Table of contents

Radiative lifetimes and g_J factors of lower-lying even-parity levels in the Yb I spectrum have been determined by time-resolved fluorescence spectroscopy. An improved source for metastable atoms, reducing magnetic stray fields, was applied in the measurements of J=3 states. The results are: 6s6d ¹D₂: tau =35.2(10) ns, g_J=1.020(7); 6s6d ³D₃: tau =23.4(11) ns, g_J=1.354(15); 6s7d ¹D₂: tau =49.8(20) ns, g_J=1.111(12); 6s7d ³D₃: tau =43.1(12) ns, g_J=1.338(7); 6s7d ³D₂: tau =66.5(36) ns, g_J=1.079(7); 6s7d ³D₁: tau =38.4(17) ns, g_J=0.508(5); 6s7s ³S₁: tau =15.9(19) ns, g_J=2.01(1); 6s8s ²S₁: tau =34.3(42) ns, g_J=2.01(1); 6s8s ¹S₀: tau =37.9(42) ns. The lifetime data have been compared with Coulomb approximation calculations and experimental values taken from measurements of transition probabilities.

Attention is drawn to the close correspondence which exists between different branches of physics in which the following situation is met, namely an 'intruder' state with distinctive properties occurs in a normal sequence of levels and mixes with them. The properties of the levels can be sharply modified and, in the right circumstances, their particle widths can exhibit a pronounced minimum, or even be zero, at a certain energy. It is shown analytically from K-matrix theory that this effect, well known in nuclear physics, occurs also in atomic physics when a Rydberg series is disturbed. An experimental case has recently been reported.

It is shown that any physical matrix element of the Coulomb Green's function can be written in a form such that its Sturmian expansion is convergent both below and above the continuum threshold.

Excitation dynamics in collisions of the Li(2s)+Na(3s) systems have been investigated theoretically in the energy region from (E_CM)=3 to 40 keV using the molecular orbital expansion method with the electron translation factor correction up to first order in the relative velocity. A pseudopotential method has been employed to obtain molecular wavefunctions and eigenenergies for the LiNa system. The first peak observed in the Li(2p) and Na(3p) excitation channels around 6.5 keV was well reproduced by the present calculation. This was attributed to the combination of increase and decrease of Sigma and Pi state cross sections. However, the present theory could not interpret the origin of the second peak found at about 22 keV, but some proposed mechanisms are given. Also, comparison of the present calculation with recent results of Nielsen et al. (1985) who used the atomic orbital expansion method is made.

The authors have measured the charge state distribution of 20 keV multiply charged Ne, Ar and Kr ions scattered from a polycrystalline tungsten target. For primary charge states up to 8+ only singly and doubly charged reflected particles are observed. However, when the primary charge state exceeds 8+, scattering into triply charged ions does occur and the shapes of the respective energy spectra also change. An explanation is given in terms of autoionisation processes occurring when the scattered particle leaves the surface in a highly excited state.

Excitation of nv₁ modes (N-H stretching) in NH₃ is dominated by a broad shape resonance centred around 7.3 eV. The angular distribution shows the dominant l=2 character of the active partial wave.

The coherent renormalised multicentre potential model including intramolecular multiple scattering has been extended, for the first time to study the vibrationally elastic and inelastic scattering of electrons by linear molecules in the intermediate energy range. The authors' calculated vibrationally elastic cross sections for e^--H₂ scattering agree very well with experiments and the vibrational excitation cross sections agree qualitatively with the measurements. Their results also show that the inclusion of the multiple-scattering effect in the calculation is important at lower incident energies.

Algebraic expressions making contributions to the specific mass shift in atomic spectra relevant to non-relativistic many-body perturbation theory have been expressed in tensorial notations. These expressions are numerically evaluated to obtain the specific mass shifts in transitions involving the 2p⁵3s and 2p⁵3p configurations of neon between the ²⁰Ne and ²²Ne isotopes. Excellent agreement is found between the experimental and theoretical specific mass shifts.

The specific mass shift contribution to the isotopic shift in ionisation energies or selected transition frequencies has been computed for a series of atoms ranging from lithium to potassium. Many-body perturbation theory within the algebraic approximation was used to compute separate expectation values of the specific mass shift Hamiltonian for upper and lower states, and the computations are complete to third order. The nuclear field shift was also considered in some cases, and computed to third order. Good agreements with experiments were obtained in most of the cases investigated. The computed residual isotopic shift (specific+field shift) between ²⁵Na and ²³Na for the ²P-²S transition is 386.5 MHz, compared with an experimental shift of 375.4(1.3) MHz. Similarly the computed residual shift in the potassium ionisation energy (⁴¹K-³⁹K) is found to be -59.8 MHz, in complete agreement with the experimental value of -61(2) MHz.

Some doubly excited autoionising states of helium atoms converging on the N=3 He⁺ threshold are calculated by use of a method of complex coordinates. Hylleraas-type wavefunctions are used for L=0 and L=1 resonances with products of Slater-orbital type wavefunctions with expansion lengths up to 319 terms for L>or=2 resonances. States of parities (-1)^L and (-1)^L+1 and with angular momenta of L<or=4 are calculated. Resonance energies and autoionisation widths are reported.

The authors report new high-resolution measurements of the 6s subshell photoabsorption spectrum of Tl I. The observations were made in the first order of a 6000 line/mm holographic grating of 3 m radius, using synchrotron radiation as the background source of continuum. All the series are extended to higher n values, and the earlier analysis is improved. A two-dimensional analysis by multichannel quantum defect theory reveals two prominent perturbations due to coupling between series converging to the ³P₀ and ¹P₁ limits and between series converging to the ³P₂ and ¹P₁ limits.

Broadening and shift parameters for the Na D lines due to collisions with atomic hydrogen have been calculated for temperatures in the range 5000-9000K using improved NaH potentials. The broadening results, while about 30% higher than the earlier calculations, still lie about 25% below the recent measurements of Lemaire et al. (1985). The value for the shift is almost twice that calculated previously and is more than twice the observed value. The primary cause of the increase in these cross sections from earlier values is the much longer-ranged potential obtained for the ionic A ¹ Sigma ⁺ NaH state in recent calculations. The fine-structure changing rate is about 50% larger than the earlier value.

The interaction between a Rydberg series of weakly autoionising states and a very broad interloping resonance gives rise to characteristic effects in experimental spectra, namely the enhancement of upper series members, distortions (involving the possible appearance of minima) of the course of intensity in a series, fluctuations of width and the 'q-reversal' phenomenon. New experimental data at high resolution are reported. New theoretical formulae are proposed. Computations of line profiles based on these formulae reproduce many features of the data, in particular 'q-reversals' which may be detuned from the resonance energy of the broad level, fluctuations in the amplitude and width of the fine structure, etc. The 'q-reversal' effect is shown to play a central role in the interpretation of the interchannel coupling.

The authors use a finite basis of square integrable functions to represent the ground state of H^- and calculate one- and two-photon detachment cross sections. The photodetachment cross sections obtained are in good agreement with previous calculations and experimental data. Two-photon detachment cross sections are found to range between 0 and 10^-10 s^-1(W cm^-2)^-2 when the frequency is varied between one- and three-photon detachment threshold.

The states lying in the discrete and the lower continuous part of the spectrum of the manifolds ^1,3S^e, ^1,3P^e,o and ^1,3D^e of the beryllium atom have been obtained by using variational wavefunctions expressed as antisymmetrised products of group functions. While the valence-shell group is described by a rather large number of configurations, only limited polarisation is admitted for the K-shell group. The calculations, made with the aim of obtaining acceptable representations of the resolvent, show satisfactory agreement with all the available experimental data and most recent theoretical calculations. The position and width of several resonances were given with good accuracy by different methods which may be employed with L² basis sets.

The two-photon transition probabilities and ionisation cross sections have been calculated from the ground state of beryllium employing the L² basis set described in the previous work by Moccia and Spizzo (1985). In addition the electric dipole polarisabilities have been calculated for several bound states.

The cross section for 2p photoionisation in atomic sodium and for the associated (2p)(4s) satellite line have been calculated using a relaxed Hartree-Fock model. The symmetry parameters associated with the ¹P and ³P final ion-core multiplets have also been determined. They are shown to have different photon energy dependencies near threshold. The results have been compared with earlier unrelaxed Hartree-Fock calculations by Theodosiou and Fielder (1982) and with available experimental data. The results are sensitive to relaxation and the inclusion of this contribution yields good agreement with experiment.

The variation of the photoelectron spin polarisation parameters ( eta and delta ) relative to the 5d, 5p and 4f subshells of mercury has been determined in the relativistic Dirac-Slater approximation. The influences of both Coulomb and non-Coulomb phaseshifts are emphasised. The authors' results are compared with other theoretical results and experimental measurements when available. They are discussed in terms of the relative importance of relativistic effects in both the initial discrete and final continuum states.

The authors present the determination of adiabatic and quasiadiabatic potential energy curves of the ground state and of some low-lying excited states of the molecular system (HeNe)²⁺. An extension of Levy's method for disadiabatisation is developed in the study of the ¹ Sigma ⁺ states. The potential curves and the electronic couplings calculated in this work are useful for the analysis of collisional processes, in particular for the charge transfer reaction Ne²⁺+He to Ne⁺+He⁺. The Einstein coefficients A_i,j needed for this study have also been calculated.

The theoretical variations of the L-shell line intensities L_{gamma 1} and L_{gamma 6} and the emission ratio ( Gamma _{gamma 6}/ Gamma _{gamma 1}) as a function of ion energy and target atomic number are given. This enables calculation of the L₁ subshell ionisation cross section by the techniques first described by Datz et al. in 1974. The ECPSSR ionisation cross sections together with the fluorescence yields of Krause (1979) and the emission rates of Salem et al. (1974) have been used. It was found that the L_{gamma 6} subtraction from the L_{gamma 236} line was most critical for ion energies around 1 MeV amu^-1 and for target atomic numbers between 74 and 96.

Thermal, Penning and charge transfer reactions of He*(2³S₁) metastable atoms and He⁺ ions with cadmium were studied in a flowing afterglow apparatus. The absolute values of the rate constants for excitation of the individual Cd⁺* levels have been measured. In Penning ionisation, only 22 and 13% of the Cd⁺ ions are produced in the upper levels of the 441.6 and 325.0 nm laser lines respectively, and 40% of them are directly formed in the Cd⁺ ground state. In charge transfer reactions, the part of the Cd⁺ ions excited in the five high-lying 9p ²P, 8d ²D, 6g ²G, 6f ²F and 9s ²S states is almost half of the product Cd⁺ ion, the remaining half being distributed between other excited levels of Cd⁺ down to a few eV below the energy of the He⁺ ion. The authors also measured the total Penning ionisation and charge transfer rate constants: k=(8.0+or-1.5)*10^-10 cm³ s^-1 and k=(5.5*1.0)*10^-10 cm³ s^-1 respectively.

By means of photo emission spectroscopy, absolute cross sections for total and state-selective electron capture for impact of 0.7-4.6 keV amu^-1 C³⁺ on atomic and molecular hydrogen have been measured. At three impact energies total cross sections have also been directly measured with a retarding field-charge state analyser. For C³⁺-H(1s), the present data are in agreement with the measurements of McCullough et al. (1983) obtained by means of translational energy spectroscopy, whereas, apart from the total cross sections, no agreement is found with the quantal calculations of Bienstock et al. (1982). The present measurements illustrate the capability of photon emission spectroscopy for unambiguous identification of the final states involved in state-selective electron capture by multi-charged ions from atomic hydrogen.

Total cross sections have been measured for the production of the metastable 1s₃ and 1s₅ states in neon at incident electron energies from threshold to 500 eV. A time of flight technique was used to determine relative cross sections which were made absolute by comparison with the known cross sections for metastable production in helium. Reasonable agreement is found between the present results and those of Phillips et al. (1981) who used different techniques to measure relative cross sections and to normalise their data. This good agreement has implications for the mechanism of the excitation of the metastable states.

The electric polarisabilities alpha , C, B and gamma for the isoelectronic species Be(¹S), B⁺(¹S) and C²⁺(¹S) have been calculated in the LCAO-SCF approximation. Extensive basis sets have been used in order to allow for a reliable estimate of these properties close to the Hartree-Fock limit. The authors' final values for alpha and gamma are, in atomic units, 45.6 and 3.91*10⁴ for Be(¹S), 11.374 and 870 for B⁺(¹S) and 4.5077 and 5 for C²⁺(¹S).

Calculations of bremsstrahlung (BS) spectra for electron (positron) scattering on muonic hydrogen (H_mu ) are presented taking into account the H_mu virtual excitations.