Table of contents

The potentials of the ground and first excited states of sodium perturbed by krypton have been obtained between 3.6 and 5.8 AA, thereby extending previous work. Agreement is found with the results of York et al. (1975) at the bottom of the upper state well but the A² Pi potential is consistently lower than that of Pascale and Vandeplanque (1974) at all values of the internuclear separation. The data taken here have also been used to connect the profiles measured at small (<10 cm^-1) and large (>500 cm^-1) detunings in the red wing.

The relation between the impact parameter capture amplitude a( rho ,v) and the quantum-mechanical one f(q,v) in charge transfer is obtained using the Brinkman-Kramers (1930) approximation. Analytical expressions for a( rho ,v) and calculated cross sections are given for some capture reactions using hydrogen-like wavefunctions.

Spectra of the optical and generalised oscillator strength for the K-shell ionisation of all free atoms with Z<or=30 have been calculated within the framework of a Hartree-Slater central potential. One of the authors' findings is that the spectra (at fixed small values of momentum transfer including the optical limit) are rich in structure, i.e. the spectral strength is non-monotonic in the kinetic energy of an ejected electron near the K threshold, for all atoms with Z>or=8. This is in sharp contrast to the hydrogenic result, according to which the spectral strength for every shell monotonically decreases with the kinetic energy. The magnitude of the near-threshold structure sometimes amounts to 30%. A partial interpretation of the structure is given and consequences of these results to applications such as electron energy-loss spectroscopy and stopping-power calculations are pointed out.

A general formula is obtained for the asymptotic expansion in R^-1 of the matrix elements of the electron repulsion operator r₁₂^-1 in diatomic two electron molecules (hydrogen-like molecules) in the adiabatic basis. As an illustration the authors applied this formula for the interaction of two atoms in the ground state of the hydrogen molecule. The authors obtain the asymptotics of the matrix element (1g1g mod r₁₂^-1 mod 1g1g) when R>>1.

Relaxed orbital binding energies have been calculated using the relativistic Hartree-Fock method for Zn, Cd and Hg. The results are in excellent agreement with relaxed orbital relativistic Hartree-Fock-Slater calculations. A residual discrepancy is found between these theoretical results and experimental values of the binding energies which, for the N shell of Hg, can be correlated with the lifetime of the core hole.

The autoionisation of doubly excited atomic states with one electron excited much more than the other is considered in the one-configuration approximation. The theory is asymptotic in the large angular momentum of the highly excited electron. The direct process results in the ejection of the high Rydberg electron whereas in exchange autoionisation the inner excited electron is ejected and the outer electron passes into the low-lying orbital. The multipole expansion of the interaction reduces the calculation of the direct autoionisation to finding the asymptotic behaviour of the one-electron matrix element between highly excited and continuum states for some power of the radial variable. Slater-type or Coulomb wavefunctions are used for the Rydberg electron and plane waves or Coulomb waves are employed for the ejected electron. The asymptotic behaviour of the two-electron matrix element is found for the exchange autoionisation and also for the exchange-type correction to direct autoionisation.

X-ray spectra of the elements iron to bromine have been observed from plasmas produced by the laser irradiation of plane solid targets. Observed wavelengths are classified as belonging to the Na, Ne, F and O I isoelectronic sequences. A comprehensive survey of the Ne-like transitions is given for these elements. Classifications are also given for the transitions 2s²2p⁵-2s²2p⁴3s, 3d, 4s, 4d and 2s²2p⁵-2s2p⁵3p in the F I isoelectronic sequence and 2s²2p⁴-2s²2p³3s, 3d, 4d in the O I isoelectronic sequence. Long-wavelength Na-like satellites to the Ne-like resonance lines are also classified for the elements gallium to bromine.

Considers a system of N coherently driven two-level atoms contained in a volume with dimensions much smaller than a wavelength. The atoms may decay by a completely cooperative mechanism which conserves (J²), alternatively they may decay by a mechanism which fails to conserve (J²). The steady-state behaviour in each case is determined using the single-atom solutions together with two limiting approximations: (a) complete atomic correlation, (b) no atomic correlation. In case (a) a behaviour characteristic of a second-order phase transition is obtained while in case (b) a first-order phase transition results.

Lineshapes of three-level resonance Raman scattering and two-level resonance fluorescence, homogeneously broadened by collisions, in the presence of arbitrarily strong coherent radiation, are studied by means of a tetradic (density-matrix) scattering theory. In the case of medium coupling strength V_L to the radiation field (where T₂^-1<or=V_L<< tau _c^-1, with T₂ and tau _c being the coherence damping and collision times, respectively) the Mollow lineshape (1977) for resonance fluorescence is rederived, with proper T₂ and population cross-relaxation effects included. In particular it is shown that relaxation through intermediate non-resonant levels can considerably alter the lineshape in resonance fluorescence. In the case of strong coupling (V_L>or approximately= tau _c^-1) the collision rates are modified by the applied field (optical collisions). It is shown that, whereas in self attenuation spectra optical collision rates can be obtained only from line intensity or line wing measurements, in resonance scattering these rates are directly obtained from linewidth measurements.

The authors report on the two-photon excitation of ¹S₀ and ¹D₂ states in mercury by pulsed ultraviolet dye laser radiation. A time-resolved registration of two-photon-excited fluorescence provided an accurate determination of the radiative lifetimes of the levels 7¹S₀ to 9¹S₀ and 6¹D₂ to 8¹D₂. In an external magnetic field Zeeman quantum beats and the zero-field level crossing signal have been observed. Furthermore the authors describe conditions which cause strong emission of amplified spontaneous radiation in transitions between highly excited mercury levels.

The polarisation and angular distribution pattern of radiation emitted from excited ions are investigated. Anisotropic electron-ion interactions are shown to play an important role for the polarisation of emitted light. Some examples are discussed.

It is demonstrated that a large set of accurate oscillator strengths of Ti II can be determined from a combination of hook and emission measurements without any assumption concerning the plasma state. Modified cascaded arcs and hollow cathode discharges have been used as plasma light sources for both hook and emission measurements. The relative f values have been converted to an absolute scale by means of literature data. The overall uncertainties of the f values are about 13-25%. Comparisons with other experimental and theoretical data are made which indicate excellent to fair agreement. Only for one of the published data sets has a wavelength-dependent discrepancy of up to a factor of two been found.

Atomic vapours of alkali metal atoms are studied with the technique of photoelectron spectroscopy, using He I and He II resonance lines. Multiplet structure due to np⁵(n+1)s³P_2,1,0 and ¹P₁ final states are well resolved for Na(n=2), K(n=3), Rb(n=4) and Cs(n=5). Additional strong satellites as a result of the interaction of np⁵nd configuration are observed for K, Rb and Cs. The intensity and the position of these satellites agree quite well with the existing empirically adjusted HF calculations for Rb and Cs. The observed ³P/¹P intensity ratio for Na deviates strongly from the theoretical HF value for 40.8 eV radiation which indicates configuration interaction between the final continuum states.

The crossed beam method was used to measure the excitation of lithium atoms to 1s2s²²S and 1s2s2p ²P terms with 22.5-500 keV proton impact. Electrons ejected at 150 degrees , 90 degrees and 30 degrees with respect to the incident beam were detected simultaneously by three electrostatic parallel-plate spectrometers. Using the plane-wave Born approximation (PWBA), excitation cross sections were calculated with a multiconfiguration approach for the target wavefunctions. Fairly good agreement between theory and experiment was obtained except for low impact energies. From an analysis of the calculation it was found that energy levels and population probabilities of the excited terms differ in their dependence on reorganisation and correlation effects.

Integral cross sections are reported for ion pair formation in collisions between hydrogen (H,D) and noble-gas (Ar,Kr,Xe) atoms from threshold to 100 eV. The energy dependence of the cross sections is strongly oscillatory. A curve crossing model provides a good semi-quantitative explanation of the results.

The energy and angular distribution of projectile electrons emitted in asymmetric collisions is calculated within the quantum mechanical electron impact approximation. The rise of the differential cross section in the backward direction is shown to follow closely the behaviour of electrons elastically scattered from the target. As an example, (H,Ne) and (H,Ar) collisions are studied and the results are compared with experiment.

The intensity distribution over the ion-excited K alpha X-ray satellite lines is strongly affected by the chemical surroundings, as the latter have a considerable influence on the fast electron rearrangement occurring prior to K alpha X-ray emission. The authors intended to provide an illustrative picture of the electron reorganisation after the creation of silicon core vacancies. The X alpha -SW method has proved to be an efficient tool for explicitly considering the chemical surroundings, and thus one can avoid the use of arbitrary effective charges. The large amount of d-type valence charge stabilised by the anisotropic environment is more likely to account for the discrepancies between energy sites and intensity distribution in the K alpha satellite spectra than interatomic electron transitions.

The L₁-shell ionisation cross sections for elements from Z=56 to 83 are calculated in the relativistic plane-wave Born approximation for incident electron energies of 1.04, 1.39 and 1.76 MeV. In this approximation initial and final states of the incident electron and the atomic electron are, respectively, described by the appropriate plane-wave solutions of the Dirac equation and the appropriate bound- and continuum-state solutions of the Dirac equation with a Coulomb potential having effective charge. The indistinguishability of two L₁ electrons in the initial state and one L₁ and one continuum electron in the final state are taken into account, and the interaction between the incident and the atomic electron is taken to be of the Moller (1931) type. Calculated results are compared with observed data, the calculation using Gryzinski's (1965) classical binary encounter approximation and the calculation of Moiseiwitsch and Norrington (1979) using a semirelativistic plane-wave Born approximation.

A model is presented for steady-state three-level resonance Raman amplification, based on an algebraic expression for single-molecule amplification (including independently variable relaxation rates) with arbitrarily strong radiative couplings, combined with a numerical solution of non-linear equations governing macroscopic collinear propagation.

Evolution under two-photon near-resonance conditions of an adiabatic Gaussian pulse is considered in Rb vapour. Theoretical results show strong self-phase modulation and pulse deformation with increasing distance of propagation and intensity at a fixed detuning. At sufficiently large distances of propagation and/or input intensity pulse deformation is seen to develop into (two-photon) optical shocks. At a higher intensity double shock fronts are predicted to occur. The previously anticipated existence of an upper bound for the output maximum and of the vanishing of the self-phase modulation at the 'interaction time' between the output and the input profiles are shown explicitly to hold for the cases considered. A quasi free-wave like evolution occurs at asymptotically higher intensities.