Browsing by Author "Liang, Kuo Kan"

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Algorithmic decoherence time for decay-of-mixing non-Born-Oppenheimer dynamics
(2008-07-14)
Cheng, Shu Chun
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Zhu, Chaoyuan
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Liang, Kuo Kan
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Lin, Sheng Hsien
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Truhlar, Donald G.
The performance of an analytical expression for algorithmic decoherence time is investigated for non-Born-Oppenheimer molecular dynamics. There are two terms in the function that represents the dependence of the decoherence time on the system parameters; one represents decoherence due to the quantum time-energy uncertainty principle and the other represents a back reaction from the decoherent force on the classical trajectory. We particularly examine the question of whether the first term should dominate. Five one-dimensional two-state model systems that represent limits of multidimensional nonadiabatic dynamics are designed for testing mixed quantum-classical methods and for comparing semiclassical calculations with exact quantum calculations. Simulations are carried out with the semiclassical Ehrenfest method (SE), Tully's fewest switch version (TFS) of the trajectory surface hopping method, and the decay-of-mixing method with natural switching, coherent switching (CSDM), and coherent switching with reinitiation (CSDM-D). The CSDM method is demonstrated to be the most accurate method, and it has several desirable features: (i) It behaves like the representation-independent SE method in the strong nonadiabatic coupling regions; (ii) it behaves physically like the TFS method in noninteractive region; and (iii) the trajectories are continuous with continuous momenta. The CSDM method is also demonstrated to balance coherence well with decoherence, and the results are nearly independent of whether one uses the adiabatic or diabatic representation. The present results provide new insight into the formulation of a physically correct decoherence time to be used with the CSDM method for non-Born-Oppenheimer molecular dynamic simulations. (c) 2008 American Institute of Physics.
Theoretical treatment of anharmonic effect on molecular absorption, fluorescence spectra, and electron transfer
(2009-03-30)
Zhu, Chaoyuan
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Liang, Kuo Kan
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Hayashi, Michitoshi
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Lin, Sheng Hsien
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應用化學系分子科學碩博班
It is well-known that the mirror image between absorption and fluorescence spectra is held for the displaced harmonic-oscillator system, and also this mirror image is independent to the chiral symmetry in which the excited-state potential energy surface is right-handed or left-handed with respect to the ground-state potential energy surface. As the first-order approximation of anharmonic correction is added into the displaced harmonic oscillator, this mirror image is broken down, and then the spectra can be depended on the chiral symmetry mentioned above. Both absorption and fluorescence coefficients are derived analytically within the first-order anharmonic approximation and numerical test is carried out to demonstrate the breaking down of the mirror image. Based on the same analysis, the electron transfer rate is derived analytically within the first-order anharmonic approximation. This rate might take the form of Arrhenius's equation but not form of Marcus's equation. Furthermore, it is found that this rate is also depending on the chiral symmetry. (c) 2009 Elsevier B.V. All rights reserved.
Theoretical treatments of ultrafast electron transfer from adsorbed dye molecule to semiconductor nanocrystalline surface
(2006-10-21)
Liang, Kuo Kan
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Lin, Chih-Kai
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Chang, Huan-Cheng
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Hayashi, Michitoshi
;
Lin, Sheng Hsien
In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system. (c) 2006 American Institute of Physics.
Theory of time-resolved sum-frequency generation and its applications to vibrational dynamics of water
(2007-09-20)
Hayashi, Michitoshi
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Shiu, Ying-Jen
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Liang, Kuo Kan
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Lin, Sheng Hsien
;
Shen, Yuan Ron
A molecular theory of time-resolved sum-frequency generation (SFG) has been developed. The theoretical framework is constructed using the coupled-oscillator model in the adiabatic approximation. This theory can treat not only the vibrational spectroscopy but also vibrational dynamics. An application of this theory is also provided for estimation of the time constants of the intermolecular vibrational energy transfer between water molecules. This approach can be used for molecular analysis of the experimental results of Shen at al. on the SFG studies of vibrational dynamics of water.