In the last 30 days

• Open/close Multiple-harmonic conversion of 1064 nm radiation in rare gases

  M Ferray et al 1988 J. Phys. B: At. Mol. Opt. Phys. 21 L31 Tag this article

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  The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 10 ¹³ W cm ^-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

• Open/close Long-range interactions and state characteristics of interacting Rydberg atoms

  S I Simonsen et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 165001 Tag this article

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  We study the long-range interaction between mutually excited hydrogen atoms in principal quantum numbers n = 4, 8, 16. The quasimolecular states are constructed from the basis of hydrogen-like product states with the configuration interaction method and with the inclusion of all order multipole moments of the total electrostatic interaction. Several common features appear, which are n-insensitive, and will thus remain relevant for the coherent control of cold Rydberg systems with higher values of n. The energy curves are shown to be attractive and repulsive, generally non-intersecting and the repulsion splitting is stronger than the attractive. The electronic probability densities of the selected states are studied as well as their relation to the single product states consisting of linear Stark states (most aligned) on each atom. One of the observed features is that the least bound states are always characterized by a high degree of polarization favoured towards the molecular centre, and therefore well approximated by a product of two Stark states. The most bound states are similarly expressed as a coherent combination of such states which secure symmetry with respect to electron exchange and parity. Coherent control between the attractive and repulsive states can be obtained with almost 100% probability in time-dependent resonant monochromatic microwave fields.

• Open/close Numerical comparison of the Kohn and Schwinger variational methods with Ladanyi's approach for potential scattering

  P Levay and B Apagyi 1988 J. Phys. B: At. Mol. Opt. Phys. 21 3741 Tag this article

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  The Kohn and Schwinger variational methods are applied to electron scattering by local spherical potentials and the numerical results are compared with those of Ladanyi's method (1970) based on the standard form of the Schrodinger equation. Both the stability and the convergence of the results are carefully analysed and, in the case of Ladanyi's method, the measure of the error of the approximate solution is investigated as well. The analysis implies that Ladanyi's method is competitive in comparison with the standard variational procedures.

• Open/close Enhancing the entanglement of a teleported state by local collective noises

  Xueyuan Hu et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 075504 Tag this article

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  We show that the entanglement of the two-qubit teleported state via a class of four-qubit entangled channel states can be increased by collective amplitude damping locally acting on one part of the channel state. Specifically, we compare the entanglement contained in the output state of teleportation before and after the action of the collective amplitude damping on the channel state, and show that for a wide range of input entangled two-qubit states, the local decoherence can result in an increase in the output entanglement. In this process, the average fidelity of the teleportation is also increased. Our result reveals that some quantum properties of the four-qubit channel state are definitely improved in the process of enhancing the fidelity by local noise.

• Open/close Enhancement of high-order harmonic generation in the presence of noise

  I Yavuz et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 135403 Tag this article

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  We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrödinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.

• Open/close Path-integral-based evaluation of the probability of hydrogen atom ionization by short photo-pulse

  A B Bichkov et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 135601 Tag this article

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  A scheme of path integral evaluation based on the saddle-point method was used for the calculation of the probability of hydrogen atom ionization by short photo-pulse. Gained results are compared with additional calculations based on time-dependent perturbation theory and the time-dependent Schrödinger equation solution by means of a finite element method and with the results of the available data based on a discrete variable representation.

• Open/close Elastic scattering of slow electrons from Y, Ru, Pd, Ag and Pt atoms: search for nanocatalysts

  Z Felfli et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 135204 Tag this article

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  Electron elastic scattering total cross sections (TCSs) for Y, Ru, Pd, Ag and Pt atoms are investigated in the electron impact energy range 0 ≤ E ≤ 7.0 eV. The complex angular momentum method that incorporates the electron–electron correlations and core-polarization interactions, crucial for the existence and stability of most negative ions, is used for the calculations. The TCSs for these atoms are found to be characterized by shape resonances (short-lived resonances), Ramsauer–Townsend minima and very sharp resonances (long-lived resonances) that correspond to the existence of stable bound states of the negative ions formed during the collision as Regge resonances. Using the configuration of the negative-ion resonances and Ramsauer–Townsend minima in the Au TCS as the benchmark for nanocatalysts, we conclude that the Y, Ru, Pd, Ag and Pt atoms represent excellent candidates for nanocatalysts individually or in various combinations. Calculated electron elastic TCSs for Y, Ru, Pd, Ag and Pt atoms are presented as illustrations.

• Open/close Real stabilization of resonance states employing two parameters: basis-set size and coordinate scaling

  Federico M Pont et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 135003 Tag this article

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  The resonance states of one- and two-particle Hamiltonians are studied using variational expansions with real basis-set functions. The resonance energies, E _r, and widths, Γ, are calculated using the density of states and an golden rule-like formula. We present a recipe to select adequately some solutions of the variational problem. The set of approximate energies obtained shows a very regular behaviour with the basis-set size, N. Indeed, these particular variational eigenvalues show a quite simple scaling behaviour and convergence when N → ∞. Following the same prescription to choose particular solutions of the variational problem we obtain a set of approximate widths. Using the scaling function that characterizes the behaviour of the approximate energies as a guide, it is possible to find a very good approximation to the actual value of the resonance width.

• Open/close Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p^{2 1}S₀ doubly excited state

  Anastasios Dimitriou et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 135001 Tag this article

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  We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two- and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p ^{2 1}S ₀ resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength- and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.

• Open/close Fast Padé transform in the theory of resonances: exact solution of the harmonic inversion problem

  Dževad Belkić and Karen Belkić 2011 J. Phys. B: At. Mol. Opt. Phys. 44 125002 Tag this article

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  We present the fast Padé transform (FPT) as a polynomial quotient for the Green or response function from signal processing, spectroscopy and resonant scattering theory. Specific illustrations are given for nuclear magnetic resonance spectroscopy within the problem of harmonic inversion, quantification or spectral analysis. Here, the input time signal points or auto-correlation functions are given via measurements or computations, and the task is to reconstruct the unknown components as the harmonic variables in terms of the fundamental complex frequencies and amplitudes. The FPT solves the harmonic inverse problem exactly by retrieving the true number of resonances with all their proper spectral parameters. This output list is finalized by means of Froissart doublets or pole-zero confluences for unequivocal disentangling of the physical/genuine from unphysical/spurious contents of the analysed time signal. Stability of investigated systems under external perturbations is especially challenged by the presence of noise. The FPT can assess the system's stability through determining the locations and distributions of spectral poles and zeros in the complex frequency plane. This permits identification of the regions that are void of noise, and gives the possibility for improved system performance under more stable conditions with full signal-noise separation. The FPT can provide a number of important biophysical and chemical quantities, including the density of states and abundance or concentrations of all the physical constituents from the investigated substance.