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Showing papers in "Journal of Physics B in 2003"


Journal ArticleDOI
TL;DR: In this article, the stability of a qubit based on the S 1/2 -D5/2 transition of single 40Ca+ ions in a linear Paul trap is investigated.
Abstract: Two-level ionic systems, where quantum information is encoded in long-lived states (quantum bits, qubits), are discussed extensively for quantum information processing. We present a collection of measurements which characterize the stability of a qubit based on the S1/2 – D5/2 transition of single 40Ca+ ions in a linear Paul trap. We find coherence times of 1 ms, discuss the main technical limitations and outline possible improvements.

141 citations


Journal ArticleDOI
TL;DR: In this article, a pseudospectral method for solving the three-dimensional time-dependent Gross-Pitaevskii (GP) equation was proposed, and it was used to study the resonance dynamics of a trapped Bose-Einstein condensate induced by a periodic variation in the atomic scattering length.
Abstract: We suggest a pseudospectral method for solving the three-dimensional time-dependent Gross–Pitaevskii (GP) equation, and use it to study the resonance dynamics of a trapped Bose–Einstein condensate induced by a periodic variation in the atomic scattering length. When the frequency of oscillation of the scattering length is an even multiple of one of the trapping frequencies along the x, y or z direction, the corresponding size of the condensate executes resonant oscillation. Using the concept of the differentiation matrix, the partial-differential GP equation is reduced to a set of coupled ordinary differential equations, which is solved by a fourth-order adaptive step-size control Runge–Kutta method. The pseudospectral method is contrasted with the finite-difference method for the same problem, where the time evolution is performed by the Crank–Nicholson algorithm. The latter method is illustrated to be more suitable for a three-dimensional standing-wave optical-lattice trapping potential.

117 citations


Journal ArticleDOI
TL;DR: In this article, the current status of inner-shell spectroscopies of free atoms and molecules using high-resolution soft-x-ray monochromators installed in the soft x-ray beamlines at the third-generation synchrotron radiation facilities is reviewed.
Abstract: This article reviews the current status of inner-shell spectroscopies of free atoms and molecules using high-resolution soft-x-ray monochromators installed in the soft-x-ray beamlines at the third-generation synchrotron radiation facilities. Beamlines and endstations devoted to atomic and molecular inner-shell spectroscopies and various types of experimental techniques, such as ion yield spectroscopy, resonant photoemission spectroscopy and multiple-coincidence momentum imaging, are described. Experimental results for K-shell excitation of Ne, O K-shell excitation of H2O and CO2, C K-shell excitation and ionization of CO2 and B K-shell excitation of BF3, obtained at beamline 27SU of SPring-8 in Japan, are discussed as examples of atomic and molecular inner-shell spectroscopies using the third-generation synchrotron radiation sources.

111 citations


Journal ArticleDOI
TL;DR: In this article, generalized cross sections for two-photon double ionization of He in the photon energy region between 40.7 and 47 eV were obtained in R-matrix Floquet theory.
Abstract: We determine generalized cross sections for two-photon double ionization of He in the photon energy region between 40.7 and 47 eV where absorption of two photons can lead to non-sequential double ionization only. The present cross sections, obtained in R-matrix Floquet theory, agree with cross sections obtained from time-dependent calculations. By examining the ratio of two-photon double ionization to two-photon single ionization, we demonstrate that core excitation effects at an intensity of 1013 W cm−2 are relatively unimportant at 45 eV, but that they are significant at other photon energies.

87 citations


Journal ArticleDOI
TL;DR: In this article, the authors discuss the evolution and applicability condition of various stabilization mechanisms of the nonstationary Schrodinger equation for strong laser field atomic stabilization, and their interpretation is considered.
Abstract: The phenomenon of strong laser field atomic stabilization is discussed. Earlier suggested models and mechanisms of stabilization are described: A- and V-type interference stabilization of Rydberg atoms, adiabatic (Kramers-Henneberger) and high-frequency stabilization of neutral atoms and negative ions, and so on. Both numerical and analytical approaches to the description of these phenomena are discussed. In this context, ab initio numerical solutions of the nonstationary Schrodinger equation, obtained by several groups of authors, are overviewed. Based on the most modern and recent solutions of this type, mechanisms of stabilization of a hydrogen atom are shown to vary with varying intensity and frequency of a laser field. Such an evolution and applicability condition of various stabilization mechanisms is described. Limitations arising due to relativistic effects are discussed. Existing experiments on strong-field stabilization are overviewed and their interpretation is considered.

86 citations


Journal ArticleDOI
TL;DR: In this paper, the spectrum, the electron density distribution and the character of three two-electron quantum systems were studied for different confinement strengths, ω, by using the quantum chemical configuration interaction (CI) method employing a Cartesian anisotropic Gaussian basis set supplemented by a quantum chemical standard Cartesian Gaussian base set, respectively, and a full CI wavefunction.
Abstract: The spectrum, the electron density distribution and the character of three two-electron quantum systems, namely the two-electron quantum dot, the negative hydrogen ion and the helium atom, confined by an anisotropic harmonic oscillator potential, have been studied for different confinement strengths, ω, by using the quantum chemical configuration interaction (CI) method employing a Cartesian anisotropic Gaussian basis set supplemented by a quantum chemical standard Cartesian Gaussian basis set, respectively, and a full CI wavefunction. Energy level diagrams and electron density distributions are displayed for low lying electronic singlet states and selected confinement parameters. The results for the three confined quantum systems are compared with each other: in general, the absolute energies of the states and the interval between states increase with increasing confinement, ω. The ordering of states may vary for different values of ω. The shape of the electron density distribution differs among the three systems. Its size increases in the order He < H− < two-electron quantum dot. Electron distributions in prolate type confinement are stretched along the z axis, while those in oblate type are compressed with respect to this axis. The CI wavefunction of most of the low lying states for the helium atom and the negative hydrogen ion is dominated by one configuration, while some of the CI wavefunctions corresponding to states of the two-electron quantum dot have more than one leading configuration. The ground state CI wavefunctions of the negative hydrogen ion and the helium atom confined in a spherical potential are dominated by harmonic oscillator functions up to 82.4 and 47.1%, respectively, for the smallest confinement of ω = 0.1. These values increase to 98.8 and 89.0%, respectively, for ω = 0.5.

85 citations


Journal ArticleDOI
TL;DR: In this article, the authors used the Bogoliubov theory of atoms in an optical lattice to study the approach to the Mott-insulator transition and derived an explicit expression for the superfluid density based on the rigidity of the system under phase variations.
Abstract: We use the Bogoliubov theory of atoms in an optical lattice to study the approach to the Mott-insulator transition. We derive an explicit expression for the superfluid density based on the rigidity of the system under phase variations. This enables us to explore the connection between the quantum depletion of the condensate and the quasi-momentum distribution on the one hand and the superfluid fraction on the other. The approach to the insulator phase may be characterized through the filling of the band by quantum depletion, which should be directly observable via the matter–wave interference patterns. We complement these findings by self-consistent Hartree–Fock–Bogoliubov–Popov calculations for one-dimensional lattices, including the effects of a parabolic trapping potential.

80 citations


Journal ArticleDOI
TL;DR: The relativistic version of the binary-encounter Bethe (BEB) model was used to calculate cross sections for K-shell ionization of atoms by electron impact as discussed by the authors.
Abstract: The relativistic version of the binary-encounter Bethe (BEB) model is used to calculate cross sections for K-shell ionization of atoms by electron impact. The BEB model requires only two atomic constants, the binding energy and kinetic energy of the K electrons. These constants are listed for carbon to antimony. Comparisons with available experimental data on N, O, Na, Al, Cl, Ca, Cu, Se and Sb show good agreement. The K-shell ionization cross sections for C, Mg, P, S, Cr, As and Cd are tabulated.

73 citations


Journal ArticleDOI
TL;DR: In this paper, the authors considered the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (0.1 )ps) laser pulse and derived the center-of-mass motion of the electron cloud driven by the strong laser field using the approximation of an incompressible medium.
Abstract: We consider the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (0.1 ?ps) laser pulse. The equation of the centre-of-mass motion of the electron cloud driven by the strong laser field is derived using the approximation of an incompressible medium. The analysis of this equation demonstrates the presence of odd harmonics of the fundamental frequency in the cluster dipole moment, and in both the internal and the scattered electric field. Both neutral and ionized clusters are considered. For clusters with radii R 100?? irradiated by a femtosecond titanium?sapphire laser with a peak intensity of I 1016?W?cm?2, the internal electric field strength near the tripled fundamental frequency is shown to be of the same order as the field of the fundamental. The reason is that both for metallic and for laser-ionized van der Waals clusters the Mie surface-plasmon energy ?M is around 5 eV, which is close to three times the energy of a titanium?sapphire laser-field quantum. On the other hand, the condition for first-order resonance with the Mie frequency is not met during the presence of the main laser pulse, but only temporarily, either at the first onset of inner ionization on the leading edge of the pulse (for van der Waals clusters) or during the subsequent Coulomb explosion. In both cases, the ion density is reduced. The presence of a strong third harmonic leads, in particular, to the enhanced production of multiply charged ions in clusters irradiated by a strong laser field, as compared with isolated atoms. This point is discussed in the light of recent experimental results on the production of multiply charged ions in laser?cluster experiments. Third-harmonic generation by a cluster in a strong laser field, as a function of both the cluster and the laser-field parameters, is also considered.

73 citations


Journal ArticleDOI
TL;DR: In this article, the authors used correlated electron-ion momentum measurements to investigate laser-induced non-sequential double ionization of Ar and Ne light intensities in a regime at and below the threshold where, within the rescattering model, electron impact ionization was expected to become energetically forbidden, yet they found Ar2+ ion momentum distributions and an electron-electron momentum correlation indicative of direct impact ionisation.
Abstract: We use correlated electron–ion momentum measurements to investigate laser-induced non-sequential double ionization of Ar and Ne Light intensities are chosen in a regime at and below the threshold where, within the rescattering model, electron impact ionization of the singly charged ion core is expected to become energetically forbidden Yet we find Ar2+ ion momentum distributions and an electron–electron momentum correlation indicative of direct impact ionization Within the quasistatic model this may be understood by assuming that the electric field of the light wave reduces the ionization potential of the singly charged ion core at the instant of scattering The width of the projection of the ion momentum distribution onto an axis perpendicular to the light beam polarization vector is found to scale with the square root of the peak electric field strength in the light pulse A scaling like this is not expected from the phase space available after electron impact ionization It may indicate that the electric field at the instant of scattering is usually different from zero and determines the transverse momentum distribution A comparison of our experimental results with several theoretical results is given

72 citations


Journal ArticleDOI
TL;DR: In this article, a molecular three-continuum approximation is employed to compute differential cross sections for the ionization of hydrogen molecules by electron impact, and the chosen final electronic wave function takes into account the molecular character of the target as well as the correlate motion between the aggregates in the final channel of the reaction.
Abstract: A recently developed molecular three-continuum approximation is employed to compute differential cross sections for the ionization of hydrogen molecules by electron impact. Within the framework of this approximation, the chosen final electronic wavefunction takes into account the molecular character of the target as well as the correlate motion between the aggregates in the final channel of the reaction. Fivefold-differential cross sections as a function of both the electron momenta in the final state and the molecular orientation are studied for different kinematical arrangements. Interference structures coming from the two-centre geometry of the molecule are predicted in this case. Integrated cross sections over all molecular orientations are also calculated. It is shown that interference patterns remain, even for this case.

Journal ArticleDOI
TL;DR: In this article, the threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron-molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum.
Abstract: The threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron–molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH4+/CH 4) = 12.65 ± 0.4 eV, IE1(CH3+/CH 4) = 13.58 ± 0.1 eV (ion pair) and IE2(CH3+/CH 4) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH4+/CH 4 and CH3+/CH 4 processes, which reflects the change in the internal energy of CH4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH2+/CH 4) = 15.1 ± 0.1 eV, IE(CH+/CH 4) = 19.8 ± 0.1 eV and IE(C+/CH 4) = 20.5 ± 0.2 eV.

Journal ArticleDOI
TL;DR: In this paper, photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between σ-and π-type orbitals, and the calculated spectra agree well with the experimental results in the outer valence region and have proved to be indispensable for interpreting the structure at higher binding energies where the single particle model of ionization breaks down.
Abstract: The valence shell photoelectron spectrum of purine has been studied experimentally and theoretically. Synchrotron radiation has been used to record spectra at photon energies of 45 and 85 eV. Photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between σ-and π-type orbitals. Vertical ionization energies and photoelectron spectral intensities have been evaluated using the many-body Green function method. The calculated spectra agree well with the experimental results in the outer valence region and have proved to be indispensable for interpreting the structure at higher binding energies where the single particle model of ionization breaks down. The photoelectron spectrum of pyrimidine has also been studied and is compared to that of purine.

Journal ArticleDOI
TL;DR: In this article, local model potentials that simulate the self-energy effects on atomic energy levels and magnetic dipole hyperfine integrals are presented for use in relativistic atomic, molecular or solid-state calculations.
Abstract: Local model potentials that simulate the self-energy effects on atomic energy levels and magnetic dipole hyperfine integrals are presented. They are proposed for use in relativistic atomic, molecular or solid-state calculations.

Journal ArticleDOI
TL;DR: In this paper, the performance of the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN has been considerably enhanced by the implementation of a magnetron phase locking mechanism.
Abstract: During the last two years, the performance of the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN (Geneva) has been considerably enhanced. Many technical improvements have been completed (i) to access nuclides that are produced in minute quantities of only 100 ions s−1, (ii) to increase the relative mass accuracy to ≤ 1 × 10−8 and (iii) to make accessible nuclei with a half-life of down to ≈ 5 ms. The major steps are presented, in particular the recent implementation of a magnetron phase locking mechanism which results in a significant reduction of the duration of ISOLTRAP's cyclotron measurements.

Journal ArticleDOI
TL;DR: In this paper, the authors presented a method to extract momentum distributions from time-dependent wavepacket calculations. But their method was limited to the ionization of model atoms and the dissociation of H2+, and they were not able to extract reliable fragment momentum distributions by using a comparatively small spatial numerical grid for the timedependent wavefunction.
Abstract: We present a new method to extract momentum distributions from time-dependent wavepacket calculations. In contrast to established Fourier transformation of the spatial wavepacket at a fixed time, the proposed 'virtual detector' method examines the time dependence of the wavepacket at a fixed position. In first applications to the ionization of model atoms and the dissociation of H2+, we find a significant reduction of computing time and are able to extract reliable fragment momentum distributions by using a comparatively small spatial numerical grid for the time-dependent wavefunction.

Journal ArticleDOI
TL;DR: In this paper, the potential energy curve and the electron density distribution for low-lying electronic states of the helium and lithium atoms confined by an anisotropic harmonic oscillator potential have been studied for different confinement parameters.
Abstract: The potential energy curve and the electron density distribution for low-lying electronic states of the helium and lithium atoms confined by an anisotropic harmonic oscillator potential have been studied for different confinement parameters (ωx, ωy, ωz) by using the quantum chemical configuration interaction (CI) method employing a Cartesian anisotropic Gaussian basis set supplemented by a quantum chemical standard Cartesian Gaussian basis set, respectively. In the case of the helium atom full CI wavefunctions have been used, and in the case of the lithium atom multi-reference CI wavefunctions have been employed. The results for the two confined quantum systems are compared to each other: parabolic-and non-parabolic-type potential energy curves are observed for states of both atoms in different type harmonic oscillator confinement. The non-parabolic potentials always appear in pairs, one with a double minimum and the other with a sharp edge at the minimum. They result from the interaction of energetically nearby states of the same symmetry. The electron density distributions of the states of the two atoms show deformation from spherical symmetry according to the symmetry and strength of the confining potential at the position of the atoms. The electron distribution of the states having non-parabolic potential energy curves is strongly deformed and occasionally shows curvilinear nodal planes of parabolic shape.

Journal ArticleDOI
TL;DR: In this paper, an R-matrix with pseudo-state (RMPS) calculation of electron-impact excitation in C2+ was performed, and the collision strengths and effective collision strengths were determined for excitation between the lowest 24 terms, including all those arising from 2s3l and 2s4l configurations.
Abstract: We have performed an R-matrix with pseudo-states (RMPS) calculation of electron-impact excitation in C2+. Collision strengths and effective collision strengths were determined for excitation between the lowest 24 terms, including all those arising from the 2s3l and 2s4l configurations. In the RMPS calculation, 238 terms (90 spectroscopic and 148 pseudo-state) were employed in the close-coupling (CC) expansion of the target. In order to investigate the significance of coupling to the target continuum and highly excited bound states, we compare the RMPS results with those from an R-matrix calculation that incorporated all 238 terms in the configuration-interaction expansion, but only the lowest 44 spectroscopic terms in the CC expansion. We also compare our effective collision strengths with those from an earlier 12-state R-matrix calculation (Berrington et al 1989 J. Phys. B: At. Mol. Opt. Phys. 22 665). The RMPS calculation was extremely large, involving (N + 1)-electron Hamiltonian matrices of dimension up to 36 085, and required the use of our recently completed suite of parallel R-matrix programs. The full set of effective collision strengths from our RMPS calculation is available at the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center web site.

Journal ArticleDOI
TL;DR: In this paper, experimental and theoretical cross sections for the single ionization of He by heavy-ion impact for electrons emitted into the scattering plane are presented, and large discrepancies between the experimental data and theoretical predictions are found, which can at least be partly attributed to the use of hydrogenic wavefunctions.
Abstract: We present experimental and theoretical fully differential cross sections for the single ionization of He by heavy-ion impact for electrons emitted into the scattering plane. Data were obtained for 2 MeV amu−1 C6+ and 3.6 MeV amu−1 AuQ+ (Q = 24, 53) projectiles, corresponding to perturbations (projectile charge to velocity ratio) ranging from 0.7 to 4.4, a regime which is inaccessible for electron-impact ionization. We observe a decreasing recoil peak intensity (relative to the binary peak) and at the same time an increasing peak in the forward direction with increasing perturbations. Large discrepancies between the experimental data and theoretical predictions are found, which can at least be partly attributed to the use of hydrogenic wavefunctions.

Journal ArticleDOI
TL;DR: In this article, the influence of ion dynamics on the profiles of the Hα and Hβ lines of the hydrogen Balmer series was investigated and it was shown that the difference between the electron density values from both the Hβ and Hα lines is about 3%.
Abstract: In this work we have performed experimental research into the influence of ion dynamics on the profiles of the Hα and Hβ lines of the hydrogen Balmer series. In order to understand this influence the electron density of a microwave plasma column at atmospheric pressure is measured from the Stark broadening of both lines. However, in this case Kepple–Griem's theory is not used, as usual, but a new computational model based on the μ-ion model that includes the effect of the ion dynamics on the profiles. The results obtained show that the difference between the electron density values from both the Hα and Hβ lines is about 3%. So, it is possible to use the Hα line for the diagnosis of the electron density in those cases in which it is not possible to use the Hβ line; for example, when the Hβ line is not intense enough.

Journal ArticleDOI
TL;DR: In this paper, the authors used N-particle wavefunctions, which follow from the number-conserving Bogoliubov theory, to predict the outcome of a single realization of the experiment where two extreme scenarios and many combinations thereof are possible.
Abstract: The dark soliton created in a Bose–Einstein condensate becomes grey in the course of time evolution because its notch fills up with depleted atoms. This is the result of quantum mechanical calculations which describe the output of many experimental repetitions of creation of the stationary soliton, and its time evolution terminated by a destructive density measurement. However, such a description is not suitable to predict the outcome of a single realization of the experiment where two extreme scenarios and many combinations thereof are possible: one will see either (1) a displaced dark soliton without any atoms in the notch, but with a randomly displaced position, or (2) a grey soliton with a fixed position, but a random number of atoms filling its notch. In either case the average over many realizations will reproduce the mentioned quantum mechanical result. In this paper we use N-particle wavefunctions, which follow from the number-conserving Bogoliubov theory, to settle this issue.

Journal ArticleDOI
TL;DR: In this paper, a weakly bound electron with an energy E 0 in the threedimensional δ-model potential (which approximates the short-range potential of a negative ion) interacting with a strong monochromatic laser field having an electric vector F (ωt) is considered.
Abstract: The quasistationary quasienergy state approach (QQES) is applied to the analysis of partial (n-photon) decay rates and angular distributions (ADs) of photoelectrons produced by an elliptically polarized laser field. The problem is formulated for a weakly bound electron with an energy E0 in the threedimensional δ-model potential (which approximates the short-range potential of a negative ion) interacting with a strong monochromatic laser field having an electric vector F (ωt) .T he resu lts presented cover weak (perturbative), strong (nonperturbative), and superstrong field regimes as well as a wide interval of frequencies ω extending from the tunnelling (¯ hω �| E0|) and multiphoton (¯ ω |E0|). For a weak laser field, exact equations for the normalization factor and for the Fourier coefficients of the QQES wavefunction at the origin (|r |→ 0) (that are key elements of the QQES approach for a δ-model potential) as well as for the detachment amplitudes are analysed analytically using both standard Rayleigh– Schr¨ odinger perturbation theory (PT) in the intensity, I ,o f the lase rfi eld and Brillouin–Wigner PT expansions involving the exact (complex) quasienergy � . The lowest-order perturbative results for the n-photon ADs are presented in analytic form, and the parametrization of ADs in terms of polarization- and angular-independent atomic parameters is discussed for the general case of elliptical polarization. The major emphasis is on the analysis of an ellipticity induced distortion of three-dimensional ADs and, especially, on the elliptic dichroism (ED) effect, i.e. the dependence of the photoelectron yield in a fixed direction n on the sign of the ellipticity (or on the helicity) of a laser field. The dominant role of binding potential effects for a correct description of ED and threshold effects is demonstrated, and the intimate relationship between atomic ED factors and scattering phases o ft hedetached electron is established for multiphoton detachment, including the above-threshold case.

Journal ArticleDOI
TL;DR: In this paper, the dynamics of a bright matter wave soliton in a quasi-one-dimensional Bose-Einstein condensate with a periodically rapidly varying time trap is considered, and the governing equation is based on averaging the fast modulations of the Gross-Pitaevskii (GP) equation.
Abstract: The dynamics of a bright matter wave soliton in a quasi one-dimensional Bose–Einstein condensate (BEC) with a periodically rapidly varying time trap is considered. The governing equation is based on averaging the fast modulations of the Gross–Pitaevskii (GP) equation. This equation has the form of a GP equation with an effective potential of a more complicated structure than an unperturbed trap. In the case of an inverted (expulsive) quadratic trap corresponding to an unstable GP equation, the effective potential can be stable. For the bounded space trap potential it is showed that bifurcation exists, i.e. the single-well potential bifurcates to the triple-well effective potential. The stabilization of a BEC cloud on-site state in the temporary modulated optical lattice is found. This phenomenon is analogous to the Kapitza stabilization of an inverted pendulum. The analytical predictions of the averaged GP equation are confirmed by numerical simulations of the full GP equation with rapid perturbations.

Journal ArticleDOI
TL;DR: In this paper, the authors discuss the prospect of creating positron-electron plasmas confined in a stellarator to facilitate efficient injection and a long confinement time of the positrons.
Abstract: The prospects of creating positron–electron plasmas confined in a stellarator are discussed. A pure electron plasma would be created before the positrons are introduced, to facilitate efficient injection and a long confinement time of the positrons. Gyrokinetic simulations are presented suggesting that a positron–electron plasma may be stable to low-frequency microturbulence if operated well below the Brillouin limit, and transport may be neoclassical. If this is the case, significant positron–electron plasma densities can be reached with positron sources that exist today.

Journal ArticleDOI
TL;DR: In this article, the authors presented calculations of the time delay between single and double ionization of helium, obtained from full-dimensionality numerical integrations of the helium-laser Schrodinger equation.
Abstract: We present calculations of the time delay between single and double ionization of helium, obtained from full-dimensionality numerical integrations of the helium–laser Schrodinger equation. The notion of a quantum mechanical time delay is defined in terms of the interval between correlated bursts of single and double ionization. Calculations are performed at 390 and 780 nm in laser intensities that range from 2 × 1014 to 14 × 1014 W cm−2. We find results consistent with the rescattering model of double ionization but supporting its classical interpretation only at 780 nm.

Journal ArticleDOI
TL;DR: In this paper, the dynamics of large clusters irradiated by intense and short laser pulses, within the framework of the nanoplasma model, were studied and the influence of electron surface collisions was considered.
Abstract: We study the dynamics of large clusters irradiated by intense and short laser pulses, within the framework of the nanoplasma model. Particular attention is paid to the influence of electron–surface collisions, which have not been considered in previous versions of the model. We show that they dominate inverse bremsstrahlung collisions when plasmon resonance occurs. The dynamics of the cluster changes considerably and the predictions of the model are significantly modified. Moreover, there is no evidence for the presence of highly charged ions and the hydrodynamic pressure is found to be smaller than the Coulomb one.

Journal ArticleDOI
TL;DR: In this paper, double ionization of Ne by 25 fs, 1.0 PW cm−2 laser pulses has been explored in a kinematically complete experiment using a reaction microscope.
Abstract: Double ionization of Ne by 25 fs, 1.0 PW cm−2 laser pulses has been explored in a kinematically complete experiment using a 'reaction microscope'. Electrons are found to be emitted into a narrow cone along the laser polarization (e), much more confined than for single ionization, with a broad maximum in their energy distribution along e. Correlated momentum spectra show both electrons being ejected into the same hemisphere, in sharp contrast to predictions based on field-free (e, 2e) recollision dynamics, but in overall agreement with recent semiclassical calculations for He.

Journal ArticleDOI
TL;DR: In this paper, the Iχ(θe,e) molecular frame photoelectron angular distributions for any orientation χ of the molecular axis with respect to the polarization are reported for direct photoionization of H2 into the H2+(2Σg+) ionic ground state at hν = 20, 28.5 and 32.5 eV.
Abstract: Dissociative photoionization of H2 induced by VUV linearly polarized synchrotron radiation P has been studied using the (VH+,Ve,P) vector correlation method. The ion–electron kinetic energy correlation diagrams obtained for the three photon excitation energies hν = 20, 28.5 and 32.5 eV enable us to identify and select the dominant dissociative photoionization processes. The Iχ(θe,e) molecular frame photoelectron angular distributions for any orientation χ of the molecular axis with respect to the polarization are reported for direct photoionization of H2 into the H2+(2Σg+) ionic ground state at hν = 20 eV and for the dominant DPI processes involving autoionization of the H2(Q1 1Σu+(1)) and H2(Q2 1Πu(1)) doubly excited states into the H2+(2Σg+) and H2+(2Σu+) continua at hν = 28.5 and 32.5 eV. They show the dominant excitation of a p σu partial wave in autoionization of the Q1(1Σu+(1)) state into the H2+(1s σg) ionic state and that of a d πg partial wave in autoionization of the Q2(1Πu(1)) state into the H2+(2p σu) continuum. A molecular frame forward–backward electron emission anisotropy is observed when ionization takes place at large internuclear distance.

Journal ArticleDOI
TL;DR: In this paper, the Mg-He and Ca-He van der Waals potential energy curves were derived using a fully iterative treatment of single, double and triple excitations in the coupled cluster calculation.
Abstract: We present coupled cluster ab initio calculations for the Mg–He and Ca–He van der Waals potential energy curves. Our calculations employ large atom-centred basis sets, diffuse interbond functions and a fully iterative treatment of single, double and triple excitations in the coupled cluster calculation, and also include corrections for core–valence correlation effects. The use of bond functions makes computations near the complete basis set limit tractable, while the coupled cluster approach gives potential energies in very good agreement with those obtained from full configuration interaction calculations. We find the Mg–He binding energy to be 5.00 cm−1 at an equilibrium distance of 5.1 A, while the Ca–He binding energy is 3.43 cm−1 at an equilibrium distance of 5.9 A. A fully iterative treatment of triple excitations increases the Mg–He binding energy by about 0.3 cm−1, or roughly 6% of the total well depth, when compared with typical perturbative treatments of triple excitations.

Journal ArticleDOI
TL;DR: In this paper, the authors examined coherent effects in a four-level Vee scheme and showed that the system can be made to display a number of different phenomena, including the inhibition of two-photon absorption and the production of multiple dark states.
Abstract: We examine coherent effects in a four-level Vee scheme. We show that the system can be made to display a number of different phenomena, including the inhibition of two-photon absorption and the production of multiple dark states. We show that this latter effect is due to the independent control of the Autler–Townes splitting of the upper and lower probe field by independent driving fields. We suggest possible experimental systems in rubidium for the observation of such effects.