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  • 1
    Electronic Resource
    Electronic Resource
    Correcting the coupled states approximation using sensitivity analysis (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5113-5116 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A sensitivity approach to the correction of the coupled states (CS) approximation, as originally proposed by Eno and Rabitz, is here examined by application. A related approach, developed by McLenithan and Secrest, is also examined. Results from the two approaches, for the rotational scattering of Ne+HD, are compared. We find that each has its preferred domain of applicability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461678
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  • 2
    Electronic Resource
    Electronic Resource
    Perturbative reactive scattering within a quasiadiabatic representation: Multichannel application (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 895-899 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Low energy reactive transition probabilities for a model multichannel collision problem, are determined within a so-called quasiadiabatic (QA) representation of the system electronic energy. The procedure involves setting up a set of coupled nonreactive surfaces (the QA representation) and then perturbatively mixing coupled-channel wave functions on the QA surfaces. It is applied to a hard-sphere-type model of the collinear A+BC reaction and for a relatively high system mass (5.0×104 a.u.). Optimization of the representation (which we have previously argued should temper maximization of the QA reactivity with a drive for balance between its diabatic and nonadiabatic components) yields results which are in very good agreement with exact ones (errors 〈10%) over a wide range of collision energies. At the same time, as the collision energy approaches the classical reactive threshold, we see evidence of QA failure; we trace this to difficulties with our particular optimization procedure when the diabatic contribution becomes dominant. "Conventional'' perturbative results are generated for the same model problem and found to be poor in general (errors (approximately-equal-to)40%–50%). It is demonstrated that the ineffectiveness of the conventional approach may be ascribed to the system's high mass.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461097
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  • 3
    Electronic Resource
    Electronic Resource
    Configuration dependent crossed beam reactive scattering in an intense laser field (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1652-1657 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We model the reaction between crossed beams of atoms A and diatoms BC in an intense nonresonant laser field. Cross sections are in particular determined for laser switching between a pair of one-dimensional potential curves which are free to rotate in space. The cross sections are dependent on the configuration or orientation of the laser field with respect to the initial relative velocity vector of the reactants; for computational simplicity, many of our calculations are performed for coincident field and relative velocity vectors. Contributions to the cross sections arising from interference between the phases of transition matrix elements, are highlighted by comparing with ones determined by random phase approximation. We also make comparison with the corresponding configuration averaged (and phase independent) cross section. Photon energy dependencies for the various cross sections are obtained at several field intensities and collision energies. We find that the shifting with photon and collision energy, of the range of partial waves from which a cross section draws, can dramatically affect the relationship between phase dependent and independent cross sections; the shifting modifies in particular the interference between transition elements. For a single field intensity and photon and collision energy, we also generate cross sections as a function of configuration angle. The structure of the configuration dependence is elucidated by decomposition into terms which are characterized by numbers of contributing photons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459093
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  • 4
    Electronic Resource
    Electronic Resource
    Photon angular momentum approximations for molecular collisions in an intense nonresonant laser field (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3579-3583 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider the problem of photon angular momentum approximation for molecular collisions in an intense nonresonant laser field. A couple of orientation averaging schemes are proposed for dealing with the absence of rotational invariance in the full (inclusive of photon angular momentum) dynamical equations. Application of the schemes is made for reactive laser switching between a pair of one-dimensional potential curves which are free to rotate in space. The preferred scheme of the two depends upon there being only a single initial or final state of interest but both are very effective over the range of intensities examined. A third approximation scheme which essentially blots out photon angular momentum effects is less satisfactory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454927
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  • 5
    Electronic Resource
    Electronic Resource
    A new perspective on local scaling analysis (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3859-3860 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a recent paper, an energy-sudden-based local scaling analysis has been introduced. We show that this local analysis can be incorporated into a much more general scaling framework−a framework which encompasses the "regular'' energy sudden scaling relations. We also make some suggestions with regard to utilizing the general framework in extending present scaling theoretic analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462917
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  • 6
    Electronic Resource
    Electronic Resource
    Mass effects and channel coupling sensitivity in vibrational energy transfer (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1711-1719 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper examines the mass dependences of vibrationally inelastic integral state-to-state cross sections for the helium-hydrogenic scattering system. The cross sections are calculated within the coupled states-exponential distorted wave (CS-EDW) approximation, and are analyzed along with the conceptually simpler distorted wave (CS-DW) cross sections. Quantitative links between the EDW and DW results are made by means of component (or channel coupling) sensitivity coefficients, which measure the sensitivity of a given EDW cross section to the potential matrix elements coupling any two states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456064
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  • 7
    Electronic Resource
    Electronic Resource
    Three-vector correlation theory for orientation/alignment studies in atomic and molecular collisions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5532-5541 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The laboratory integral cross section under crossed-beam conditions is examined for collisions between a structured and an unstructured species, where the orientation (or alignment) of the initial and final states is determined with respect to arbitrarily configured laboratory axes. Three parameters are necessary to characterize the mutual orientation of the relevant axes in this case and these parameters then also characterize the cross section. The laboratory cross section is expanded in terms of a more fundamental (angle-independent) set of cross sections as determined in the so-called collision frame, where projections of the structured species are taken with respect to the direction of the initial relative velocity vector. Drawing upon a number of symmetry relations, we count the numbers of fundamental cross sections for arbitrary initial and final angular momenta. Then we consider the explicit dependence on angular parameters which can be anticipated for the laboratory cross section. Finally, we raise a number of experimental considerations in attempting to determine the fundamental cross sections. These considerations are "fleshed out'' by applying our cross-section analysis to a recent three-vector correlation experiment involving collisions between Ca and He.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463786
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  • 8
    Electronic Resource
    Electronic Resource
    Laser preparation and probing of initial and final orbital alignment in collision-induced energy transfer Ca(4s5p, 1P1) +He→Ca(4s5p, 3P2)+He (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 8212-8224 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a crossed beam atomic energy transfer experiment, relative cross sections are measured between initial and final magnetic substates of atomic orbitals in a three vector correlation experiment. A pulsed laser beam prepares Ca(4s5p 1P1) in a single magnetic sublevel ||j1mi〉 with respect to the laser polarization vector. Subsequent collision with He at a well-defined relative velocity yields Ca(4s5p 3P2). The near-resonant Ca(4s5p 3P2) is probed by a second polarized pulsed laser, revealing its magnetic sublevel ||j2mf〉 distribution with respect to the probe laser polarization vector. The experiment is analyzed in the collision frame where the direction of the initial relative velocity vector serves as the quantization axis. In this frame, the initial and final Ca states are characterized by substates ||j1m1〉 and ||j2m2〉, respectively. Fourteen collision frame cross sections are needed to describe the energy transfer completely. Eight of these cross sections are real and positive (conventional type—describing population transfer) and six are complex (coherence type—containing phase information). By symmetry, only 15 unique parameters for the real and imaginary parts of the cross sections are required, nine of which are obtained here using linear polarizations and collinear laser beams.Possible cases for circularly polarized light are also tabulated; measurement of these cases would provide several more parameters. For initial parallel preparation of the 1P1 p orbital with respect to the relative velocity (m1=0), the final 3P2 state is highly aligned in the m2=±1 and m2=±2 sublevels. Initial preparation of the p orbital perpendicular to the relative velocity vector (m1=±1) produces an aligned 3P2 state with greater population in the m2=0 and m2=±1 sublevels compared to the m2=±2 sublevels. Using the coherence information and symmetry, orientation cross sections are obtained into the m2=±1 sublevels (i.e., m preserving 1→1 and m-changing 1→−1) and are nearly identical. Preparation of the Ca p orbital perpendicular to the initial relative velocity enhances the state-specific collisional transfer of 1P1→3P2 by a factor of 2.2±0.2 over initial parallel preparation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462326
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  • 9
    Electronic Resource
    Electronic Resource
    A new photon angular momentum approximation for molecular collisions in intense nonresonant laser fields (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 1658-1664 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We here explore a new scheme for dealing with the photon angular momentum effects which arise in the analysis of collisions in intense nonresonant laser fields. The scheme involves approximating the system total angular momentum so as to create a reduced set of dynamical equations depending parametrically upon the relative orientation of laser field axis and system transition dipole moment. The equations are solved as a function of orientation and results are collected using numerical quadrature. We examine the scheme in application to a model collision problem. Comparing with the results of exact calculations we find that the new scheme is very effective in determining the magnitudes of S-matrix elements but not the corresponding phases. The scheme is also compared with an earlier one which involves predynamical orientation averaging of laser–particle coupling elements. We find that the new scheme is at least as accurate as the earlier and is much more tractable computationally. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468897
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  • 10
    Electronic Resource
    Electronic Resource
    Multimode Floquet theory for atomic and molecular laser-modified collision processes (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 1344-1349 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We outline a multimode-Floquet-based approach to the analysis of atomic and molecular laser-modified collision processes. The approach depends upon Fourier decomposition of the radiation field and identification of time-independent laser-interaction elements in the Floquet representation. The corresponding time-independent Floquet dynamical equations are solved and S-matrix elements between the various Floquet channels collected to determine the transition probabilities of interest. The approach is examined for laser excitation in a model energy transfer system. Results are compared with ones obtained conventionally, i.e., by explicitly following the time dependence of the field amplitude. We conclude that the Floquet method nicely complements the conventional. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470791
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