As part of a systematic, quantitative study of the angle dependence of core-level-electron excitation by electron impact, we have determined the generalized oscillator strength (GOS) for the carbon 1s(2${\mathrm{\sigma }}_{\mathit{g}}$→2${\mathrm{\pi }}_{\mathit{u}}$) transition in ${\mathrm{CO}}_2$. The experimental results were obtained at an impact energy of 1290 eV, in the angular range of 2°–14°, with an energy resolution of 0.9 eV. Theoretical values for the GOS were also obtained, using ab initio Hartree-Fock molecular wave functions and allowing for the relaxation of all the molecular orbitals in the determination of the excited-state wave function.

Accurate measurements, within 1.6%, of the cross section of K radiation emission of Mo, Cd, and Sn at 59.54-keV photon energy were carried out. The results are compared with those of other experiments, and they confirm the great reliability of the combination of up-to-date tabulations of K fluorescence yields and photoeffect cross-section values calculated by relativistic quantum-mechanics models.

A finite-volume variational principle based on the Dirac Hamiltonian is formulated in a way similar to the Kohn variational principle for the logarithmic derivative used in nonrelativistic electron-atom scattering. As in R-matrix methods, the essential idea is to solve the Dirac equation variationally within a finite reaction volume for a given total energy of the system. In contrast to previous methods, we do not need to impose any unphysical boundary conditions on the trial function. The continuation of the variational solution into the outer region is represented by a standard close-coupling expansion. The K matrix is obtained by matching the variational solutions with the solutions in the outer region on the surface enclosing the reaction volume.

In ion-atom collisions where the projectile is ionized, target electrons act not only coherently by screening the target nucleus but they may also act incoherently by directly ejecting a projectile electron. This electron-electron interaction should be relatively most important for targets that have a low nuclear charge, since the cross section for a neutral target is roughly proportional to ${\mathit{Z}}_{\mathit{t}}^2$+${\mathit{Z}}_{\mathit{t}}$, where ${\mathit{Z}}_{\mathit{t}}^2$ is the contribution due to the target nucleus and ${\mathit{Z}}_{\mathit{t}}$ comes from the target electrons. In order to investigate the electron-electron interaction, we have measured and calculated cross sections for ${\mathrm{Li}}^{2+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{O}}^{7+}$ on ${\mathrm{H}}_2$ and He, ${\mathrm{Au}}^{52+}$ on ${\mathrm{H}}_2$, He, C, and ${\mathrm{N}}_2$, ${\mathrm{Au}}^{75+}$ on ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$, ${\mathrm{U}}^{86+}$ on ${\mathrm{H}}_2$ and He, and ${\mathrm{U}}^{90+}$ on ${\mathrm{H}}_2$. The collision energies range from 0.75 to 405 MeV/nucleon. The calculations have been performed in the plane-wave Born approximation. We demonstrate that for energies where the target electrons have sufficient kinetic energy in the projectile frame to ionize the projectile, the electron-electron interaction can lead to a significant increase in the total electron-loss cross section.

The probability of pion capture by hydrogen was measured for ${\mathrm{CH}}_3$OH, ${\mathrm{CD}}_3$OH, and ${\mathrm{CH}}_3$OD as a function of temperature from -150 to +250 °C. The nuclear capture of a pion by a proton was identified using the pion charge-exchange reaction. The coincident photons from the ${\mathrm{\pi }}^0$ decay were detected by TRIUMF’s large NaI spectrometers. We have found a relative difference of 1.79±0.11 in the pion capture probability in solid and supercritical ${\mathrm{CD}}_3$OH that is interpreted in terms of the hydrogen-bond-breaking model.

Using synchrotron radiation and photoelectron spectrometry, we have examined the 4s4${\mathit{p}}^6$5p resonance region in krypton and the 5s5${\mathit{p}}^6$6p resonance region in xenon. We have obtained partial and total cross sections, intensity ratios, and photoelectron angular distribution parameters for the energy regions 20.6–21.5 eV in xenon and 24.6–25.3 eV in krypton. We also report Shore parameters for all cross-section data taken. In addition to the anticipated single-electron transition, we clearly resolve features attributable to two-electron transitions in both species. Characteristics of these autoionizing states may differ considerably in the two available exit channels.

A systematic investigation of ionization enhancement due to collisional excitation followed by autoionization (EA) is presented for Ga i–like rare-earth elements. Both nuclear charge and temperature dependence are analyzed and compared with direct-impact ionization (DI) rates. Collisional excitation and autoionization rates were calculated in the distorted-wave factorization-interpolation method. DI rates were calculated by a modified plane-wave Born approximation method. The rates for the combined EA process for selected Ga i–like ions from Mo xii to Dy xxxvi are presented for the relevant temperature ranges. It is shown that indirect ionization is dominant for Mo through Pr but is reduced gradually with Z and approaches zero at Dy. A density diagnosis is provided by the opening of EA channels that are not active at low densities.

The factorization-interpolation model, developed to compute collisional excitation rate coefficients efficiently in dense plasma, is applied here to autoionization and radiationless electron capture. Results using a parametric atomic central potential and factorization interpolation for autoionization rates and dielectronic recombination rate coefficients in Ne-like Fe are compared to multiconfigurational Dirac-Fock calculations. Agreement is very good. As a further application, we treat the problem of indirect ionization via collisional excitation followed by autoionization in Zn-like Mo.

The electronic-energy-straggling values for 50–250-keV ${\mathrm{H}}^{+}$ beams penetrating Cu, Ag, and Pt have been measured using backscattering. The Cu, Ag, and Pt layers with a thickness of about 30 nm were deposited onto Si, ${\mathrm{SiO}}_2$, ${\mathrm{Al}}_2$${\mathrm{O}}_3$, and ${\mathrm{ZrO}}_2$ substrates with electron beams or by sputtering. The uniformity and grain growth of the thin films were observed by cross-section transmission electron microscopy (TEM) and diffraction. The thickness fluctuation determined by TEM is less than 2 nm. The backscattering spectra were obtained with an electrostatic toroidal analyzer with an energy resolution (standard deviation) of 150–200 eV, and the computer-simulated spectrum analysis has given the best-fit values of the energy straggling. The derived straggling values are compared with the theoretical values calculated from the extended Lindhard-Winther theory using the local electron density of the Hartree-Fock-Slater atomic model. Discussion is also made in terms of the short-range correlation effect proposed by Sigmund [Phys. Rev. A 26, 2497 (1982)].

Using the linear-response dielectric theory, we have investigated the electronic stopping power for slow atoms in solids. The effect of the electrons bound to the projectile is considered according to the Brandt-Kitagawa (BK) model [Phys. Rev. B 25, 5631 (1982)]. Using a local-field-correction dielectric function, the correlation and exchange interaction of the electron gas in solids is also taken into account. An analytical formula for the stopping power is obtained, and various theoretical results are compared with each other and with available experimental data.

Complex bound and autoionizing spectra of strontium, barium, and radium are treated at two different levels of approximation using multichannel-quantum-defect techniques (MQDT) and eigenchannel R-matrix calculations. In the first level, the R-matrix calculations are conducted entirely in LS coupling without explicitly including any spin-orbit terms in the Hamiltonian. A subsequent frame transformation to jj coupling is then carried out prior to the MQDT treatment. In the second level, the entire R-matrix calculation is conducted in jj coupling, including spin-orbit terms in the Hamiltonian explicitly within the R-matrix box. The two methods give nearly identical results even for atoms as heavy as barium, each showing good agreement with experiment, but significant differences begin to appear between the two levels of approximation for atomic radium.

The threshold photoelectron spectrum of HCl has been investigated in the 980–760-Å region. The various autoionization processes leading to the observed resonances are discussed. From the resonance positions located at the successive vibrational thresholds, spectroscopic constants of the ${\mathit{X}}^2$Π state of ${\mathrm{HCl}}^{+}$ have been determined.

Satellite spectra of the Kα line of heliumlike nickel, Ni xxvii, recorded from tokamak-fusion-test reactor (TFTR) Ohmic plasmas with well-defined experimental conditions and central electron temperatures in the range 2–5 keV, have been compared with theoretical predictions. The relevant plasma parameters that determine the spectral features, i.e., the electron and ion temperatures, and the relative abundances of Ni xxv, Ni xxvi, and Ni xxvii have been obtained from least-squares fits of synthetic spectra to the experimental data. Both the dielectronic satellites and the satellites which are produced by collisional inner-shell excitation are well described by the theory given by Bombarda et al. [Phys. Rev. A 37, 504 (1988)] and Vainshtein and Safronova (unpublished). The electron-temperature results derived from the fits are in good agreement with electron-temperature measurements from independent diagnostics. However, the values obtained for the relative abundances of Ni xxv, Ni xxvi, and Ni xxvii are larger by factors of 1.3–2 compared to recent coronal-equilibrium predictions of Zastrow, Källne, and Summers [Phys. Rev. A 41, 1427 (1990)]. Plasma-modeling calculations performed with the multi-ion-species-transport [R. A. Hulse, Nucl. Technol. Fusion 3, 259 (1983)] code show that these deviations can be explained by radial ion transport assuming for the ion-transport diffusion coefficient D values in the range 1–2.5 ${\mathrm{m}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$.

However, the observed deviations may be partially ascribed to theoretical uncertainties of the ionization and recombination rate coefficients used for the coronal-equilibrium calculations. Systematic discrepancies are found to exist between the predicted and observed intensity ratios, x/w, y/w, and z/w, of the heliumlike lines w, 1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$; x, 1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$; y, 1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$; and z, 1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2s ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$.

The occurrence of superfluorescence in the near-infrared regime has been investigated between high-lying levels of Sr, Ba, and Na under the influence of a static homogeneous external electric field. The time inequalities for single-pulse superfluorescence were fulfilled. The measurements show for all the investigated superfluorescent transitions that the square root of the peak intensity of the superfluorescent pulse decreases with the square of the electric-field strength. If one takes into account the dependence of the number of excited atoms, the wavelengths, and the line strengths of the superfluorescent pulses on the electric-field strengths, the experimental findings cannot be explained by well-known theories of superfluorescence.

The lowest np Rydberg series of triatomic hydrogen has been studied using optical-optical double-resonance excitation from the metastable 2p2 A2″ level. Bound states, detected by field ionization as well as autoionizing states of the p series are characterized using rotational multichannel quantum-defect theory. Deviations from the calculations appear when vibrational interactions give rise to predissociation below threshold or autoionizing interlopers above threshold. The np Rydberg manifold of H3 provides an almost complete panorama of channel interactions in a polyatomic molecule: rotational and vibrational interactions and autoionization with rich Fano profile structure as well as predissociation.

The optical pumping parameters that modify the pure-state pseudo-Stokes parameters in an electron-superelastic-scattering experiment are calculated for the 3 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$(F=2)–$3^2$${\mathit{P}}_{3/2}$(F) transitions of Na. A QED model is used to describe the laser excitation. It is found, that while the magnitude of the pumping parameter K’ associated with circularly polarized excitation is close to unity for other than weak laser intensities, the value of the parameter K, associated with linearly polarized light, differs significantly from unity and is dependent on such experimental conditions as laser intensity, laser detuning, laser beam profile, and the Doppler width of the atomic transition. Methods for measuring K are suggested.

An electron-beam ion trap has been used to study the spectra of highly ionized barium. Spectra of N-, O-, and F-like barium were produced by 10-keV electron-impact excitation and ionization and were observed using a von Hámos crystal spectrometer. Calibration lines were obtained by injection of Ti, V, Cr, and Zn into the trap and observation of H- and He-like transitions. A number of lines were identified for each of the ionization stages, and their energies were measured with a typical accuracy of about 150 ppm. These measurements have been compared to calculations using a multiconfigurational Dirac-Fock code. Agreement with the data is best for the F-like transitions, and diminishes for transitions in the more highly charged ions.

Optical isotope shifts in the 4${\mathit{f}}^4$5d ${}_{}{}^6{}_{}{}^{}$${\mathit{K}}_{9/2}$–(23 537${)}_{9/2}^0$ transition (5702 Å) of Nd ii have been measured using collinear fast-beam-laser spectroscopy. [The level notation (23 537${)}_{9/2}^0$ includes the term value of the level, its parity, and total angular momentum.] From the measured optical isotope shifts, changes in mean-square nuclear charge radii δ〈${\mathit{r}}^2$〉 (${\mathrm{fm}}^2$) among the five even-even neodymium isotopes with mass numbers 142–150 have been deduced: (142,144), 0.283(9); (144,146), 0.257(8); (146,148), 0.284(9); and (148,150), 0.385(12). And configuration admixtures of the previously unclassified (23 537${)}_{9/2}^0$ level were quantitatively analyzed to be 4${\mathit{f}}^4$6p, 4${\mathit{f}}^3$5${\mathit{d}}^2$, and 4${\mathit{f}}^3$5d6s with mixing probabilities 15%, 84%, and 1%, respectively.

K-shell photoabsorption spectra of CO, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}_{}^{18}{}_{}{}^{}$O, NO, ${\mathrm{O}}_2$, ${\mathrm{CO}}_2$, ${\mathrm{N}}_2$O, ${\mathrm{C}}_2$${\mathrm{H}}_2$, ${\mathrm{C}}_2$${\mathrm{D}}_2$, ${\mathrm{C}}_2$${\mathrm{H}}_4$, ${\mathrm{C}}_2$${\mathrm{D}}_4$, ${\mathrm{C}}_2$${\mathrm{H}}_6$, ${\mathrm{C}}_2$${\mathrm{D}}_6$ measured with unprecedented energy resolution and signal-to-noise ratio are presented. These spectra reveal many new features in core-excited valence and Rydberg states. Detailed vibrational structures are observed in these spectra, providing valuable information on the geometrical and vibrational properties of the core-excited molecules. In addition, C 1s and N 1s core-hole lifetimes are found to be ∼120 and ∼135 meV in these molecules with little dependence on their bonding environments. These results and the tentative peak assignments are discussed briefly in terms of the equivalent core model, multielectron excitations, exchange interactions, and the geometry of the excited molecules.

We have studied the ionization of Na ns and nd Rydberg atoms by ∼1-μs pulses of 10-MHz, 670-MHz, 2-GHz, and 4-GHz fields, bridging the gap between the previous measurements of ionization by ∼1-μs rise-time pulses and 8- and 15-GHz microwave fields. We have also studied the ionization of Li ns and nd Rydberg atoms by 4-, 8-, and 15-GHz fields, which, due to the smaller Li quantum defects, represents the high-frequency limit of nonhydrogenic microwave ionization. The Na measurements show that ionization at 10 MHz occurs without change in n at fields of 1/16${\mathit{n}}^4$ and 1/9${\mathit{n}}^4$. At 670 MHz ‖m‖=0, 1, and 2 states are all ionized at fields between 1/3${\mathit{n}}^5$ and 1/9${\mathit{n}}^4$, indicating that transitions to higher-n states are occurring. As the frequency is raised further, to 2 and 4 GHz, the ‖m‖=0 and 1 states exhibit an increasingly sharp threshold for ionization at E=1/3${\mathit{n}}^5$, but the ‖m‖=2 states are unlikely to ionize at this field, with ionization occurring mostly at higher fields, 1/3${\mathit{n}}^5$<E<1/9${\mathit{n}}^4$, approaching the 15-GHz behavior of ionization at E=1/9${\mathit{n}}^4$. At 4 GHz, Li n=20 ‖m‖=0 and 1 states ionize at E=1/3${\mathit{n}}^5$, but at n≃45, ‖m‖=1 states ionize at E=1/9${\mathit{n}}^4$, and the ‖m‖=0 states ionize at E>1/3${\mathit{n}}^5$. The ‖m‖=2 states appear to ionize at E=1/9${\mathit{n}}^4$. As the frequency is raised to 8 and then to 15 GHz, the transition of ‖m‖=0 and 1 states to ionization at E=1/9${\mathit{n}}^4$ occurs at lower n. When a small static field is applied, the Li ionization threshold fields drop from 1/9${\mathit{n}}^4$ to fields somewhat above 1/3${\mathit{n}}^5$. Together the Na and Li measurements show that microwave ionization depends on a rate-limiting step, which is a resonant multiphoton transition.