• 한국진공학회:학술대회논문집
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• 한국진공학회 2010년도 제39회 하계학술대회 초록집
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• pp.93-93
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• 2010

In this talk, I will introduce a reactive force field (ReaxFF) molecular dynamics (MD) simulation. In contrast to common MD simulations with empirical FFs, we can predict chemical reactions (bond breaking and formation) in large scale systems with the ReaxFF simulation where all of the ReaxFF parameters are from quantum mechanical calculations such as density functional theory to provide high accuracy. Accordingly, the ReaxFF simulation provides both accuracy of quantum mechanical calculations and description of large scale systems of atomistic simulations at the same time. Here, I will first discuss a theory in the ReaxFF including the differences from other empirical FFs, and then show several applications for studying chemical reactions of SiHx radicals on Si surfaces, which is an important issue in Si process.

• Bulletin of the Korean Chemical Society
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• 제30권4호
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• pp.857-862
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• 2009

We model lattice-mismatched group III-V semiconductor $In_{x}Ga_{1-x}$ alloys with the three-parameter anharmonic Kirkwood-Keating potential, which includes realistic distortion effect by introducing anharmonicity. Although the potential parameters were determined based on optical properties of the binary parent alloys InAs and GaAs, simulated dielectric functions, reflectance, and Raman spectra of alloys agree excellently with experimental data for any arbitrary atomic composition. For a wide range of atomic composition, InAs- and GaAs-bond retain their respective properties of binary parent crystals despite lattice and charge mismatch. It implies that use of the anharmonic Kirkwood-Keating potential may provide an optimal model system to investigate diverse and unique optical properties of quantum dot heterostructures by circumventing potential parameter searches for particular local structures.

• Nuclear Engineering and Technology
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• 제39권3호
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• pp.221-230
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• 2007

Wavelet theory was applied to detect a singularity in a reactor power signal. Compared to Fourier transform, wavelet transform has localization properties in space and frequency. Therefore, using wavelet transform after de-noising, singular points can easily be found. To test this theory, reactor power signals were generated using the HANARO(a Korean multi-purpose research reactor) dynamics model consisting of 39 nonlinear differential equations contaminated with Gaussian noise. Wavelet transform decomposition and de-noising procedures were applied to these signals. It was possible to detect singular events such as a sudden reactivity change and abrupt intrinsic property changes. Thus, this method could be profitably utilized in a real-time system for automatic event recognition(e.g., reactor condition monitoring).

• Bulletin of the Korean Chemical Society
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• 제22권7호
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• pp.721-726
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• 2001

The vibrational transition probability expressions for the forced Morse oscillator have been derived using the commutation relations of the anharmonic Boson operators. The formulation is based on the collinear collision model with the exponential repulsive potential in the framework of semiclassical collision dynamics. The sample calculation results for H2+ He collision system, where the anharmonicity is large, are in excellent agreement with those from an exact, numerical quantum mechanical study by Clark and Dickinson, using the reactance matrix. Our results, however, are markedly different from those of Ree, Kim and Shin's in which they approximate the commutation operator I。 as unity, the harmonic oscillator limit. We have concluded that the quantum number dependence in I。 must be retained to get accurate vibrational transition probabilities for the Morse oscillator.

• Bulletin of the Korean Chemical Society
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• 제16권9호
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• pp.850-858
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• 1995

Golden-rule like formulas have been used without theoretical basis to calculate the resonance lifetimes and final state distributions in the predissociation of van der Waals molecules. Here we present their theoretical basis by extending Fano's configuration interaction theory. Such extensions were independently done by Farnonux [Phys. Rev. 1985, 25, 287] but his work, unfortunately, was not well known outside some small group of people in the field of Auger spectroscopy. Since my extension is easier to understand than his, it is presented here. Theoretical basis of Golden rule like formulas used in the predissociation of van der Waals molecules was obtained by using such extensions. Factors responsible for several aspects of predissociation dynamics, such as variations of dynamics as functions of resonance lifetimes, or variations in shapes of final quantum state distributions of photofragments around resonances, were identified. Parameters, or dynamical information that could be obtained from the measurement of partial cross section spectra were accordingly determined. The theory was applied to the vibrational predissociation of triatomic van der Waals molecules and its result was compared with those calculated by close-coupling method. An example where Golden-rule like expression fails and branching ratios vary greatly around a resonance was considered.

• Bulletin of the Korean Chemical Society
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• 제21권5호
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• pp.497-502
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• 2000

Quantum mechanical analysis is presented for the photodissociation dynamics of the v'=4 levels of the $A^2Σ^+$ state of the OH molecule. We focus on the effects of the multichannel interactions between the asymptotically degenerate states in the recoupling region to see how they affect the dynamics near the predissociating resonances. Both the scalar (total cross section and branching ratios) and the vector properties (angular distributions and alignment parameters) of O($^3P_j, j=0, 1, 2) are treated. The resonances are predicted to be highly Lorentzian, and the branching ratios do not change much across them. Vector properties, however, show very delicate effects of the multichannel interactions and overlapping near the isolated and overlapping resonances. Computed resonance lifetimes agree reasonably well with experimental results.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2008년도 International Meeting on Information Display
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• pp.1441-1444
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• 2008

We developed ultra-accelerated quantum chemical molecular dynamics and spectroscopic characterization simulators for development of PDP materials. By combination of these simulators, realistic structure of PDP materials is drawn on the computer. Furthermore, based on the structures, various properties such as cathode luminescence spectrum and secondary electron emission, is successfully evaluated. The strategy of "Experiment integrated Computational Chemistry" using developed simulators will presented that has the potential in being powerful tool for designing the PDP materials.

• Bulletin of the Korean Chemical Society
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• 제18권12호
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• pp.1274-1280
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• 1997

The photodissociation dynamics of CH2I2 has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin-orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of CH2I2 at this wavelength. These processes are found to occur via B1 ← A1 type electronic transitions. The quantum yield of I*(2P1/2) is determined to be 0.25, indicating that the formation of ground state iodine is clearly the favored dissociation channel in the 304 nm wavelength region. From the angular distribution of dissociation products, the anisotropy parameters are determined to be β(I)=0.4 for the I(2P3/2) and β(I*)=0.55 for the I*(2P1/2) which substantially differ from the limiting value of 1.13. The positive values of anisotropy parameter, however, show that the primary processes for I and I* formation channels proceed dominantly via a transition which is parallel to I-I axis. The above results are interpreted in terms of dual path formation of iodine atoms from two different excited states, i.e., a direct and an indirect dissociation via curve crossing between these states. The translational energy distributions of recoil fragments reveal that a large fraction of the available energy goes into the internal excitation of the CH2I photofragment; < Eint > /Eavl=0.80 and 0.82 for the I and I* formation channels, respectively. The quantitative analysis for the energy partitioning of available energy into the photofragments is used to compare the experimental results with the prediction of direct impulsive model for photodissociation dynamics.

• Nuclear Engineering and Technology
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• 제56권3호
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• pp.893-899
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• 2024

In a secondary cooling system of a sodium-cooled fast reactor (SFR), rapid detection of hydrogen due to sodium-water reaction (SWR) caused by water leakage from a heat exchanger tube of a steam generator (SG) is important in terms of safety and property protection of the SFR. For hydrogen detection, the hydrogen detectors using atomic transmission phenomenon of hydrogen within Ni-membrane were used in Japanese proto-type SFR "Monju". However, during the plant operation, detection signals of water leakage were observed even in the situation without SWR concerning temperature up and down in the cooling system. For this reason, the study of a new hydrogen detector has been carried out to improve stability, accuracy and reliability. In this research, the authors focus on the difference in composition of hydrogen and the difference between the background hydrogen under normal plant operation and the one generated by SWR and theoretically estimate the hydrogen behavior in liquid sodium by using ultra-accelerated quantum chemical molecular dynamics (UA-QCMD). Based on the estimation, dissolved H or NaH, rather than molecular hydrogen (H₂), is the predominant form of the background hydrogen in liquid sodium in terms of energetical stability. On the other hand, it was found that hydrogen molecules produced by the sodium-water reaction can exist stably as a form of a fine bubble concerning some confinement mechanism such as a NaH layer on their surface. At the same time, we observed experimentally that the fine H₂ bubbles exist stably in the liquid sodium, longer than previously expected. This paper describes the comparison between the theoretical estimation and experimental results based on hydrogen form in sodium in the development of the new hydrogen detector in Japan.

• Bulletin of the Korean Chemical Society
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• 제30권12호
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• pp.2962-2968
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• 2009

The photodissociation dynamics of 1,2-bromochloroethane ($C_2H_4BrCl$) was investigated near 234 nm. A two-dimensional photofragment ion-imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain speed and angular distributions of the nascent Br($^2P_{3/2}$) and Br${\ast}($^2P_{1/2}$) atoms. The total translational energy distributions for the Br and Br${\ast}$ channels were well characterized by Gaussian functions with average translational energies of 100 and 84 kJ/mol, respectively. The recoil anisotropies for the Br and Br${\ast}$ channels were measured to be ${\beta}$ = 0.49 ${\pm}$ 0.05 for Br and 1.55 ${\pm}$ 0.05 for Br${\ast}$. The relative quantum yield for Br${\ast}$ was found to be ${\Phi}_{Br{\ast}}$ = 0.33 ${\pm}$ 0.03. The probability of nonadiabatic transition between A' states was estimated to be 0.46. The relevant nonadiabatic dynamics is discussed in terms of interaction between potential energy surfaces in Cs symmetry.