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  • 1985-1989  (28)
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  • 1
    Electronic Resource
    Electronic Resource
    Inelastic collisions of OH(X 2Π) with para-H2: Λ-doublet and hyperfine-structure transitions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 6931-6937 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this article we report Λ-doublet and hyperfine-structure resolved close-coupled and coupled-states cross sections for collisions of OH (X 2Π) with para-H2 at thermal collision energies relevant to kinetic modelling of OH emission in astrophysical sources. Our calculations predict a degree of selective excitation of the Λ-doublet levels in agreement with earlier work of Schinke and Andresen a a higher collision energy. That is, collisions of OH with H2 lead to preferential population of the lower Λ-doublet (ε=+1,e) in the Ω=3/2 manifold, while the upper Λ doublet (ε=−1,f) is preferentially populated in the Ω=1/2 manifold. In contrast to a purely stastistical model, we observe here that, for a given JΩε→JΩε' transition, the hyperfine resolved cross sections are consistent with a strong ΔF=0 collisional propensity rule. This collisional propensity is a manifestation of a fundamental tendency to conserve the orientation of the rotational angular momentum vector during collisions of molecules with closed-shell targets. This propensity rule implies that cross sections for transitions across a hyperfine doublet will be small.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454390
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Quantum close-coupled studies of collisions of NO(X 2Π) with a Ag(111) surface (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3218-3231 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Close-coupled calculations of transition probabilities for rotationally inelastic scattering of NO(X 2Π) by a rigid, uncorrugated Ag(111) surface are presented. These calculations explicitly include the two adiabatic potential energy surfaces of V+ and V− symmetry, which are required in a fully quantum mechanical treatment of the collision dynamics. This enables us to provide the first theoretical description of the dependence of the transition probabilities on the spin–orbit and Λ-doublet states of the scattered molecules, which are a direct manifestation of the open-shell character of the 2Π electronic ground state of NO. A comparison is made with the results of experiments by Luntz, Kleyn, and Auerbach at IBM, and Zare and co-workers at Stanford.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453010
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) transitions in collisions with He: Integral cross sections and alignment effects (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 4790-4800 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The fully quantum theory is developed for 1P→3P transitions in collisions of electronically excited (nsn'p) alkaline earth atoms with closed shell atoms. Spin-changing transitions occur by means of the small spin–orbit mixing between the 1P and 3PJ=1 states of the isolated atom, and are facilitated by the crossing between the 1Π and 3Σ potential curves which correlate, respectively, with the 1P and 3P asymptotes. Close-coupled calculations are carried out for the Ca(4s5p)+He system, based on four different choices of the necessary interaction potentials. Particular attention is devoted to the simulation of the recent experiment of Hale, Hertel, and Leone [Phys. Rev. Lett. 53, 2296 (1984)], in which the 1P→3P cross section was determined in a crossed-beam experiment as a function of the orientation of the initially excited 5p orbital. This polarization dependence depends critically on the long-range splitting between the 1Π and 1Σ curves. A fully adiabatic description of the collision dynamics is used to interpret the results of the quantum scattering calculations. No clear-cut theoretical justification is found for the "orbital following'' models which have been developed to interpret prior experimental studies of collisions involving excited atoms in P electronic states. Rather, a picture emerges in which the initially selected orientation of the Ca p orbital correlates, at short range, with equal probability to Σ-like and Π-like potential curves. Variations in the 1P→3P cross sections are due to long-range Coriolis coupling between the electronic orbital and nuclear orbital momenta and may reflect quantum interference effects between the Π-like and Σ-like adiabatic potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452701
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Rotationally inelastic collisions between a molecule in a 2Σ electronic state and a 2S atom: Sudden factorization, scaling, and symmetry relations (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 1859-1865 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Sudden factorization and scaling relations are developed for the scattering of molecules in 2Σ electronic states by open-shell 2S atoms, with particular emphasis given to the influence of the open-shell character of the atomic perturber. The molecule-state-resolved cross sections are expressed as a sum of spin-independent and spin-correlated terms. Both terms can be expressed as a weighted sum of spin-free tensor opacities. In the sudden limit the tensor opacities satisfy factorization relations formally equivalent to those first derived for 1Σ–1S collision systems. However, in the collision of two open-shell species these dynamical factors cannot be identified with inelastic scattering off a single adiabatic potential energy surface. The breakdown in the scaling and symmetry relations, which are valid in the energy sudden limit, for 2Σ-state molecules scattering off closed-shell targets provides a qualitative measure of the strength of the electrostatic exchange forces which arise from the open-shell nature of both the molecule and the atomic target.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451188
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Collision-induced transitions between molecular hyperfine levels: Quantum formalism, propensity rules, and experimental study of CaBr(X 2Σ+)+Ar (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2191-2200 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The general quantum treatment of collisions of a 2Σ+ molecule with hyperfine structure is presented. The recoupling technique introduced by Corey and McCourt into the field of molecular collisions [J. Phys. Chem. 87, 2723 (1983)] allows us to represent hyperfine-state-resolved tensor opacities, and hence cross sections, in terms of the corresponding nuclear- and also electron-spin-free quantities. The formalism also predicts (independent of the dynamical limit) that the largest F→F′ cross sections will be those for which ΔF=ΔJ, a rule well known for radiative transitions. Hyperfine-state-resolved scattering involving collisions of CaBr(X 2Σ+) with Ar is also studied here experimentally by electric quadrupole state selection and cw dye laser fluorescence detection. The relative final F′ distributions were determined for the N=3,e→N=5,e and N=2,e→N=1,e collisional transitions. These results clearly exhibit the ΔF=ΔJ propensity rule. Moreover, the F′ distributions were predicted with nearly quantitative accuracy using our previously determined CaCl(X 2Σ+)-Ar tensor opacities. By contrast, the MJ- randomization model, first proposed to treat the influence of hyperfine structure in atomic collisions, is shown to disagree with both our experimental data and theoretical predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449311
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  • 6
    Electronic Resource
    Electronic Resource
    Inelastic collisions of CaCl(X 2Σ+) with Ar: A collaborative theoretical and experimental study (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 556-566 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate rotationally inelastic cross sections of CaCl(X 2Σ+) with Ar at a collision energy of 0.24 eV. Theoretical cross sections, determined by coupled states calculations based on an electron-gas description of the potential surface, are compared with experimental cross sections, determined in a molecular beam apparatus involving initial state selection by an electric quadrupole field and final state detection by laser-induced fluorescence. The agreement between theoretical and experimental cross sections is excellent, except for the e → e transitions with ΔN=even, which suggests a residual inaccuracy in the theoretical description of the second Legendre moment of the anisotropy in the potential. Both the theoretical and experimental cross sections clearly confirm a propensity toward conservation of the spectroscopic e/ f label. The sets of experimental and theoretical cross sections can be well fit by the sudden scaling relation, although the entire set of base cross sections can not be well represented by a simple power law.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449521
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    A stable linear reference potential algorithm for solution of the quantum close-coupled equations in molecular scattering theory (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 2044-2050 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show how the linear reference potential method for solution of the close-coupled equations, which arise in inelastic scattering theory, can be reformulated in terms of an "imbedding-type'' propagator. Explicit expressions are given for the blocks of the propagator matrix in terms of Airy functions. By representing these functions in terms of moduli and phases, in both classically allowed and classically forbidden regions, one can evaluate the propagator without any numerical difficulty. The resulting algorithm is tested on a highly pathological problem—the rotationally inelastic scattering of a polar molecule by a spherical ion at extremely low kinetic energy—and found to be completely stable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452154
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  • 8
    Electronic Resource
    Electronic Resource
    Collisional effects in Stark spectroscopy of molecules in 1Π electronic states (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 3340-3348 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the theoretical framework necessary to describe inelastic collisions between Stark mixed Λ-doublet levels of a molecule in a 1Π electronic state, and the subsequent effect on fluorescence intensities. It is convenient to work with the spherical tensor moments of the excited state density matrix. For weak fields and short times after excitation the ratio of the fluorescence intensities for emission on "forbidden'' and allowed 1Σ+←1Π lines is predicted to vary linearly with time and quadratically with field strength. Simulation studies are presented, based on collisional parameters suggested by previous theoretical studies of inelastic collisions of molecules in Π electronic states. The present article provides the foundation for the accurate interpretation of laser diagnostic studies in plasma environments, such as those described by Mandich, Gaebe, and Gottscho in the accompanying article.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449195
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  • 9
    Electronic Resource
    Electronic Resource
    Collision induced transitions between 2Π and 2Σ states of diatomic molecules: Quantum theory and collisional propensity rules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 100-113 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop the exact quantum description, free of any dynamical approximations, of rotationally inelastic collision induced transitions between 2Π and 2Σ electronic states of a diatomic molecule. An explicit connection is made between the matrix elements of the electrostatic coupling, described in an asymptotically exact diabatic basis, and the results of an ab initio calculation of the appropriate atom–molecule adiabatic electronic wave functions of A' and A‘ symmetry. Analysis of the quantum close-coupled equations demonstrates that the use of Franck–Condon approximations in the description of E → E energy transfer is unjustified and, furthermore, that in collisions involving homonuclear diatomic molecules the s/a permutation-inversion symmetry of the molecular wave functions will be rigorously conserved. The extension of the infinite-order sudden approximation to electronically inelastic 2Π → 2Σ processes allows us to predict two new collisional propensity rules: (a) When Δ J=0 the cross sections will become vanishingly small for transitions which conserve the e/f symmetry index of the molecular wave function. (b) In a high-J Hund's case (b) limit transitions from either the F1 or F2 2Π-state manifolds will populate only one of the Σ-state spin-doublet levels, consistent with a physical model in which the electronic spin S is a spectator so that the relative orientation of N and S is preserved during the collision.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450831
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  • 10
    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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