Table of contents

Triple differential cross sections have been calculated for a range of incident energies and momentum transfers. The necessity of using correlated initial- and final-state wavefunctions for helium and of going beyond the first order is demonstrated. Excellent agreement has been obtained with recent absolute experimental measurements in helium.

Cross sections for multiphoton detachment from the negative ion of fluorine are calculated by applying the complex dilatation method with a finite basis of square integrable wavefunctions. The ion ground state is represented by a Hartree-Fock wavefunction. Excited states are described in the frozen core approximation. The author discusses the validity of representing excited states by a core plus a free electron to calculate multiphoton detachment cross sections. Two-photon detachment cross sections are compared with previous calculations.

The vibrational and rotational properties of the electronic ground state of the H₂⁺ molecular ion in a strong, external magnetic field are investigated theoretically for field strengths up to 10⁶ T. The hindered rotation of the magnetically dressed H₂⁺ system is examined in detail.

The charge exchange collision between Ar⁷⁺ (1.75 KeV a.m.u.^-1) and He has been studied by VUV spectroscopy. In the single-electron capture process, Ar⁶⁺(nl) is observed, n=4 being populated. Absolute cross sections have been measured. Double capture is identified and populated levels are assigned.

The modified Glauber approximation is employed to calculate the differential cross sections for electron-potassium elastic scattering at intermediate energies. The results are found to reproduce the two 'dips' (kinks) observed in experimental data by Buckman et al. (1979). At low scattering angles, the modified Glauber values agree better with the experimental data than those of other Glauber calculations.

The authors report numerical computations of differential cross sections for rotational transitions in electron-Na₂ scattering at collisional energies between 25 and 300 eV. In addition a simple spectator model is proposed, which leads to a closed-form expression for the rotational transition probabilities. The model is found to be in excellent agreement with numerical computations. All results reproduce recent experimental measurements of rotational-rainbow structures for backward scattering.

The authors have measured pick-off annihilation rates of orthopositronium localised in density fluctuations in fluid ethane as a function of microwave frequency in the range 90-100 GHz. Significant reductions in the annihilation rates are observed at 92 and 96.2 GHz. Possible mechanisms for microwave absorption and for the resulting decrease in the local density of electrons around Ps are discussed.

The generation of a squeezed state via non-degenerate four-wave mixing in a system of three-level atoms is discussed. The condition for receiving a nearly perfect squeezing is shown.

The 5p absorption spectrum of laser-excited atomic barium has been investigated in order to probe the changes in correlation between the 6s and 5d subshells. A pulsed tunable dye laser was used to populate the 6s5d states via the resonantly excited 6s6p ¹P₁ level. The 5p absorption spectrum between 50 and 85 nm was recorded at two different delay times, using as a pulsed continuum source a laser-generated plasma, formed by focusing a ruby laser onto a samarium target. The excited spectra show a very large number of new absorption features, most of them sharp but a few of them broad and quasimolecular in appearance.

For the high-lying doubly excited states of H^- and He, the authors have quantitatively examined diabaticity at close avoided crossings between two hyperspherical adiabatic potentials and have confirmed that the close avoided crossings in the diagrams of the adiabatic potential curves can be reconnected 'diabatically' for the N manifolds investigated here. Using these diabatically connected hyperspherical potentials, they have calculated the energy levels of the high-lying doubly excited states, i.e. ¹S^e states from N=7-9 hydrogenic thresholds for H^-, as well as those of ¹S^e states with N=6 and 7, and of ¹P⁰ and ¹D^e states with N<or=4 for He. Their results agree reasonably with other elaborate calculations based on the complex coordinate rotation method within a few percent accuracy.

Escape factors for upper and lower limits to the source function appropriate for spherical geometry have been evaluated using a set of Stark-broadened line profiles, computed with different approximations, for the Lyman- alpha line of argon XVIII. The method used to compute the escape factors, which is based on a general formalism can be applied to any kind of line profile and is suitable for any geometry. Comparisons are made with previous calculations that used Holtsmarkian line profiles and significant differences are noted.

An analysis, based on the quantum mechanical impact theory, is made of the electron-produced width and shift of the 2l2l' dielectronic satellite lines. In particular, the screening effects of the spectator electron on the dielectronic lineshift are investigated and discussed in connection with the parent lineshift. Numerical calculation has been carried out for the line parameters of the two helium-like transitions (2p²)¹D to (1s2p)¹P and (2s2p)¹P to (1s2s)¹S in laser plasmas. It is shown that the perturbation effects in the upper and lower levels of these transitions are of equal importance and also that the inelastic cross section contribution in line broadening completely masks the elastic one.

Experimental studies of the collisional broadening and shift of the 655.0 and 671.4 nm thallium lines perturbed by neon and argon were performed. The results are compared with those calculated on the basis of the van der Waals and the Czuchaj-Sienkiewicz potentials (1984). These results demonstrate that the Lennard-Jones model potential is completely inadequate for the description of pressure effects caused by neon on higher thallium states. The measured values of the broadening and shift by argon are in poor agreement with those computed for the van der Waals potential. Calculations based on realistic potentials reported by Czuchaj and Sienkiewicz give values of the broadening coefficient in good agreement with the experimental data.

The authors study the effect of level degeneracy and of the polarisation of the driving field on the correlated evolution of the translational states and of the number of fluorescent photons for atoms in a monochromatic radiation field. The evolution equations are formally solved for the case of deflection of an atomic beam by a laser beam. They give explicit solutions for the distribution of deflection angles in several special cases.

The authors consider a system of optically active two-state atoms immersed in a buffer gas and irradiated by a light beam. The corresponding Bloch equations are augmented by Keilson-Storer (KS) collision kernels (1952) which account for velocity-changing collisions. Expansion of the density operator elements in the eigenfunctions of KS kernels allows one to obtain exact solutions for the Laplace transforms of the density operator elements which may be expressed as rational approximants with any prescribed degree of accuracy. As an example they apply this method to discussion of the light-induced drift effect. In particular they give analytic expressions for the drift velocity, which are illustrated by numerical results. Finally, they briefly discuss possible generalisations and applications of the presented method.

The C(3)¹ Sigma ⁺ state of NaK has been studied by two complementary techniques: laser-induced fluorescence, recorded at high resolution by Fourier transform spectrometry; and two-photon resonant ionisation spectroscopy. Constants for the state, which dissociates to Na(3p ²P₁2/)+K(4s ²S₁2/), are T_e=17787.3 cm^-1, omega _e=69.66 cm^-1 and r_e=4.445 AA. These values are compared with the results of recent ab initio calculations.

Using monochromatised synchrotron radiation, the valence band and F 2s photoelectron spectra of XeF₂ have been recorded at photon energies of between 50 and 110 eV, below and above the Xe 4d ionisation edge. Branching ratios and partial cross sections have been obtained from the experimental spectra and compared with theoretical results from Ms X alpha calculations. There is generally reasonable agreement between theory and experiment. The 5 pi _u cross section is enhanced at a pre-edge resonance, while only the 9 sigma _g orbital (about 80% Xe 5s) character shows strong interchannel correlation. The results show that this type of interchannel correlation may only be important for non-bonding valence levels.

SCF-CI calculations of the photodissociation probability of the CN radical both with and without Rydberg functions in the basis set are presented. They have shown that there is no Rydberg state under the interstellar cut-off limit (13.6 eV above the ground state v=0). A value of 7.4*10^-10s^-1 has been determined, which is in close agreement with the interstellar-medium models.

Analytic expressions are obtained for Coulomb and exchange terms in inert-gas potentials, incorporating the appropriate asymptotic behaviour. The author has also deduced suitable damping factors for the van der Waals terms. This allows one to obtain accurate values for the position and depth of the minimum potential as well as potentials over a wide range of separations.

By means of differential energy-loss measurements, autoionisation processes have been studied mostly at a centre-of-mass collision energy of E=93 eV, around which two-electron excitation due to potential curve crossing begins to arise. Additional experiments were done at E=107 and 163 eV. The velocity of the scattered particles was analysed by a time-of-flight technique. In the energy-loss spectra, two peaks of the autoionised Ne⁺ ions located at energy losses of Q approximately=45 and 37.5 eV were observed. The former loss peak is attributed to the reaction Na⁺+Ne to Na⁺+Ne(2s²2p⁴(¹D)3s²¹D)-45.2 eV Ne⁺+e^- and the latter peak can be interpreted in terms of the rainbow effect appearing in the molecular autoionisation process. The two-electron excitation process described above can be well understood as proceeding through two successive one-electron excitation processes Ne(2s²2p⁶) to (Ne(2s²2p⁵3s)) to Ne(2s²2p⁴3s²).

A quantum mechanical description of the sputtering of diatomic molecules off an ion-bombarded surface is given. The single-collision mechanism is treated where an oriented molecule receives momentum from a collision with a substrate atom and is consequently emitted from the surface. Special attention is given to the excitation of rotational and vibrational degrees of freedom as a consequence of a sudden collision. An approximately factorising excitation probability, Poisson distributed for vibrational excitation, and approximately Gaussian distributed for rotational excitation, is found. The quantisation of the vibrational level is compared with a quasiclassical scheme; deviations for the low vibrational states are pointed out.

The authors report a comparative study of the quantal and classical treatments of single-electron charge transfer from the L and K shells of Li(1s²2s) in collision with He²⁺ ions at intermediate energies. Quantal calculations have been performed using coupled-state atomic orbital expansions with up to 43 pseudostates in the basis, within the impact parameter formalism, for impact energies of 8 to 2000 keV lab. Classical trajectory Monte Carlo calculations for L-shell capture, using a model potential for the active electron, have been performed for impact energies from 50 to 400 keV, and using an effective-charge model for K-shell capture at impact energies from 600 to 2000 keV lab.

The authors study the elastic scattering of electrons by the metastable states H(2s), He(2¹S) and He(2³S) at intermediate energies within the framework of the third-order optical potential theory. The first-, second- and third-order contributions to the optical potential are analysed separately, and are compared with the corresponding quantities for e^-H(1s) and e^-He(1¹S) elastic scattering. Results for positron impact scattering are also presented.

The interaction of slow electrons (collision energies up to about 20 eV) with polyatomic targets is examined for the specific case of the methane molecule in order to study the effect on total cross sections of various local approximations for the exchange interaction between bound and continuum electrons. It is found that the tail of such an interaction, outside the 'hard-core' region of the static potential, plays an important role at very low collision energies and ultimately controls the correct appearance of the experimentally found Ramsauer-Townsend (RT) minimum in the integral elastic cross section. Very good agreement between computed and measured values for the cross sections is found when using the modified semiclassical exchange approximation (MSCE) proposed. The broad resonance, which appears at experimental energies of around 8 eV, is also satisfactorily reproduced by the present methods.

The R-matrix method has been extended to enable electron impact ionisation processes to be studied. A 'fast' ionising electron is described by a distorted wave and both the initial target state as well as the continuum state consisting of the final ion and a 'slow' ejected electron, are expanded in terms of an R-matrix basis. The method is programmed for a general atomic system and some test calculations for total cross sections including ionisation plus simultaneous excitation are presented for electron impact ionisation of neutral helium.

A multichannel effective range theory (ERT) of the electron impact electronic excitation via a shape resonance is presented and applied to the excitation of the a¹ Delta _g and b¹ Sigma ⁺_g states of O₂ through the O₂^-2 Pi _g shape resonance. All the parameters required for the ERT treatment are extracted from an ab initio calculation on the O₂^-(² Pi _g) system. The X³ Sigma ^-_g, a¹ Delta _g and b¹ Sigma ⁺_g O₂ states can be considered as parents of the X ² Pi _gO₂^- resonance and the corresponding multichannel structure is analysed. Although the O₂^-(² Pi _g) resonance is very narrow (8*10^-5 eV) and located at very low energy (0.09 eV) at the O₂ equilibrium position, it dominates the electronic excitation process in the few eV energy range. Theoretical results are presented in the 0-12 eV energy range and compared with previous experimental and theoretical results.

Energy and angular dependences of differential cross sections for elastic scattering and vibrational excitation of the fundamental vibrations and some strongly excited overtones for OCS and CS₂ are presented. The impact energy ranges from 0.3 to 5 eV and scattering angles are measured between 12.5 and 138 degrees . Integral elastic, vibrationally inelastic and total cross sections are tabulated. The total cross sections are compared with theoretical results of Lynch et al. (1979). With few exceptions good agreement is found. The differential cross sections for elastic and inelastic scattering are compared with first Born approximations for dipole, quadrupole and polarisation potential interactions. A body-frame phaseshift analysis has been applied to the experimental data in the energy range below 1.2 eV.

The total cross sections for 1-400 eV positrons and electrons colliding with NH₃ and H₂O molecules were measured using a TOF method. The cross sections were obtained by a normalisation method. The results for e⁺-NH₃ and e⁺-H₂O at low energy were much lower than the theoretical values of Jain (1986). The cross sections of positronium formation in these gases were measured.

The author presents an analytic kinetic model available for all particle-beam-induced atomic plasmas, without any restriction on the distribution of electronic levels. The method is an iteration of the already known solution available only for the distribution of atomic levels as in the rare gases. The author recalls a universal atomic kinetic model which, independently of its applications to the study of efficient laser systems, might be a first step in the analytic investigation of molecular problems. Then, the iteration is systematically applied to all possible atomic structures whose number is increased by the non-local character of inelastic processes. The author deduces a general analytic representation of the 'tail' of the electron distribution function as a ratio between non-local source terms and a combination of inelastic cross sections, from which the author exhibits a physical interpretation and essential scaling laws. The theory is applied to sodium which is an important element in the research of efficient laser systems.