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  • 1
    Electronic Resource
    Electronic Resource
    Adiabatic time evolution of atoms and molecules in intense radiation fields (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2657-2665 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We derive the condition for a time dependent quantum system to exhibit an exact or higher order adiabatic time evolution. To this end, the concept of adiabaticity is first analyzed in terms of the transformation properties of the time-dependent Schrödinger equation under a general unitary transformation Uˆ(t). The system will follow an adiabatic time evolution, if the transformed Hamiltonian, Kˆ(t)=Uˆ°HˆUˆ−i(h-dash-bar)Uˆ°Uˆ, is divisible into an effective Hamiltonian hˆ(t), defining adiabatic quasistationary states, and an interaction term Ωˆ(t), whose effect on the adiabatic states exactly cancels the nonadiabatic couplings arising from the adiabatic states' parametric dependence on the time. This decoupling condition, which ensures adiabaticity in the system's dynamics, can be expressed in a state independent manner, and governs the choice of the unitary operator Uˆ(t), as well as the construction of the effective Hamiltonian hˆ(t). Using a restricted class of unitary transformations, the formalism is applied to the time evolution of an atomic or molecular system in interaction with a spatially uniform electromagnetic field, and gives an adiabatic approximation of higher order to the solutions of the semiclassical Schrödinger equation for this system. The adiabatic approximation so obtained exhibits two properties that make it suitable for the studies of intense field molecular dynamics: It is valid for any temporal profile of the field, and improves further as the field intensity increases, as reflected in the weakening of the associated residual nonadiabatic couplings with increasing field strength.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455963
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  • 2
    Electronic Resource
    Electronic Resource
    Nonperturbative wave packet dynamics of the photodissociation of H2+ in ultrashort laser pulses (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5497-5515 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The wave packet dynamics of the photodissociation of H2+ under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the coupled-channel wave functions in momentum space. These structures appear in the channels' momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser-induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric-field (EF) gauge, in which the matter–field interaction is of the length form and the radiation-field (RF) gauge, in which it is of the velocity form.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463783
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  • 3
    Electronic Resource
    Electronic Resource
    Dynamical quenching of laser-induced dissociations of heteronuclear diatomic molecules in intense infrared fields (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 4737-4749 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This article explores the influence of permanent dipole moments, i.e., of direct vibrational excitations, on the dynamical dissociation quenching (DDQ) effect, a mechanism for laser-induced vibrational trapping in the infrared (IR) spectral range which was recently demonstrated for the homonuclear H2+ ion, and was shown to result from a proper synchronization of the molecular motions with the oscillations of the laser electric field [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)]. To this end, the wave packet dynamics of the HD+ and, to a lesser extent, the HCl+ molecular ions are considered in an intense IR laser field of variable frequency. Variations in the absolute phase of the laser electric field, a form of variations in the initial conditions, reveal new signatures of the DDQ effect due to the presence of nonzero permanent dipole moments in these molecules. The added permanent dipole/field interaction terms induce a discrimination between parallel and antiparallel configurations of the aligned molecule with respect to the laser's instantaneous electric field. As a result, molecules that are prepared antiparallel to the field at peak intensity find their dissociation quenched most efficiently, while those that are prepared parallel to the field are strongly dissociative. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478361
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular dynamics in intense fields. III. Nonadiabatic effects (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2840-2850 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that coupled equations for multiphoton processes can be written with either momentum transition moments or dipole transition moments. In perturbation (low field intensity) theories, nonadiabatic corrections are necessary to make the adiabatic momentum electronic transition probabilities nonzero and equivalent to electronic dipole transition probabilities in infrared transitions. For strong fields, nonadiabatic interactions can become important due to multiphoton resonances, and both approaches necessitate full coupled equations. In the case of electronic excitations, adiabatic, nuclear dipole transition probabilities become nonzero and equivalent to nuclear momentum transitions probabilities only after nonadiabatic corrections are included. These corrections should become important at strong fields for electronic states coupled nonadiabatically. It is shown further using a representation which introduces field modified electronic functions and electronic potentials that unusual field effects will occur at photon frequencies in resonance with adiabatic electronic potential energy separations at pseudo (avoided) crossings. Thus, in such cases, electronic energy gaps are increased by the field and nonadiabatic couplings at these avoided crossings are reduced by the field. Strong resonant fields should therefore quench nonadiabatic couplings at avoided crossings, rendering nonadiabatic reactions more adiabatic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449234
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  • 5
    Electronic Resource
    Electronic Resource
    Higher order adiabatic separation of strongly coupled systems. I. Two-body systems and dressed hydrogen atom (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 7224-7231 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The phase corrected adiabatic separation method proposed recently by Marechal [J. Chem. Phys. 83, 247 (1985)] is reexamined. We have been able to show that a state independent differential equation exists, which governs the phase coupling function f of this method, if f further satisfies a set of constraints, Eqs. (7) of the present paper. The method is illustrated by its application to three simple, exactly soluble two-body systems: the free hydrogen atom, the linearly forced hydrogen atom, and the system of two harmonically coupled oscillators. We show that for these systems, the present method, which employs an adiabatic separation procedure, after phase correcting the Hamiltonian, leads to exact results, suggesting its utility for more general problem, involving nonseparable potentials. A nonseparable system of particular interest is the dressed hydrogen atom, which is a representative of all dressed systems, molecular or atomic, with respect to radiative couplings. The method of phase corrected adiabatic separation is shown to give, for field frequencies lying below an IR threshold, a better representation for the exact eigenstates of this dressed systems, as ensuing residual couplings are estimated to be of second order in the original radiative coupling constant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451359
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