Normalized emission spectra of the optically excited Rb resonance lines (7800 Å, 7948 Å) have been measured in the presence of He, Ne, Ar, Kr, and Xe for about 50 Å on either side of the lines. The perturber gas pressures were in the range 100-1100 Torr and the temperature was 318 °K. The spectra, normalized to the total emission, are shown to be due to binary collisions. Shifts and widths of the Lorentzian cores of the lines were obtained, at pressures sufficiently low for impact theory to apply. The widths are corrected for the Rb hfs and the instrument function. Various power-law fits to the near-wing intensities are presented and compared to theoretical expectations. The red satellites reported by other authors are found to be, in the low-pressure limit, mere shoulders on the line wing for Ar and Kr, while Xe barely produces an actual peak. The measured linewidths and shifts are compared to previous measurements.

The decay rate $R$ for the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}+\mathrm{one}\mathrm{photon}$ transition in heliumlike ions is studied for large $Z$. We write $R=R^{\mathrm{}\left(0\right)\mathrm{}}(1+\frac{b_1}{Z}+\cdots )$ and evaluate $b_1$ exactly with the help of Schwinger's integral representation of the Coulomb Green's function. The result supports the accuracy of the variational wave functions obtained by Drake. In view of the results of recent theoretical work on radiative and recoil corrections, we conclude that the rates predicted by pure quantum electrodynamics are known to considerable precision. The discrepancy between the theory and experiment for several values of $Z$ is briefly discussed.

We propose a variational scheme for calculating the energy eigenvalues and eigenfunctions of the hydrogen atom in the presence of a strong magnetic field. Numerical calculations were performed for several of the lowest states, and for the wavelengths of some allowed transitions. We discuss briefly the equivalence of the problems related with solid-state physics and astrophysics, and compare our results with previous calculations. Because the suggested scheme converges very fast for fields up to approximately ${10}^9$ G this is a convenient manner in which to treat problems where the interest is concentrated in the range of variation of the magnetic field from zero up to approximately ${10}^9$ G.

Measurements of the mean life of the 27 887-${\mathrm{cm}}^{-1\mathrm{}}$ level in ${\mathrm{U}}_{\mathrm{i}}$ have been made at a single vapor density using electronic excitation and a method of delayed coincidence. The lifetime values were obtained by optically detecting the decay of the 3584.9-Å resonance transition. Using branching ratios obtained from known relative $f$ values, the average of the measured lifetimes was corrected for imprisonment of the resonance radiation, and absolute $f$ values were derived from the corrected lifetime giving the following results: ${\tau }_0=7.3\mathrm{}\pm 1.1$ nsec, $f_{3585}=0.18\mathrm{}\pm 0.03$, and $f_{4620}=0.20\mathrm{}\pm 0.03$. The error given with each quantity is the standard deviation as determined from the dispersion of the individual measurements. A systematic error of 1% due to possible nonlinearities in the time scale of the system is also assigned to the results. Unaccounted for systematic errors affect only the correction in the measured lifetime, which is small compared to the statistical error. The lifetime and $f$ values are presented in comparison with the experimental results of another group of workers.

Relative $\mathrm{gf}$ values for 49 prominent ${\mathrm{U}}_{\mathrm{II}}$ lines and 21 of the strongest ${\mathrm{U}}_{\mathrm{I}}$ lines have been measured. The ${\mathrm{U}}_{\mathrm{II}}$ measurements were made using a wall-stabilized arc into which the uranium was introduced in the form of U${\mathrm{F}}_6$. The ${\mathrm{U}}_{\mathrm{I}}$ values were measured with a flow-stabilized arc which is essentially a free-burning arc stabilized by streaming argon around one of the electrodes which is formed from a molten ball of uranium held in a tungsten cup. Temperatures in the wall-stabilized and flow-stabilized arc were 10 500 K and 5500 K, respectively. The ${\mathrm{U}}_{\mathrm{I}}$ relative $\mathrm{gf}$ values were placed on an absolute scale employing a recent lifetime determination for the 27 887-${\mathrm{cm}}^{-1\mathrm{}}$ level of ${\mathrm{U}}_{\mathrm{I}}$. This absolute scale was extended to the ${\mathrm{U}}_{\mathrm{II}}$ values by measuring the relative intensity of a ${\mathrm{U}}_{\mathrm{I}}$ and a ${\mathrm{U}}_{\mathrm{II}}$ line in the wall-stabilized arc.

Calculations of the Stark broadening of hydrogen lines treat the radiating atoms and the perturbing ions as quasistatic. The present experiment represents an attempt to determine whether the possible breakdown of this approximation near the center of the line can account, at least partially, for the existing discrepancies between theoretical and experimental profiles in the core of the Balmer lines. The central regions of $H_{\alpha }$, $H_{\beta }$, $H_{\gamma }$, and $H_{\delta }$ profiles have been measured in a wall-stabilized arc over a range of atom-ion relative velocities by varying the atom-ion reduced mass. The cores of all four lines exhibit a significant dependence on the reduced mass. With increasing reduced mass, the experimental profiles gradually show more structure, but still less than the theories predict. Extrapolation of the results for $H_{\alpha }$ and $H_{\beta }$ to infinite reduced mass, i.e., to the static case, gives results that agree quite well with recent calculations.

The structure of heavy atoms in strong magnetic fields of the order of ${10}^{12}$ G, such as are generally presumed to be present on pulsars, has been previously described in terms of a magnetic Thomas-Fermi (TF) model, suitably adapted from the normal TF model. We present here a model which is analogous to the well-known screening-theory approach for ordinary, "laboratory" atoms—the electrons occupy various orbits with a transverse dimension characterized by the radius of magnetic cyclotron orbits and an over-all dimension characterized by Bohr orbits. Not only does this model give results similar to, though quantitatively more reliable than, the magnetic TF model, but it provides more insight into the structure of atoms in strong magnetic fields. For instance, the number of electrons in the $n\mathrm{th}$ shell is seen to be proportional to $n^4$ instead of the usual $2n^2$ in the laboratory situation. This accounts for the tighter binding of such atoms, a result already derived in the magnetic TF model, but also goes further in suggesting a new kind of shell structure and a new Periodic Table for atoms in strong magnetic fields. Quantum mechanically, our model is based on energy minimization with a simple analytical wave function and this represents the first time a simple wave function has been suggested for heavy atoms in strong magnetic fields. The wave function consists of an antisymmetrized product of single-particle orbitals, each of which is a product of an ordinary hydrogenic radial function and a magnetic Landau orbital so that both the spherical Coulombic and cylindrical magnetic symmetry of the problem are taken into account. However, in analogy with Bohr theory, we motivate and discuss many of the results without explicit use of this wave function. An approximate electron density function is trivially established which is itself physically more reasonable than the corresponding magnetic TF function. Further, our model is valid over a broader range of magnetic-field values and its variational-bound character allows for both successive improvements in its predictions and for a knowledge of the sign of the error in the energy estimate. Useful scaling laws are presented so that, starting from the knowledge of the ground-state energy for some $Z$ and $B$, the energy for other combinations of $Z$ and $B$ can be estimated. A second major result of this paper is that the single-particle wave function that arises during the course of the development of the many-electron wave function is of interest in itself, combining as it does aspects of both spherical and cylindrical symmetry. This simple wave function seems to describe well the ground state of the hydrogen atom at all values of the magnetic field.

Theory and experiment are compared for the $L_{23}\mathrm{MM}$ principal and satellite spectra of argon. For the principal spectrum it is shown that theory and experiment are in excellent agreement if the doublet conventionally assigned to $L_{23}-M_1{M}_1$ is reassigned to $L_{23}-M_1{M}_{23}$ (${}_{}{}^1{}_{}{}^{}P$). It is hypothesized that the splitting of the ${}_{}{}^1{}_{}{}^{}P$ term is due to configuration interaction with the ${\mathrm{}\left(3p\right)\mathrm{}}^3{}_{}{}^2{}_{}{}^{}D\mathrm{}\left(3d\right)\mathrm{}{}_{}{}^1{}_{}{}^{}P$ term of ${\mathrm{Ar}}^{+2\mathrm{}}$. The reassignments enable us to locate, tentatively, several unknown singlet levels in ${\mathrm{Ar}}^{+2\mathrm{}}$. An examination of the satellite spectrum confirms Mehlhorn's assignment of the terms in the ${\mathrm{Ar}}^{+2\mathrm{}}$ ${\mathrm{}\left(2p\right)\mathrm{}}^5{\mathrm{}\left(3s\right)\mathrm{}}^2{\mathrm{}\left(3p\right)\mathrm{}}^5$ configuration. The calculated satellite spectra agree with experiment in spectral shape (the location of peaks) and agree to a factor of 2 in individual satellite peak intensity. The satellite calculations support Mehlhorn's conclusion that theory considerably overestimates the $L_1-L_{23}{M}_1$ Auger transition rate.

Transition-rate expressions are developed to analyze the Auger cascade leading to multiply charged ions following inner-shell vacancy production. In general, our aim is to predict the intensity and spectral distribution of the soft x-ray emission occurring at the termination of the Auger cascade, to determine when and if there can be an inner-shell population inversion, and at what pumping level one will obtain amplified spontaneous emission. Aluminum is examined in detail starting with an initial $K$-shell vacancy, and it is found that there can be significant amplified spontaneous emission in the 160-170-Å region providing one has a hard x-ray flux of about ${10}^{11}$ W/${\mathrm{cm}}^2$ at energies greater than the Al $K$-shell binding energy. It is hypothesized that the recent observations of Jaegle et al., indicating gain at 117.4 Å in a two-plasma experiment, can be accounted for via x-ray coupling between the plasmas. An experiment to check the hypothesis is advanced.

Absolute total apparent electron impact cross sections for excitation of helium $4{}_{}{}^1{}_{}{}^{}S$, $5{}_{}{}^1{}_{}{}^{}S$, $3{}_{}{}^1{}_{}{}^{}P$, $4{}_{}{}^1{}_{}{}^{}P$, $5{}_{}{}^1{}_{}{}^{}P$, $3{}_{}{}^1{}_{}{}^{}D$, $4{}_{}{}^1{}_{}{}^{}D$, $5{}_{}{}^1{}_{}{}^{}D$, $4{}_{}{}^3{}_{}{}^{}S$, $3{}_{}{}^3{}_{}{}^{}P$, $3{}_{}{}^3{}_{}{}^{}D$, and $4{}_{}{}^3{}_{}{}^{}D$ levels have been measured. The incident electron energies begin at 50 eV and extend to 500 eV for most levels studied and extend to 800 eV for the $3{}_{}{}^1{}_{}{}^{}P$, $4{}_{}{}^1{}_{}{}^{}S$, and $4{}_{}{}^1{}_{}{}^{}D$ levels. The $1\sigma$ errors for most of the cross sections measured are ± 10%. The important $3{}_{}{}^1{}_{}{}^{}P$ cross section is 15% below the Bethe approximation (Kim and Inokuti) at 200 eV and is within 6% of the Bethe approximation at 500 eV. The results are reported as polarization-corrected, apparent cross sections. Cascade-corrected cross sections are also given at selected energies. In addition the pressure dependencies of the apparent cross sections are presented for representative levels. Considerable attention is given to the description of the experimental apparatus, the treatment and analysis of the data, the techniques employed in absolute calibration, and the assessment of the errors in the experiment.

The interaction between an atom and ion of the same element leads to gerade and ungerade states of the diatomic molecular ion. The energy splittings between the gerade and ungerade states determine the cross section for resonant charge transfer. Using the JWKB approach these energy splittings are derived from the asymptotic forms of the wave functions for the isolated atom and ion. Pseudopotential calculations of the splittings are reported for ${\mathrm{Li}}_2^{+}$, ${\mathrm{Na}}_2^{+}$, ${\mathrm{Rb}}_2^{+}$, and ${\mathrm{Cs}}_2^{+}$, and are used together with previous ab initio and model-potential calculations to test the JWKB method. The comparison shows that the method is sufficiently reliable to facilitate accurate calculations of the cross sections for resonant charge transfer at low energies.

Energy-loss distributions of ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}$, ${}_{82}{}^{}{}_{}{}^{}\mathrm{Pb}$, ${}_{80}{}^{}{}_{}{}^{}\mathrm{Hg}$, and ${}_{64}{}^{}{}_{}{}^{}\mathrm{Gd}$ after passage through hydrogen are reported in the energy interval $100<E<\mathrm{}500\mathrm{}$ keV. The first three moments were extracted and compared with theoretical values of Lindhard et al. and reasonable agreement was obtained. Calculations of the energy-loss distributions were performed for the case where the projectiles lose their energy by elastic collisions with target atoms. Examples of calculated distributions are shown together with measured distributions.

$K$-shell photoelectric cross-section measurements at 36.818 and 74.409 keV, in 15 elements in the range $33\le Z\le 74$, have been made using Sood's method with minor modifications which are necessitated because of the nonavailability of clean and strong sources of low-energy photons. No discrepancy between theory and experiment similar to the one reported for the $L$-shell case has been found.

Elastic and inelastic collisions in the asymmetric He-$X$ ($X=\mathrm{N}\mathrm{e},\mathrm{A}\mathrm{r},\mathrm{K}\mathrm{r},\mathrm{X}\mathrm{e}$) systems are studied at low-keV energies (0.8-3.0 keV). The experimental method involves energy-loss measurements on the scattered particles using either a time-of-flight technique (for neutrals) or electrostatic energy analysis (for ions). Differential cross sections are reported and the results interpreted in a quasimolecular treatment of the collision. Of particular significance is the very low excitation and ionization of the helium projectile relative to excitation of the target. An understanding of this requires a modification of the usual diabatic correlation rules, and implies the promotion of the outer filled molecular orbital. Predictions made from the modified correlation diagram, and calculated potential-energy curves are found to agree with the experimental results.

We present the results of a Dirac-Hartree-Fock calculation of Rayleigh scattering from He atoms in the energy range 0-500 Ry. The second-order $S$ matrix is used to describe the scattering; the dominant correlation corrections are extracted from the fourth-order $S$ matrix. The scattering amplitude is decomposed into a multipole series and numerical methods are employed to study the multipole amplitudes. For low photon energies the scattering is accurately described by a single electric-dipole amplitude; in this low-energy region correlation corrections modify the second-order amplitude significantly. At higher energies many multipoles contribute to the scattering; above 100 Ry the absorptive part of the second-order amplitude together with the fourth-order correlation corrections are negligibly small. For these very high energies the scattering is accurately described by a scattering form factor.

The $R$-matrix formalism developed by the author to treat scattering from nonspherically symmetric potentials is applied to the scattering of electrons from ${\mathrm{H}}_2$ in the static exchange approximation. Calculations were performed for ${\Sigma }_g$, ${\Sigma }_u$, ${\Pi }_g$, and ${\Pi }_u$ symmetries and phase shifts extracted for $s$, $p$, and $d$ partial waves. The inclusion of $p$ orbitals on each center was found to be essential in representing $d$-wave contributions to the phase shift and to have a non-negligible effect on $p$-wave scattering. The results are in good agreement with other calculations based on the static-exchange model using entirely different numerical techniques. We conclude by discussing the outlook for the method and how it may be extended to treat polarization, correlation, and inelastic processes.

A minimum and a maximum in the generalized oscillator strength are observed as a function of momentum transfer for the [$np\left)\right(n+1\mathrm{})s$] transition in Ne, Ar, Kr, and Xe using 25-keV incident electrons. The results indicate that the occurrence of these extrema is due to zeros in the first Born scattering amplitude, and the positions of the extrema are found to be in excellent agreement with available Hartree-Fock calculations.

The extrapolation method of Ochkur, valid for intermediate energies (∼50 eV), is applied to the exchange form of the Glauber amplitudes. In the case of elastic scattering of electrons from hydrogen atoms at 54.4 eV the "post" and "prior" forms of the exchange amplitude are equivalent, whereas for the case of inelastic scattering there is a minute discrepancy between the two forms of the amplitude. The results are compared with the close-coupling calculation. The investigation is expected to be useful for optically forbidden exchange-allowed transitions due to electron impact at intermediate energies.

An effective single-electron potential is proposed which is appropriate for a realistic description of the electronic motion in quasimolecular systems formed in near-adiabatic ion-atom collisions. The potential is represented as a sum of two spherical potentials, each centered about one of the colliding nuclei. For finite internuclear distances, these potentials are obtained by smoothly interpolating the screening parameters of suitable atomic potentials (Thomas-Fermi or phenomenological potentials) between the united-atom and separated-atom limits ("variable-screening model"). The use of phenomenological potentials entails a great accuracy in describing the atomic states and opens up the possibility of treating ionized quasimolecular systems which were hitherto inaccessible to molecular-orbital Hartree-Fock calculations. Molecular correlation diagrams calculated from the variable-screening model for various neutral and ionized systems are presented. For the neutral cases the agreement with the results of Hartree-Fock calculations is surprisingly good. The scaling properties of the variable-screening model are discussed.

Cross sections have been measured as a function of electron energy (1.4 to 7.5 eV) for production of $L_{\alpha }$ photons from the dissociative recombination of electrons and ${\mathrm{D}}_2^{+}$. Ions made in a low-pressure ion source by electron bombardment and formed into a beam are reacted at right angles with a magnetically confined electron beam. Recombination was observed by detecting $L_{\alpha }$ photons from resultant excited atoms. It was demonstrated that a large fraction of the observed light arises from cascade to the $2p$ state from levels of higher principal quantum number. The measured cross section is thus presented as an upper limit for formation of $\mathrm{D}\mathrm{}\left(2p\right)\mathrm{}$ in the dissociative recombination process. The results, represented approximately by the expression $\sigma \mathrm{}\left(2p\right)<(7.6\mathrm{}\times {10}^{-17\mathrm{}})E^{-1.25\mathrm{}}$ ${\mathrm{cm}}^2$, indicate that recombinations leading to formation of a $\mathrm{D}\mathrm{}\left(2p\right)\mathrm{}$ are a small fraction of the total recombination measured by Peart and Dolder.