277 research outputs found

    Laser-induced nonsequential double ionization: kinematic constraints for the recollision-excitation-tunneling mechanism

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    We investigate the physical processes in which an electron, upon return to its parent ion, promotes a second electron to an excited state, from which it subsequently tunnels. Employing the strong-field approximation and saddle-point methods, we perform a detailed analysis of the dynamics of the two electrons, in terms of quantum orbits, and delimit constraints for their momentum components parallel to the laser-field polarization. The kinetic energy of the first electron, upon return, exhibits a cutoff slightly lower than 10Up10U_p, where UpU_p is the ponderomotive energy, as in rescattered above-threshold ionization (ATI). The second electron leaves the excited state in a direct ATI-like process, with the maximal energy of 2Up2U_p. We also compute electron-momentum distributions, whose maxima agree with our estimates and with other methods.Comment: 13 pages, 4 figure

    Quantum interference in laser-induced nonsequential double ionization in diatomic molecules: the role of alignment and orbital symmetry

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    We address the influence of the orbital symmetry and of the molecular alignment with respect to the laser-field polarization on laser-induced nonsequential double ionization of diatomic molecules, in the length and velocity gauges. We work within the strong-field approximation and assume that the second electron is dislodged by electron-impact ionization, and also consider the classical limit of this model. We show that the electron-momentum distributions exhibit interference maxima and minima due to the electron emission at spatially separated centers. The interference patterns survive the integration over the transverse momenta for a small range of alignment angles, and are sharpest for parallel-aligned molecules. Due to the contributions of transverse-momentum components, these patterns become less defined as the alignment angle increases, until they disappear for perpendicular alignment. This behavior influences the shapes and the peaks of the electron momentum distributions.Comment: 12 pages, 7 figures; some discussions have been extended and some figures slightly modifie

    The quantum brachistochrone problem for non-Hermitian Hamiltonians

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    Recently Bender, Brody, Jones and Meister found that in the quantum brachistochrone problem the passage time needed for the evolution of certain initial states into specified final states can be made arbitrarily small, when the time-evolution operator is taken to be non-Hermitian but PT-symmetric. Here we demonstrate that such phenomena can also be obtained for non-Hermitian Hamiltonians for which PT-symmetry is completely broken, i.e. dissipative systems. We observe that the effect of a tunable passage time can be achieved by projecting between orthogonal eigenstates by means of a time-evolution operator associated with a non-Hermitian Hamiltonian. It is not essential that this Hamiltonian is PT-symmetric
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