We have measured and calculated doubly differential single-ionization cross sections for 50–150-keV p+He collisions as a function of the projectile energy loss and scattering angle. The measured cross sections were put on an absolute scale by normalizing the integrated cross sections to known total cross sections. Clear effects of the postcollision interaction (PCI) were observed in the scattered projectile spectra. Our singly differential cross sections as a function of projectile energy loss do not agree with corresponding cross sections obtained from electron spectroscopy. These discrepancies are attributed to the PCI. Our calculations including the PCI reproduce the shape of our data quite well; however, there are discrepancies in the absolute magnitude. © 1996 The American Physical Society.

We examine the structure of the hidden crossings of the adiabatic potential curves E(R) in the problem of two Coulomb centers with charges ${\mathit{Z}}_2$ and ${\mathit{Z}}_1$, when ${\mathit{Z}}_2$≫${\mathit{Z}}_1$. In this case we observe a new type of hidden crossing between the states (n,l,m)-(n,l+1,m), where n, l, and m are the spherical quantum numbers of the united atom. This leads to the appearance of a mechanism of nonadiabatic transitions in hydrogen—multicharged ion collisions. As an example of this mechanism we present results of a calculation of inelastic transitions in ${\mathrm{O}}^{7+}$(1s)+H collisions. © 1996 The American Physical Society.

Distorted-wave methods have been used to calculate cross sections for electron-impact ionization of sodiumlike ${\mathrm{Ar}}^{7+}$ for collision energies ranging from near threshold to 1 keV. We included contributions due to excitation-autoionization and resonant excitation–double autoionization (REDA), as well as the contributions from direct ionization of the 3s electron and the n=2 inner-shell electrons. We find that the effects of radiative damping and loss channels reduce the REDA contributions by a factor of 4. We compare our calculated results to experimental results and to theoretical cross sections obtained in a close-coupling approximation. © 1996 The American Physical Society.

We apply a single-channel distorted-wave approximation in conjunction with the independent-event model to calculate the double-capture cross sections by ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, and ${\mathrm{B}}^{5+}$ from neutral He. The capture probabilities of the two electrons in two successive independent events are calculated by a single-channel distorted-wave approximation with realistic screening charge parameters. Our T matrix has been evaluated analytically and therefore the present method is much simpler than other available rigorous calculations. Our results are compared with the measurements and other calculations. It is found that the present method gives fairly good agreement with the measurements. © 1996 The American Physical Society.

Low-energy (${\mathit{E}}_0$=15–40 eV), high-energy-resolution, electron-energy-loss spectra of nitric oxide (NO) have been measured and spectrally deconvolved. This process yielded direct absolute differential cross sections (DCS) for electron-impact excitation of the electronic states of the NO molecule. Integration of these DCS gives integral cross sections (ICS), which confirm earlier estimates that the magnitudes of the NO ICS are one to two orders smaller than the analogous transitions in ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$. In this paper we present clear evidence for the observation of the L′${\mathrm{}}^2$Φ electronic state, showing a ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{r}}^{}{}_{}{}^{}$${\to }^2$Φ excitation transition in an electron-scattering experiment. © 1996 The American Physical Society.

The electron-loss cross section of ${\mathrm{He}}^{+}$ ions impinging upon noble-gas targets (2⩽${\mathit{Z}}_2$⩽36) is calculated by using the coupled-channel method for the active projectile electron in the static screened field of the target atom. The calculations show a saturation of the projectile-electron-loss cross section with increasing target atomic number. This saturation effect due to neutral target systems is much more pronounced than for ionization or excitation by charged particles. Comparison with experimental data indicates a small electron-loss contribution from electron-electron interaction processes for heavy targets at intermediate velocities. Remaining discrepancies in the data are discussed in the light of the approximations involved in our theoretical treatment. © 1996 The American Physical Society.

We present a method for calculation of differential ionization cross sections from theories that use the close-coupling expansion for the total wave function. It is shown how, from a single such calculation, elastic, excitation, and ionization cross sections may be extracted using solely the T-matrix elements arising from solution of the coupled equations. To demonstrate the applicability of this formalism, the convergent close-coupling theory is systematically applied at incident energies of 150–600 eV to the calculation of e-He ionization. Comparison with available measurements is generally very good. © 1996 The American Physical Society.

We have studied spontaneous electron-ion recombination processes taking place in the electron cooler of a storage ring by a systematic investigation of the lifetimes of a stored beam for several choices of the cooler electron beam density. The measurements were performed with 8- and 21-MeV/amu ${\mathrm{D}}^{+}$ and, in this case, it is found that the main contributor to the beam lifetime is radiative recombination. Contributions to the lifetime from other processes such as three-body recombination and single elastic scattering are examined. © 1996 The American Physical Society.

L x-ray production and subshell ionization cross sections of Au and Bi have been measured for the bombardment of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ ions in the energy range 30–57 MeV and 36–84 MeV, respectively. Comparison of the Lα x-ray production cross sections of Au with the earlier data by Malhi and Gray [Phys. Rev. A 44, 7199 (1991)] shows reasonable agreement for ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ impact and their data are consistently higher for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at all energies. The measured Lα line energy shows a shift towards higher energy, which appears to be proportional to the square of the projectile atomic number at all impact energies. This shift suggests the presence of multiple ionization in the L and M shells with a simultaneous production of four M holes in Au at the impact energy of 3 MeV ${\mathrm{u}}^{\mathrm{-}1}$ of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. The L-subshell ionization cross sections obtained from the measured x-ray production cross sections have been compared with the semiclassical approximation and perturbed stationary state theory with energy loss, Coulomb deflection, and relativistic correction for the electron motion calculations, which show large deviations for the ${\mathit{L}}_1$ subshell. The ${\mathit{L}}_2$- and ${\mathit{L}}_3$-subshell ionization cross sections are underestimated by both the theories by a factor of 1.2–4.0 for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ impact whereas for the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ case there is reasonable agreement for the ${\mathit{L}}_3$ cross section but the ${\mathit{L}}_2$ cross section is underestimated by 20–30 %. © 1996 The American Physical Society.

We present a systematic experimental and theoretical study of Na(3p←3s) excitation by slow (v<1 a.u.) singly and multiply charged ions. In particular, the scaling behavior of the respective excitation cross section ${\mathrm{\sigma }}_{\mathrm{NaI}}$ with projectile ion charge state q is investigated. Due to the dominance of the competing electron capture channels at low collision energies E, the excitation cross sections deviate significantly from a commonly applied σ/q=f(E/q) cross-section scaling relation. © 1996 The American Physical Society.

Charge transfer in ${\mathrm{B}}^{2+}$${(}^2$S${,}^2$P)+He and in ${\mathrm{B}}^{3+}$${(}^1$S)+He collisions is studied theoretically by using a semiclassical molecular representation with 8 and 12 molecular channels for ${\mathrm{B}}^{2+}$ and ${\mathrm{B}}^{3+}$ on He systems, respectively, at collision energies between 200 eV and 200 keV for the former and between 600 eV and 50 keV for the latter. The ab initio potential curves and nonadiabatic coupling matrix elements are obtained from the multireference single- and double-excitation configuration-interaction (MRD-CI) calculations for the ${\mathrm{B}}^{2+}$-He system and a pseudopotential-modified configuration-interaction method for the ${\mathrm{B}}^{3+}$-He system. The present cross sections for charge transfer by the ground state ${\mathrm{B}}^{2+}$ ions are found to have a broad maximum with a magnitude as large as 2×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ at 100 keV and those by an excited ${\mathrm{B}}^{2+}$${(}^2$P) state are found to be larger by a factor of 6 than those by the ground state in the same energy regime. ${\mathrm{B}}^{2+}$-excitation cross sections are smaller than those for charge transfer below 1 keV, while they increasingly dominate above this energy. The present total charge-transfer cross section for ${\mathrm{B}}^{3+}$ in collisions with He is similar to that obtained in earlier work by Gargaud et al. [J. Phys. B 27, 3985 (1994)] both in magnitude and energy dependence, but is found to show slightly different ${\mathrm{B}}^{2+}$(2s) and ${\mathrm{B}}^{2+}$(2p) production ratio. © 1996 The American Physical Society.

An approximate solution of the Dirac equation is obtained for particles scattering in arbitrary static potentials. Both short-range and long-range potentials are considered. © 1996 The American Physical Society.

We present numerical ab initio calculations for a quantum system that is a one-dimensional analog of electron-hydrogen scattering at low energies. This approach allows for an exact treatment of electron correlation. The exchange probability can be computed directly from the full unsymmetrized two-electron wave function. One can thus determine without approximation the relative importance of the direct versus the exchange process. We find that, in this low-energy regime, the exchange probability can be quite high and can exceed the probability of the direct process. We then take into account the exchange probability in a calculation of the single-electron effective potential for the free electron. © 1996 The American Physical Society.

We measured the probability ${\mathit{R}}_{\mathit{s}}^2$ for two electrons to be captured and to remain on the projectile ion during collisions between slow ${\mathrm{C}}^{6+}$ ions and rare-gas targets. The dependence of ${\mathit{R}}_{\mathit{s}}^2$ values of these systems on the incident ion velocities has been studied. For He, Ne, and Xe targets, we have obtained high ${\mathit{R}}_{\mathit{s}}^2$ values at a projectile energy of 12 keV and a decrease of ${\mathit{R}}_{\mathit{s}}^2$ value for increasing projectile energy. The measured ${\mathit{R}}_{\mathit{s}}^2$ values in collisions on He are compared with the data evaluated from Auger electron spectra. © 1996 The American Physical Society.

The time dependence, from the transient to the stationary regime, of a quantum state initially prepared as a cutoff plane wave impinging on a one-dimensional square barrier is analyzed in detail using a decomposition based on a discrete set of critical points in the complex momentum plane. The motion and shape of the main wave front, the forerunners, and asymptotic (large time) features of the wave are discussed by means of the exact solution and in terms of velocities and characteristic times associated with the partial contributions of the critical points. © 1996 The American Physical Society.

The experimental measurement of electron-impact K-shell ionization cross sections of Cu and Co elements within the energy region 8–25 keV was reported. The influence of a substrate of thin targets on ionization cross sections has been corrected by a method based upon a bipartition model of electron transport [Z. M. Luo, Phys. Rev. B 32, 812 (1985); 32, 824 (1985)]. The measured cross sections are in good agreement with existing data and are well reproduced by the empirical formula of Green and Cosslett [Proc. Phys. Soc. London 78, 1206 (1961)]. © 1996 The American Physical Society.

Ab initio calculations of the total dielectronic recombination (DR) rate coefficients for ten ions along the Ni I isoelectronic sequence in the ground state (${\mathrm{Mo}}^{14+}$, ${\mathrm{Ag}}^{19+}$, ${\mathrm{Xe}}^{26+}$, ${\mathrm{Pr}}^{31+}$, ${\mathrm{Gd}}^{36+}$, ${\mathrm{Dy}}^{38+}$, ${\mathrm{Ta}}^{45+}$, ${\mathrm{Au}}^{51+}$, ${\mathrm{At}}^{57+}$, and ${\mathrm{U}}^{64+}$) have been performed using the HULLAC computer code package. Resonant and nonresonant stabilizing radiative transitions were included. Collisional transitions following electron capture were neglected. The present level-by-level calculations include the contributions of all the levels (over 17 000) belonging to the following Cu-like inner-shell excited configurations: 3${\mathit{d}}^9$4ln′l′ (n′≤9), 3${\mathit{p}}^5$3${\mathit{d}}^{10}$4ln′l′ (n′≤5), and 3s3${\mathit{p}}^6$3${\mathit{d}}^{10}$4ln′l′ (n′≤5). The configuration complexes with a hole in the 3p inner shell contribute about 10% to the total DR rate coefficients and the complexes with the hole in the 3s inner shell about 1%. The contributions of 3${\mathit{d}}^9$4ln′l′ for n′≳9 were evaluated by extrapolation, applying an ${\mathit{n}}^{\mathrm{-}3}$ scaling, which was checked for the specific ${\mathrm{Ta}}^{45+}$ case. It is shown that at electron temperatures higher than half the ionization energy ${\mathit{E}}_{\mathit{I}}$(Cu) of the Cu-like ion, the Burgess-Merts (BM) semiempirical formula can provide DR results with an accuracy better than ±20% for the relatively heavy ions (Z≳54), whereas for the lighter ions it leads to an underestimation of up to a factor 2 (for Mo). On the other hand, at low electron temperature [${\mathit{kT}}_{\mathit{e}}$<0.3${\mathit{E}}_{\mathit{I}}$(Cu)] the BM approximation underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate. © 1996 The American Physical Society.

We present an improved version of the wave function derived by Alt and Mukhamedzhanov [Phys. Rev. A 47, 2004 (1993)] that satisfies the Schrödinger equation up to terms of order O(1/${\mathrm{\rho }}_{\mathrm{\alpha }}^2$) in the region where the pair α=(β,γ) remains close, while the third particle α moves to infinity (${\mathrm{\rho }}_{\mathrm{\alpha }}$→∞). The new wave function contains the zeroth- and all the first-order O(1/${\mathrm{\rho }}_{\mathrm{\alpha }}$) terms, and matches smoothly Redmond’s asymptotics and the Redmond-Merkuriev wave function when all three particles are well separated. © 1996 The American Physical Society.

Absolute cross sections have been determined for the dissociative recombination and dissociative excitation of ${}_{}{}^4{}_{}{}^{}\mathrm{HeH}_{}^{+}{}_{}{}^{}$ for electron energies below 40 eV. The dissociative recombination cross section is in semiquantitative agreement with recent theoretical results by Sarpal, Tennyson, and Morgan [J. Phys. B 27, 5943 (1994)] and Guberman [Phys. Rev. A 49, R4277 (1994); in XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, AIP Conf. Proc. No. 360, edited by L. J. Dube, J. B. A. Mitchell, J. W. McConkey, and C. E. Brion (AIP, New York, 1995), p. 307]. The calculated resonant structure below a collision energy of 1 eV was not fully reproduced by the experiment. The quantum states of the dissociative recombination products at 0 eV collision energy have been determined; ground-state helium and excited hydrogen atoms (n=2) are dominantly formed, in agreement with recent predictions by Guberman. The dissociative excitation has an onset around 10 eV and follows the shape of the dissociative recombination cross section, illustrating that both processes start with the formation of doubly excited neutral states that lie in the ionization continuum as well as in the dissociation continuum. The dissociative excitation cross section is in quite good agreement with recent calculations by Orel and Kulander. © 1996 The American Physical Society.

An experimental and theoretical study of ground-state photoionization of neon is presented in the photon energy range between 44 and 53 eV. This portion of the spectrum is characterized by a singly excited Rydberg series 2s2${\mathit{p}}^6$np, and by overlapping doubly excited Rydberg series 2${\mathit{s}}^2$2${\mathit{p}}^4$3snp and 2${\mathit{s}}^2$2${\mathit{p}}^4$3pnl (l=s,d). With the use of synchrotron radiation of ≃3 meV spectral resolution, numerous hitherto unobserved resonances were resolved, including some which exhibit relativistic effects. To identify resonances observed here and in earlier works, we employed numerical calculations, which combine the eigenchannel R-matrix method, multichannel quantum defect theory, and the recoupling frame transformation. These nearly ab initio methods account for most of the observed features in the spectrum, including a class of spin-induced relativistic effects. © 1996 The American Physical Society.