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  • 1990-1994  (27)
  • 1985-1989  (28)
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  • 1
    Digitale Medien
    Digitale Medien
    On the generation of preferential Λ-doublet populations in the collisional relaxation of highly rotationally excited CH(X 2Π) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7468-7479 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: By means of full quantum close-coupling and coupled states calculations based on an ab initio potential energy surface for the Ar–CH system, we confirm a propensity seen experimentally by Hancock, Stuhl, and their co-workers. During the rotational relaxation of high rotational levels of the CH(X 2Π) radical, produced by photolysis of a suitable precursor, there appears a clear population imbalance in favor of the Λ-doublet levels of Π(A‘) symmetry. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of both the temporal evolution and the pressure dependence of this Λ-doublet asymmetry. This asymmetry is a consequence of both an enhanced depletion of high N Π(A') levels and the enhanced formation of Π(A‘) levels in the next lower (N−1) manifolds. The physical origin of this propensity involves a crossing between two adiabatic bender potentials which follow, respectively, the A' and A‘ potential energy surface (PES). This crossing occurs only for the "helicopter-like'' approach of the CH molecule, in which its rotational angular momentum is aligned along the initial relative velocity vector. Thus, a strong v, N correlation in the reactant channel results in a strong Λ, N correlation in the product channel.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.468269
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  • 2
    Digitale Medien
    Digitale Medien
    Potential energy surfaces for the interaction of CH(X 2Π,B 2Σ−) with Ar and an assignment of the stretch-bend levels of the ArCH(B) van der Waals molecule (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4547-4560 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the CH radical in its ground (X 2Π) and second excited (B 2Σ−) electronic states. These potential energy surfaces are then used in an adiabatic analysis of the rovibronic levels of the ArCH(X) and ArCH(B) van der Waals complexes. A qualitative discussion of the expected features in the B˜←X˜ electronic spectrum of ArCH is presented, and these are compared with the experimental spectrum reported earlier by Lemire et al. [J. Chem. Phys. 99, 91 (1993)].
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.467442
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  • 3
    Digitale Medien
    Digitale Medien
    Potential energy surfaces for the interaction of BH(X 1Σ+,A 1Π) with Ar and a theoretical investigation of the stretch-bend levels of the ArBH(A) van der Waals molecule (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2887-2902 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the BH radical in its ground (X 1Σ+) and first excited (A 1Π) electronic states. These potential energy surfaces are then used with an adiabatic bender model for the calculation of the vibrational energy levels of the ArBH van der Waals complex in its ground and first excited singlet electronic states. Comparison of vibrational energies calculated using this adiabatic bender model with computed exact vibrational energies indicates that the former provides a very useful description of the bound levels of the ArBH complex. A qualitative discussion of the expected features in the A 1Π−X 1Σ+ electronic spectrum of ArBH is also presented, to facilitate comparison with the experimental ArBH spectrum reported in the following paper [E. Hwang and P. J. Dagdigian, following paper, J. Chem. Phys. 101, 2903 (1994)]. The most strongly bound ArBH(A) levels, with Ar–BH separations less than in the ground state ArBH(X) complex, correspond to motion described primarily by the more attractive VA‘ potential energy surface and to a helicopterlike internal motion of the BH moiety. For the more weakly bound states supported by higher bender curves, the vibrational motion cannot be described as occurring on either the VA‘ or VA' potential energy surfaces separately.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.468431
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  • 4
    Digitale Medien
    Digitale Medien
    Theoretical studies of He(1S)+CH(X 2Π). II. Fully ab initio cross sections for the inelastic scattering and comparison with experiment (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1338-1349 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report a series of full close-coupling calculations of integral cross sections for fine structure resolved, rotationally inelastic transitions of CH induced by collisions with He. These calculations use the necessary two CH(X 2Π)+He potential energy surfaces as determined by a variety of ab initio techniques described in the preceeding paper. The calculated N=1→N=2−7 cross sections confirm a previous prediction of preferential population of final state levels in which the electronic wave function of the CH molecule is antisymmetric with respect to reflection in the plane of rotation of the molecule. A generally good overall agreement is found between the energy-dependent cross sections determined in earlier experiments of Macdonald and Liu and appropriate averages of the calculated cross sections. However, small, systematic, qualitative discrepancies persist between theory and experiment. Diagnostic calculations were carried out to identify the cause of these discrepancies. These calculations explored the influence of restrictions in basis set, configuration interaction, and functional forms used in fitting the potential energy surfaces. They also explored the consequences of the dynamical approximations of CH as a rigid rotor and the independence of the CH spin–orbit constant on the approach of the He partner. All these diagnostic calculations generally confirm the anticipated marginal influence of these approximations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466611
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  • 5
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.454390
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  • 6
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.453010
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  • 7
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.452701
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  • 8
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.451188
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  • 9
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.449311
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  • 10
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.449521
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