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Time- and space-resolved extreme ultraviolet (XUV) photoabsorption spectra of a laser-produced silicon plasma have been recorded using a dual-laser-produced plasma technique. Under appropriate experimental conditions we have been able to measure structure due to 2p-subshell excitation from metastable quartet states of the configuration. The experimental spectrum is compared to a spectrum synthesized with computed data from the Cowan suite of atomic structure codes. Eigenvector data from the atomic structure calculations performed in LS coupling are used to assign most of the observed resonance structures and a number of the 2p-subshell ionization limits of atomic silicon are inferred from the experimental data.

The angular distribution of the ionic fragments from multiphoton dissociative ionization of diatomic molecules shows a pronounced anisotropy. Calculations within the field-ionization, Coulomb explosion model show that the experimental results (pulses of 50 fs at an intensity of ) can be explained without invoking reorientation of the molecular axis. The fragments of the lighter and molecules, on the other hand, show clear signs of being forced into alignment.

The effects of the pump phase noise on the linewidth of the -laser without inversion are studied. Although the finite pump phase noise is added to the spontaneous emission noise, the linewidth of the -laser without inversion can be reduced. Physically this is interpreted as the enhancement of the atomic-coherence-induced noise correlation via the pump phase noise.

A mesh computational method for calculations of complex energy eigenvalues of quantum systems described by Schrödinger equations with non-separable spatial variables is developed. Complex eigenvalues of the energy are calculated for the ground state of the hydrogen atom and for a hydrogen-like system with a screened Coulomb interaction potential in strong electric fields by means of solving Schrödinger equations in cylindrical coordinates with the outgoing wave boundary condition. The method is also applied to the simplest diatomic molecule . Complex eigenvalues of the energy E are calculated for the ground and two excited states of a molecular ion in strong electric fields F directed along the molecular axis. The real parts of the energy and half-widths of a level are obtained as functions of the internuclear distance R. The equilibrium internuclear distance for the ground state of is obtained as a function of F from F = 0 up to when a minimum on potential curve E(R) disappears. These results are in good agreement with experimental data on in intense laser beam fields.

A hydrogen atom confined in a spherical box with impenetrable surface is investigated. The critical cage radius , at which a state is no longer bound is calculated for levels up to and including n = 10. Regularities present in these radii are pointed out. An empirical equation is proposed between and n for a fixed . The total number of bound states is also studied as a function of .

The electronic structures of titanium chloride and its cation have been analysed by means of both ligand field (LFT) and density functional (DFT) theories. Despite the discrepancy between the DFT and LFT concerning the assignment of the ground state of , an overall agreement is seen for TiCl. The observed ground state is found to result from our calculated ground state by a ionization process with an energy of .

We have made relativistic calculations of oscillator strengths in the isoelectronic sequence of neon for application to the modelling of x-ray laser systems. We find that the level ordering can change at certain atomic numbers, and that this is often associated with a vanishing oscillator strength in one lasing channel and an enhancement of the oscillator strength in a competing channel. It will be necessary to include the effect of plasma microfields to properly model emission spectra of a dense plasma.

2s electron single photoionization, shake-up and shake-down cross sections and asymmetry parameters are calculated for Na atoms in the ground and excited 3p, 3d, 4s, 4p, 4d and 4f states. The relaxed orbital single-configuration Hartree-Fock approximation is used. A strong dependence of calculated cross sections on the atomic orbit that the valence electron occupies is found.

Double and single Auger transition rates for the L shell of Li-like Kr, Nb and Gd ions are calculated using multiconfiguration Dirac-Fock bound-state wavefunctions. From these rates, the ratio of the cross sections for the corresponding resonant electron-transfer double-excitation and resonant electron-transfer excitation processes are derived. The values we obtained for these ratios are of the order of , consistent with recent experimental results.

We study by numerical simulation two important effects in the mechanism of multiphoton exchange between a diatomic molecule and an intense laser field: the dephasing and rephasing of the molecular dipole with respect to the driving field and the loss of coherence of the molecular wavepacket due to anharmonicity. We restrict the analysis to times of the order of the first recurrence period of the process and to the case where few photons are absorbed. We take the example of the HF molecule as a representative case. We find that a representation of the induced dipole by a simple waveform is useful for predicting the different scenarios that can occur. In particular, we focus on a `throttling' phenomenon due to anharmonicity of the vibrations or to rotational motion.

A simple model potential for the electron-Ca-atom interaction is used to describe the process of charge transfer in Rydberg-atom-calcium-atom collisions. The interaction of the final ionic state with degenerate manifolds of Rydberg covalent states is treated by inclusion of the negative ion decay due to the Coulomb field of the positive ion. This effect leads to a strong reduction of the charge transfer cross section and shifting of the value of the principal quantum number n corresponding to the cross section maximum towards higher n. The rates averaged over the experimental velocity distribution agree quite well with the experimental data and with independent calculations using the Landau-Zener model, although the results of the latter are higher than the present for the charge transfer from s states. The distortion of the classical ion trajectories due to the Coulomb interaction plays a minor role in the calculation of the negative-ion decay probability.

We present the results of quantal calculations of cross sections and rate coefficients for rovibrational transitions in ortho- and para-, induced by collisions with ground state para-. Rovibrational levels up to were included in the calculations, and rate coefficients are available for temperatures . Comparison is made with previous calculations and with measurements of the rate coefficient for vibrational relaxation . Agreement is found to be good at both high and low temperatures, but the measurements exceed the calculations at intermediate temperatures. Large discrepancies are found with previous calculations, which employed a semiclassical method.

We present a quantum mechanical method for calculating the ejected electron probability distribution in ion-atom collisions. The time-dependent Schrödinger equation, formulated with a classical straight-line trajectory for the internuclear motion, is integrated directly on a two-centre momentum space grid. The time-dependent wavefunction at large internuclear separation is analysed to extract the ejected momentum distribution as well as excitation and charge transfer amplitudes. Calculations have been performed at collision velocities of 1, 2 and 5 au for proton and antiproton collisions with atomic hydrogen, at a few impact parameters. Excitation and charge transfer probabilities are also calculated to check against results obtained from the close-coupling method. The ejected electron momentum distributions are shown.

Absolute cross sections of the associative ionization in the collision of with or have been measured in a merged beam experiment over a relative velocity interval ranging from 0.4 to . At a given relative velocity, the two cross sections significantly differ at low velocities where the cross section for formation exceeds that of by as much as 35%. The difference, however, decreases with the velocity and falls in the experimental error above . In both cases the energy dependence becomes compatible at low energy with an law.

Electron transmission experiments are reported involving the measurement of absolute total cross-sections for scattering of electrons by for energies between 20 meV and 9.5 eV, with an energy resolution in the incident electron beam of between 3 and 4 meV full-width half-maximum. These data represent the first absolute values of scattering cross-sections recorded for . Results are also reported for scattering in the presence of an axial magnetic field of 20 G over an energy range which extends from 16 meV to 250 meV.

Strong evidence is found for electron attachment at very low impact energies, in agreement with the results of other groups. Data suggest that attachment occurs through the p-partial wave. A resonance in the form of a doublet in the total scattering cross-section has been found between 70 and 200 meV. This structure is absent in the spectrum obtained in the presence of an axial magnetic field.

A qualitative mechanism is suggested for the formation of the observed structure involving virtual excitation of the first vibrational quantum of . The doublet nature of the resonance is attributed to the intervention of the excited -states of . The plausibility of the proposed mechanism could be checked by accurate calculations of excited state potentials for and by measurement of the yield of ions as a function both of the electron energy and the temperature of the target gas.

Coincidence measurements in coplanar asymmetric geometry are reported for electron impact ionization of helium leaving the residual ion either in its ground state or in an n = 2 excited state. Cross sections are placed on an absolute scale by extrapolation of the generalized oscillator strength to the optical limit. Scattered electron energies of 570 and 1500 eV are considered with ejected electron energies of 10, 20 and 40 eV. Comparison is made with theoretical calculations in the first Born approximation using parametric potentials. Clearly, certain features of the experimental results can only be explained by multiple-scattering effects.

The electron-impact single ionization of low-charged Ti ions was studied both experimentally and theoretically. Absolute cross sections for ions in charge states q = 1 to 6 except 3 were measured using a crossed-beams technique as well as calculated with the configuration-average distorted-wave method, including both direct ionization and excitation-autoionization contributions. For charge states q = 4, 5 and 6, the measurements are in good agreement with the calculations considering that large components of the incident ion beam were in metastable configurations. The cross sections for single ionization of and were determined in part with a high-resolution energy-scan method. Besides contributions of excitation-autoionization mechanisms, a remarkable number of structures arising from resonant-excitation double-autoionization (REDA) processes could be observed.

Using the crossed-beams technique the following absolute cross sections have been measured for the reaction :

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double ionization

triple ionization

quadruple ionization .

Measurements for high charge states and ionization degrees, respectively, show significant contributions of inner-shell processes which partly dominate the respective cross section. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Tawara, respectively.

We have measured and calculated differential cross sections for electron-impact excitation of the level in ytterbium at collision energies of 5, 10, 20, 40 and 80 eV. Calculations were made in the unitarized distorted-wave approximation. Measurements of the relative differential cross section, taking account of volume correction effects, were placed on an absolute scale by normalization to the theoretical integral cross section at 80 eV with extrapolation to lower energies based on the measured excitation function of Shimon et al (1981 Opt. Spektrosk. 50 571). Excellent agreement between theory and experiment is obtained in the regime of low momentum transfer.

We employed the fast-neutral-beam technique in a measurement of the absolute partial cross section for the formation of ions following controlled electron-impact ionization of the fullerene from threshold to 200 eV. Our measurement is the first report of an absolute measurement of this cross section which does not employ any normalization procedure and which does not rely on the use of controversial vapour pressure versus temperature data for to determine the number density in the ion source. All quantities that determine the ionization cross section were measured directly in our apparatus. We found a peak value for the parent ionization cross section of in the energy range between 70 and 80 eV. This value is in very good agreement (to within 5%) with the cross section reported earlier by Matt et al (1996 J. Chem. Phys. 105 1880) who employed a novel normalization procedure. Our cross section is significantly lower than the cross sections reported by several other investigators who relied on vapour pressure data in their absolute calibration.

The de-excitation cross sections of H atoms colliding with molecular hydrogen were determined in the velocity range of 2-. At a mean velocity of the de-excitation cross sections are , and . The velocity dependence of the de-excitation cross sections observed in the experiment agrees with the quadrupole-H dipole interaction and with the quantum character of the molecular rotation.

Positronium (Ps) formation in positron-helium scattering has been investigated in different partial waves at medium energies including the Ore gap region using the close-coupling approximation with realistic wavefunctions for the following states: He(1s1s), He(1s2s), He(1s2p), He(1s3s), He(1s3p), Ps(1s), Ps(2s), Ps(2p). Calculations are reported of rearrangement cross sections to Ps(1s), Ps(2s) and Ps(2p) states for incident positron energies up to 200 eV. The present partial cross sections are in good agreement with experimental results and a variational calculation in the Ore gap region.

We demonstrate that the electron energy loss spectrum in collision with metal clusters or fullerenes depends strongly on the scattering angle of the electron. This results from the fact that the excitation probability of the surface plasmon modes of different multipolarity is correlated with the scattering angle of the electron. We derive the kinematic conditions under which the plasmon excitation with the given angular momentum dominates in the cross section. We perform our treatment in the resonance plasmon approximation and also in the random phase approximation with exchange using the wavefunctions of the Hartree-Fock jellium model. We compare the predictions of these two quite different approaches for the cluster and report their qualitative agreement. Theoretical results are compared with the available experimental data for the electron-fullerene collision. Reasonable agreement between theoretical and experimental results is reported.

The third-order nonlinear optical polarizability of -derived fullerene tubes has been studied theoretically. The results indicate that the carbon atom number, symmetry and cap have great effects on the nonlinear optical properties of fullerene tubes. When a fullerene tube has a high carbon atom number the major dynamical response peaks in its spectrum concentrate on a narrow region with lower energy and larger values. In the case with similar atom numbers, the tube without a cap and an armchair tube have larger values than the tube with a cap and a zigzag tube, respectively. In addition, we point out that the tube may be very important in optical device applications because it may have a larger value than a pure carbon nanotube and its value may be controlled by its composition.

Please note that the quantities plotted in figure 1 of our paper are in fact , and not as stated on the graphs, in the figure caption and in the text. This in no way changes any of the discussion or conclusions made in the paper, since the correct plots of are qualitively similar to those of . We apologize to our readers for this inaccuracy.