Table of contents

The Compton profiles (CP) of H⁺₂(² Sigma ⁺_g) are calculated using the exact Born-Oppenheimer (BO) wavefunction for various internuclear distances R. By using the rotational-vibrational (RV) state distribution factors, calculated according to the Franck-Condon (FC) principle or measured from experiment, the vibrationally averaged values of CP for H⁺₂(² Sigma ⁺_g) and D⁺₂(² Sigma ⁺_g) in the adiabatic approximation are also calculated. Various theoretical values are given for comparison with the CP measured from H⁺₂-Ar collisions. The experimental data are most compatible with the exact BO values at R=2.0 au but the data, with an error of 15%, fail to resolve various theoretical values at either small or large momenta q. It is important to have accurate experimental data (of absolute measurements) with an error of less than 10% in order to determine the accuracy of the theoretical values reported here.

Observations are reported of resolved components of deuterium Balmer emission in neutral beam heated JET plasmas. The features are interpreted as Doppler-shifted emission from deuterium in the neutral beams spectrally resolved into components by the nu *B motional Stark electric field. The mechanisms of excitation of the deuterium are elucidated and the emission modelled. Attention is drawn to the diagnostic potential of the observations for measurement of local magnetic fields, deuteron density and effective plasma ion charge.

Double ionisation of xenon by 40.8 eV photons has been studied using coincidence techniques. Photoionisation at this wavelength is found to populate all energetically accessible states of the doubly charged ion. Indirect double ionisation contributes significantly to the doubly-charged-ion yield.

An L² discretisation of the radial two-body Coulomb problem has been given by Yamani and Reinhardt using a non-orthogonal Laguerre basis. The Fourier coefficients form a set of orthogonal polynomials characterised by a positive-definite linear functional for all repulsive coupling but only for a limited range of attractive strengths. It is shown here that this behaviour is due to the choice of the original basis set alone and is not intrinsic to the Coulomb Hamiltonian. A new expansion is presented, the Fourier coefficients of which are kernel polynomials of the Yamani-Reinhardt ones. They possess the positive-definiteness property for arbitrary coupling strengths. Hence the new expansion is a natural one to use for equivalent quadrature formulations and finite-basis approximations.

Potassium vapour, contained together with a buffer gas in a glass fluorescence cell at a controlled temperature, was irradiated with pulsed dye-laser radiation which excited the K atoms to the 5²P_1/2 or 5²P₃/₂ state. The effective decay rate of the 5²P atoms was determined in relation to the buffer gas densities by methods of fluorescence spectroscopy with time resolution. The experiments yielded the total quenching (depopulation) cross sections of 19, 94 and 120 AA² for collisions with H₂, N₂ and CH₄ molecules, respectively.

The results of high accuracy calculations on a number of ground and excited states of the deuterium and tritium mesomolecules dd mu , tt mu and dt mu are presented. The absolute errors of computed binding energies of the ground and excited states in most cases of these mesomolecules do not exceed 1*10^-6 eV.

The authors derive integral equations for a relativistic system of two particles, interacting via the Coulomb force, from the underlying quantum field theory. Three cases, where particles are charged spinless scalars or spin-1/2 fermions, are considered. They discuss the ground-state solutions of these equations in the perturbative and strong-coupling (high-Z) regime for arbitrary masses and charges of the particles. In the limit where the mass of one of the particles becomes infinite comparison is made with the Dirac-Coulomb and Klein-Gordon-Coulomb equations.

The spectrum of ionised rubidium from a laser-produced plasma has been photographed in the 250-500 AA region. Lines arising from 4p-5s and 4p-4d transitions in ion stages Rb IV to RB IX were found to dominate the spectrum. Some 4p-5s transitions in Rb VI together with the strongest lines of the 4p-5s in Rb V were identified by isoelectronic comparison and Dirac-Fock calculations. Considerable configuration mixing occurs in Rb V and leads to a major redistribution of intensity between 4p-5s and 4p-4d transitions.

The symmetrised form of the operator Van Vleck perturbation theory is considered (the irreducible tensorial set theory is used as the mathematical basis of this consideration). General formulae which define spectroscopic rotational, centrifugal and resonance constants of molecules as functions of reduced matrix elements of vibrational operators are obtained. Algorithms for the ES-1055 computer have been derived on the basis of the formulae obtained, and a system of analytical calculations using the language 'REDUCE' has been created. The dyad and pentad of the CH₄-type molecule are considered for illustration. Formulae are obtained which connect spectroscopic and intramolecular constants not only for isolated but also for interacting vibrational states.

The use of scattering states permits the incorporation of collisional interactions directly into the coupling coefficients between electromagnetic fields and colliding atoms. In the scattering state representation, the coefficients are dependent on the energy and relative momentum exchanges of the atoms. Collisionally induced radiative transition channels are associated with the partially broken translation symmetry.

State-selective single electron capture in He²⁺-Li collisions has been investigated experimentally by the two complementary methods of photon spectroscopy and translational energy spectroscopy. The present work deals primarily with the population of He II (n=4), which is also of interest for alpha particle diagnostics in tokamak plasmas. A critical comparison of the authors' results with advanced theoretical studies is presented.

Mutual neutralisation of N⁺/O^- and O⁺/O^- ions has been measured by an inclined-beams technique for interaction energies between 33 and 2090 eV. At the lower limit the energy range almost overlaps that of the merged-beams experiments described in 1975 by Moseley et al. The present results are at least three times smaller than those of Moseley et al and possible reasons for the discrepancies are discussed. At interaction speeds of greater than 2*10⁵ m s^-1 the cross sections for N⁺/O^- and O⁺/O^- converged with recent measurements by Peart and Foster for Na⁺/O^-.

The creation of an e^--e⁺ pair as an intermediate mechanism of electron capture in collisions between bare heavy ions at relativistic impact velocities is analysed. A distorted-wave model is developed to study the process. A two-centre Sommerfeld-Maue spinor is introduced to represent the positron as moving in the simultaneous presence of the projectile and target nuclei. Single differential cross sections are calculated as a function of positron energy. Comparisons are given with results obtained by using other theoretical approximations.

An additive Mittag-Leffler type of partial-wave S-matrix expansion based on resonance poles, their corresponding residues plus a background integral is presented. A particular decomposition of this expression is studied in detail, and the authors emphasise the employment of exterior complex dilatation methods in connection with its investigation. They further give a convenient and simple formula for the residues of the partial-wave S matrix at mirror poles expressed in terms of the residues at the corresponding resonance poles. The restrictions on its applicability are explicitly shown in terms of conditions on the interaction potential. Furthermore, the analytic properties of the various parts of the partition of the expansion are discussed. The considerable advantages and the strength of the formal development are demonstrated by accurate numerical calculations on previously studied electron-atom scattering models using Be, Mg and Ca as targets. The outcome of these investigations is, among other things, a rather appealing (so called) reduced partial-wave S-matrix expansion which may be employed in order to obtain precise scattering information in a very time-saving manner.

Electron impact excitation of the Hg(6³P₁) sublevels has been studied by combining the techniques of electron-photon coincidences and polarised electron scattering. A light polarisation filter allowed the separate observation of the M_j=0 and M_j=+or-1 sublevel excitation. Spin-up-down asymmetries were measured at collision energies of 8 and 15 eV and scattering angles ranging from 5 to 120'. A variety of parameters and observables have been calculated from experimentally determined state multipoles which yield detailed information about the influence of different spin-dependent interactions on electron impact excitation.

A joint experimental and theoretical study of electronic excitation in low-energy e^--O₂ collisions is presented. Experimentally, the total cross sections for the excitation of the (. . pi ³_u pi ³_g) c ¹ Sigma ^-_u, A ³ Sigma ⁺_u and A'³ Delta _u group of states are measured in the 10-20 eV energy range on a crossed-beam spectrometer. Theoretically, the contribution of the resonant excitation process through the (² Pi _g)O^-₂ resonance is studied in the extension of the effective range approximation, developed in the authors' previous study on the a ¹ Delta _g and b ¹ Sigma ⁺_g states. Resonant excitation of the a and b states only involves a rearrangement of the outer pi _g shell, whereas excitation of the ( pi ³_u pi ³_g) states also involves the more inner pi _u shell. Comparison between the experimental and theoretical results show that this resonance process is dominating the excitation of the c, A and A' states.

The problem of the interaction of radiation with arbitrary polarisation and intensity with the resonance transitions 1/2-1/2 and 1-0 is solved. Exact expressions for the orientation vectors of the ground and excited states for the transition 1/2-1/2 and for the orientation vector and alignment tensor of the transition 1-0 have been obtained. In the case of longitudinal propagation the system of equations for the absorption of an elliptically polarised wave is obtained. It is shown that, irrespective of the initial state, the polarisation at the transition 1-0 tends either to the linear or to the circular form as it propagates depending on the times of collisional relaxation of orientation and alignment and on the magnetic field value. In the case of transverse propagation the dependence of the saturation intensity of a linearly polarised wave on the magnetic field value and on relaxation widths is obtained.

The authors have analysed the optical double resonance signal to second order in the modulation of the amplitudes of the two lasers at high intensities. Resonant modulation in the populations of the upper level of the transitions are induced at the effective Rabi frequency alpha ₀, whenever the modulation frequencies themselves or a combination of them becomes equal to O, +or- alpha ₀, +or-2 alpha ₀. It is proposed that by performing experiments similar to those done by Miyazaki et al such Rabi resonances in the populations of the levels may be detected by optogalvanic technique-i.e., by monitoring the oscillations in the current or the voltage at the sum or the difference of the modulation frequencies of the two lasers.

Spectra of elements belonging to the iron group, emitted from a low inductance, high voltage vacuum spark, have been recorded in the 10-20 AA range by means of a slitless RbAP crystal spectrometer. From the space- and energy-resolved spectra the authors conclude that the Ne I-like ions emit their line radiation from a relatively extended plasma region, the 'pre-pinch'. In similar discharges, titanium spectra have been recorded in the 10-100 AA range by a 2 m grazing incidence Schwob-Fraenkel spectrometer. The experimental intensities of the Ti XIII, Ne I-like 2p⁶-2p⁵3l (l=s, d) and 2s²p⁶-2s2p⁶3p El transitions have been compared with intensities predicted by a non-LTE collisional-radiative atomic model of the Ne I-like emission. The model includes the effect of recombination from F I-like ions and inner-shell ionisation of Na I-like ions, in addition to electron impact and radiative transitions. Based on the spatially imaged results and the comparison of experimental and theoretical line intensities, the authors estimate an electron density of approximately 6*10¹⁹ cm^-3 in the pre-pinch phase of the vacuum spark plasma. The estimated electron temperature of the emitting plasma is 200-300 eV and its spatial extent is determined to be of the order of 200 mu m.