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    A critique of the two level approximation
    (Springer New York, 2006) Hakioǧlu, Tuǧrul; Savran, Kerim; Meşe, Emine
    The conditions in favor and necessity of a realistic multileveled description of a decohering quantum system is examined. Under these conditions approximate techniques to simplify a multileveled system by its first two levels is unreliable and a realistic multilevel description in the formulation of decoherence is unavoidable. In this regard, the first crucial observation is that, the validity of the two level approximation of a multileveled system is not controlled purely by sufficiently low temperatures. That the type of system-environment coupling and the environmental spectrum have a dominant role over the temperature is demonstrated. Particularly, zero temperature quantum fluctuations induced by the Caldeira-Leggett type linear coordinate coupling can be influential in a wide energy range in the systems allowed transitions. The second crucial observation is that the decoherence times being among the system's short time scales are found to be dominated not by the resonant but non-resonant processes.
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    Öğe
    The quasienergy spectrum of argon from 400 to 800 nm
    (Iop Publishing Ltd, 2007) Meşe, Emine; Potvliege, R. M.; 0009-0001-0120-4721
    The Floquet quasienergy spectrum of argon atoms exposed to a strong laser field is presented for incident wavelengths of 400, 600, and 800 nm and electron excursion amplitudes up to 30 a.u. The calculation is performed by diagonalizing the Floquet Hamiltonian on a discrete basis of Sturmian functions, the atom being described by a one-electron model potential. How the quasienergies of the dressed bound states change as functions of both the wavelength and the intensity of the incident field is studied. The spectrum of the dressed excited states varies slowly with these two parameters, apart for a number of changes between direct and avoided crossings and for less frequent, more profound reorganizations. The spectrum converges at high intensity towards that predicted by the high-frequency Floquet theory.