A 17-state basis containing seven exact atomic states and ten pseudostates is employed in a calculation of cross sections for the electron-impact excitation of a hydrogen atom from the 1s to the n=2 and n=3 states. The incident energies range from 14 to 100 eV. The results are compared with some other recent calculations and with experiment. Improved effective (thermally averaged) collision strengths are presented for the excitation of the n=3 states.

The process of charge exchange with ion excitation is investigated theoretically. The multielectron and multichannel model for the process under consideration is proposed. The formulas for two-electron exchange coupling are obtained and discussed. The model is applied to the investigation of charge exchange with ion excitation in collisions of helium ions with mercury atoms at thermal energies. The matrix elements of exchange coupling, the adiabatic potential energies, the partial cross section, and the partial rate constant for the process under consideration are calculated. The matrix elements of exchange coupling in this case are mainly determined by electron-core interactions. The effective potentials are used for these interactions. The calculated partial rate constant for excitation of ${\mathrm{Hg}}^{+}$(7p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$) agrees with experimental data.

The properties of the laser-induced first resonance state in the photodissociation of ${\mathrm{H}}_2^{+}$ are obtained from calculations in the radiation-field (RF) gauge, also called the velocity gauge, as a function of intensity and wavelength of the field within an adiabatic electronic-field representation, and the results are compared to the corresponding ones obtained using a diabatic RF gauge representation and an adiabatic electric-field gauge representation. Gauge transformation within the coupled equations used for these calculations is also discussed.

We report calculations of differential and integral cross sections for positron (${\mathit{e}}^{+}$) scattering from He, Ne, Ar, Kr, and Xe rare gases. An optical-potential approach is employed in which the repulsive Coulombic interaction is calculated exactly at the Hartree-Fock level and the attractive polarization and correlation effects are included via a model potential determined from the use of a local density-functional theory (DFT). These model calculations are further compared with the results from two other local potentials, one based on determining the short-range correlation energy, ${\mathit{E}}_{\mathrm{C}}$, for a positron in an homogeneous electron gas and the other from the correlation-polarization potential of an electron interacting with a free-electron gas. We found that the present DFT-based, correlation-polarization treatment is fairly simple to implement computationally and appears to be the most accurate of all the models examined here. Our results are in fact compared with recent measurements of differential and integral cross sections for positron scattering with rare gases and are found to be remarkably close to both sets of experiments.

Electron-scattering cross sections for the 3s→3p dipole transition in ${\mathrm{Ar}}^{7+}$ are calculated in both the quantal distorted-wave approximation and the classical-trajectory Monte Carlo method. The two approaches are in substantial agreement for the angular differential cross section at 20-, 50-, and 100-eV incident electron energy. This result indicates the degree to which classical scattering influences the dipole excitation. Further distorted-wave calculations are made for the 3s→3d and 3s→4s transitions in ${\mathrm{Ar}}^{7+}$ and the 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$S→3s3p ${}_{}{}^{3,1}{}_{}{}^{}$P transitions in ${\mathrm{Ar}}^{6+}$. The forward-backward asymmetry in the angular distribution of scattered electrons is examined for each transition. All transitions exhibit strong backward scattering at energies near the excitation threshold.

We present an experimental study of the electron emission induced by the collisional ionization of Ne gas by 106-keV/u ${\mathrm{H}}^{+}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ projectiles. Cross sections, doubly differential in electron energy ${\mathit{E}}_{\mathit{e}}$ and angle θ, were measured throughout the ranges 0.5≤${\mathit{E}}_{\mathit{e}}$≤440 eV and 0°≤θ≤180°. We show that measurements at electron energies down to 0.5 eV were possible after a reduction of the physical extension of the gas target to a minimum. We find a remarkable dependence of the measured distribution of soft electrons with the extension of the target. We try a model according to which this effect is attributed to the scattering of electrons into the acceptance cone defined by the detector. Our measurements enable us to perform a quantitative analysis of the shape of the soft-electron peak. Contrary to current belief, this peak is found to be strongly asymmetric. The magnitude of this asymmetry is comparable to that of the cusp resulting from electron capture into the continuum. This puts into evidence that, also for these low-energy electrons, the interaction with the two Coulomb centers, projectile and residual target ion, is essential. By analyzing the ratio of the emission obtained with ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ and ${\mathrm{H}}^{+}$, we also study the dependence of the doubly differential cross section on the projectile charge.

The rate coefficients for mixing between He(2 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{\mathit{J},}$${\mathit{M}}_{\mathit{J}}$) levels during collisions with ground-state helium atoms and for conversion of He(2 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{\mathit{J}}$) atoms to ${\mathrm{He}}_2$(b ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$) molecules via three-body reactions in helium gas have been investigated over the temperature range 1.6–300 K. The measured rate coefficients for collisionally induced P-state mixing decrease slowly with decreasing temperature, from (1.8±0.5)×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$ at 300 K to (4.5±0.5)×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$ at 4.2 K. The rate coefficients for the production of ${\mathrm{He}}_2$(b ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$) molecules via three-body reactions are observed to increase with decreasing temperature and are described by the relation ${\mathit{k}}_{\mathit{P}}$≃(2.5+267${\mathit{T}}^{\mathrm{-}1}$)×${10}^{\mathrm{-}32}$ ${\mathrm{cm}}^6$ ${\mathrm{s}}^{\mathrm{-}1}$. This behavior, which is very different from that noted in earlier studies of the conversion of He(2 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$) atoms to ${\mathrm{He}}_2$(a ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$) molecules through three-body reactions, suggests that the reaction is not thermally activated.

Time-resolved photoabsorption spectra of gallium plasmas have been photographed in the 50–450-Å region using the dual-laser-produced-plasma technique. The absorbing plasmas were produced by focusing the output of a Q-switched ruby laser onto slab targets while the background continuum was produced by focusing the output of a Nd:YAG oscillator-amplifier system (where Nd:YAG denotes neodymium-doped yttrium-aluminum-garnet) onto a samarium or hafnium target. At 130-ns delay between the two pulses the spectrum recorded was due almost exclusively to ${\mathrm{Ga}}^{+}$. We have observed transitions due to both 3d and 3p excitation. In the former case we identified 3${\mathit{d}}^{10}$4${\mathit{s}}^{2\mathrm{-}}$3${\mathit{d}}^9$4${\mathit{s}}^2$nf and 3${\mathit{d}}^9$4${\mathit{s}}^2$np series converging on the ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{5/2}$ limits of ${\mathrm{Ga}}^{2+}$. In the 3p case no strong transitions were observed because of line broadening by super-Coster-Kronig decay of the 3p hole. The identifications were made by comparison with the predictions of Hartree-Fock and Dirac-Fock atomic-structure codes. Many-body calculations were also performed which proved invaluable in estimating the effects of different decay processes. The theoretical predictions are compared with the experimental data.

An energy approach based on consistent QED theory is outlined which presents the energy of any decaying atomic state in complex form with the imaginary part responsible for different types of decays. The aim is to study, in a uniform manner, elementary processes responsible for emission-line formation in plasmas. The energy approach is generalized here for the collisional problem. Numerical results on electron collisional strengths and rate coefficients for Ne-like ions are presented. It is shown that collisional rate coefficients are close to their maxima at an electron temperature ∼0.30${\mathit{E}}_{\mathrm{threshold}}$ for iron and ∼0.15${\mathit{E}}_{\mathrm{threshold}}$ for argon.

Single-, double-, and triple-photoionization processes of carbon monoxide (CO) have been studied in the photon-energy region of 37–100 eV by use of time-of-flight mass spectrometry and a photoion-photoion-coincidence method together with synchrotron radiation. The single-, double-, and triple-photoionization cross sections of CO are determined. Ion branching ratios and the partial cross sections for the individual ions respectively produced from the precursors ${\mathrm{CO}}^{+}$ and ${\mathrm{CO}}^{2+}$ are determined separately at excitation energies where the molecular and dissociative single- and double-photoionization processes compete. The threshold for the molecular double photoionization was found to be 41.3±0.2 eV. Furthermore, in single photoionization, the production of ${\mathrm{CO}}^{+}$ is dominant whereas with double photoionization dissociation becomes dominant.

Momentum distributions and spectroscopic factors are obtained in a high-resolution electron-momentum spectroscopy study of neon at 1500 eV. The shapes and relative magnitudes of the momentum profiles are in good agreement with the results of calculations made within the distorted-wave Born approximation (DWBA) and target Hartree-Fock approximation frameworks. The DWBA accurately describes the relative magnitudes of the 2p and 2s manifold cross sections as well as the shape of the 2s cross section. Results for the momentum profiles belonging to excited ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ manifolds are also presented. Spectroscopic factors for transitions belonging to the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ manifolds are assigned up to a binding energy of 95 eV. The spectroscopic factor for the lowest 2s transition is 0.85±0.02, whereas that for the ground-state 2p transition is 0.92±0.02. Comparison of the present binding energies and spectroscopic factors are made against the results of several many-body calculations and photoelectron-spectroscopy (PES) results. The ramifications of the present work to earlier PES classifications of the correlation satellites are also considered.

A fast beam of sulfur Rydberg atoms is formed by charge exchange from an 11-keV beam of ${\mathrm{S}}^{+}$ ions in a crossed thermal rubidium beam, laser-excited to the 10F or 8F levels. Population of specific n=9,10 sulfur Rydberg levels is studied by selective excitation with a Doppler-tuned ${\mathrm{CO}}_2$ laser followed by field ionization. Complete spectral resolution of levels with L≥4 allows a direct study of the populations in specific nL levels and reveals distinctly nonstatistical distributions of population in L. The results are compared with predictions of a classical Monte Carlo three-body collision theory, revealing some similarities, but also clear differences.

Elastic scattering of photons from solid samples of Pb, Pt, and W was measured in order to investigate the limits within which scattering experiments can be described as being due to free atoms. The experiments were performed with photons of 22.1 keV from an x-ray tube and from an ${}_{}{}^{241}{}_{}{}^{}\mathrm{Am}$ (59.54 keV) γ source, providing a momentum transfer ranging from x=0.1 to 2.0 A${\mathrm{̊}}^{\mathrm{-}1}$. It is shown that the low-momentum-transfer limit for the free-atom approximation will depend on the scatterer, temperature, geometrical resolution, and on the accuracy of the experiment.

We derive the nuclear-spin relaxation rate of gaseous ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ due to the magnetic-dipole interaction between the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclear spins. This dipolar relaxation rate is numerically evaluated for temperatures from 0.1 K to 550 K. At room temperature, the relaxation time for a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ density of 10 amagats is 74.4 h. We have made a series of high-density (4–12 amagat) ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ samples for which nulcear relaxation is limited by the magnetic-dipole interaction. Both our theoretical and experimental results are particularly important for the growing use of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, polarized through spin exchange with optically pumped Rb vapor.

The charge-state dependence of the binary-encounter-electron (BEE) double-differential cross section (DDCS) at 0° with respect to the beam direction resulting from collisions of 1 MeV/amu ${\mathrm{H}}^{+}$, ${\mathrm{C}}^{\mathit{q}+}$, ${\mathrm{N}}^{\mathit{q}+}$, ${\mathrm{O}}^{\mathit{q}+}$, ${\mathrm{F}}^{\mathit{q}+}$, ${\mathrm{Si}}^{\mathit{q}+}$, and ${\mathrm{Cl}}^{\mathit{q}+}$, and 0.5 MeV/amu ${\mathrm{Cu}}^{\mathit{q}+}$ with ${\mathrm{H}}_2$ is reported. The data show an enhancement in the BEE DDCS as the charge state of the projectile is decreased, in agreement with the data reported by Richard et al. [J. Phys. B 23, L213 (1990)]. The DDCS enhancement ratios observed for the three-electron isoelectronic sequence ${\mathrm{C}}^{3+}$:${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{4+}$:${\mathrm{N}}^{7+}$, ${\mathrm{O}}^{5+}$:${\mathrm{O}}^{8+}$, and ${\mathrm{F}}^{6+}$:${\mathrm{F}}^{9+}$ are about 1.35, whereas a DDCS enhancement of 3.5 was observed for ${\mathrm{Cu}}^{4+}$. The BEE enhancement with increasing electrons on the projectile has been shown by several authors to be due to the non-Coulomb static potential of the projectile and additionally to the e-e exchange interaction. An impulse-approximation (IA) model fits the shape of the BEE DDCS and predicts a ${\mathit{Z}}_{\mathit{p}}^2$ dependence for the bare-ion cross sections. The IA also predicts a binary peak energy that is independent of q and ${\mathit{Z}}_{\mathit{p}}$ and below the classical value of 4t, where t is the energy of electrons traveling with the projectile velocity. We observed a BEE energy shift ΔE (ΔE=4t-${\mathit{E}}_{\mathrm{peak}}$, where ${\mathit{E}}_{\mathrm{peak}}$ is the measured energy at the peak of the binary encounter electrons) that is approximately independent of q for the low-${\mathit{Z}}_{\mathit{p}}$ ions, whereas the measured ΔE values for Si, Cl, and Cu were found to be q dependent.

In this paper we present the results of eigenchannel R-matrix calculations of the photoionization cross section of Sc from the 3d4${\mathit{s}}^2$ ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2,5/2}$ ground electronic state. The calculations were performed in LS coupling using configuration interaction for the target states as well as for the full atomic wave function. The spin-orbit interaction was incorporated through a recoupling frame transformation. We focus on the gross dynamics of the atom that need to be described accurately to obtain good agreement with experiment.

In this paper we use the calculated photoabsorption cross section and scattering parameters to classify most of the autoionizing lines reported by Garton et al. [Proc. R. Soc. London Ser. A 333, 1 (1973)]. This study proves the capabililty of eigenchannel R-matrix calculations to provide spectroscopically useful information on the complicated spectra of transition-metal atoms. We present cross sections for the six optically allowed combinations 4${\mathit{s}}^2$3d ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2,5/2}$→$1^{\mathit{o}}$/2,$3^{\mathit{o}}$/2,$5^{\mathit{o}}$/2,$7^{\mathit{o}}$/2 from near the (4s3d${)}^3$D thresholds to the (3${\mathit{d}}^2$${)}^3$F thresholds.

We have carried out a relative measurement of the photoionization cross section of the excited Sc (3${\mathit{d}}^3$${)}_3^2$${\mathit{D}}_{3/2}$ state which lies 36 276.63 ${\mathrm{cm}}^{\mathrm{-}1}$ above the ground state. These experimental results are compared with a calculated photoionization cross section. The theoretical cross section was obtained from scattering parameters and dipole matrix elements that were calculated prior to completion of the experiment and were not adjusted after we obtained the experimental data. The calculated cross section thus demonstrates the predictive power of this theoretical method. The observed features are accurately reproduced in a complicated region of the Sc autoionizing spectrum where numerous interacting channels converge to their respective ionization thresholds.

The large DD-fusion-yield enhancement reported earlier by our group [Phys. Rev. Lett. 63, 1292 (1989); J. Phys. Chem. 94, 7665 (1990)] in the bombardment of deuterated targets by large heavy-water clusters was shown by postacceleration magnetic and electrostatic filtering to be due to small-ion impurities produced in the acceleration column. With the filtering arrangement in place we have carefully studied DD-fusion rates with small (${\mathrm{D}}_2$O${)}_{\mathit{n}}$${\mathrm{D}}^{+}$ and (${\mathrm{H}}_2$O${)}_{\mathit{n}}$${\mathrm{H}}^{+}$ ions (n≤10) which produced detectable fusion rates. Formation of carbon films on the polydeuteroethylene surfaces caused the observed fusion rates to decrease rapidly with time. This problem was solved by using ${\mathrm{D}}_2$O ice targets which are ‘‘self-cleaning.’’ No enhancement was observed for the fusion rates of the small ${\mathrm{D}}_2$O clusters after the oxygen knock-on corrections were made. However, the fusion yields for knock-on processes produced by (${\mathrm{H}}_2$O${)}_{\mathit{n}}$${\mathrm{H}}^{+}$ clusters (4≤n≤10) showed approximately a twofold enhancement over the yields for ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ ions at the same velocity. The stopping power of clusters was measured and shown not to be responsible for this enhancement.

The stopping cross sections of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, He, and Ne for hydrogen projectiles in the energy range 3–20 keV per nucleon have been measured by time of flight. We compare our experimental result to the sum of the individual contributions due to excitation and ionization of the target and of the projectile, respectively, and due to charge exchange, using published cross-section data. Satisfactory agreement is found only for the He target and only at moderate projectile velocities, whereas for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ the calculated values are about 30% too low. A Monte Carlo program allows us to simulate the measured time-of-flight spectra and to explain minor trends in the experimental data: for increased Ne gas pressure, an increased specific energy loss has been found that can be traced to different regions of impact parameters selected in our transmission geometry. This also explains, in part, the increased specific energy loss for deuterons compared to protons of equal velocity that is most evident for Ne. In contrast, a decrease of the specific energy loss with increasing pressure for He may be explained by impurities in the target gas. If we correct for the effect of impurities, the stopping cross section of He at 4 keV per nucleon is slightly smaller (0.60×${10}^{\mathrm{-}15}$ eV ${\mathrm{cm}}^2$) than published earlier (0.72×${10}^{\mathrm{-}15}$ eV ${\mathrm{cm}}^2$) and depends on the 3.8th power of projectile velocity.