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  • Artikel  (109)
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  • 11
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3476-3487 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Canonical Van Vleck perturbation theory is used to transform curvilinear and rectilinear normal coordinate vibrational Hamiltonians of HCN, C2H2, and CO2 to block-diagonal effective Hamiltonians. Accurate energies as high as 11 000 cm−1 above the zero point are reported for all three molecules. In the absence of off-diagonal coupling terms in the effective Hamiltonians, these two coordinate systems yield identical perturbative expansions for the vibrational energies. Only when coupling terms are introduced do differences between the calculated energies in the two representations become apparent. In CO2, where there is pronounced configuration interaction between nearly degenerate states, we find that the perturbative energies obtained from the curvilinear normal coordinate Hamiltonian are converging significantly faster than those obtained in the rectilinear normal coordinate representation.
    Materialart: Digitale Medien
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  • 12
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 414-430 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Three-dimensional potential energy and dipole moment surfaces have been calculated for the 24 electron triatomics O3, CF2, NO−2, and NF+2 using complete active space self-consistent field wave functions (CASSCF) and a basis set of 87 (99 for NO−2) contracted Gaussian-type orbitals (cGTOs). The analytical potential energy functions (PEFs) have been used in perturbation and variational calculations of anharmonic spectroscopic constants and vibrational energy levels. The results for O3 and CF2 are compared to the available experimental data, and predictions of the rotational and vibrational spectra of NO−2 and NF+2 have been made by comparison to these species. The equilibrium geometries of NO−2 and NF+2 are predicted to be re=1.260 A(ring), θe=116.5° and re=1.242 A(ring), θe=108.0°, respectively. The fundamental vibrational frequencies (ν1,ν2,ν3) and absolute band intensities (at 300 K) of these two ions are predicted to be 1286 cm−1/31 cm−2 atm−1, 782 cm−1/13 cm−2 atm−1, 1232 cm−1/2900 cm−2 atm−1 (NO−2) and 1272 cm−1/181 cm−2 atm−1, 711 cm−1/25 cm−2 atm−1, 1210 cm−1/1230 cm−2 atm−1 (NF+2). Band intensities for several low-lying vibrational transitions have been calculated for all four species from the CASSCF dipole moment functions, and comparisons to experiment have been made for O3 and CF2.
    Materialart: Digitale Medien
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  • 13
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 4378-4390 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The highly excited vibrational states of polyatomic molecules are investigated using canonical Van Vleck perturbation theory, implemented in a superoperator framework. This approach is used to transform a vibrational Hamiltonian to a new representation which has a form ideally suited to the study of the dynamics of interest. The key advantage is that the solution to the full problem is obtained in the new representation using significantly smaller basis sets than are needed to obtain the solutions in the original representation. The transformations are applied to the Hamiltonian operator itself, not the Hamiltonian matrix; this superoperator approach obviates the need for large basis sets. The tedious and complex algebra, that is required to perform these transformations, is readily implemented with FORTRAN codes. Combining these two features has enabled the investigations of vibrational dynamics in energy regimes and densities of states, unattainable by standard methods. These methods are applied to two model problems and to the study of the highly excited overtones of CHD3 with up to five quanta of excitation in the CH bond.
    Materialart: Digitale Medien
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  • 14
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 937-944 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The vibrational states of a planar model of acetylene are obtained using a discrete variable representation. This representation takes advantage of the dynamic separability of high frequency stretching motions and low frequency bending motions. We obtain bend states up to 8770 cm−1 above the zero point energy converged to within ±2 cm−1.
    Materialart: Digitale Medien
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  • 15
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 38 (1997), S. 4815-4831 
    ISSN: 1089-7658
    Quelle: AIP Digital Archive
    Thema: Mathematik , Physik
    Notizen: An orthogonal function set called distributed Gaussian orthogonal polynomials is constructed from equally spaced Gaussians and the corresponding quadrature is studied. The infinite chain limit of both polynomials and the quadrature is studied and analytical expressions are found for polynomials, quadrature points, weights, and Lagrange functions. The connections to Wannier functions are explored, and a way of constructing Wannier functions from Gaussians is proposed. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
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  • 16
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8718-8724 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a‘1 or 3a‘2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m30 transition as 3a‘1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S0 and D0 on solid ground. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 17
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 480-487 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor-series expansion of the potential-energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (≤7600 cm−1) for 138 states with an absolute mean deviation of 1.5 cm−1. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
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  • 18
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 459-468 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dynamics and spectroscopy of (J=0) acetylene bending degrees of freedom are investigated using a reduced dimensional Hamiltonian. This Hamiltonian is obtained by applying an adiabatic approximation to average the vibrational Hamiltonian over the ground state in the three stretch coordinates. Within this approximation, an effective bend force field is obtained by adjusting force constants in the adiabatic potential to improve agreement between experimental and theoretical eigenvalues. With minor modification, a global bend force field is determined that qualitatively describes the vinylidene vibrations and quantitatively describes the acetylene vibrations. This surface is compared to the results of a recent ab initio calculation. A dispersed fluorescence spectrum out of the excited A˜ state, calculated from this model, is found to agree well with results of a recent experimental study. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
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  • 19
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 469-478 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dynamics and spectroscopy of an adiabatic Hamiltonian, derived by McCoy and Sibert [J. Chem. Phys. 105, 459 (1996)], describing the bending motions of acetylene are presented and discussed. The resulting eigenfunctions of this model are interpreted using classical, semiclassical, and quantum mechanical descriptions of the vibrations. Using perturbation theory, the Hamiltonian describing the bends is reduced to two coupled hindered rotor Hamiltonians. This simple Hamiltonian predicts that local mode dynamics of the bending motion first occurs at about 6000 cm−1 of excitation. This prediction is confirmed by the quantum mechanical studies. The hindered rotor Hamiltonian also predicts that l-type doubling leads classically to stable periodic orbits corresponding to planar motion. The extent of planar type motion is quantified using both semiclassical and quantum mechanical models. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
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  • 20
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1717-1724 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Anharmonic force fields and ro-vibrational spectroscopic properties of AlF3 andSiF3+ have been investigated in detail, using the coupled cluster method with single and double substitutions augmented by a perturbative treatment of triple excitations [CCSD(T)] with a triple zeta basis set of 124 contracted Gaussian-type orbitals (cGTOs). A complete set of ro-vibrational spectroscopic constants for each species has been calculated using second order perturbation theory. The geometry only was calculated with a quadruple zeta basis set (224 cGTOs). Our best estimates of the equilibrium bond distances re (Al–F) and re (Si–F) are 1.624 Å and 1.508 Å, respectively, based on the quadruple zeta CCSD(T) bond distances and corrections derived from spectroscopically related known molecules. The CCSD(T) fundamental frequencies are689 cm−1(ν1),301 cm−1(ν2),951 cm−1(ν3), and241 cm−1(ν4) forAlF3 and853 cm−1(ν1),357 cm−1(ν2),1187 cm−1(ν3), and307 cm−1(ν4) forSiF3+. Energies of a number of low lying vibrational states(≤2400 cm−1) have been calculated by three methods: (1) standard second order perturbation theory formulas; (2) Van Vleck perturbation theory based on raising and lowering operators to second order and to fourth order; and (3) a variational method. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
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