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Energy- and angle-resolved photoionization spectra of He irradiated by linearly polarized intense 810 nm laser radiation and several of its XUV odd harmonics are investigated. The angular distribution of the odd-order peaks, produced by single-photon ionization by one harmonic, is, surprisingly, broadened by the IR field. The even-order ones, due to two-colour, two-photon ionization, show at 90° lobes which depend on the relative IR–XUV phase. Application to the characterization of attosecond pulses is suggested.

double-vacancy states undergo strong amplification in relativistic self-trapped plasma channels on 3d → 2p transitions in the λ = 2.78–2.81 Å region. The ²P_3/2 → ²S_1/2 component at λ ≅ 2.786 Å exhibits saturated amplification demonstrated by both (1) the observation of spectral hole-burning in the spontaneous emission profile and (2) the correlated enhancement of 3p → 2s cascade transitions (²S_1/2 → ²P_j; j = 1/2, 3/2) at λ = 2.558 Å and λ = 2.600 Å. The condition of saturation places a lower limit of ∼10¹⁷ W cm⁻² on the intensity of the x-ray beam produced by the amplification in the channel. The anomalous strength of the amplification signalled by the saturation mirrors the equivalently anomalous behaviour observed for all 3d → 2p transitions corresponding to single-vacancy Xe^q+ arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The conspicuous absence of amplification involving states with double-vacancy configurations suggests the operation of a selective interaction that enhances the production of states. Overall, the generation of double-vacancy states of this genre demonstrates that an excitation rate approaching ∼1 W/atom for ionic species is achievable in self-trapped plasma channels.

An analytical expression for the electron peak intensity of above-threshold ionization (ATI) in an intense laser field is obtained using strong field approximation (SFA) and pole approximation for the rescattering amplitude. Numerical calculations of angular distributions of ATI electrons in a two-colour laser field are performed. The calculations have shown the presence of intermediate sidelobes inside all energy spectra of ATI electrons. Also we obtain that intermediate sidelobes always transform to two sidelobes and finally in one back-directed lobe near cut-offs of the direct, rescattering monochromatic and bichromatic plateaux.

The Stark broadening parameters of 26 lines of Au II have been measured in a plasma produced by ablation of a gold target with a Nd:YAG laser. In order to obtain suitable experimental conditions for the homogeneity and stability of the plasma, a study of the temporal plasma evolution in helium, argon and nitrogen at different pressures was made. The broadening parameters were obtained with the gold target placed in molecular argon at 6.5 Torr, which provides appropriate measurement conditions. A Boltzmann plot was used to obtain the plasma temperature [16 200 ± 1400 K] and by means of the Saha equation the electron density [(1.45 ± 0.23) × 10¹⁷ cm⁻³] and the plasma composition were determined. The local thermodynamic equilibrium condition was checked. Contributions to broadening arising from a mechanism different from the Stark broadening as well as self-absorption influence were estimated for every studied line.

Using the R-matrix Floquet approach, we have investigated multiphoton ionization of the 1s2s ¹S state of He. We compare emission rates for the 1s and 2s electrons after absorption of N photons, where N is the minimum number of photons needed to emit the 1s electron. For N = 1 and 2, we find that emission of the 1s electron dominates, while for N = 4 emission of the 2s electron dominates. For N = 3, both processes are of comparable magnitude. In the absence of resonances, emission of the 1s electron typically becomes less likely by a factor of 5 for each additional photon absorbed. However, for N = 4, emission of the 1s electron is strongly enhanced due to the intermediate 2s2p ¹P resonance.

A method for solving the time-dependent Schrödinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

This paper extends the momentum-transfer theory of drift tubes to ion traps and similar devices where the electric fields vary with both position and time. The collision frequencies that are incorporated into two-temperature and multi-temperature moment theories are given precise microscopic definitions instead of being treated as adjustable parameters.

The empirical diffusion equation and the two-temperature moment theory of ion motion are applied to cylindrical devices used in field-asymmetric ion mobility spectrometry, where the external fields vary with position and time. Both approaches show that ion focusing occurs under the conditions where it is observed experimentally. The two-temperature theory provides new, quantitative information about the ion temperature as a function of position and time in the apparatus.

The two-temperature moment theory of ion motion in devices where the external fields vary with position and time is applied to ideal quadrupole ion traps. In first approximation, the theory gives differential equations with collision frequencies that vary with the effective temperature characterizing the relative kinetic energy of the ion–neutral collisions. Solutions of the set of coupled differential equations provide the ion number density, average velocities, average energy and average temperature as functions of time and of position in the apparatus. That information also enables the effective temperature for the ion ensemble and the position-dependent energy density to be determined as a function of time. Solutions of the coupled equations are discussed for the Maxwell model, rigid spheres and general ion–neutral interactions.

New experimental and theoretical data for the degree of linear polarization of Lyman-α emission induced by proton and antiproton impact on atomic hydrogen in the 1–1000 keV impact energy range are presented. For the calculations a two-centre extension of the basis generator method is used to solve the time-dependent Schrödinger equation. In the experimental part we present new measurements for the polarization fraction of the impact induced Lyman-α radiation in the energy range from 13 keV to 800 keV for the collision system p + H. The calculated polarization fractions are in very good agreement with the experimental data.

Elastic integral, differential and momentum transfer cross sections for electron collisions with an O₃ molecule have been calculated in a 16-state R-matrix approach. The 16 target states have symmetries X¹A₁, 1³B₂, 1³B₁, 1³A₂, 1¹B₁, 1¹A₂, 2³B₂, 2¹A₁, 1¹B₂, 2³A₂, 2¹A₂, 2³B₁, 2¹B₂, 2¹B₁, 3¹A₁ and 1³A₁ and have been represented by configuration interaction (CI) wavefunctions. In our CI model, we keep the core 6 electrons frozen in doubly occupied molecular orbitals 1a₁, 2a₁ and 1b₂. The complete active space consists of 18 valence electrons that are allowed to move freely among 12 molecular orbitals: 3a₁, 4a₁, 5a₁, 6a₁, 7a₁, 1b₁, 2b₁, 2b₂, 3b₂, 4b₂, 5b₂ and 1a₂. This CI model gave an adequate description of the vertical spectrum of these excited states which span the energy range 0–10 eV and also gave a good representation of the charge cloud of the ground state at its equilibrium geometry which provided a dipole moment of 0.61 D in good accord with the experimental value 0.53 D. Our calculations detect one bound O⁻₃ state (²B₁) at the equilibrium geometry of the O₃ molecule. We also find two broad shape resonances in ²A₁ and ²B₂ symmetries out of which the ²A₁ resonance supports dissociative electron attachment when an O–O bond is stretched beyond 3.1 a₀. Born correction is applied for the elastic and the dipole allowed transitions to account for partial waves higher than l = 4 that are excluded in the R-matrix calculation. Elastic and excitation cross sections are presented for incident electron energies up to 15 eV and our results are compared with the other theoretical and experimental works.

The idea of coherent states has proved very useful in the study of the quantum nature of electromagnetic radiation. Coherent states are also interesting because their evolution in time comes closest to classical dynamics. So they are the best approximation in quantum mechanics to classical behaviour. In recent years, attempts have been made to generalize the concept of coherent states to systems other than those made up of the simple harmonic oscillator (such as light). In one such attempt, Klauder introduces a set of physical criteria that generalized coherent states should have (Klauder J R 1996 J. Phys. A: Math. Gen.29 L293). These criteria have been applied to the hydrogen atom and several other systems. In this paper, we apply Klauder's criteria to identify generalized coherent states for the quadratic Hamiltonian H = ℏ(f₁a†a + f*₂a² + f₂a^†2). We check if two-photon coherent states of light satisfy these criteria. We find that they do not, which is expected: their non-classical properties are well known. We then use Klauder's criteria to find a set of states that are associated with the quadratic Hamiltonian. We find that these states too display non-classical behaviour even for large values of the quantum numbers. This non-classical behaviour is revealed in the form of sub-Poisson photon statistics. We will also briefly consider the one-photon Hamiltonian, apply Klauder's criteria to it and comment on its statistics.

The interaction of keV H⁺, He⁺ and He²⁺ ions with gas-phase H₂O molecules leads to formation of H₂O^q+ ions which subsequently can undergo dissociation into various fragment species. From coincident determination of the fragment-ion kinetic energies, kinetic energy releases (KER) for the different dissociation channels are obtained. We present the KER values for the various dissociation channels at different projectile kinetic energies. For He⁺, extraordinarily large relative dissociation yields are observed as compared to H⁺ and He²⁺. For both singly charged projectiles we observe 14.5 eV H⁺ fragments, most probably due to the two-step H₂O²⁺→ HO²⁺ + H → H⁺ + O⁺ + H process predicted 5 years ago (Nobusada K and Tanaka K 2000 J. Chem. Phys.112 7437).

The fields of two linearly-polarized, co-propagated laser beams differing slightly in frequency are modelled by those of a tightly focused Gaussian beam. When a single electron is injected sideways into or near the common focus of the two beams, it gets captured and violently accelerated, reaching a maximum energy gain in excess of 2 GeV from beams of present-day power. Our simulations show that acceleration takes place typically over a fraction of a millimeter and that a maximum energy gradient of over 12 TeV m⁻¹ may be achieved.

The interaction of matter-wave solitons in elongated Bose–Einstein condensate with time-dependent parabolic trap is investigated using the perturbation theory based on the inverse scattering transform. Regimes of parametric and main resonances in solitons interactions are investigated for harmonic trap potentials. The predictions of the theory are confirmed by the numerical simulations of the quasi-one-dimensional Gross–Pitaevskii equation.

Excitation of the 3 ¹P state of magnesium by 20 eV electrons has been studied experimentally using the scattered-electron–polarized-photon correlation method over a wide range of scattering angles (10°–120°) and theoretically using the convergent close-coupling and R-matrix with pseudo-states methods. The measured linear Stokes parameters, and the circular polarization calculated from these parameters, assuming coherent excitation, are generally well produced by these theoretical models as well as by some previous theories. Relative differential cross sections for elastic scattering and excitation of the 3 ¹P and 3 ³P states were also measured over the angular range 10°–140°. The results are compared with those from previous experiments and with present and previous theoretical predictions.

We have performed large-scale CIV3 calculations of excitation energies from the ground state for 48 fine-structure levels as well as of oscillator strengths and radiative decay rates for all electric-dipole-allowed and intercombination transitions among the (1s²2s²2p⁶)3s²(¹S), 3s3p(^1,3P^o), 3s3d(^1,3D), 3s4s(^1,3S), 3s4p(^1,3P^o), 3s4d(^1,3D), 3s4f(^1,3F^o), 3p²(¹S, ³P, ¹D), 3p3d(^1,3P^o, ^1,3D^o, ^1,3F^o), 3p4s(^1,3P^o) and 3d²(¹S, ³P, ¹D) states of Br XXIV. These states are represented by extensive configuration-interaction (CI) wavefunctions obtained using the CIV3 computer code of Hibbert. The relativistic effects in intermediate coupling are incorporated by means of the Breit–Pauli Hamiltonian which consists of the non-relativistic term plus the one-body mass correction, Darwin term, and spin–orbit, spin-other-orbit and spin–spin operators. Small adjustments to the diagonal elements of the Hamiltonian matrices have been made so that the energy splittings are as close as possible to the experimental values. Our calculated excitation energies, including their ordering, are in excellent agreement with the available experimental results except that the levels ¹D₂ and ³P₂ belonging to the same configuration 3p² interchanged their positions compared to the experiment. This interchange in our calculation is discussed and explained through eigenvector compositions of the two levels. From our radiative decay rates, we have calculated radiative lifetimes of some fine-structure levels. Our calculated lifetimes of the levels 3s3p(³P₁) and 3s3p(¹P₁) are found to be in good agreement with the experimental and other theoretical results. In this calculation we also predict new data for several fine-structure levels where no other theoretical and experimental results are available.

A tunable Fourier-transform-limited narrow-band vacuum-ultraviolet (VUV) laser system has been developed that is entirely based on solid-state laser technology. The possibility of generating radiation pulses of adjustable duration enables the production of VUV radiation with a bandwidth of less than 100 MHz. The tunability and the narrow bandwidth of the VUV laser are demonstrated in a high-resolution measurement of the (5p)^{6 1}S₀ → (5p)⁵7d[3/2](J = 1) transition of the seven most abundant isotopes of Xe, in which the isotopic shifts could be determined with an accuracy of better than 20 MHz and the hyperfine structure of the transitions of ¹²⁹Xe and ¹³¹Xe could be resolved. An analysis of the line profiles reveals that the full-width at half-maximum of 100 MHz of the transitions is limited by the Doppler effect.

The alignment parameter A₂₀ and the orientation parameter O₁₀ for the 4p⁴(¹D)5p²F_7/2 and the 4p⁴(¹D)5p²D_5/2 states of Kr⁺ populated in resonant Auger transitions from 3d⁹5p/6p photo-excited states of Kr were measured at BESSY II for photon energies between 91 eV and 92.8 eV. The method of fluorescence polarimetry was applied. A bandwidth of the exciting photons of 10 meV allowed to study A₂₀ and O₁₀ within the natural line width. Interference effects for resonances overlapping due to lifetime broadening were predicted and observed even when separated by 15 times the natural line width. The relative contribution of the three allowed electron partial waves was derived from A₂₀ and O₁₀ as a function of the exciting-photon energy and compared with the calculated contributions. The resulting good overall agreement between experiment and prediction is considered as a noticeable improvement of the understanding of excitation and decay of the prominent Kr 3d⁹5p resonances.