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  • 1
    Electronic Resource
    Electronic Resource
    Ab initio study of the energetics of the spin-allowed and spin-forbidden decomposition of HN3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3307-3318 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe an investigation of the energetics of the dissociation of ground state hydrazoic acid HN3. The study is limited to the lowest energy spin-allowed and spin-forbidden decomposition pathways, namely HN3(X˜ 1A')→N2(X 1Σg+) +NH(a 1Δ,X 3Σ−) and HN3(X˜ 1A') →N3(X˜ 2Πg)+H(2S). Complete active space SCF and multireference configuration interaction calculations with large basis sets are used (a) to determine the NNN–H and NN–NH bond dissociation energies of HN3, (b) to locate the geometry of the transition state for the spin-forbidden decomposition and the corresponding activation energy, and (c) to investigate the magnitude and origin of the exit channel barrier in the spin-allowed decomposition channel.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458811
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  • 2
    Electronic Resource
    Electronic Resource
    Quantum scattering studies of electronically inelastic collisions of CN (X 2Σ+, A 2Π) with He (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5425-5439 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using recent ab initio interaction potential energy surfaces for the CN (X 2Σ+, A 2Π)+He system [H.-J. Werner, B. Follmeg, and M. H. Alexander, J. Chem. Phys. 89, 3139 (1988)], we have calculated fully quantum cross sections for inelastic transitions between individual rovibrational levels of the A 2Π and the X 2Σ+ states of CN. We have concentrated on the transitions studied experimentally by Dagdigian and co-workers for CN+Ar, namely transitions between the rotational levels of the A, v=8 and X, v'=12, the A, v=7 and X, v'=11, and the A, v=3 and X, v'=7 vibrational manifolds. In the case of the 8→12 and 7→11 transitions the cross sections are large (0.1–1 A(ring)2), and the dependence on initial Λ doublet level and on final rotational quantum number displays the same subtle alternations as seen experimentally. In the case of the 3→7 transitions, for which the vibrational levels are energetically much more separated, the calculated cross sections for CN+He are extremely small (10−5 A(ring)2), far smaller than observed experimentally for CN+Ar. In order to resolve this discrepancy, we have carried out some additional ab initio calculations for the CN+Ar system, but the change in the interelectronic coupling potential appears not to be large enough to explain the magnitude of the experimental cross sections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457570
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  • 3
    Electronic Resource
    Electronic Resource
    A log-derivative propagation scheme for the exact solution of two-state curve crossing problems (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2388-2395 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is presented for the exact quantum solution of certain two-state curve crossing problems, where electronic state ||1〉 is energetically open at one end of the range of integration, while state ||2〉, and, possibly, state ||1〉, is energetically open at the other end of the range of integration. The method involves the use of log-derivative propagators, but differs from the usual log-derivative integration scheme in that one must propagate through the range of integration not only the log-derivative matrix but also a transformation matrix which permits one to reconstruct the initial wave function after the integration is completed. The method is numerically stable and, in a "solution following'' approximation to the log-derivative propagators, converges as the fourth power of the step size. Application is made to several model problems. In one case the exact results are compared with the predictions of earlier semiclassical analyses [P. V. Coveney, M. S. Child, and A. Bárány, J. Phys. B 18, 457 (1985)]. The method is completely general, and can be applied to arbitrary potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456996
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  • 4
    Electronic Resource
    Electronic Resource
    Clarification of the electronic asymmetry of Λ doublets in 3Π electronic states of diatomic molecules (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 7118-7124 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reflection symmetry of the spatial part of the electronic wave function for 3Π diatomic molecular states is examined carefully for the individual Λ doublet levels by means of an approach presented earlier [M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 80, 4325 (1984)]. The results are: For a 3Π molecule in Hund's case (a) the electronic wave function in the Ω=1 (F2) e levels will be antisymmetric and, in the the f levels, symmetric with respect to reflection of the spatial coordinates of the electrons in the plane of rotation of the molecule. The electronic wave functions in the F1 and F3 levels will not have a defined plane of symmetry. By contrast, in the Hund's case (b) high J limit, the electronic wave function in the F1e, F2 f, and F3e levels will be antisymmetric and, in the F1 f, F2e, and F3 f levels, symmetric with respect to reflection. Thus, the symmetry of the wave functions in the F2Λ-doublet levels reverses with the passage from case (a) to case (b). In the case (b) limit, the main branch P and R lines of a 3Π–3Σ− transition will probe antisymmetric levels, irrespective of the Fi level of the 3Π state, while the main branch Q lines will probe symmetric levels. This will be reversed for a 3Π–3Σ+ transition. At low J, in the case (a) limit, in a 3Π–3Σ− transition the P2i and R2i lines with i odd and the Q2i lines with i even will probe antisymmetric levels, while the P2i and R2i lines with i even and the Q2i lines with i odd will probe symmetric levels; the other 18 rotational branches probe levels with no well-defined reflection symmetry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453356
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  • 5
    Electronic Resource
    Electronic Resource
    HF–HF differential scattering cross sections (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3983-4002 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Close-coupled scattering calculations have been carried out for collisions between HF molecules in their ground vibrational state at a total energy of 1480 cm−1. State-to-state integral and differential cross sections were determined for transitions j1j2→j'1j'2, which refer to collisions HF(j1)+HF(j2)+ΔE→HF(j'1) +HF(j'2), where ΔE is the energy gap. Particularly large cross sections are obtained for the j j+1→j+1 j transitions, which are both resonant and coupled to first order by the dipole–dipole interaction. The relationship between the calculated cross sections and the results of molecular beam scattering experiments is discussed. Total differential scattering measurements for HF–HF collisions are also reported and compared with the calculated results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458784
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  • 6
    Electronic Resource
    Electronic Resource
    Rotational energy transfer in HF: A computational study (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7563-7589 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Close-coupled scattering calculations have been carried out for rotationally inelastic collisions between HF molecules. Integral cross sections were determined for vibrationally elastic transitions j1 j2→j '1 j 2(v1v2), which refer to collisions HFa (v1, j1)+HFb (v2, j2)+ΔE→HFa (v1, j '1 )+HFb (v2, j 2), where ΔE is the energy gap. Total energies between 241.1 and 3900 cm−1 (0.03 and 0.48 eV) were considered. When v1=v2, the rotational exchange transitions j1 j2→j2 j1(v1v2) are energetically resonant (i.e., ΔE=0), and the corresponding cross sections increase as the collision energy decreases. Particularly large cross sections are obtained for the j j+1→j+1 j(vv) transitions, which are both resonant and coupled to first order by the dipole–dipole interaction. When v1≠v2, the j j+1→j+1 j(v1v2) transitions are no longer resonant, and the corresponding cross sections are greatly reduced at small to moderate collision energies. The dependence of the j1 j2→j '1 j 2(v1v2) cross sections upon the vibrational states of the molecules, the collision energy, and the interaction potential used is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457280
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  • 7
    Electronic Resource
    Electronic Resource
    Quantum theory and collisional propensity rules for rotationally inelastic collisions between polyatomic molecules (NH3 and CO2) and an uncorrugated surface (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 790-800 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the general quantum theory of collisions of a symmetric top molecule with an uncorrugated surface. The similarities between the description of collisions of a molecule with a structureless atom and a flat surface allow us to exploit earlier gas-phase results. We then derive several collisional propensity rules: (1) In experiments in which both inversion states in the initial J,K doublets of para-NH3 are equally populated, both inversion states of all collisionally excited levels must also be equally populated. If, however, the initial inversion level can be state selected, then unequal populations will be observed in collisionally excited inversion doublets. (2) For transitions from the J=0 level of ortho-NH3 into rotational levels of the K=3 stack, a strong propensity will exist toward conservation of the inversion symmetry for transitions into levels with J' odd, but toward a change in the inversion symmetry for transitions into levels with J' even. (3) If the odd terms in the angular expansion of the potential dominate, then for transitions out of rotational levels with J〉0 in the K=0 stack of ortho-NH3 into rotational levels of the K=3 stack, a strong propensity will exist toward population of the upper level of the inversion doublet if the initial state has even J, and toward population of the lower level if the initial state has odd J. Using the similarities between the wave functions of a symmetric top and those of a linear polyatomic molecule with degenerate bending modes, we derived several propensity rules for the specific case of collisions of CO2 (0000) with an uncorrugated surface. In collisions which excite the low-lying (0110) bending vibration, if the initial rotational quantum number is small, then we predict that the probability of transition into a final state with J' odd will be much larger than for transition into a final state with J' even.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455202
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  • 8
    Electronic Resource
    Electronic Resource
    Inelastic collisions of CaF(A 2Π) with He and Ar: Quantum calculations and adiabatic analysis (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3581-3589 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present coupled-states (CS) calculations for collisions of CaF(A 2Π) with He and Ar. These calculations are based on potential energy surfaces determined by use of the Gordon–Kim electron–gas method [J. Chem. Phys. 56, 3122 (1972)]. The theoretical CS cross sections for rotationally inelastic transitions are compared with the experimental results of Dufour and co-workers [Chem. Phys. 98, 315 (1985)]. We obtain theoretical cross sections significantly greater than the experimental values. In order to discuss and interpret the results of our scattering calculations, we make use of a fully adiabatic description of the collision dynamics. This adiabatic analysis shows that the long-range part of the potential plays a key role in determining the sizes of the theoretical cross sections, and in underlying the failure of the energy sudden scaling relation to describe the calculated cross sections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453907
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum studies of inelastic collisions of O2(X 3Σ−g) with He: Polarization effects and collisional propensity rules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2726-2737 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate rotationally inelastic cross sections of O2(X 3Σ−g) with He at a collision energy of 27 meV. Theoretical cross sections obtained from close-coupled (CC) calculations are compared with results from the infinite-order sudden (IOS) approximation. Both the CC and IOS fine-structure state-resolved cross sections exhibit a strong ΔN=ΔJ Fi conserving collisional propensity. An analysis of the general expression for state-resolved cross sections in terms of spin-independent tensor opacities clearly establishes, without the introduction of dynamical approximations, the direct connection between this propensity rule and the collisional propensity for the conservation of the orientation of the nuclear rotational angular momentum vector N. In the low-N limit, Fi changing O2–He collisions are much more strongly depolarizing than collisions that conserve the Fi symmetry level. This enhanced collisional depolarization of an initial distribution of the total molecular angular momentum vector J is related to the collisional reorientation of N that must occur in Fi changing transitions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451029
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  • 10
    Electronic Resource
    Electronic Resource
    Mechanism of and alignment effects in spin–changing collisions involving atoms in 1P electronic states: Ca(4s5p 1P)+noble gases (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 5373-5385 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a detailed study of the mechanism of spin–changing transitions between 1P and 3P electronic states corresponding to the nsn'p Rydberg state of an alkaline earth atom in collision with a structureless, spherical partner. This type of process takes place on four potential curves and involves collisional coupling between the electronic angular momentum L of the electronically excited atom, the spin S of the atom in the final 3P state, and the orbital angular momentum l associated with the relative motion of the collision partners. We review the use of Hund's case coupling schemes in describing the collision and present a careful analysis of the initial state preparation under conditions of laser excitation in a crossed beam experiment. This underlies the development of a new statistical model for the dependence of the spin–changing cross section on the alignment of the pump laser. Finally, we describe a new technique to follow the redistribution of flux onto locally adiabatic states as the initially excited atom is approached by the closed-shell partner. This is used to show that dynamical corrections to our simple statistical model are small.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456444
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