Table of contents

The authors address the question of how classical and quantal degrees of freedom interact in time-dependent situations. From a variational principle, a set of self-consistently coupled Schrodinger (electronic)-Hamilton (nuclear) equations are derived and discussed. The equations conserve total energy and momentum and reduce to the Born-Oppenheimer approximation in the adiabatic limit. In their formulation, the nuclear trajectories carry quantal information in the WKB sense. Current applications to simulations of atomic collisions are mentioned.

The vector polarisability of the atomic hydrogen ground state, which arises due to relativistic corrections, is found in the static limit. The dynamic P-odd polarisability of hydrogen, caused by the weak electron-nucleus interaction, is also calculated.

It is shown that an optimised version of Brillouin-Wigner perturbation theory is closely related to the iteration-variation method; the bound-state energy in the former and the phaseshift in the latter are both expressible as Pade approximants.

Including the probability of autoionisation and radiative decay the authors have estimated the total electron impact excitation-autoionisation cross section for lithium-like ions. They present results for Ne⁷⁺ and Fe²³⁺.

A novel and analytic expression is derived for mean excitation energies of arbitrarily stripped ions.

The ground-state energies of the three small molecules H₂O, BH and FH are calculated by two different mean-field methods. A comparison of the results shows that the g-Hartree approach, recently introduced and amply applied to atoms, substantially overestimates the magnitude of the correlation energies. The results are discussed in the context of a general Fock operator with effective electronic charges.

The bound states of the Hellmann potential, which is the superposition of the attractive Coulomb potential (-A/r) and the Yukawa potential (B e^-Cr/r) of arbitrary strength B and screening parameter C, have been re-examined in the light of first-order Rayleigh-Schrodinger perturbation theory. Instead of working with the usual Coulomb potential as the unperturbed Hamiltonian, the authors propose that an effective Coulomb potential with a modified strength represents the lowest-order approximation to the Hellmann potential. An arbitrary parameter appearing in the new Coulomb potential is than determined from the notion of the virial theorem and intuitive physical arguments. It is found that the accuracy of the predicted energy eigenvalues for the bound states improves significantly even when C approximately=1, for which the usual Coulomb perturbations give poorer results. In spite of the simplicity of their approach, the numerical results compare very well with those of Adamowski (1985) obtained through a ten-parameter variational calculation.

Algebraic expressions are derived to second order in perturbation theory for the contributions to the effective two- and three-electron orthogonal operators which act within the configurations s²p^Nd and owe their origin to the excitations of the types d to d', d to s', p to p', sd to p², s to d, s to s', sd to pp', sd to pf and s² to p². The parameters defining the strengths of the orthogonal operators are obtained by least-squares fits to the experimental levels of 2s²2p^N3d and are compared with the values calculated from Hartree-Fock integrals and configuration energies for the third spectra of O, F, Ne, Na and Mg, and for the fourth spectra of F, Ne, Na, Mg and Al. The agreement, which ranges from excellent to moderate, is discussed in terms of higher-order contributions and omitted excitations.

A recently introduced set of orthogonal operators for pNd configurations has been used to fit the experimental energy levels of the 2p^N3d configurations in III and IV spectra of the first row elements. The 2p¹3d to 2p⁵3d configurations have been considered and the isoionic development of the parameters is presented. The two-, three- and four-electron parameters include electrostatic configuration interaction effects to infinite order in perturbation theory and very small mean errors are obtained in the parametric fitting. The effect of the 2p^N4s configurations which overlap the 3d configurations is included explicitly. Of the magnetic interactions, the spin-orbit interaction of the 2p electron is included in combination with an empirical correction for the magnetic interactions in the core. The fitted values of the two-electron parameters are compared with HF predictions. In an appendix, the matrix elements of the four-electron operators in p³d are given.

One-electron properties of the OCS molecule have been obtained from SCF and CI wavefunctions using extended Gaussian basis sets. While most properties are in good agreement with experiment, the electric quadrupole moment is an exception. The effect of additional diffuse s-, p- and d-type functions is examined. By an appropriate choice of d exponents it has been possible to obtain improved agreement with experiment for the quadrupole moment at the CI level. In particular, for the quadrupole moment of OCS: theta _SCF=-5.27*10^-40 Cm² theta _CI=-3.58*10^-40 Cm² as compared with theta _EXP=(-2.93+or-0.50)*10^-40 Cm² experimentally.

Theoretical calculations have been carried out on the ground and excited electronic states of NeH. Electric dipole transition moments have been obtained and radiative lifetimes have been estimated. Radial coupling matrix elements between the ² Sigma ⁺ states have been also computed. It has been found that the magnitude of the A-X interaction in NeH is much smaller than that of the corresponding interaction in HeH, in accord with the existence of fluorescence from the A ² Sigma ⁺ state of NeH but its absence in HeH.

The L X-ray fluorescence cross sections ( sigma _{L alpha} , sigma _{L beta} and sigma _{L gamma} ) and relative intensities (I_{L beta} /I_{L alpha} and I_{L gamma} /I_alpha ) for La, Pr, Sm, Eu, Gd, Tb and Dy have been measured at various photon energies in the range of 11-41 keV. In addition the revised measured values of L XRF cross sections and relative intensities for Ba, Ce and Nd are also reported. A reasonable agreement is found between the present measured results and the theoretically calculated values of L XRF cross sections and relative intensities using various physical parameters based on relativistic theory. To the best of the authors' knowledge, the L XRF cross sections and relative intensities for these elements at different energies, except those of Nd, Sm, Gd, Tb and Dy at 17.8 and 25.8 KeV are being reported for the first time.

The L X-ray production cross sections. sigma _{L alpha} , sigma _{L beta} and sigma _{L gamma} for Gd and Tb obtained by proton impact in the energy range 250-400 keV have been measured using an HPGe detector. The theoretical calculations for these X-ray production cross sections have been performed by perturbed stationary state (PSS) theory with relativistic (R), energy loss (E) and Coulomb deflection (C) corrections (ECPSSR theory) using the theoretical values for X-ray emission rates, subshell fluorescence yields and Coster-Kronig transition probabilities. Due to the lack of other experimental data, the present results have been compared with the theoretical predictions and are found to be in reasonable agreement. The intensity ratios I_{L beta} /I_{L alpha} and I_{L gamma} /I_{L alpha} are also presented.

The authors have studied the fluorescence spectrum of a two-level atom interacting with a resonant smooth laser pulse. Assuming a filter width larger than the Fourier width of the pulse, an approximate analytic closed form for the power spectrum valid for any pulse shape is obtained. In these conditions the power spectrum exhibits a time-developing three-peak structure, while the energy spectrum presents a single line, unless the pulse is near rectangular. Theoretical energy spectra are in qualitative agreement with measurements of the resonance fluorescence emitted by strontium atoms irradiated by short laser pulses.

The authors have observed dramatic changes in both the magnitude and the lineshape of Faraday rotation on an allowed E1 transition in Sm I (4f ⁶6s²⁷F₀-4f ⁶6s6p ⁷G₁) as a function of laser intensity and buffer-gas pressure. The effect is ascribed to the development of optical and Zeeman coherences and a theoretical treatment, based on the solution of steady-state density matrix equations is given. Effects of optical pumping on the odd isotopes ¹⁴⁷Sm and ¹⁴⁹Sm, both with I=7/2, are also considered.

The hydrogen atom Zeeman effect is treated by using simple basis functions of the form r^N e^{- beta r} Y^M_L. The Schrodinger eigenvalue equation is transformed into a recurrence relation, which yields accurate energy levels when solved by a new shooting-relaxation technique.

The relative ion yields for Rbⁿ⁺ (n=1-3) and Srⁿ⁺(n=1-4) have been determined using a combination of a time of flight mass spectrometer and monochromatised synchrotron radiation crossed with an atomic beam. The incident photon energies are around the threshold of 3d ionisation: 112-126 eV for Rb and 136-146 eV for Sr. For both Rb and Sr, the ion yield curves for multiply charged ions show prominent peaks corresponding to the excitation of 3d electrons to bound np (n=5, 6) states, while those for singly charged ions have no noticeable structure. At the resonance energies, Rb²⁺ dominates among Rbⁿ⁺ (n=1-3) ions and Sr³⁺ among Srⁿ⁺ (n=1-4). It was found that excitation of the 3d electron followed by two or three successive autoionisations is an important possible route for the production of multiply charged ions.

Measurements are reported of absolute photoionisation cross sections of Zn⁺ and Ga⁺ ions by VUV radiation. Both cross sections were dominated by autoionisation resonances. For Ga⁺ the largest resonances had cross sections of 3*10^-16 cm² and were centred about 21.88 eV. This is similar to the energy of 21.9 eV calculated by Pindzola et al (1982) for 3d¹⁰4s² to 3d⁹4s²4p ³P₁. Comparisons were also made with the spectrum of atomic zinc measured by Marr and Austin (1969). For Zn⁺ some photoionisation resonances could be related to autoionising transitions in the electron impact ionisation cross sections reported by Rogers et al (1982) but the strongest electron-impact-induced transition does not appear in the photoionisation spectrum.

Auger spectra of atomic aluminium caused by 2p photoionisation have been measured at selected photon energies. The information obtained from these spectra together with information from the photoelectron spectra (initial state of the Auger transitions) and from optical data (for the final states of the Auger transitions) provided the basis for the classification of practically all observed Auger lines.

Parity non-conserving (PNC) optical rotation has been measured by laser polarimetry in the 648 nm magnetic dipole transition (6p³ J=3/2 to 6p³ J'=5/2) in atomic bismuth. The experiment involves finding the small differences in rotation between selected frequency points in the vicinity of the F=6 to F'=7 hyperfine component. Faraday rotation, which can be distinguished from PNC rotation by its wavelength dependence, is used in locking the laser frequency and calibrating the PNC effect. Results obtained over a six-year period are summarised; a detailed discussion of error sources and associated tests is given. The final result for the PNC parameter of the 648 nm transition is R=(-9.3+or-1.4)*10^-8. This is an agreement with the measurements of Birich et al (1984) but not with those of Barkov and Zolotorev (1980). It is also consistent with the standard model of the electroweak interaction, but the uncertainty in the atomic theory is now the limiting factor in the comparison.

Absorption from the Ar₂(1³ Sigma ⁺_u) state to the Ar₂(2³ Pi _g) states was investigated between 490 and 530 nm using a dye laser as probe light. The complicated spectrum is attributed to two closely spaced molecular levels (separated by 68 cm^-1) which are assigned to the 2_g and 1_g states. The vibrational constants of the 1³ Pi ⁺_u state were determined to be omega _e=293+or-4 cm^-1 and omega x_e=2+or-1 cm^-1, while for the 2_g and 1_g levels omega _e=350+or-10 cm^-1 and omega _e=295+or-4 cm^-1 respectively, were obtained.

The photoabsorption and fluorescence cross sections of GeH₄ were measured in the 50-200 nm region using synchrotron radiation as a light source. The fluorescence spectra were dispersed and used to identify the emitters as excited Ge and GeH. Weak vibrational structure superimposed on the absorption continuum was observed in the 70-80 and 105-115 nm regions. The assignment of the observed absorption bands to Rydberg transitions is discussed.

Classical three-body calculations have been performed to predict total and doubly differential ionisation probabilities and cross sections for 10 keV to 100 MeV H⁺+H and 100 keV H⁺+He collisions. Total and differential electron capture cross sections have been calculated from 3 keV to 1 MeV for H⁺+H. The results are in good agreement with measured cross sections and with other theoretical results known to be relatively good approximations in specific impact energy regimes. The validity of the classical approximation especially at high projectile velocities is discussed.

The K-shell ionisation cross sections, sigma _K, have been measured for ₆₀Nd, ₆₂Sm, ₆₅Tb, ₆₇Ho and ₇₃Ta targets in the incident proton energy range from 2.7 to 10.0 MeV. The absolute sigma _K values have been determined by normalisation to the known Coulomb excitation cross section of nuclear collective states. The experimental results are discussed with reference to the theoretical models and to the other proton-induced ionisation data.

The authors analyse the 'elastic' scattering, accompanied by the transfer of l photons, of fast electrons by atoms in the presence of a strong laser field. They focus their attention on the region of small momentum transfers, where the 'dressing' effect due to the dipole distortion of the target by the laser field is shown to produce important modifications of the differential cross section. Detailed calculations are presented for both atomic hydrogen and helium targets at an incident electron energy of 100 eV and for various geometrical configurations.

Absolute electron scattering cross sections for argon, krypton and xenon have been measured at low electron energies using the powerful technique of photoelectron spectroscopy. The measurements have been carried out at electron energies varying from 0.7 to 10 eV with an accuracy of +or-2.7%. The cross sections obtained have been compared with other recent measurements and theoretical computations.

Spin polarisation parameters have been calculated for the elastic scattering of electrons from mercury atoms at incident energies up to 180 eV. These results were obtained by solving the Dirac equations, which included relativistic Dirac-Fock static and non-local exchange potentials plus a non-relativistic polarisation potential. A comparison with recent experiments of the results for the spin polarisation parameters S, T and U as well as for the difference in phase between the direct and exchange scattering amplitudes yields very good agreement at energies above 15 eV. The authors also present results for elastic differential, total and momentum transfer cross sections.

The authors have used the polarisation-correlation technique to obtain a complete set of target parameters for the 3³P state of helium excited by electron impact. The linear and circular polarisation of the 3³P to 2³S decay radiation (388.9 nm) is measured in coincidence with the inelastically scattered electrons for an incident energy of 60 eV as a function of the electron scattering angle. From the measured time spectra the authors deduce a lifetime of the 3³P state of 93.6+or-1.0 ns. The depolarisation due to the fine-structure interaction in the ³P state is accounted for. They present the experimental results in terms of the angular momentum transfer L_{perpendicular to} and the alignment angle gamma of the excited electron cloud and a comparison is made with a recent FOMBT calculation. The excitation process is found to be completely coherent, indicating the absence of spin-dependent interactions during the excitation process.

Electron-polarised-photon coincidence techniques are used to determine linear and circular polarisation correlations for the differential electron impact excitation of the VUV resonance transitions in neon, argon and krypton at impact energies of 60 and 80 eV and a range of electron scattering angles up to 45'. Where possible, comparisons with previous experimental and theoretical work are made. The measurements indicate that excitation of fully spin-orbit coupled states occurs. Transfer of angular momentum perpendicular to the scattering plane is positive for all the scattering angles studied and is similar to what was observed previously in He at the same impact energies. Significant differences between helium and the other rare gases occur when the alignment of the excited-state charge cloud is considered.

The Cavalleri electron density sampling technique (1969) has been used to measure the rate of loss of thermal electrons in mixtures of CH₃Br and H₂ at temperatures in the range 293-500 K. From these data attachment coefficients for thermal electrons in CH₃Br have been derived with an uncertainty of less than +or-4% enabling the temperature dependence and activation energy to be determined. In addition the diffusion coefficient for thermal electrons in CH₃Br at room temperature has been determined from which an estimate of the e^--CH₃Br momentum transfer cross section has been made for energies less than 100 meV.

Electron attachment cross sections are reported in the electron energy range 0-160 meV, and at energy resolutions of 6.5-7.0 meV (FWHM) for the molecules CH₂Br₂ (dibromomethane), 1,1,1-C₂Cl₃H₃ (1,1,1-trichloroethane), 2,3-butanedione and 2,3-pentanedione. Use is made of the Kr photoionisation method. Attachment lineshapes, deconvoluted from the spectral slit (electron energy) function, are converted to cross sections by normalisation through thermal attachment rate constants at 300 K. Rate constants as a function of mean electron energy in are calculated from the cross sections using a Maxwellian electron energy distribution function, and a simple expression given for the energy of the peak attachment rate constant.

Saturation and power broadening of the calcium singlet resonance line (422.7 nm) was found to show strong pressure dependence for helium, argon and xenon. This was ascribed to transfer of excitation from the (4s 4p)¹P₁ level through the sequence of levels ¹D₂, ³D_J, ³P_J by predominantly collisional processes. The first step of this cascade, ¹P₁-¹D₂, was identified as the slowest process and cross sections are presented. The temperature independence of the process for xenon indicates that the large cross section for this gas may be due to a favourable potential curve crossing. Quenching of the ³P_J levels is also detected and cross sections are given for argon and xenon.