We present empirical scaling laws, as a function of the projectile charge state, for single and double electron capture in slow collisions between highly charged ions and He atoms at impact velocities of 0.1 and 0.5 a.u. The fitting parameters are shown to be suitable for predicting the populated states in single and double electron capture. The scaling law for single capture is found to be nearly independent of the projectile velocity in the range 0.1–0.5 a.u. The same fitting procedure is followed for double electron capture at the velocity of 0.5 a.u. since independent monoelectronic transitions, due to electron-nucleus interactions, are dominant. At this velocity, the scaling law for the projectile charge dependence of double electron capture cross sections is found to be similar to that for single electron capture. At the lower velocity of 0.1 a.u., where dielectronic processes caused by the electron-electron interaction gain importance, the charge dependence of double capture cross sections is strongly modified.

The shake-off mechanism, known in photon impact, has been applied to the analysis of two-electron transitions in slow ion-atom collisions. The mechanism consists of relaxation of the electronic state of an outer target electron upon relocalization of the inner-electron wave function from the target to projectile nucleus. Based on this mechanism, a method is proposed to estimate cross sections of two-electron processes. The method is demonstrated by calculating cross sections of double capture, transfer excitation, and transfer ionization for collisions of the excited He $\mathrm{}\left(1s2l\right)\mathrm{}$ atom with the ${\mathrm{B}}^{5+}$ ion. The dependence of the cross sections of double capture and transfer excitation on the principal quantum number $n$ of the final state is presented.

Theoretical results for the generalized oscillator strength and optical oscillator strength (OOS) have been obtained for the excitations from the ground vibronic state $X^1{\Sigma }^{+}$ $(\nu =0)$ to the vibrational levels $v^{\prime }=0,1\mathrm{}$ of the $B^1{\Sigma }^{+}$, $C^1{\Sigma }^{+}$, and $E^1\Pi$ electronic states within the first Born approximation. The target electronic states were determined using the configuration-interaction method, with a Hartree-Fock basis for the occupied molecular orbitals, and improved virtual orbitals for the virtual space. We discuss recent optical and electron impact measurements and reexamine previous OOS results for these processes.

We find the solution of the Schrödinger equation for the case of a central square well (or barrier) plus a centrifugal tensor well potential for a system of two spin one-half particles. The wave function is derived and from it a scattering amplitude is obtained explicitly.

A deconvolution procedure is proposed to determine the resonant width $\Gamma$, the asymmetry parameter $q$, and the background cross section ${\sigma }_b$ of a Fano-type resonance in the absence of an ultrahigh-energy resolution. This procedure enables a direct extrapolation to infinite energy resolution using a set of explicit analytical relations in terms of the ratio between the width $\Gamma$ and the experimental energy resolution $\Omega$ in the limit of $\Gamma /\Omega \ll 1$.

Periodic variation in the $L{\beta }_1-\mathrm{to}-L\alpha$ x-ray intensity ratio of I and Ag projectiles has been observed as a function of the atomic number $Z_2$ of the collision partner. It is shown that this periodic behavior is linked to the enhanced intensity of the $L{\beta }_3$ and $L{\beta }_4$ x-ray components, which are not resolved from the dominant $L{\beta }_1$ x ray. The enhancement of the $L{\beta }_3$ and $L{\beta }_4$ components is attributed to the increased $L_1$-subshell ionization via molecular orbitals. The x-ray intensity ratio is also found to be nearly the same, in a few selected cases studied, irrespective of the role of the emitter as target or projectile.

The relative partial photoionization cross sections ${\sigma }_i$ and photoelectron angular distribution parameters ${\beta }_i$ are measured for all possible final ionic states of ${\mathrm{Br}}^{+}$ between the ${}^3{P}_2$ and ${}^1{S}_0$ thresholds. The decay patterns of the autoionizing ${4p}^4{}^3{P}_{1,0}n\mathcal{l}$, ${4p}^4{}^1{D}_{2}n\mathcal{l}$, and ${4p}^4{}^1{S}_{0}n\mathcal{l}$ Rydberg series arising from the $4\overrightarrow{p}ns,md$ excitations are observed at the fine-structure level in all available channels. For each Rydberg series, the energies, quantum defects, and photoelectron angular distribution parameters are determined, as well as the widths and shape parameters of the low-lying members. Relative total cross sections are derived from the partial cross sections and from ion-yield measurements over the lowest members of the ${}^3{P}_{1,0}$ and ${}^1{D}_2$ series. All major spectroscopic and dynamic properties of these series are reported for this open-shell atom. Conclusions are drawn from the comprehensive data sets in comparison with other halogen atoms and the neighboring closed-shell rare gas atoms. Our results, encompassing the entire autoionization regime, are compared with other experimental data and theoretical calculations, where available.

Recent measurements of the photoionization of Na by ejection of a $3s,$ $2p,$ or $2s$ electron give branching ratios, β parameters, and absolute partial cross sections for a number of excitation processes, allowing one to perform the full partition of the total photoabsorption cross section into its components. In parallel, recent developments of the $R$-matrix code, together with extensive configuration and close-coupling expansions, make it possible to calculate the relevant quantities over the continuum energy range (40–140 eV) as well as in the resonant energy region corresponding to the excitation of a $2p$ or $2s$ inner-shell electron onto empty optical orbitals. A critical comparison is made of our experimental results with previous and present theoretical calculations. Valuable information is obtained for the ionization energies and relative cross sections. The main limitation in testing the validity of the various theoretical approximations is the lack of accuracy of the experimental photoabsorption cross section measured only to about 25%.

Ions with charge states as high as $80+,$ produced in the Lawrence Livermore National Laboratory electron beam ion trap were extracted and transferred to a Penning ion trap (RETRAP). RETRAP was operated at cryogenic temperature in the field of a superconducting magnet. The stored low-energy ions collided occasionally with ${\mathrm{H}}_2$ molecules in the ultrahigh-vacuum environment of the trap, capturing one or two electrons and reducing the charge state of the ions. The number of ions was monitored nondestructively by ramping the axial oscillation frequencies of the ions through resonance with a tuned circuit composed in part of trap capacitance and an external inductor. This produced resonance signals whose square is proportional to the number of ions in each charge state. These signals were recorded vs storage time to determine the electron-capture rates. From these rates the relative electron-capture cross sections were obtained using estimates of the mean ion energies based on modeling the ion storage, and with the aid of a density calibration measurement using ${\mathrm{Ar}}^{11+}.$ The measured total electron-capture cross sections are consistent with a linear increase with charge state $q.\mathrm{}$ The cross-section data for the highest charge states lie above the predictions of the absorbing sphere model, but agree within uncertainties in both experiment and theory. The true double-capture cross-section fraction for $q>\mathrm{}35\mathrm{}$ is near 25%. The results are discussed with relation to measurements on lower charge states, and with theory.

Absolute cross sections and rate coefficients have been determined for dissociative recombination of electrons and ${\mathrm{CO}}^{+}$ ions for energies from 1 meV to 54 eV. We found values of $4\times {10}^{-12}$ ${\mathrm{cm}}^2$ at 1 meV and ${10}^{-15}$ ${\mathrm{cm}}^2$ at 1 eV, with an essentially $1/E$ energy dependence. Branching ratios over the final atomic product states have been determined using a position- and time-sensitive imaging system. At zero eV collision energy the predominant yield is to ground-state atomic fragments (76%). At higher collisional energies the branching ratio to the ground-state limit is reduced. A new limit, $\mathrm{O}{(}^{1}D)+\mathrm{C}{(}^{1}D),$ opens up and branching to the $\mathrm{O}{(}^{3}P)+\mathrm{C}{(}^{1}D)$ limit increases. Cross sections are also determined for dissociative excitation of ${\mathrm{CO}}^{+}.$ Thermal rate coefficients are deduced from the dissociative recombination (DR) data, and compared with measurements in the literature. Consideration of both the theoretical and spectroscopic data in the literature giving information about the potential curves along which DR may take place reveals both a paucity and disparity of the data.

A quasiclassical analysis of the differential inelastic scattering of oriented $\mathrm{Ca}{(4s5p,}^1{P}_1)$ atoms on He at a collision energy of 1 eV is presented. Inelastic channels correspond to spin-changing events populating different Zeeman sublevels of three fine-structure states of $\mathrm{Ca}{(4s5p,}^3{P}_j),$ $j=0,$ 1, and 2. The magnitude of the right-left scattering asymmetry in the helicopter plane is shown to depend on the locking dynamics. The relative values of different Zeeman-specific cross sections are determined by the diffraction scattering on the attractive ${}^1\Pi$ state in the entrance channels, by the Coriolis mixing between degenerate components of the repulsive ${}^3{\Sigma }^{+}$ intermediate state, and by the radial coupling between 1-1 and $0^{-}{-0\mathrm{}}^{-}$ fine-structure components of the ${}^3{\Sigma }^{+}{-}^3{\Pi }_{\Omega }$ states in the exit channels. The predicted large right-left scattering asymmetry suggests that it can be measured experimentally.

Electron capture in ${\mathrm{C}}^{2+}+\mathrm{H}$ collisions is studied theoretically by using a semiclassical molecular representation with nine molecular states for the doublet manifold at collision energies above 10 eV. The ab initio potential curves and nonadiabatic coupling matrix elements for the ${\mathrm{CH}}^{2+}$ system are obtained from the multireference single- and double-excitation configuration interaction method. The adiabatic potential curves show no bound state for the $1{}^2{\Sigma }^{+}$ state, but a very shallow well in the $2{}^2{\Sigma }^{+}$ potential, suggesting a possibility of a bound state of ${\mathrm{CH}}^{2+}.$ The corresponding total and partial cross sections for charge transfer are found to be in a reasonable agreement with experiment in shape, but the present magnitude is found to be larger by nearly a factor of 2 at the high-energy end.

The double-ionization Compton profile $J^{2+}$ of a two-electron system is studied using highly correlated configuration-interaction–type wave functions for the initial state. For the case of helium we calculate the double-ionization profile using an uncorrelated double-continuum final state, which is a good approximation for the kinematic conditions of high-momentum transfer. Excellent agreement with the predictions of the impulse approximation (IA) is obtained. However, the use of a double-continuum wave function accounts for an asymmetry in the profile, in contrast with the symmetric peak predicted by the IA. Results for other two-electron systems are presented within the IA, and the high-$Z$ scaling property of the profile is found. The different momentum regions of the profile are discussed in terms of the physical mechanism of double ionization.

The interaction of fast charged particles with a semi-infinite medium is described using both quantum-mechanical and semiclassical dielectric formulations. We consider the coupling with bulk and surface excitations for arbitrary trajectories of the particle and apply the solutions to several cases of special interest. Interference effects in bulk and surface excitations for some particular trajectories are described. We analyze in detail the process of reflection-electron-energy-loss spectroscopy, obtaining results for the probabilities of multiple bulk- and surface-plasmon excitations. The calculations are compared with available experimental results, showing a good description of the angular dependence of the excitation phenomenon.

It is shown that within the local plasma approximation, the experimental logarithmic mean excitation energy that occurs in the expression for the stopping power is directly related to the Shannon entropy of the electronic charge density. This energy is calculated from the Shannon entropy of ab initio wave functions for several atoms and molecules. The results are consistent with experimental and theoretical values calculated by independent methods. The dependence of the mean excitation energy on the quality of the wave functions is examined. Furthermore, it is shown that the maximum entropy principle can be used as a guide in evaluating the local plasma model.

The stability of multicharged ${\mathrm{C}}_{60}^{q+}\hspace{0.5em}(3\mathrm{}<~q<~6)$ fullerene ions, produced by slow $(v=0.2\mathrm{}$ a.u.) ${\mathrm{Xe}}^{8+}$-ion impact, has been studied using coincidence measurements between outgoing projectiles, recoil ion fragments, and ejected electrons. Evidence was found for sequential evaporation of ${\mathrm{C}}_{60}^{3+}$, for competition between evaporation and fission of ${\mathrm{C}}_{60}^{4+}$, and for dominant asymmetric fission of ${\mathrm{C}}_{60}^{5+}$ into triply and quadruply charged even-numbered fullerene and low-mass singly charged and neutral carbon clusters.

We have investigated the ground-state geometries of aluminum-doped sodium clusters ${\mathrm{}\mathrm{Na}}_n\mathrm{Al}$ $\mathrm{}(n=1,10\mathrm{})\mathrm{}$ using an ab initio molecular-dynamics method. It is seen that a single Al impurity atom affects significantly the geometries of small ${\mathrm{}\mathrm{Na}}_n$ $(n<~6)$ clusters, whereas the effect is less pronounced for large clusters. Our results show an early appearance of nonplanar ground-state geometries and it is observed that for $n>~6$ the Al atom gets trapped inside the Na cage. The stability of these clusters has been examined from the systematic analysis of energetics. This indicates ${\mathrm{}\mathrm{Na}}_5\mathrm{Al}$ and ${\mathrm{}\mathrm{Na}}_7\mathrm{Al}$ having 8 and 10 valence electrons to be the stable clusters.

We have classically simulated the ionization and dissociation dynamics of a one-dimensional model of molecular hydrogen ${\mathrm{H}}_2$ under intense ultrashort laser pulses. The relative yields as well as the corresponding rates of the fragments have also been calculated numerically for different wavelength laser pulses ($\lambda =1064,$ 532, and 266 nm) that have the same intensity $(I=2.0\mathrm{}\times {10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2)$ and duration ${(T}_d=35\mathrm{}\mathrm{fs}).\mathrm{}$ We find that the intense ultrashort pulses with shorter wavelength favor the simultaneous ionization of two electrons in ${\mathrm{H}}_2.$

The radial electric dipole moments of bound-bound, bound-free, and free-free transitions of high-lying $\mathrm{}(n>\mathrm{}15\mathrm{})\mathrm{}$ Rydberg states and low-lying continuum states $(\epsilon <400\mathrm{}$ ${\mathrm{cm}}^{-1})$ are calculated for hydrogen and the alkali-metal atoms. All dipole matrix elements for the transitions between low-$l$ $(s$, $p$, $d$, and $f)$ states of H, Li, and Rb are presented. Results of similar calculations for the other alkali-metal atoms (Na, K, and Cs) are summarized. These numerical results, using quantum defect theory, are compared with semiclassical approximations. The dipole matrix elements cross zero at certain combinations of initial and final states. The matrix elements at these so-called Cooper minima in Li, K, Rb, and Cs are presented in detail.

The short-pulse dynamics of excitation and stimulated emission between highly excited states $\mathrm{}(15<n<\mathrm{}60\mathrm{})\mathrm{}$ is studied with picosecond far-infrared pulses (40–60 $\mu \mathrm{m}$) originating from a free electron laser. We measured the excitation of a Rydberg wave packet in rubidium around $n=40\mathrm{}$ excited from a deeper bound $\left(n\approx 20\right)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary $n=40\mathrm{}$ state we found that upon irradiating with a short far-infrared laser pulse, adjacent Rydberg states are populated by resonant Raman transfer via a deeper $\mathrm{}(n=20\mathrm{})\mathrm{}$ bound state. Finally, we discuss the dynamics of wave-packet excitation at a Cooper minimum in lithium, where sign change of the excitation matrix elements causes excitation at large distances from the atomic core. Possible measurement methods for this effect are discussed and found to yield different information.