In an ion-atom collision electron capture and loss depend, in general, on the beam energy, projectile charge state, and target Z. While most fundamental theories of capture and loss are formulated in terms of a single electron moving in the fields of the pertinent nuclei, various scaling laws have been developed to account for partially stripped ions and multielectron targets. The purpose of the present work is to systematically examine the dependence of capture and loss cross sections on target Z. Cross sections for single-electron capture and loss for 1 MeV/u oxygen ions passing through gas targets were measured as a function of target Z for incident projectile charge states of 5+, 6+, 7+, and 8+. The targets used were ${\mathrm{D}}_2$, He, Ne, Ar, and Kr. The electron-capture measurements are generally in reasonable agreement with existing theoretical and empirical scaling rules. The electron-loss cross sections differ appreciably from predictions of the plane-wave Born approximation, particularly for the heaviest targets studied.

Relativistic configuration-interaction calculations are carried out including the spin-orbit term for the transition moments for energy transfers in Ar-${\mathrm{Cl}}^{+}$ collisions. Our calculations indicate that once the bound ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state of ${\mathrm{ArCl}}^{+}$ is formed through Ar${(}^1$$S_0$)+${\mathrm{Cl}}^{+}$${(}^1$${\mathrm{D}}_2$) collisions, the transition moment for the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state to dissociate into ground-state species is so small that ${\mathrm{ArCl}}^{+}$ would be long lived in the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state. This is consistent with the experimental findings.

Energy deposition in matter by atoms and partially stripped ions is studied. Expressions for the collisional and restricted collisional energy deposition rates are derived for high-velocity nonrelativistic projectiles, using Bethe theory. These formulas are intended for the calculation of amounts of energy deposited into a target at depths less than the mean path for change in electronic state of the projectile. Calculations are presented for hydrogen atoms, protons, and electrons on aluminum and graphite targets.

Absolute cross sections are presented for single and multiple ionization of He, Ne, Ar, and Kr resulting from ${\mathrm{He}}^{+}$ impact between 15 and 2000 keV. The cross sections are measured for collisions in which the projectile charge state is unchanged (direct ionization), decreases (single-electron capture), and increases (single-electron loss). From these data total cross sections for target-ion production, for free-electron production, and for single-electron capture are determined. The data are used to demonstrate the relative importance of the various ionization channels as they contribute to the production of free electrons. In addition, from the electron-loss data, it is deduced that electron loss by the projectile generally results in ionization of the target as well.

Low-energy multiply charged ions of the refractory elements tungsten and molybdenum have been produced and stored using a novel technique that couples laser ablation with a radio-frequency ion trap. The charge states of the ions stored range from q=+1 to +4 for tungsten and from q=+1 to +6 for molybdenum. Approximately ${10}^5$ ions with energy less than 5×${10}^{\mathrm{-}3}$ eV/amu are stored for a period in excess of 2 s. The technique can be used to store low-energy multiply charged ions of almost any element.

The orientation and alignment parameters of atomic excited states of hydrogen and helium atoms formed in collisions with electrons, positrons, protons, and antiprotons are examined. For the orientation parameter 〈$L_y$〉 (defined to be the expectation value of the electronic angular momentum perpendicular to the collision plane), it is found that the signs of 〈$L_y$〉 for proton and positron impact are negative, consistent with the prediction of the classical model with a repulsive force. For electron and antiproton impact, except for electron scattering at large angles, the signs of 〈$L_y$〉 are positive, consistent with the classical model with an attractive force. A simple semiempirical scaling of 〈$L_y$〉 for electron-impact excitations to 2p states was found. The orientation of 2p states formed in electron capture in proton–hydrogen-atom collisions and in positron–hydrogen-atom collisions are also studied and the angular dependences of 〈$L_y$〉 are found to show similar behavior. We also examine the orientation parameters for excitation and capture to 3d states and the alignment angles for excitation and capture to 2p states. It is found that in the latter cases these parameters do not show similar simple dependences with scattering angles.

We have performed new crossed-beam measurements and quantum-mechanical calculations on transitions between the short-lived ${\mathrm{Ne}}^{\mathrm{*}\mathrm{*}}${(2p${)}^5$(3p)${\mathrm{\}}}_{\mathit{k}}$≡{α${\mathrm{\}}}_{\mathit{k}}$ states with k=4,5,6,7 (Paschen numbering), induced by collisions with ground-state He atoms at energies between 70 and 140 meV. The {α${\mathrm{\}}}_4$,5,6,7 multiplet is distinguished by the presence of several avoided crossings between the adiabatic potentials $V_k^{\Omega }$(R)—a sign of strong, localized radial coupling. This has inspired a simple, semiclassical model for the ${\mathrm{Ne}}^{\mathrm{*}\mathrm{*}}$-He collision process, which has the following ingredients: (i) straight-line trajectories with hard-sphere scattering at the classical turning point $R_T$; (ii) rotational coupling for R>$R_L$ and ‘‘locking’’ of the electronic angular momentum J to the internuclear axis for R≤$R_L$, with $R_L$ the locking radius; (iii) Landau-Zener–type curve-crossing transitions near the crossing radius $R_C$. This model goes a long way toward explaining the experimental polarization effects, i.e., differences between polarized cross sections $Q_{l\leftarrow k}^{\Vert M_k\Vert }$ for the {α${\mathrm{\}}}_k$→{α${\mathrm{\}}}_l$ transition depending on the asymptotic orientation of the total electronic angular momentum J, as specified by the magnetic quantum number $M_k$. These polarization effects are at times very large. For example, at a center-of-mass collision energy E≃100 meV, we find ${\mathit{Q}}_{\mathit{l}\leftarrow 5}^{\mathrm{\Vert }0\mathrm{\Vert }}$/${\mathit{Q}}_{\mathit{l}\leftarrow 5}^{\mathrm{\Vert }1\mathrm{\Vert }}$=0.5 and 3.5 for l=6 and 7, respectively, ${\mathit{Q}}_{\mathit{l}\leftarrow 6}^{\mathrm{\Vert }0\mathrm{\Vert }}$/$Q_{\mathit{l}\leftarrow 6}^{\Vert 1\Vert }$=0.41 and 1.3 for l=5 and 7, and ${\mathit{Q}}_{\mathit{l}\leftarrow 7}^{\mathrm{\Vert }0\mathrm{\Vert }}$/${\mathit{Q}}_{\mathit{l}\leftarrow 7}^{\mathrm{\Vert }1\mathrm{\Vert }}$=0.08 and 8.9 for l=4 and 5.

The complex Kohn variational method, which is an anomaly-free algebraic variational procedure, is implemented for the case of collisions of electrons with polyatomic molecules. The complex Kohn method requires only Hamiltonian matrix elements and, in this formulation, does not require any exchange matrix elements involving continuum functions. Direct matrix elements of the scattering potential(s) which involve continuum functions are evaluated by an adaptive three-dimensional quadrature scheme. The entire procedure is applied to elastic electron-methane scattering at the static-exchange level and proves to be both efficient and accurate. No Ramsauer-Townsend minimum is found at low energies, and it is concluded that this feature of the experimental cross section cannot be described theoretically without the inclusion of target polarization.

Electron-attachment cross sections and rate constants have been measured and calculated for the dissociative attachment processes e+${\mathrm{CFCl}}_3$→${\mathrm{Cl}}^{\mathrm{-}}$+${\mathrm{CFCl}}_2$ and e+${\mathrm{CCl}}_4$→${\mathrm{Cl}}^{\mathrm{-}}$+${\mathrm{CCl}}_3$. Good agreement over the electron-energy range 1–200 meV is found in energy dependence between present calculated cross sections and experimental (krypton photoionization) cross sections at 300 K. The same calculation, with suitable adjustment of thermal populations, was used to calculate electron-attachment rate constants k(ɛ¯) in the range 50–600 K. Experimental rate constants for ${\mathrm{CFCl}}_3$ and ${\mathrm{CCl}}_4$ were measured at temperatures of 294, 404, and 496 K (${\mathrm{CFCl}}_3$) and 294, 400, and 500 K (${\mathrm{CCl}}_4$) using the Cavalleri electron-density sampling method. Good agreement is found between present measurements and calculations, poor agreement with flowing-afterglow Langmuir-probe (FALP) data in ${\mathrm{CFCl}}_3$ at the higher temperatures, and reasonable agreement with FALP data for ${\mathrm{CCl}}_4$.

We present a procedure to subtract the contribution of the first Born approximation from ‘‘exact’’ transition amplitudes of projectile plus target collision complexes. Following this preliminary step, only a small set of amplitudes requires accurate calculation, these amplitudes are labeled by low values of angular momentum and are invariant under coordinate rotations.

Relativistic distorted-wave Born (RDWB) cross sections of Be-like ions for excitations from the ground state 2$s^2$ ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ to the 2s2p ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ states by electron impact are reported for ${\mathrm{Ne}}^{6+}$ through ${\mathrm{U}}^{88+}$. Multiconfiguration Dirac-Fock wave functions were used to describe the target-ion states, and the relativistic continuum wave functions were calculated in the field of (frozen) target-ion charge distribution with the configuration-average exchange potential. We found, however, that cross sections hardly changed even if the exchange terms were omitted. In contrast, adding the 2$p^2$ configuration to the ground-state wave function reduced cross sections by 10–60 %. RDWB cross sections were calculated from thresholds to T=10 keV, where T is the incident electron energy. For Z≃25 and above, where Z is the nuclear charge, intermediate coupling mixes the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ states such that the transition to the former becomes practically spin- and electric-dipole allowed, changing the high-T behavior of the excitation cross section. Cross sections are presented in compact fitting formulas that allow interpolations to determine cross sections with high precision for arbitrary Be-like ions (10≤Z≤92) and arbitrary incident energies (T≤10 keV).

Intensity decays of the 3s3$p^2$ ${}_{}{}^2{}_{}{}^{}\mathrm{S}$${,}^2$P${,}^2$D, 3$p^3$ ${}_{}{}^2{}_{}{}^{}\mathrm{P}$${,}^2$D, 3s3p${(}^3$P)3d ${}_{}{}^2{}_{}{}^{}D$, 3s3p${(}^1$P)3d ${}_{}{}^2{}_{}{}^{}F$, 3$s^2$3d ${}_{}{}^2{}_{}{}^{}D$, 4s ${}_{}{}^2{}_{}{}^{}S$, 4p ${}_{}{}^2{}_{}{}^{}P$, 5p ${}_{}{}^2{}_{}{}^{}P$, 4$d^2$D, and 4f ${}_{}{}^2{}_{}{}^{}$F states in S S IV have been recorded using the beam-foil technique. The arbitrarily normalized decay curves method for cascade corrections has been applied to obtain accurate lifetimes for 11 of the 18 levels studied. Theoretical lifetimes for these levels, obtained in a superposition-of-configurations calculation, are also reported, and several interesting examples of both long- and short-range interactions are discussed.

It has been known for some time how to construct an approximation to the amplitude for non- relativistic potential scattering in a low-frequency laser field that is correct to first order in the frequency and that depends only on the on-shell field-free scattering amplitude. The next term in the expansion, of second order in the frequency, is determined here. In addition to the physical field-free scattering amplitude, a knowledge of the single-photon spontaneous bremsstrahlung amplitude (up to terms of first order in the frequency) is required. The improved low-frequency approximation obtained here is used to derive an improved sum rule. It is found that when the cross section (correct to second order) is summed over final photon states, the result depends only on the physical field-free cross section in a form which is identical to that obtained from a classical description of the motion of the particle in the field, with the collision taking place instantaneously and without influence from the field. A relativistic version of this sum rule was given earlier by Brown and Goble [Phys. Rev. 173, 1505 (1968)], who based their analysis on general field-theoretic considerations.

Absorption oscillator strengths for dipole-allowed transitions among the 2$p^4$ ${}_{}{}^3{}_{}{}^{}P$, 3s ${}_{}{}^5{}_{}{}^{}$,3S°, 3p ${}_{}{}^5{}_{}{}^{}$,3P, 4s ${}_{}{}^5{}_{}{}^{}$,3S°, 3d ${}_{}{}^5{}_{}{}^{}$,3D°, 4p ${}_{}{}^5{}_{}{}^{}$,3P, and 3s’ ${}_{}{}^3{}_{}{}^{}\mathrm{°}$ states of O i are calculated in length and velocity formulations using extensive configuration-interaction wave functions. We also include these 12 states in our close-coupling calculations of the inelastic cross sections for O i. Cross sections for excitation of the 3s ${}_{}{}^5{}_{}{}^{}\mathrm{°}$, 3p ${}_{}{}^5{}_{}{}^{}P$, 4s ${}_{}{}^3{}_{}{}^{}S$°, 4p ${}_{}{}^3{}_{}{}^{}\mathrm{P}$, 3d ${}_{}{}^3{}_{}{}^{}D$°, and 3s’ ${}_{}{}^3{}_{}{}^{}D$° states by electron impact are presented in the energy region from 13.87 to 100 eV and these are compared, where available, with other calculations and experiments. The present results are in good agreement with the recent experiments at energies 13.87, 20, and 30 eV for the ${}_{}{}^3{}_{}{}^{}\mathrm{p}$ ${}_{}{}^5{}_{}{}^{}\mathrm{P}$ transition and at energies 13.87 and 16.5 eV for the ${}_{}{}^3{}_{}{}^{}\mathrm{s}$ ${}_{}{}^5{}_{}{}^{}\mathrm{°}$ transition. However, the present cross sections for the ${}_{}{}^3{}_{}{}^{}\mathrm{s}$ ${}_{}{}^5{}_{}{}^{}\mathrm{°}$ transition at 20 and 30 eV are large by a factor of 2 compared to experiment. There are discrepancies between theory and experiment in magnitude for the dipole-allowed ${}_{}{}^3{}_{}{}^{}\mathrm{d}$ ${}_{}{}^3{}_{}{}^{}\mathrm{°}$ and ${}_{}{}^3{}_{}{}^{}\mathcal{’}$ ${}_{}{}^3{}_{}{}^{}\mathrm{°}$ transitions.

The two-body collision-induced light-scattering spectrum of gaseous ${\mathrm{CH}}_4$ was studied as a function of temperature from 295 K to that of the saturated vapor, 130 K. Models to explain both the induced translational and rotational scattering worked well over this wide temperature range. The validity of a fixed-parameter expression for the pair polarizability important to the translational scattering was shown to hold approximately. Values of the dipole-quadrupole and dipole-octopole polarizabilities were consistent with earlier determinations and served to confirm the applicability of the long-range model of the induction mechanism giving the rotational scattering. The contribution of bound dimers to the spectrum was identified and its temperature evolution studied.

We report the ionization of hydrogenlike Li and Na atoms of 30<n<70 by 15-GHz microwave electric fields. As n increases above 40, the ionization threshold field falls increasingly far below the 1/9$n^4$ value expected for direct field ionization of the initially excited state. These results are similar to those obtained with H at 9.9 GHz, indicating the generality of such observations. While the H experimental results have been reproduced by calculations based on classical chaos, we present here a simple alternative interpretation based on more familiar quantum-mechanical notions.

The variationally stable procedure for Nth-order perturbative-transition-matrix elements introduced by the authors [Phys. Rev. Lett. 61, 404 (1988)] is presented here in detail. Its key features are that it is noniterative, involves only two unknown functions regardless of the value of N, is stable even near intermediate-state resonances (due to the presence of energy numerators rather than energy denominators), and uses the inverse of the perturbation operator for N≥3. Explicit formulas are presented for application to high-order multiphoton processes in atomic hydrogen. Numerical results for multiphoton-ionization cross sections (for two, three, and seven photons), for the frequency dependence of the nonlinear susceptibilities for harmonic generation (for the third, fifth, and seventh harmonics), and for harmonic-generation transition rates up to 11th order for λ=1064 nm are presented for the atomic hydrogen ground state and compared with results of others by more standard procedures. We also present detailed analyses of the Z scaling of our results for hydrogenic systems, of the use of the imaginary part of the appropriate 2N-photon nonlinear susceptibility to obtain N-photon-ionization cross sections, and of the relation of the variational principle presented here to variational principles for scattering processes of Nuttall and Cohen [Phys. Rev. 188, 1542 (1969)] and of Schwinger [Phys. Rev. 72, 742 (1947)].

Errors in the Rydberg-Klein-Rees potentials for the ground and B-excited states of LiNa and LiK and in the ground-state potential of RbCs are detected using the reduced-potential curve (RPC) method and shown to be due to errors or inaccuracies in the analysis of the spectrum. Using the rules of the RPC scheme, estimates for the left limbs of the ground- and B-state potentials of LiNa and LiK are given which are quite accurate for the ground states. With the use of the RPC method, we also give estimates of the ground-state potentials of ${\mathrm{Rb}}_2$ and RbCs up to the dissociation limit. The RPC method is further employed for a calculation of accurate internuclear potentials from inaccurate theoretical ab initio potential curves for a series of molecules. These procedures could be useful for a prediction of the spectrum. The examples should illustrate the predictive and constructive power of the RPC method.

Measurements of the autoionization rates of 6$p_{1/2}$nk states of Ba in the presence of static and 9-GHz microwave fields have been performed using a multistep laser excitation technique. For field amplitudes below 1/3$n^5$, which defines the Inglis-Teller limit, the measured rates in both static and dynamic fields are in good agreement with a simple Stark-effect model. The rates show an $n^{\mathrm{-}4}$ scaling rather than the familiar $n^{\mathrm{-}3}$ dependence observed in zero field. We also report the first observation of forced autoionization by microwaves. Connections between our measurements and the physical situation in plasmas are discussed, including implications for dielectronic recombination.

We have measured the 1S-2S transition frequency in atomic hydrogen with a precision of 7 parts in ${10}^{10}$ by continuous-wave Doppler-free two-photon spectroscopy. We employ cavity-enhanced multimilliwatt radiation near 243 nm produced by sum-frequency generation, and we observe the 1S-2S transition in a low-pressure hydrogen-helium cell with a resolution of 3 parts in ${10}^9$. For a frequency comparison we detect an optical heterodyne signal at the difference frequency between the 243-nm light used to excite the 1S-2S transition and the second harmonic of a reference laser locked to an interferometrically calibrated ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}_2^{}{}_{}{}^{}$ absorption line near 486 nm. After determining systematic corrections due to the pressure shift of the F=1 hyperfine component in a 0.7 vol. % hydrogen–99.3 vol. % helium gaseous mixture, we obtain the energy-level separation f(1S-2S) =2 466 061 413.2(18) MHz. Choosing a value of the Rydberg constant measured independently by high-resolution spectroscopy of the hydrogen Balmer-β transition, we find the hydrogen ground-state Lamb shift to be $f_{\mathrm{LS}\left(1\mathrm{}\mathrm{S}\right)=8173.9(19\mathrm{}}$) MHz, in good agreement with the theoretical value of 8172.89(9) MHz.