Table of contents

The experimental observation of the laser-induced satellites in emission of hydrogenic ions is reported. The measured profiles of the Lyman line of Al XIII exhibit peaks in the wings consistent with theoretical predictions of the spectral line modification by strong single-frequency electric fields.

The Cl(2p) photoelectron spectra of HCl and DCl have been measured with high resolution. Careful data analysis with a least-squares fitting method enabled us to determine the spectroscopic parameters for the three spin-orbit and molecular field split components of the Cl(2p)^-1states. The experimental results were compared with ab initiocalculations. The values of 84±9 meV, 71±13 meV, and 103±10 meV for the lifetime line widths of ²_1/2 , ²_1/2⁺ , and ²_3/2states were extracted from the experiment. An anomalous photoionization cross section ratio (60%) between the ²_1/2⁺and ²_3/2states was observed.

Positronium formation in positron-hydrogen collisions is studied using Schwinger's variational principle at intermediate energies. A total of 12 correlated functions in the basis set expansion is found to be sufficient to predict accurate cross sections in agreement with the experimental and theoretical values available in the literature.

High-resolution cold-target recoil-ion momentum spectroscopy (COLTRIMS) has been used to study single-electron capture processes in collisions of Ar⁶⁺ions with He at an impact energy of 9 keV and scattering angles between 0 and 3 mrad. The energy-gain spectra show that the capture occurs mainly into the 4s state with a significant contribution involving capture into the 4p state. Reaction channels associated with transfer excitation into the 3s3p3d states of Ar⁵⁺are also observed for the first time, accounting for 6(±0.4)% of the total cross sections for single-electron capture. Experimental results are found to be in good agreement with the two-electron atomic orbital close-coupling calculations.

A novel feature of the polarizational bremsstrahlung (BrS) emitted in a fast collision of a heavy charged projectile with an atom is described. The peculiarity in the cross section of the process originates from the kinematics of atomic electrons in the polarizational bremsstrahlung process and is similar to that which occurs in inelastic scattering, where it is known as the Bethe ridge. In the polarizational BrS process the Bethe peculiarity manifests itself as an additional maximum in the velocity dependence of the cross section.

The technique of double translational energy spectroscopy has been used for the first time with an atomic hydrogen target. We have carried out measurements of state-selective one-electron capture in 6 keV C²⁺ -H(1s) collisions which avoid the ambiguities in interpretation of previous experiments and allow separate identification of the collision channels associated with either C²⁺ (2s² )¹ S ground-state or C²⁺ (2s2p)³ P^ometastable primary ions. In a theoretical counterpart, partial cross sections for both reactions have been calculated using a semiclassical molecular approach. There is good agreement between experiment and calculations.

We have investigated the influence of rotational excitation of the perturber molecule on the rate coefficient for vibrational relaxation of para-H₂ . Comparison is made with our previous (quantum mechanical) calculations, which assumed the perturber molecule to be constrained to its rotational ground state, and with measurements of the rate coefficient. We also compare with the results of recent semi-classical calculations by Zenevich and Billing (Zenevich V A and Billing G D 1999 J. Chem. Phys.1112401).

We review topics of current interest in the physics of electronic, atomic and molecular scattering in the vicinity of thresholds. Starting from phase space arguments, we discuss the modifications of the Wigner law that are required to deal with scattering by Coulomb, dipolar and dispersion potentials, as well as aspects of threshold behaviour observed in ultracold atomic collisions. We employ the tools of quantum defect and semiclassical theories to bring out the rich variety of threshold behaviours. The discussion is then turned to recent progress in understanding threshold behaviour of many-body break-ups into both charged and neutral species, including both Wannier double ionization and three-body recombination in ultracold gases. We emphasize the dominant role that hyperspherical coordinate methods have played in understanding these problems. We assess the effects of external fields on scattering, and the corresponding modification of phase space that alters the Wigner law. Threshold laws in low dimensions and examples of their applications to specific collision processes are discussed.

A three-body classical trajectory Monte Carlo method has been used to investigate the process of electron capture to the continuum (ECC) by dipole particles impinging on hydrogen atoms. The projectile consisted of two particles of opposite charge separated by a fixed distance. The orientation of the dipole was also fixed. The calculations were carried out for an impact velocity of 1 au. It was found that the energy spectrum of the electron ejected via the dipole interaction exhibits a cusp peak at 0° observation angle. The cusp formation was studied as a function of the length and orientation of the dipole. A particularly intense cusp was observed when the positive charge was the leading particle in the dipole. Analysis of individual electron trajectories revealed that quasi-stationary two-centre orbits play an important role in the formation of the ECC peak.

Absolute total cross sections for electron capture between slow, highly charged ions and alkali targets have been recently measured. It is found that these cross sections follow a scaling law with the projectile charge which is different from the one previously proposed which is based on a classical over-barrier model (OBM) and verified using rare gases and molecules as targets. In this paper we develop a `semiclassical' (i.e. including some quantal features) OBM in an attempt to recover experimental results. The method is then applied to ion-hydrogen collisions and compared with the result of a sophisticated quantum mechanical calculation. In both cases the present method is found to underestimate the correct result by a factor of two, but, where comparison can be made, it is superior to other OBMs. A qualitative explanation for the discrepancies is also given.

We determine the electronic structure of the metallofullerene La@C₆₀for different configurations of La, namely 5d6s² , 5d² 6s and 4f5d6s. The fullerene environment is modelled by a confining potential V_c (r ), equivalent to an attractive spherical square well of depth -8.22 eV and width 1.89 au. The atomic orbitals of La are obtained by solving numerically the Dirac-Fock equations including V_c (r ). For both configurations 5d6s²and 4f5d6s, La donates two electrons to the fullerene molecule, while in 5d² 6s, it is demonstrated that only a partial transfer of the 5d electron to the molecule occurs. For specific values of the well depth, we observe a collapse of the 5d orbital inside the hollow cage region. This phenomenon is interpreted by means of double-valley potentials.

The relative cross section for the excitation of the 2p_ustate of H₂⁺as a function of the orientation of the internuclear axis is measured and calculated. 400 eV electrons bombard an H₂target. Electrons scattered at 18° with an energy loss of 41 eV are detected in coincidence with H⁺fragment ions of energy 4.9 eV. Measurement of the scattering angle and energy loss establishes the momentum transfer vector of the collision Kand the scattering plane. The angular distribution of 4.9 eV H⁺ions relative to Kis recorded. Because the rotation time of H₂is long compared to the dissociation time, it follows that the H⁺ions leave the collision region along the line of the internuclear axis at the time of the collision. The relative cross section peaks when the internuclear axis is perpendicular to K. Calculations predicting the shape of the angular distribution fit the data.

The resonant state of the Coulomb three-body system d + (µ³ He) (J= 0) below the (µ d)_1s+ ³ He threshold is investigated using the adiabatic hyperspherical (AHS) approach. The effective numerical method for the scattering problem in the AHS basis is developed. This allows one to study in detail the energy dependence of the three-body wavefunction, its local characteristics (G - and -factors) and the phase shift in the resonance range. The resonance position E₀= 70.82 eV and the width = 3.8 ×10^-4eV are obtained as the parameters of the Breit-Wigner formula which reproduces the numerical results for the regularized normalization constant N² (E ). The numerically calculated G -factor depends strongly on Ewithin the resonance width. Its Edependence is reproduced with high accuracy by a three-parameter formula. In contrast to the G -factor, the numerically obtained -factors do not depend on Enoticeably in the resonance range.

Electron-impact ionization of Ca atoms from metastable states has been investigated using a crossed atomic and electron beam technique. The value of the total ionization cross section from the 4s4p ³ P_Jmetastable states at an electron energy of 4-20 eV was determined. It has been found that ionization cross sections from the metastable and ground states differ considerably. Such a discrepancy is mainly due to the different mechanisms of ion formation from the metastable and ground states. The obtained results are compared with those calculated using the classic mechanics binary approximation.

The resolution of the spectrum of ³ P H^-up to the n= 5 threshold has been achieved to very high accuracy via the state-specific solution of the complex eigenvalue Schrödinger equation, using as a cut-off limit resonance states of widths of about 10^-9au. In this energy region and for this cut-off value, we identified one discrete state, the 2p²³ P and 31 resonance states. We show how an appropriate analysis of the energies, widths and wavefunction characteristics leads to the classification of series of states into unperturbed and perturbed, using the Gailitis-Damburg dipole resonance approximation as the zero-order model. Two lonerstates are present: the discrete state below the n= 2 threshold and a resonance state below the n= 4 threshold, which overlaps the third member of the dipole resonance series.

We show that quantum interference from spontaneous decay in a three-level -type system can be realized in the dressed-state picture of coherently driven systems. The physical realization relaxes the stringent conditions upon the level spacing and quantum numbers of the lower-level doublet. Since the physical realization means the equivalence between the systems with quantum interference and the driven systems, the equivalence may be conveniently used to illustrate the physical pictures of some phenomena. We give two interesting examples. The first is the dynamically induced population inversion, which originates from quantum interference between dressed states. The second is the disappearance of a dark state in a bare system with quantum interference, which is due to a disparity in coupling intensities on two transitions of the equivalent Vsystem.

Isotope shifts of three lines of Hf I, lying in the red spectral region, have been measured by means of saturated-absorption spectroscopy in a hollow cathode. Combined with experimental data, previously given in the literature, and pseudo-relativistic Hartree-Fock estimates the field shifts (FS) of six relevant configurations are deduced: FS(5d² 6p² ) = -84 mK, FS(6p² 6s² ) = 36 mK, FS(5d6p² 6s) = -28 mK, FS(5d³ 6p) = -90 mK, FS(5d⁴ ) = -94 mK and FS(5d6s² 6p) = 12 mK, referred to 5d² 6s² . For these three studied spectral lines, connecting the configurations 5d² 6s²and 5d6s² 6p the specific mass shifts are estimated to lie between 5 and 8 times the normal shifts, using a King plot and pseudo-relativistic estimates for the Vinti integrals, respectively.

The threshold photoelectron (TPE) spectrum of SF₆has been recorded over the photon energy range 25-140 eV using synchrotron radiation and a penetrating-field electron spectrometer. In addition, the photo-double ionization spectrum of SF₆has been obtained over the energy range 30.7-49.3 eV using the threshold photoelectrons coincidence technique. The TPE spectrum in the inner-valence ionization region is found to be significantly different to the conventional photoelectron spectrum in the same binding energy region. Indirect autoionization of neutral Rydberg and shape-resonance states of SF₆dominate the formation of the TPE spectrum. Inner-valence SF₆⁺ion states appear to play a major role in the formation of the SF₆²⁺ion states. The onset of double ionization in SF₆was found to occur at 31.98±0.02 eV.

The CH₃⁺ +H₂ CH₅⁺ +hradiative association process has been studied by means of an ab initiotreatment including the determination of the potential energy surfaces and dipole moments. Calculated rate constants compare positively with experimental measurements.

An atom in a box with penetrable or impenetrable potential walls is shown analytically to move under the influence of the repulsive forces from the boundaries of the cavity when all the atom-medium interactions are described by the Dirichlet boundary conditions or effective one-particle `box' potentials. Simple geometrical methods allow one to find the energetically optimal position of the atom in the cavity. For instance, the ground state energy of the hydrogen atom is proved to be minimal when the nucleus is situated at the centre of the cube or sphere. Some applications for molecules are also mentioned.

Mechanisms of two-electron excitation of the (2s² )¹ S, (2p² )¹ D and (2s2p)¹ P autoionizing states of helium are studied both experimentally and theoretically. It is shown that an explicit introduction of a kinematic factor, with a process-specific phase leads to a productive parametrization of experimental cross sections of ionization, allowing one to extract cross sections of excitation of autoionizing states. Using a new fitting procedure together with the proposed parametrization made it possible to obtain the excitation cross sections and magnetic sublevel population from electron spectra as well as, for the first time, to resolve the contribution of resonance and interference components to resonance profiles. Interference with direct ionization is shown to contribute significantly into resonance formation even for backward ejection angles. We demonstrate theoretically that the excitation cross sections thus extracted from experimental electron spectra hold information about the interaction of autoionizing states with an adjacent continuum.

A theoretical model of hyperfine spectra of nuclear spin conversion of molecular isomers has been developed. The model takes into account the nuclear spin-spin and spin-rotation interactions, as well as the saturation of intramolecular mixing of molecular ortho and para states. The model has been applied to hyperfine spectra of nuclear spin conversion in ¹³ CH₃ F molecules subjected to an external electric field. Conditions under which the hyperfine structure in the spectra can be resolved have been determined.

We have carried out extensive R -matrix close-coupling calculations of the electron-impact excitation of C³⁺and O⁵⁺ . We have determined effective collision strengths for transitions between the lowest nine terms and cross sections between the ground state and selected excited terms of these ions from 41-state R -matrix with pseudo-states (RMPS) calculations that employ basis sets consisting of nine physical states and 32 pseudo-states. In order to investigate the dependence of electron-impact excitation on coupling of the bound states to the target continuum states and also the highly excited bound states included in these RMPS calculations, we have compared these results with those determined from our 9-state and 13-state R -matrix calculations without pseudo-states. As one would expect, this additional coupling is in general less important in O⁵⁺than in C³⁺ ; however, these effects vary significantly with the type of transition and are complicated by the resonance contributions. This makes it difficult to draw any general conclusions regarding their dependence on ionization stage. These results complement earlier work on the Li-like ions Be⁺and B²⁺and provide improved sets of excitation data for these ions.

Coherent light interacting with a nonlinear nonabsorbing medium of inversion symmetry is modelled as a quartic anharmonic oscillator if the lowest (i.e. third) order of nonlinearity is considered. The equation of motion of the quadrature operators of such an oscillator is solved approximately in a closed analytical form. The corresponding solution is used to investigate the possibility of finding the continuous squeezed states. It is found that the amount of squeezing is increased with the increase of intensity of the applied field. An improvement of the squeezing is also possible if the coupling strength is increased. We report the squeezing effect of the input vacuum field. The possibility of generating continuous squeezing in an absolute sense (i.e. 100%) of the input radiation field is also discussed.

Cross-sections are reported for total integral and backward scattering of electrons from chlorine dioxide (OClO) for energies in the range 20-500 meV. The measurements were made in transmission experiments using a synchrotron radiation photoionization apparatus with an energy resolution in the incident electron beam of ~1.0 meV (FWHM). Integral and backward scattering cross-sections are reported. The present integral cross-section values are a correction to those reported in an earlier publication (Gulley et al1998a J. Phys. B: At. Mol. Opt. Phys.315197) which have been found to be too low due to the presence of an impurity in the original OClO samples. The absolute accuracy of the integral cross-sections is limited by experimental difficulties with respect to strong rotationally inelastic forward scattering due to the dipolar nature of the target molecules.

Backward scattering data provide good evidence that dissociative attachment occurs through p-wave attachment. Comparison is made with predictions of the first Born pure dipole approximation for both total and backward scattering with excellent agreement above 100 meV with integral scattering data, but showing a significant underestimate for backward scattering cross-sections. Behaviour at energies below 100 meV represents competition between inelastic scattering and attachment and is not well understood.

The decay rate of orthopositronium (o-Ps) formed and thermalized in eight different gases is systematically investigated as a function of temperature. The o-Ps collisional quenching rate is observed to increase with temperature, T , for He, Ne, Ar, N₂ , ethane, methane, isobutane and neopentane. All of the gases except ethane and methane increase linearly over the investigated temperature range, 300 K<T <600 K. Recent theoretical work for noble gases at elevated temperatures suggests a linear increase in the collisional quenching rate with temperature at the low densities used herein where collective phenomena such as bubble formation and density fluctuations are not present. When comparisons can be made with previous experimental results the agreement is generally poor. Precision o-Ps decay rate measurements using gases are also discussed concerning systematic effects due to the energy dependence of the collisional quenching rate.

We discuss the calculation of ionization rates of helium using time-dependent solutions of the full-dimensional Schrödinger equation in conjunction with time-dependent solutions of a single active electron (SAE) model of helium. The SAE model is a one-electron atom with a non-Coulombic effective potential that can be tuned in certain limits to give near quantitative agreement with the results of the full-dimensional integration. We show how the tuned SAE model can be used to improve the accuracy and reliability of the calculations of ionization cross sections. In addition we consider a case in which failure of the SAE model can be attributed to strong correlation effects in laser-driven helium, specifically interaction with an intermediate doubly excited (autoionizing) state of helium.

We present a combined study of single and double K-K electron transfer cross sections along with the single and double K-shell ionization of Ar induced by Si projectiles in the energy range 0.9-4.0 MeV u^-1 . The charge-state dependence of the normal and hypersatellite x-rays was used to derive the cross sections for the one- and two-electron processes, respectively. The enhancement in the fluorescence yields due to multiple vacancies was measured from the energy shifts and intensity ratios of the characteristic x-ray lines to derive K-shell vacancy production cross sections from x-ray production cross sections. The ratio of double to single K-K transfer cross sections is found to be quite large for this nearly symmetric collision system, whereas the ratio of double to single ionization cross sections is quite small. The measured single K-K transfer cross sections are reproduced very well by the two-centre close-coupling calculations whereas the double K-K transfer data are underestimated by the theory based on the independent-electron approximation (IEA). The K-shell ionization cross sections are found to deviate strongly from the calculations based on the continuum distorted wave eikonal initial state (CDW-EIS) and ECPSSR models. The CDW-EIS calculations along with the IEA model grossly underestimate the double ionization cross sections. It is stressed that in the case of two-electron processes the independent-electron model breaks down and the possible role of correlations between K-electrons is discussed.

The effect of the choice of the exchange correlation potential on the cross section and asymmetry parameter profiles is analysed. The VWN potential is compared with respect to the LB94, which displays the correct Coulomb tail. The effect of the two possible choices of the electron configuration (ground state, GS, or transition state, TS) is also considered. The comparison with respect to the experimental data shows that in the outer-valence region the VWN TS and the LB94 GS are the best choices for first-row hydrides and N₂ , while for core ionization LB94 GS performs much better. The discrepancies in the second-row hydrides are not recovered by the LB94 potential, and are ascribed to the screening effect which can be taken into account by a time-dependent extension of the theory. Because of accuracy and computational economy, LB94 GS appears to be a decidedly superior choice in comparison with the VWN TS potential.

The total cross section, , for electron capture by Na⁺ions from oriented coherent elliptic states (CES) of Li with principal quantum number n= 25 was studied experimentally for impact perpendicular to the minor axis of the elliptic orbit. The remaining geometrical parameters of the CES, which are the eccentricity, e , and the angle, , between the major axis and the beam direction, were varied in the course of the experiments, as was the reduced impact velocity v_r= nv , where vis the projectile velocity in atomic units. Several representative cuts were chosen within the parameter space (v_r ,e , ) = (0.74-2.09, 0-1, 0-2 ). The velocity range includes the region of matching velocities (v_r 1) where for given geometrical parameters attains its maximum value and the high-velocity region where is a strongly decreasing function of v_r . The cross section depends sensitively on each geometrical parameter, and the dependences change dramatically as v_ris varied. The spatial distribution of the CES apparently governs in some region of parameter space (v_r ,e , ) and in another the momentum distribution prevails.

Li₂ , HF, and H₂and other light diatomics at high vibration-rotational excitation exhibit an unusual form of vibration-rotation transfer generally associated with near energy resonance. This quasi-resonant transfer (QRT) gives rise to narrow rotational distributions. Near resonance in angular momentum is also a necessary requirement for the occurrence of QRT. However, the underlying physical processes differ little from those governing the more common forms of collisional transfer which, along with QRT, can be rationalized via the mechanism of linear to angular momentum interconversion within boundary conditions set by energy conservation. Velocity-jplots illustrate that these boundary conditions moderate the mechanism in a unique fashion in the case of QRT since they are sharply defined around a limited set of jvalues. The occurrence of QRT will be widespread in the high lying states of light diatomic molecules, the hydrides for example, and may readily be identified using plots of the energy and angular momentum conservation relations. Energy conservation forces a reduction in the maximum available torque arm in the angular momentum mechanism for all but a narrow range of jtransitions. This analysis of the primary physical mechanism is confirmed via multiellipsoid Monte Carlo calculations for Li₂and for H₂ . In HF-Ar we show that QRT is a much more likely process than pure rotational transfer giving rise to collisional pumping which will be enhanced in a multicollision environment.