A static-exchange formalism is proposed based on the assumption that the scattering electron undergoes the same interaction as the extra electron in the negative ion of the target atom. In this method the wave function of the negative ion of the target atom is calculated, and the wave function of the frozen neutral core of the negative ion is used in the calculation of the interaction of scattering electron with the target atom. The screening effect of the incident electron on the target atom is approximated by that of the absorbed extra electron on the neutral core of the corresponding negative ion. The spin-polarization effect of the incident electron on the closed-shell target atoms is also considered by spin-polarized self-consistent field calculations for the negative-ion structures. A parameter-free spin-polarized correlation-polarization potential is included to represent the correlation and polarization effects in both the structure and scattering calculations. In order to account for the relativistic effects for heavy targets, a quasirelativistic approach is applied to the calculations of the negative-ion structures and the scattering processes. The results of low-energy electron scattering with Ca, Sr, Ba, and Ra atoms are given for energies between 0.01 and 5 eV.

Results are presented of a Monte Carlo simulation of the passage of fast ${\mathrm{H}}_2^{+}$ ions through thin carbon foils, with emphasis on the transmitted fraction. An essential feature of the calculation is that the Coulomb interaction between the constituent protons and the multiple scattering of each in the foil is treated simultaneously. The agreement between previous experimental data and the simulation is excellent.

The rapid relativistic distorted-wave method of Zhang, Sampson, and Mohanty [Phys. Rev. A 40, 616 (1989)] for excitation, which uses the atomic-structure data of Sampson et al. [Phys. Rev. A 40, 604 (1989)], has been extended to ionization. In this approach the same Dirac-Fock-Slater potential evaluated using a single mean configuration is used in calculating the orbitals of all electrons bound and free. Values for the cross sections Q for ionization of various ions have been calculated, and generally good agreement is obtained with other recent relativistic calculations. When results are expressed in terms of the reduced ionization cross section ${\mathit{Q}}_{\mathit{R}}$, which is proportional to ${\mathit{I}}^2$Q, they are close to the nonrelativistic Coulomb-Born-exchange values of Moores, Golden, and Sampson [J. Phys. B 13, 385 (1980)] for hydrogenic ions except for high Z and/or high energies. This suggests that fits of the ${\mathit{Q}}_{\mathit{R}}$ to simple functions of the impact electron energy in threshold units with coefficients that are quite slowly varying functions of an effective Z can probably be made. This would be convenient for plasma-modeling applications.

The electron-impact ionization cross section for highly charged ions is obtained in a level-to-level multiconfiguration approximation. The target atomic ion is described by a Dirac-Fock approximation while the interaction between electrons includes static, magnetic, and retardation effects. Level-to-level cross-section results for several highly charged ions are presented to illustrate the computational method.

The radiative deexcitation cross section for ${\mathrm{He}}^{\mathrm{*}}$(2 ${}_{}{}^1{}_{}{}^{}$S) in collisions with He has been measured in the collision energy range from 40 to 110 meV. The experimental results agree well with quantal distorted-wave calculations of Zygelman and Dalgarno [Phys. Rev. A 38, 1877 (1988)]. The measured radiative quenching cross sections for the systems ${\mathrm{He}}^{\mathrm{*}}$(2 ${}_{}{}^1{}_{}{}^{}$S)+Ar,Kr,Xe manifest a scaling law for fixed collision energy with ${\mathrm{\alpha }}^2$ as the scaling parameter, where α is the static dipole polarizability of the heavier rare gas.

The method of zero-degree Auger spectroscopy was used to measure cross sections for the production of K and L Auger electrons by double electron capture in 60-keV ${\mathrm{C}}^{6+}$+He collisions. Corresponding atomic-structure calculations were performed to determine the related Auger and x-ray transition rates. The resulting Auger yields are found to deviate significantly from unity. The Auger yields were applied to evaluate cross sections for the production of the configuration 3lnl’ (n=3 to 5) and 2lnl’ (n=3 to 8). These data are found to compare well with experimental results obtained previously for the isocharged system ${\mathrm{O}}^{6+}$+He. Comparison is also made with recent calculations including the electron-electron interaction. It is shown that electron-correlation effects play a significant role for double electron capture in the studied collision system.

An ab initio method for the treatment of metastable negative ions is presented, which is applicable to larger and especially polyatomic molecules. This method is based on Feshbach’s projection-operator formalism. The ‘‘discrete component’’ Φ of the resonance is thereby represented by a large-scale configuration-interaction expansion, calculated as the ground-state eigenfunction of a perturbed Hamiltonian related to the negative system. The real part of the complex potential is given by the energy expectation value of Φ and the level shift Δ, and the imaginary part is the negative half width Γ/2 of the resonance. Level shift and resonance width are calculated energy-dependently by using a Green’s-operator approach. The Lippmann-Schwinger equation for this Green’s operator is defined by the free Green’s operator and the background potential. The latter includes correlation effects of the target system as well as the polarization of energetic lower orbitals according to the additional captured electron. The complex potential contains all information for calculation of vibrational excitation in resonant electron-molecule scattering processes.

A possibility of describing distribution reactions in quantum-mechanical systems of three charged particles within a semiclassical Faddeev-Hahn equation formalism is investigated. To solve these equations a modified strong-channel-coupling method is applied with consideration of a linear Stark effect.

From spectra of a very weak barium hollow-cathode discharge obtained with a Fourier-transform spectrometer, the branching ratio between the visible resonance line at 553.5 nm and the infrared line at 1500 nm of the barium atom was determined to be 485±40. Given the known lifetime and branching fractions between the infrared lines originating in the resonance level, it is now possible to present definitive branching fractions and transition probabilities for all transitions out of the 6s6p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1^{\mathrm{°}}$ barium resonance level.

The natural radiative lifetimes of the 5${\mathit{d}}^9$6${\mathit{s}}^2$ ${(}^2$${\mathit{D}}_{5/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$) metastable states of ${\mathrm{Hg}}^{+}$ as well as of the 5${\mathit{d}}^9$6s${(}^3$${\mathit{D}}_2$) metastable state of ${\mathrm{Hg}}^{2+}$ have been measured by counting for equal time intervals either the 282-, 198-, or 217-nm photons emitted when the respective metastable ions decay to the ground state. The metastable Hg ions are produced inside a cylindrical electrostatic ion trap by electron bombardment of background Hg vapor maintained at pressures ranging from 4 to 50×${10}^{\mathrm{-}8}$ Torr. The trap consists of a 5.0-cm-diam, 7.5-cm-long cylinder with end caps plus a concentric 0.003-cm-diam central cylinder maintained at a negative potential of 140 V. Following electron impact of the Hg vapor, some of the photons emitted by the decaying metastable ion population are focused onto a narrow-bandwidth interference filter. The photons transmitted by the filter are detected by a photomultiplier tube as a function of time and generate a decay curve. However, the signal detected with the interference filter used to monitor the ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ metastable-state population consists of two exponentially decaying components, indicating the presence of another metastable state decaying through the emission of a photon within the bandpass of the filter. The second component is assumed to arise from a cascade originating with the 5${\mathit{d}}^9$6s6p (J=9/2) metastable state of ${\mathrm{Hg}}^{+}$. Therefore, as dictated by the composition of the photon decay curve, the pressure-dependent decay rates of the metastable states are obtained from a least-squares fit of either a single- or double-component exponential decay to the natural logarithm of the decay curve.

Mean decay rates extrapolated to zero pressure of the background Hg vapor using a straight-line least-squares fit, and the inverse of the zero-pressure intercept yields the natural radiative lifetime of the relevant metastable state. Our results for the lifetimes of the ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{5/2}$, ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$, and J=9/2 metastable states of ${\mathrm{Hg}}^{+}$ are 87±4, 8.8±0.4, and 60±6 msec, respectively, while the lifetime of the ${\mathrm{Hg}}^{2+}$ ${(}^3$${\mathit{D}}_2$) metastable state is 34±3 msec.

A model for calculating the profile of spectral lines emitted by multielectron emitters in a hot plasma is described. The Stark broadening is included in the model by using the static ion approximation and an impact approximation for the electrons. The atomic data required for the line-shape calculation are extracted from an atomic structure code and prepared as data necessary for the excited and ground levels of the radiative transition. For the cases where electron broadening is much smaller than the average ionic Stark shift, an approximation is proposed to obtain rapidly a diagonal form of the evolution operator for the emitter. Line shapes of lithiumlike and berylliumlike ions have been calculated under the conditions of recent experiments performed in laser-produced plasmas.

By x-ray bombardment of metal single crystals, x rays of KLL radiative Auger electrons are observed on the low-energy side of the Kα lines. Unlike the usual radiative Auger process where x-ray continua result, we find x rays in the form of sharp lines that originate from a hitherto unobserved virtual Raman process. To our surprise this is the dominant radiative Auger process in the investigated single crystals.

Double-resonance spectroscopy of ${\mathrm{N}}_2$ via the ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathrm{g}}^{}{}_{}{}^{}$, v’=5 level has enabled the definitive assignment of a number of intense and heretofore unidentified bands observed in the single-photon spectrum in the region between the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ ${\mathit{v}}^{+}$=0 and 1 ionization thresholds. The results resolve several long-standing controversies concerning the assignment of Rydberg states of low principal quantum number that converge to the ${\mathit{A}}^2$${\mathrm{\Pi }}_{\mathit{u}}$ and ${\mathit{B}}^2$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ states of the ion and indicate that significant revisions of previous assignments are necessary. The spectra are discussed in the context of the entire manifold of states in this region, which includes high vibrational levels of the b’ ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ valence state as well as Rydberg states that converge to the ${\mathit{X}}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$, ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$, and ${\mathit{B}}^2$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ states of the ion.

The angular distribution of ${\mathit{L}}_3$ x rays of Th and U ionized by 1.5-MeV protons has been measured and the anisotropy parameters of these lines have been deduced. The ratios of the anisotropy parameters of the different ${\mathit{L}}_3$ lines were found to disagree with the theoretical predictions. For U, the alignment of the ${\mathit{L}}_3$ subshell has been determined from the angular distribution of the L${\mathrm{\alpha }}_2$ line, and from the measured anisotropy of the other ${\mathit{L}}_3$ x-ray lines the magnetic-quadrupole–electric-dipole mixing ratios were deduced. The experimental mixing ratios for the ${\mathit{ns}}_{1/2}$-2${\mathit{p}}_{3/2}$ transitions are significantly larger than the theoretical values.

The hyperfine structure and radiative lifetime of the 4${\mathit{d}}^{10}$6p ${}_{}{}^2{}_{}{}^{}$P states of neutral silver were determined in pulsed level-crossing, optical-double-resonance, and time-resolved experiments using high-power frequency-tripled radiation from a dye laser. For the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ state, we obtained τ=78(5) ns for the lifetime and ‖a‖=9.05(25) MHz for the magnetic-dipole interaction constant of ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}$. The lifetime and the a factor for the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ state were determined to be τ=95(6) ns and ‖a‖=38.7(10) MHz, respectively, using time-resolved laser spectroscopy. The experimental results are compared with theoretical calculations using the multiconfiguration Hartree-Fock method.

We have studied the laser intensity dependence of the multielectron ionization of ${\mathrm{N}}_2$ in the range of ${10}^{13}$–5×${10}^{15}$ W/${\mathrm{cm}}^2$ (laser wavelength 305 and 610 nm; pulse duration 2 ps). At 610 nm, we detect four charge states of atomic fragments (${\mathrm{N}}^{+}$, ${\mathrm{N}}^{2+}$, ${\mathrm{N}}^{3+}$, and ${\mathrm{N}}^{4+}$). The fragment kinetic energies and population intensity dependences indicate that they are produced by a sequential ionization occurring during the dissociation of the molecular ions (from ${\mathrm{N}}_2^{2+}$ to ${\mathrm{N}}_2^{8+}$) and/or formed by subsequent atomic ionization. In the first case our findings imply that the ionization occurs at well-defined internuclear distances. Moreover, the initial kinetic energies of the fragments are independent of the laser intensity. The results are interpreted in terms of laser-pulse development and volume effects. At 305 nm the dissociation processes show a different tendency. We detect two charge states of nitrogen fragments (${\mathrm{N}}^{+}$ and ${\mathrm{N}}^{2+}$) arising from a vertical excitation of the ${\mathrm{N}}_2$ molecules.

The 6${\mathit{p}}_{1/2,3/2}$np and 6${\mathit{p}}_{3/2}$nf (J=1) series in barium were excited in a two-step pulsed laser experiment starting on the 6s5d ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_1$ metastable state via the 5d6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_0$ as intermediate level. Use was made of the 6${\mathit{s}}_{1/2}$6${\mathit{p}}_{1/2}$ character mixed into the 5d6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_0$ level to excite the 6pnp series, which appear to have extremely low autoionizing rates compared to other 6pnl series. The observed spectra were analyzed using a seven-channel multichannel quantum-defect theory analysis. Evidence of configuration interaction between the 6${\mathit{p}}_{1/2}$np and 6${\mathit{p}}_{3/2}$nf series was found.

A theoretical study of laser-induced autoionization of an atomic system in interaction with a time-dependent excitation is presented. Interference effects between the two ionization channels of the atom are found to be responsible for coherent phenomena, such as the enhancement of the photon yield, for a pulsed excitation. An analysis of the pulse-shape effects shows that pumping mechanisms of the proper dressed state, occurring during the rise time of flat-top pulses, can result in an enhancement of the photon yield. An adiabatic approximation based on the use of time-dependent complex dressed states is shown to be adequate to describe the interaction with smooth pulses.

We have made high-resolution measurements of the photoionization and photodissociation spectra of molecular hydrogen in the energy region near the first ionization limit in a strong, static, external electric field. We find that below the classical field-ionization energy ${\mathit{E}}_{\mathit{c}}$=-2 √F electric-field-induced predissociation is a dominant decay mechanism for many of the highly excited Stark components, while above the classical field-ionization energy there is a competition between field-induced ionization and dissociation. We have also observed forced autoionization of the (1sσ7pσ) ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$(ν=1, J=1 and 2) levels through their coupling with the structured continuum of highly excited Stark states, leading to enhanced ionization and dissociation through those Stark states that are nearly degenerate with them.

We present a systematic experimental study of the dissociation and ionization of molecular hydrogen in intense laser fields at three different wavelengths: 1.064 μm, 532 nm, and 355 nm. The light intensity ranges from ${10}^{13}$ W/${\mathrm{cm}}^2$ to ${10}^{15}$ W/${\mathrm{cm}}^2$. Measurements include electron spectra and angular distributions, kinetic-energy spectra and angular distributions of ion fragments, and ion dissociation fractions. We report a number of interesting and novel phenomena, including the following: above-threshold ionization to various vibrational states in the molecular ion, production of very-high-energy electrons from ionization at 1.064 μm, enhanced dissociation of the ${\mathrm{H}}_2^{+}$ molecular ion via the mechanism of bond softening, above-threshold dissociation, and evidence for the alignment of the molecule in a high-intensity laser field.