Browsing by Author "Mebel, A. M."

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Anharmonic effect on unimolecular reactions with application to the photodissociation of ethylene
(2007-07-26)
Yao, L.
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Mebel, A. M.
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Lu, H. F.
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Neusser, H. J.
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Lin, S. H.
The importance of anharmonic effect on dissociation of molecular systems, especially clusters, has been noted. In this paper, we shall present a theoretical approach that can carry out the first principle calculations of anharmonic canonical and microcanonical rate constants of unimolecular reactions within the framework of transition state theory. In the canonical case, it is essential to calculate the partition function of anharmonic oscillators; for convenience, the Morse oscillator potential will be used for demonstration in this paper. In the microcanical case, which involves the calculation of the total number of states for the activated complex and the density of states for the reactant, we make use of the fact that both the total number of states and the density of states can be expressed in the inverse Laplace transformation of the partition functions and that the inverse Laplace transformation can in turn be carried out by using the saddle-point method. We shall also show that using the theoretical approach presented in this paper the total number of states and density of states can be determined from thermodynamic properties and the difference between the method used in this paper and the thermodynamic model used by Krems and Nordholm will be given. To demonstrate the application of our theoretical approach, we chose the photodissociation of ethylene at 157 and 193 nm as an example.
Dissociation Rate Constant of the Hydrogen Fluoride Dimer by the ab Initio Anharmonic RRKM Theory
(2009-12-31)
Yao, L.
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Mebel, A. M.
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Lin, S. H.
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應用化學系
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Department of Applied Chemistry
The dissociation rate constants for hydrogen fluoride dimers; are calculated using the YL method, proposed by Yao and Lin (Yao, L.; Mebel, A. M.; Lu, H. F.; Neusser, H. J.; Lin, S. H. J.. Phys. Chem. A 2007, 111, 6722). The dividing surface method and RRKM theory are also used to obtain pseudo-transition states and rate constants, respectively. For the dissociation of HF dimers, the anharmonic rate constants are around 3.02 x 10(10) to 3.46 x 10(12) s(-1), while the harmonic rate constants are in the range of 2.93 x 10(10) to 1.66 x 10(13) s(-1), at a temperature range of 243-1000 K, for the canonical case. For the microcanonical case, the anharmonic rate constants are in the range of 1.91 x 10(12) to 1.32 x 10(13) s(-1) and the harmonic rate constants are in the range of 1.89 x 10(12) to 4.93 x 10(13) s(-1), with a total energy range of 1338-4500 cm(-1). Both harmonic and anharmonic rate constants are only comparable to the experimental results 0.5 x 10(10) s(-1) and 1 x 10(11) s(-1) in an inert gas bath at room temperature. In this case the harmonic and anharmonic rate constants show similar results in this calculation. The results of this paper indicate that the YL method is suitable for calculating dissociation rate constants of small flexible HF dimers and the anharmonic effect should be included.
Experimental and theoretical investigation of high-power laser ionization and dissociation of methane
(2007-09-27)
Sharifi, A.
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Kong, F.
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Chin, S. L.
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Mineo, H.
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Dyakov, Y.
The main purpose of this paper is to report the high-power laser ionization-dissociation of CH4 at various femtosecond (fs) laser intensities (from 1 x 10 14 W/cm(2) to 2 x 1015 W/cm(2)) with a laser pulse duration of 48 fs. The generalized molecular Keldysh theory has been applied to calculate the ionization yields for CH4+ and CH4++. Outside the influence of the A intense laser, we propose to calculate the mass spectra due to the decomposition of CH4+ and CH4++, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The agreement between the experimental mass spectra and calculated mass spectra seems to be reasonable.
Experimental and theoretical investigations of ionization/dissociation of cyclopentanone molecule in a femtosecond laser field
(2008-11-28)
Wang, Qiaoqiao
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Wu, Di
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Jin, Mingxing
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Liu, Fuchun
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Hu, Feifei
The ionization/dissociation mechanism of cyclopentanone has been experimentally investigated in molecular beam by irradiating with intense 394 and 788 nm laser fields with pulse duration of 90 fs. The range of laser intensities varied from 3 X 10(13) to 4 X 10(14) W/cm(2). For both wavelengths, the singly charged parent ion is observable while the doubly charged one cannot be found easily, although the fragmentation pattern supports its presence. Meanwhile, the extent of fragmentation at 788 nm is less than that in the 394 nm case. We quantitatively analyze the ionization processes of cyclopentanone in intense femtosecond laser by comparing the calculation results of ionization rate constants obtained from Ammosov-Delone-Krainov, Keldysh, and Keldysh-Faisal-Reiss (KFR) theories based on hydrogenlike atom model. We also compare the experimental and theoretical results; the generalized KFR theory is found to be useful in predicting the ionization yields of singly and doubly charged cyclopentanone ion. To interpret the dissociation patterns of the cyclopentanone ions, we have used the Rice-Ramsperger-Kassel-Marcus theory with the potential surfaces obtained from the ab initio quantum chemical calculations. (c) 2008 American Institute of Physics.
Ionization and Dissociation Processes of Pyrrolidine in Intense Femtosecond Laser Field
(2009-07-09)
Wang, Qiaoqiao
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Wu, Di
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Zhang, Dongdong
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Jin, Mingxing
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Liu, Fuchun
Ionization and dissociation mechanisms of pyrrolidine in intense 800 nm laser field (10(13) to 10(14) W/cm(2)) have been experimentally investigated by using a method of molecular beam and time-of-flight mass spectrometer. Singly charged parent ion and numerous fragment ions are observed in the mass spectra, which are investigated as a function of laser intensity and polarization. In order to understand the details of the ionization processes of pyrrolidine in intense femtosecond laser field, we quantitatively calculate the rate constants and ion yields by means of generalized Keldysh-Faisal-Reiss theory, and the excitation probabilities of the excited states are also calculated by using Floquet theory. The results suggest that the ionization might occur partially through the excited states of neutral pyrrolidine. Comparing with linearly polarized (LP) laser field, we observe some enhancement of fragmentation with a circularly polarized (CP) laser field above the saturation threshold intensity which might be explained by the active energies of the pyrrolidine molecular ions are different under CP and LP laser irradiated. To interpret the dissociation patterns of the pyrrolidine ions, we have used the Rice-Ramsperger-Kassel-Marcus theory with the potential surfaces obtained from the ab initio quantum chemical calculations.
Ionization/dissociation processes of methyl-substituted derivates of cyclopentanone in intense femtosecond laser field
(2013-10-24)
Wang, Qiaoqiao
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Dyakov, Yuri A.
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Wu, Di
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Zhang, Dongdong
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Jin, Mingxing
Ionization and dissociation of 2- and 3-methyl cyclopentanones have been investigated in molecular beams by their irradiation with intense 394 and 788 nm laser fields with pulse duration of 90 fs and intensity of 3 x 10(13)-4 x 10(14) W/cm(2). The analysis of the resulting mass spectra allowed us to discern the effects of methyl substitution and its position on the outcome of ionization/dissociation processes induced by the intense femtosecond laser field. Generalized Keldysh-Faisal-Reiss (g-KFR) and ab initio G3(MP2, CCSD)//B3LYP/6-31G*/RRKM theoretical calculations helped to uncover the formation mechanism of major ionic fragments observed in the mass spectra including C5H10+, C4H6O+, C3H3O+, C3H4O+, C3Hx+, and C2Hx+. (C) 2013 Elsevier B.V. All rights reserved.
On the calculation of the dissociation rate constant of the water dimer by the ab initio anharmonic RRKM theory
(2009-03-05)
Yao, L.
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He, R. X.
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Mebel, A. M.
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Lin, S. H.
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應用化學系
The dissociation rate constant of the water dimer was calculated by using the method proposed by Yao and Lin (YL method). The dividing surface method and RRKM theory are used to calculate the pseudo transition state and the rate constant, respectively. For the canonical case at temperature range of 243-1000 K and for the microcanonical system at total energy 1411-4000 cm(1), the anharmonic rate constant, around 10(9)-10(11) s(1), is close to the experimental prediction. The present results indicate that the anharmonic effect should be included and YL method is suitable for calculating dissociation rate constants of such small flexible water cluster. (C) 2009 Elsevier B. V. All rights reserved.
Photodissociation of 1,3,5-triazine: An ab initio and RRKM study
(2007-09-27)
Dyakov, Y. A.
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Mebel, A. M.
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Lin, S. H.
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Lee, Y. T.
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Ni, C.-K.
The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of 1,3,5-triazine at different wavelengths of the absorbed photon. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level, and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that, after being excited by 275, 248, or 193 nm photons, the triazine molecule isomerizes to an opened-ring structure on the first singlet excited-state potential energy surface (PES), which is followed by relaxation into the ground electronic state via internal conversion. On the contrary, excitation by 285 and 295 nm photons cannot initiate the ring-opening reaction on the excited-state PES, and the molecule relaxes into the energized ring isomer in the ground electronic state. The dissociation reaction starting from the ring isomer is calculated to have branching ratios of various reaction channels significantly different from those for the reaction initiating from the opened-ring structure. The existence of two distinct mechanisms of 1,3,5-triazine photodissociation can explain the inconsistency in the translational energy distributions of HCN moieties at different wavelengths observed experimentally.
Potential energy surfaces for the lowest excited states of the nitrogen-vacancy point defects in diamonds: A quantum chemical study
(2008-09-10)
Zyubin, A. S.
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Mebel, A. M.
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Chang, H. C.
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Lin, S. H.
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應用化學系
Quantum chemical calculations of geometry relaxation in lowest excited states of neutral (NV(O)) and negatively charged (NV) nitrogen-vacancy point defects in diamond have been performed employing the CASSCF method with a finite NC(19)H(28) model cluster. Vibrations activated by the electronic transitions are determined by comparing calculated atomic displacements in the excited states with normal mode vectors, and the activated frequencies are evaluated as similar to 600 cm (1). The barrier for N migration through the vacancy in NV is estimated at the TD-B3LYP level and no significant decrease of this barrier (similar to 5 eV in the ground state) is found due to electronic excitations. (c) 2008 Elsevier B. V. All rights reserved.
Quantum Chemical Modeling of Photoabsorption Properties of Two- and Three-Nitrogen Vacancy Point Defects in Diamond
(2009-06-18)
Zyubin, A. S.
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Mebel, A. M.
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Hayashi, M.
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Chang, H. C.
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Lin, S. H.
Quantum chemical calculations of the geometric and electronic structures and vertical transition energies for several low-lying excited states of the neutral and negatively charged vacancy-related point defects in diamond containing two and three nitrogen atoms (N(2)V(0), N(2)V(-), and N(3)V(0)) have been performed employing various theoretical methods (time-dependent density functional theory, equation-of-motion coupled cluster, and multireference perturbation theory) and different basis sets and using C(21)H(28), C(35)H(36), and C(51)H(52) finite model clusters. In the;round states, the vacancy-related atoms are found to be shifted away from the vacancy center by similar to 0.1 angstrom, whereas the positions of atoms from the second layer around them vacancy remain nearly unchanged, indicating a local character of geometry relaxation due to defects. The lowest excited states are formed with participation of the stretched (N(2)V) or broken (N(3)V) C-C bond and nonbonding combinations of nitrogen lone pairs as donors, with the C-C antibonding molecular orbital (MO) in N(2)V(0), broken C-C bond in N(3)V(0), and diffuse vacancy-related MOs serving as acceptors. Normally, the first excited states have a valence character, but the diffuse states are rather close in energy, especially for N(3)V(0) (2(2)A(1) and 1(2)E excited states). The first optically active excitation in the N(2)V(0) defect with the calculated energy of similar to 2.6 eV (in close agreement with the experiment) is formed by the electronic transition from the stretched C-C bond to the antibonding C-C MO, with an additional contribution from the combination of nitrogen lone pairs. For the negatively charged N(2)V(-) system, the lowest excitation to the 1(2)A(1) state is predicted to occur from the singly occupied antibonding b(1) MO to the empty diffuse a(1) orbital, but the CASPT2 calculated excitation energy, similar to 0.9 eV, underestimates the experimental zero phonon line observed at 1.26 eV. The lowest excited states of N(3)V(0), 2(2)A(1), and 1(2)E correspond to transitions from the singly occupied MO (SOMO) to the diffuse lowest vacant orbital and from the nonbonding combination of nitrogen lone pairs to SOMO, respectively, and have similar energies of about 3.1-3.3 eV, in agreement with the experimental photoabsorption band maximum at similar to 3 eV.
Quantum Chemical Modeling of Photoadsorption Properties of the Nitrogen-Vacancy Pint Defect in Diamond
(2009-01-15)
Zyubin, A. S.
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Mebel, A. M.
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Hayashi, M.
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Chang, H. C.
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Lin, S. H.
Quantum chemical Calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis Sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with tile first absorption band calculated at similar to 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0) we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (similar to 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (similar to 3.2 eV and higher). (C) 2008 Wiley Periodicals, Inc. J Comput Chem 30: 119-131, 2009
Quantum Chemical Modeling of Photoadsorption Properties of the Nitrogen-Vacancy Pint Defect in Diamond
(2009-01-15)
Zyubin, A. S.
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Mebel, A. M.
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Hayashi, M.
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Chang, H. C.
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Lin, S. H.
Quantum chemical Calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV0 and NV-) have been performed employing various theoretical methods and basis Sets and using finite model NCnHm clusters. Unpaired electrons in the ground doublet state of NV0 and triplet state of NV- are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV- are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with tile first absorption band calculated at similar to 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV0 we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (similar to 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (similar to 3.2 eV and higher). (C) 2008 Wiley Periodicals, Inc. J Comput Chem 30: 119-131, 2009
Structure and predissociation of the 3p sigma(u)D (3)Sigma(+)(u) Rydberg state of N(2): First extreme-ultraviolet and new near-infrared observations, with coupled-channels analysis
(2008-11-28)
Wang, Qiaoqiao
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Wu, Di
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Jin, Mingxing
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Liu, Fuchun
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Hu, Feifei
The 3p sigma(u)D (3)Sigma(+)(u) Rydberg state of N(2) is studied experimentally using two high-resolution spectroscopic techniques. First, the forbidden D (3)Sigma(+)(u)-X (1)Sigma(+)(g) transition is observed for the first time via the (0,0) band of (14)N(2) and the (1,0) band of (15)N(2), using 1 extreme-ultraviolet +1 ultraviolet two-photon-ionization laser spectroscopy. Second, the Rydberg-Rydberg transition D (3)Sigma(+)(u)-E (3)Sigma(+)(g) is studied using near-infrared diode-laser photoabsorption spectroscopy, thus extending the previous measurements of Kanamori [J. Chem. Phys. 95, 80 (1991)], to higher transition energies, and thereby revealing the (2,2) and (3,3) bands. The combined results show that the D(v=0-3) levels exhibit rapidly increasing rotational predissociation as v increases, spanning nearly four orders of magnitude. The D-state level structure and rotational predissociation signature are explained by means of a coupled-channels model which considers the electrostatically coupled (3)Pi(u) Rydberg-valence manifold, together with a pure-precession L-uncoupling rotational interaction between the 3p sigma(u)D (3)Sigma(+)(u) and 3p pi(u)G (3)Pi(u) Rydberg p-complex components.
Structure and predissociation of the 3p sigma(u)D (3)Sigma(+)(u) Rydberg state of N(2): First extreme-ultraviolet and new near-infrared observations, with coupled-channels analysis
(2008-11-28)
Wang, Qiaoqiao
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Wu, Di
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Jin, Mingxing
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Liu, Fuchun
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Hu, Feifei
The 3p sigma(u)D (3)Sigma(+)(u) Rydberg state of N(2) is studied experimentally using two high-resolution spectroscopic techniques. First, the forbidden D (3)Sigma(+)(u)-X (1)Sigma(+)(g) transition is observed for the first time via the (0,0) band of (14)N(2) and the (1,0) band of (15)N(2), using 1 extreme-ultraviolet +1 ultraviolet two-photon-ionization laser spectroscopy. Second, the Rydberg-Rydberg transition D (3)Sigma(+)(u)-E (3)Sigma(+)(g) is studied using near-infrared diode-laser photoabsorption spectroscopy, thus extending the previous measurements of Kanamori [J. Chem. Phys. 95, 80 (1991)], to higher transition energies, and thereby revealing the (2,2) and (3,3) bands. The combined results show that the D(v=0-3) levels exhibit rapidly increasing rotational predissociation as v increases, spanning nearly four orders of magnitude. The D-state level structure and rotational predissociation signature are explained by means of a coupled-channels model which considers the electrostatically coupled (3)Pi(u) Rydberg-valence manifold, together with a pure-precession L-uncoupling rotational interaction between the 3p sigma(u)D (3)Sigma(+)(u) and 3p pi(u)G (3)Pi(u) Rydberg p-complex components.
Theoretical study of multiphoton ionization of cyclohexadienes and unimolecular decomposition of their mono- and dications
(2008)
Zyubina, T. S.
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Mebel, A. M.
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Hayashi, M.
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Lin, S. H.
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應用化學系
Quantum chemical calculations of the geometric structure, vertical excitation energies, and ionization potentials for the isomeric pair of 1,3- and 1,4-cyclohexadienes and their mono- and dications have been performed employing a variety of theoretical methods and basis sets. The computed ionization potentials and electronic excitation energies are used to evaluate the range of internal energies available for fragmentation of the cations following multiphoton resonance ionization of the cyclohexadienes in intense laser field. The conditions governing the competition between multiple ionization and decomposition of the ions are also discussed. Calculations of stationary points on the potential energy surfaces for various fragmentation channels and relative product yields at different available internal energies are then utilized to analyze the trends in branching ratios of major dissociation products of the 1,4-cyclohexadiene(2+) dication, which include C3H3+ + C3H5+, C2H3+ + C4H5+, and C4H3+ + C2H5+.