Table of contents

Bremsstrahlung spectra for a 0.2-10 keV electron on Ba are calculated for the photon frequencies above the 4d-threshold: omega =100-200 eV. The polarization bremsstrahlung is taken into account. A simple method is proposed for the approximate calculation of the polarizational bremsstrahlung cross section.

Using monochromatized extreme-UV radiation from the undulator beamline BW3 at HASYLAB we have measured at high resolution (E/ Delta E approximately 7000) the Li⁺ photoion yield spectrum at excitation energies corresponding to transitions to double K-shell vacancy states. A number of such resonances including single-photon-three-electron excitations, e.g. 1s²2s(²S) to 2p³(²p), have been observed for the first time. The spectrum is tentatively classified with the aid of Hartree-Fock calculations.

In the concluding paragraph of a recent paper on two-state atoms driven by transients, Vitanov (1994) raised two questions. First, he queried whether or not, at non-zero detuning, there exist asymmetric pulses of arbitrary smoothness that can restore the system to its initial level. Second, he asked whether one could find symmetric pulses that fail to restore the system to its initial state, regardless of pulse area. In this letter, we respond to these speculations. We show that pulses with shapes that are hyperbolic secants modified by powers of the hyperbolic tangent can arbitrarily approach the restoring condition, while the existence of a symmetric pulse incapable of restoration implies unitarity violation. We also draw a distinction between the infinitely large set of restoring pulse areas that exist for symmetric pulses, as compared to the small number (typically no more than one) that can be found in the asymmetric case.

Accurate calculations are reported for the photoionization of the ground and the excited states of Fe I with an extensive coupled channel expansion using the R-matrix method. A total of 52 LS terms of the core ion Fe II dominated by configurations 3d⁶4s, 3d⁷, 3d⁶4p, 3d⁵4s², 3d⁵4s4p are included; these eigenstates thereby account for the photoionization of the outer 4s subshell as well as the open inner 3d subshell of the ground state 3d⁶4s²(⁵D) of Fe I. The coupling of all relevant photoionization channels give rise to extensive autoionization structures and ensure that no large and sharp discontinuity at the 3d inner-shell ionization edge is present. The new results differ by up to several orders of magnitude from earlier data in central field type approximations that are currently employed in plasma modelling of astrophysical and laboratory sources.

Laser-induced fluorescent lines in the (3) ¹Π to B(1) ¹Π (0-0) and (1-1) bands of scandium monochloride are observed spectroscopically for both isotopomers, ⁴⁵Sc³⁵Cl and ⁴⁵Sc³⁷Cl. Rotational analysis of the spectra allows the characterization of the B(1) ¹Π state. Preliminary values of the principal constants at equilibrium in this state for ⁴⁵Sc³⁵Cl are, in cm^-1: T_e = 6020.3, ΔG_1/2 = 379.1, B_e = 0.1551.

The excitation function of the dissociative photoionization excitation of N₂ has been measured with the detection technique of coincidence between photoion and fluorescence photon using extreme-UV synchrotron radiation as an excitation source. Coincidence spectra obtained at the excitation photon energy in the region of 36.5-44.3 eV show at least two components of the dissociative photoionization excitation, one of which is ascribed to the dissociation of the 2 sigma _g^-12 Sigma _g⁺ state.

Two independent calculations have been carried out to obtain the integral cross sections for the excitation of n=2 and 3 states from the ground state of hydrogen at electron impact energies lying between n=3 and n=4 thresholds. The first is the 15-state R-matrix calculation of Fon et al. (1992, 1994) and the second is the algebraic variation method of Wang and Callaway (1994). It is shown that in this energy range the contribution of the ionization continuum to the excitation cross sections is estimated to range from 12 to 15%, and that the convergent J-matrix results of Konovalov and McCarthy (1994) have not yet completely converged.

We present the first application of the CCC method to electron scattering from the metastable 2^3,1S states of helium at energies of 35.2, 40 and 50 eV relative to the ground state. Due to unitarity of the close-coupling formalism the reliability of the e-He(2^3,1S) results may be tested not only by comparison with available e-He(2^3,1S) experimental data, but also by comparing with the e-He(1¹S) results arising from the same calculation with appropriate experiments. In the case of electron-impact ionization of He(2³S), we find that the CCC results are in quantitative agreement with the ionization spin asymmetries, but are almost a factor of two lower than the total ionization cross section measurements. However, the corresponding total ionization cross sections from the ground state are also in quantitative agreement with experiment. Agreement with available differential cross section measurements is only fair.

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

The use of the quark configuration (s+f)⁴ to represent the states of the atomic f shell calls for a simultaneous transformation of operators. Operator equivalences are developed for z₀, a two-electron operator associated with spin-spin interactions, and t₁₁, a scalar three-electron operator appearing in third order in perturbation theory. Both belong to the irreducible representation (IR) (111) of Racah's group SO(7), and can be represented by combinations of the single-quark operators of the types (f_theta ^Dagger f_theta )⁽²) and (f_theta ^Dagger f_theta )⁽⁰⁾-7¹2/s_theta ^Dagger s_theta , where theta distinguishes the four quarks. The group SO(7)' (which plays the same role for the f quarks as SO(7) does for the f electrons) is used to establish relations between matrix elements of z₀ and those of other electronic operators such as U⁽²⁾ and W⁽¹²⁾, thereby accounting for various unexpected connections and proportionalities that go beyond what would be expected from straightforward applications of the Wigner-Eckart theorem.

Atomic beam laser spectroscopy was used to measure the isotope shifts (IS) and hyperfine structures of all stable titanium isotopes in eight Ti I spectral lines. Six of the lines investigated cover all allowed transitions between the lower 3d²4s² a ³P and the upper 3d³4p y ³D^o terms; two of them are between the 3d²4s² a ³P_1,2 and 3d²4s4p z ³P₂^o levels. J-dependences of the IS in the a ³P and y ³D^o terms were established. The contributions of the field and mass shifts to the J-dependence were separated using model-independent data on muonic atom IS and electronic scattering experiments. Evidence for mass shift domination of the J-dependence was provided.

Recently observed new fluorescence lines in Kr are assigned. The initial states are Kr III states of the configurations 4p³6s and 4p³5d which are populated following selective excitation of the Kr I 3d_5/2⁹np resonances. Probabilities of various decay channels of the Kr I 3d_5/2⁹np resonances leading to the initial states of the fluorescence lines are theoretically estimated. In the case of the Kr I 3d_5/2⁹5p resonance, these estimations are compared with the experimentally determined probabilities.

Spontaneous radiation by three-level atoms in a dissipative medium (such as plasma) has been investigated by Langevin equations and the modified Weisskopf-Wigner approximation. General formulae for calculating spontaneous emission coefficients, i.e. Einstein A coefficients, are presented. Numerical calculation shows that the dissipation of plasmas can modify the spontaneous emission coefficients significantly and causes a pit in the coefficient-density curves for relatively low temperature plasmas, and also a pit in the coefficient-temperature curves.

The multiphoton emission spectrum of a pair of coupling two-level atoms interacting with two modes of the radiation field is studied. A general expression for the two-mode multiphoton emission spectrum is presented. The non-degenerate two-photon emission spectra of two atoms are calculated when the two modes of the field are in various number states and some new results are obtained. It is found that the two-atom emission spectrum in the two-mode vacuum field reflects the atomic dipole-dipole interaction more sensitively than in the case of a single-mode vacuum field.

Cross sections sigma (m₁m₂)^nl for the energy pooling processes in the collision between two excited sodium atoms are calculated for the case when the partners are prepared in the 3p-excited states with the definite orbital momentum projections m₁ and m₂. As a result of the process one of the colliding atoms comes to the ground state and the other partner is excited to nl state with nl=5s, 4d, 4f. The collision velocity is equal to or higher than thermal. The Landau-Zener-locking model (LZ-locking) is developed: the sudden change of the type of the coupling between the electron orbital momentum and the internuclear axis (the locking approximation) is combined with the multicrossing Landau-Zener model. The validity of the LZ-locking model is analysed. The results show strong cross section dependence on the initial quantum numbers m₁ and m₂, especially in the 5s channel.

High resolution double-electron capture translational energy-loss spectra have been obtained for Kr³⁺ in Ar and Xe³⁺ in Ar and Kr at a collision energy of 6 keV. For the Kr³⁺ in Ar spectrum the transitions are assigned to excited states in both the projectile and target ions. The Xe³⁺ in Ar spectrum displays eight well resolved peaks which are attributed to four reaction channels and their corresponding spin-orbit coupling states. The spectrum for Xe³⁺ in Kr is remarkably similar, however the reaction channels are formed from capture into higher excited states of the target ion confined by energetic and adiabatic potential energy curve crossing parameters.

A systematic calculation for the total and differential cross sections for electron impact excitation of helium from its initially excited metastable 2³S state to higher states namely n^1.3S and n^1.3P (with n=2-4) states is reported. Distorted wave approximation theory has been used for the calculation. The effect of different distortion potentials as well as the effect of target polarization by the projectile is explicitly considered. For comparison similar first Born level results are also calculated. Results are compared with the available earlier calculations and experimental data.

A variant of the Born approximation is used to calculate doubly differential and total ionization cross sections for the (e, 2e) interaction between an electron and the alkali atoms, lithium through rubidium. The calculation is done for the ionization of the valence electron. A primary purpose of the work is to establish the basic features of the doubly differential cross sections. The total cross section results compare well with the few experimental data available.

We have carried out calculations for the excitation of copper from the 4s ground state to the 4p J=1/2, 3/2 fine-structure states by electron impact at intermediate energies using the relativistic distorted-wave method. These are the first reported results of generalized spin parameters for this atom. We also report extensive results for the Stokes parameters for this process. Comparisons of the cross sections with experimental measurements and previous theoretical calculations are made.

We consider the rotationally resolved excitation of heteronuclear diatomic molecules. Particular emphasis is on the selective population of the Lambda -doublet states and their coherence properties produced during the collision. The general theory is developed and the relevant selection rules are derived. Using recently obtained T-matrix elements for the a³ Pi excitation of CO we have calculated the relevant parameters of interest for several energies as a function of the scattering angle. The results show that the epsilon =+1 members of the Lambda -doublet states are preferentially excited for small and large scattering angles. It is shown that a considerable amount of coherence between the Lambda -doublet states is produced which is responsible for a nonvanishing electric dipole moment induced by the collision.

We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).

Cross section ratios sigma ²⁺/ sigma ⁺ for double and sigma ³⁺/ sigma ⁺ for triple relative to single ionization of argon, krypton and xenon atoms by positron impact were measured from threshold up to 1000 eV. All sigma ²⁺/ sigma ⁺ ratios exhibit double peak structures of which the high-energy structure is attributed to inner-shell processes, in agreement with theoretical estimates.