidates as devices for nanoelectronics and photonics due to their unique properties. However, the most relevant spectroscopic analyses by far have been performed in the positive time range, with the pump pulse coming before the probe. We studied the real-time coherent phonon in SWNTs using 7.1-fs pump-probe measurements for negative time delays in which vibronic coherence between the ground state and exciton state can be observed. Coherent vibrations corresponding to the radial breathing mode caused by the wave packet generated in the exciton state were detected, in contrast to the ground-state wave packet dynamics for positive delays previously reported. The present results show that excited-state wave packets contribute greatly to radial breathing modes in four, (6,5), (6,4), (7,5), and (8,3), chiral systems, but very little to G modes, even though the oscillation period of the latter is 20 fs, which is much shorter than the 100 fs of the exciton life. We discussed the differences in dephasing times in the four different chiral systems.
Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer
(2014-08-14)
Nakata, Kazuaki
;
Tokunaga, Eiji
;
Du, Juan
;
Xue, Bing
;
Miyazaki, Jun
K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having T-c = 395 K (= and the crystal has T-c = 395 K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 cm(-1). This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.
Ultrafast dynamics of uracil and thymine studied using a sub-10 fs deep ultraviolet laser
(2016-07-07)
Xue, Bing
;
Yabushita, Atsushi
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Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Single 9.6 fs deep ultraviolet pulses with a spectral range of 255-290 nm are generated by a chirpedpulse four-wave mixing technique for use as pump and probe pulses. The electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of uracil and thymine over the full spectral range using a 128-channel lock-in amplifier detector. Two probe photon energy-dependent lifetimes gradually increasing with the probe photon energy are obtained from the decay dynamics data. Ultrafast decay dynamics through the conical intersection is assigned from the first excited pi pi* to the final ground state involving the n pi* states. Vibrational modes of the electronic ground state and excited states can be observed, which are strongly coupled to the decay dynamics of the electronic excited state.