In this paper we give a self-contained account of some general spectral and dissipative properties of open spin systems. These properties are discussed in the framework of completely positive quantum dynamical semigroup equations. Special classes of such open systems are studied in detail. The effects of the environment symmetry and the spin-surrounding interaction multipolarities on the shiftings and the splittings of energy levels and relaxation parameters are explained. The physical meaning of relaxation parameters is found. © 1996 The American Physical Society.

The absolute differential cross sections and generalized oscillator strengths of resolved 3${\mathit{p}}^6$${\mathrm{}}^1$${\mathit{S}}_0$→3${\mathit{p}}^5$${(}^2$${\mathit{P}}_{1/2}$)4s, 3${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)4s′ transitions in argon were determined by angle-resolved electron-energy-loss spectrometer at an incident electron energy of 1500 eV and scattering angles from 2° to 11.5°. Corrections of the experimental results for the angular factor and pressure effect have been made. These results are compared with other experimental and theoretical results. The absolute differential cross sections and generalized oscillator strengths of unresolved nondipole excitations 3${\mathit{p}}^6$→3${\mathit{p}}^5$4p+4p′ are reported. The profile of the generalized oscillator strength curve is similar to that of neon atoms. © 1996 The American Physical Society.

We study the analytical properties of the exact ground-state energy E(λ) of a two-electron atomic ion of nuclear charge Z as a function of the parameter λ=1/Z. We find that E(λ) has a second-order pole at the point λ=∞ of a λ complex plane. The principal part of the Laurent expansion of E(λ) about this point can be found analytically: E(λ)=-4-${\lambda }^2$/4+O(1/${\lambda }^2$), λ→∞. We find a new singularity of E(λ) at the point ${\lambda }_1$≊9.41 of a λ complex plane. © 1996 The American Physical Society.

The relative photodetachment cross section of ${\mathrm{Al}}^{\mathrm{-}}$ has been measured in the wavelength range 2420–2820 nm (0.440–0.512 eV), using a coaxial ion-laser beams apparatus, in which a 2.98-keV ${\mathrm{Al}}^{\mathrm{-}}$ beam is merged with a beam from an F-center laser. The cross-section data near the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{0,1,2}$${\to }^2$${\mathit{P}}_{1/2,3/2}$ photodetachment threshold have been fitted to the Wigner threshold law and to the zero-core-contribution theory of photodetachment. The electron affinity of aluminum was determined to be 0.44094(+0.00066/-0.00048) eV, after correcting the experimental threshold for unresolved fine structure in the ground states of ${\mathrm{Al}}^{\mathrm{-}}$ and Al. The new measurement is in agreement with the best previous measurement (0.441±0.010 eV) and is 20 times more precise. The Wigner law agrees with experiment within a few percent for photon energies within 3% of threshold. A proposed leading correction to the Wigner law is discussed. © 1996 The American Physical Society.

Proceeding towards the evaluation of the complete set of radiative corrections of order ${\mathrm{\alpha }}^2$ to the energy levels of hydrogenlike systems, the second-order self-energy–vacuum-polarization contribution to the Lamb shift of bound electrons is derived and calculated. We focus on the Uehling part of the vacuum-polarization insertion in the effective photon interaction. Additional energy corrections of order ${\mathrm{\alpha }}^2$ are also calculated. We present a brief summary of various energy contributions for K- and L-shell electrons in hydrogenlike and lithiumlike high-Z ions. © 1996 The American Physical Society.

Dispersive photoelectron spectra and constant-initial- (ionic-) state spectra are presented for the known band systems of jet-cooled ${\mathrm{Xe}}_2$ that dissociate to Xe${(}^1$${\mathit{S}}_0$)+Xe*(6p,5d) between approximately 74 627 and 80 849 ${\mathrm{cm}}^{\mathrm{-}1}$. In many instances, the dominant molecular ion-core assignment for each gerade Rydberg state deduced from the photoelectron data agrees well with predictions made from angular momentum arguments. Differences are attributed to either local excited-state homogeneous perturbations or strong predissociation. © 1996 The American Physical Society.

The dispersion coefficients ${\mathit{C}}_6$, ${\mathit{C}}_8$, and ${\mathit{C}}_{10}$ for the interactions among H, He, and Li are calculated using variational wave functions in Hylleraas basis sets with multiple exponential scale factors. With these highly correlated wave functions, significant improvements are made upon previous calculations and our results provide definitive values for these coefficients. © 1996 The American Physical Society.

The total ion and/or electron yield spectra of rare gases Xe, Kr, and Ar have been recorded around the 4${\mathit{d}}^{\mathrm{-}1}$, 3${\mathit{d}}^{\mathrm{-}1}$, and 2${\mathit{p}}^{\mathrm{-}1}$ ionization thresholds, respectively, with very high photon energy resolution at the Finnish beam line at the MAX I storage ring. Linewidths of the excited states are determined for higher excitations than earlier. The widths are observed to differ slightly for each core-excited state. © 1996 The American Physical Society.

A simple nonvariational wave function for two electron atoms is described. The radial part is derived from the asymptotic one-electron wave functions that are modified to account for the proper core boundary conditions. The electron-electron correlation is described by an ansatz which has the correct behavior for ${\mathit{r}}_{12}$→0 and ${\mathit{r}}_{12}$→∞. The additional parameter in this ansatz is calculated from a perturbation calculation. This wave function yields ground-state energies for the heliumlike atoms with Z=1 (${\mathrm{H}}^{\mathrm{-}}$) to 10 (${\mathrm{Ne}}^{8+}$) within 0.0021–0.0068${\mathit{E}}_{\mathit{h}}$ of the exact energies, which is significantly better than obtained with previous models. We also calculated 〈${\mathit{r}}^{2\mathit{n}}$〉 and multipolar polarizabilities and performed other tests on this wave function. © 1996 The American Physical Society.

Energies of the fifteen (2l2${\mathit{l}}^{\prime }$2${\mathit{l}}^{\mathrm{″}}$) states for boronlike ions with Z = 5–100 are evaluated to second order in relativistic many-body perturbation theory. Second-order Coulomb and Breit-Coulomb interactions are included. Corrections are made to lowest order for the frequency-dependent Breit interaction and for the Lamb shift. A detailed discussion of the various contributions to the energy levels is given for boronlike iron (Z=26). Comparisons of the calculated energy levels with available experimental data are made for the entire sequence. © 1996 The American Physical Society.

All-electron basis-set Dirac-Fock calculations for the lowest-lying ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state of the AgH molecule are reported. The Dirac-Coulomb Hamiltonian for point nuclei and kinetically balanced four-component spherical spinor basis functions are used. The large-component radial variation is of Gaussian form, ${\mathit{r}}^{\mathit{ne}\mathrm{-}\xi {\mathit{r}}^2}$, with ξ an element of the increasing geometric sequence a,ab,${\mathit{ab}}^2$,... . An (uncorrelated) ground-state energy of -5315.1083 hartrees is obtained; the corresponding bond length and dissociation energy are, respectively, 1.6908 Å and 0.0529 hartrees (1.44 eV). © 1996 The American Physical Society.

The problem of the minimum value of the dipole moment needed to assure the existence of a bound electronic state has been addressed for the electrostatically screened Coulomb interaction of the Yukawa type. Our variational calculation demonstrates that the value of the minimum dipole moment increases as the screening parameter increases. More interestingly, we have found that the dipole’s length has a remarkable effect on the minimum dipole moment; a feature not found for the unscreened case. This effect yields a rapid increase of the value of the minimum dipole moment, particularly for large values of the screening parameter, with the increasing dipole moment’s length. © 1996 The American Physical Society.

The angular distribution of Auger electrons originating from the decay of resonantly excited 4${\mathit{d}}^{\mathrm{-}1}$6p states in Xe has been studied with very high photon and electron energy resolution. The resolution enhancement via the Auger resonant Raman effect has enabled to resolve the β parameters for individual resonant Auger transitions. The results are compared to previous experiments and to theoretical predictions calculated by using the multiconfigurational Dirac-Fock approach. © 1996 The American Physical Society.

The 2${\mathit{p}}_{1/2,3/2}$ electrons of Ar have been excited selectively to the 4s orbital and the deexcitation has been recorded by utilizing the Auger resonant Raman effect. Intensity distribution resulting from the mixing between 3${\mathit{p}}^4$4s and 3${\mathit{p}}^4$3d final ionic states of the resonant Auger transitions has been clearly revealed. Experimental results are compared with the results of multiconfigurational Dirac-Fock calculations. © 1996 The American Physical Society.

A theory of inelastic n,l-changing and quasielastic l-mixing collisions of Rydberg atoms with the heavy rare-gas atoms is developed. It is based on the semiclassical impact-parameter approach combined with the normalized perturbation theory. Semiclassical formulas for scaled transition probabilities and cross sections as functions of the inelasticity parameter and the collision strength are analyzed. This approach gives a general description of collisional nl→${\mathit{n}}^{\prime }$ transitions independently of the specific values of the principal quantum number, the relative velocity, and the type of colliding atoms. The energy and angular dependence of the electron-atom scattering amplitude is included in the theory by incorporating the impulse-approximation results in the weak-coupling limit. The Ramsauer-Townsend effect affects significantly the values of the cross sections, especially for the inelastic transitions with large energy transfer. The results obtained are used in calculations of the quenching cross sections for the Rb(nS,nD,nF) atoms in collisions with Ar, Kr, and Xe. Comparison of theory and experiment is made in a wide range of the principal quantum numbers and transition energy defects. © 1996 The American Physical Society.

The R-matrix theory of vibronic processes in electron-molecule scattering is developed from a generalized variational formalism, valid for any enclosed system of interacting particles with different mass parameters. The question of choice of variational basis functions is discussed, examining practical reasons for using basis functions that do not have specified boundary values or normal gradients on the boundary surface of the enclosed volume. A nonadiabatic theory is derived in which the vibronic R matrix, including channels describing dissociation processes, is deduced from the fixed-nuclei electronic R matrix, given as a function of electronic energy and internuclear distance. © 1996 The American Physical Society.

We develop the distorted-wave and adiabatic theories of the ion-pair formation in collisions of a Rydberg atom ${\mathit{A}}^{\mathrm{*}\mathrm{*}}$ with a ground-state atom B at thermal collision velocities. The e-B interaction is modeled by a zero-range potential. Both theories give results that are in qualitative agreement with the standard Landau-Zener model, although their prediction for m-changing transitions is different, where m is the projection of the angular momentum. To incorporate these transitions into the adiabatic theory, we take into account the rotation of the internuclear axis before and after a nonadiabatic transition. The m-averaged cross sections are substantially higher than those following from the standard Landau-Zener model, and the distorted-wave results are slightly higher than the adiabatic results with the internuclear-axis rotation taken into account. We apply our results to a description of the McLaughlin and Duquette experiment on ${\mathrm{Ca}}^{+}$-${\mathrm{Ca}}^{\mathrm{-}}$ pair formation. The calculated n dependence of the rate for the ion-pair formation is too flat as compared to the experimental observations and does not agree with known values of the electron affinity of Ca. We discuss other possible mechanisms responsible for the ion-pair formation in this process. © 1996 The American Physical Society.

Measurements on dissociative ionization of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ by fast (16–25 MeV) protons are carried out in an effort to understand two-electron processes in collisional ionization. The high velocity limit of the ratio of cross sections, R, of two-electron processes to one-electron processes obtained from these measurements along with earlier data is used to obtain a universal scaling law for R as a function of projectile charge and energy, irrespective of the sign of the projectile charge. © 1996 The American Physical Society.

A versatile electron-impact spectrometer, equipped with three independent electrostatic analyzers, has allowed the simultaneous investigation of the excitation and ionization and Auger decay of the Xe 4d states via electron-electron coincidence experiments. Energy and angular dependence of the final-state continuum interactions, the postcollision interactions, has been studied in both the (e,e′ Auger) and (e,2e) binding energy spectra up to 30 eV above the 4d thresholds. The experimental results have been compared with the theoretical predictions of Kuchiev and Sheinerman (Usp. Fiz. Nauk 158, 353 (1989) [Sov. Phys. Usp. 32, 569 (1989)]). A fairly good agreement between the experiments and theory has been found in all the studied cases. Also the decay of the Xe 4${\mathit{d}}^{\mathrm{-}1}$np inner-shell excited states and the energy dependence of the 4${\mathit{d}}_{5/2}$:4${\mathit{d}}_{3/2}$ branching ratios have been studied and compared with analogous data from photoionization experiments. © 1996 The American Physical Society.

Double, single, and total ionization cross sections have been calculated for the single ionization process of an electron-helium atom system for the incident energy range (50–300 eV) including the electron exchange effect. The final-state wave function involves a correlated three-body Coulomb continuum, and satisfies the three-body asymptotic boundary condition. The electron exchange effect is found to play a dominant role for near-symmetric and symmetric geometries. All results have been compared with available experiments, and reasonably good agreement has been noted except for low incident energies (e.g., 50 eV) and low ejected energies (in the double-differential or single-differential cross-section level). © 1996 The American Physical Society.