The exchange scattering of electrons by atomic hydrogen is considered in the eikonal approximation. The six-dimensional integrals for the "post" and "prior" forms of the eikonal exchange amplitudes are reduced to two-dimensional integral expressions. A comparison of the eikonal-exchange cross section, nonexchange Glauber, and experimental results for 50-, 100-, and 200-eV electron-hydrogen elastic scattering is given. Numerical convergence problems were not encountered in the evaluation of the scatering amplitudes and the procedure demonstrates that eikonal exchange calculations are almost as easily done as Glauber calculations in the single active electron approximation.

Alternative methods for summing the Born series for a scattering process are derived and compared. These include Padé and unitary approximations of the scattering operator and their combinations. The various approximations are applied to an attractive Yukawa potential, and it is shown that most of the unitary and Padé approximations converge much more rapidly than the Born series. A comparison of the methods provides a practical test for convergence at a particular order of the perturbation theory.

Differential scattering measurements for a beam of metastable argon (Ar*) incident on an argon target have been made in the center-of-mass energy range 10-75 eV. In the angular range beyond the rainbow region, there is a strong series of gerade-ungerade symmetry-exchange oscillations which is the dominant structured component of the scattering. These oscillations are identified with coherent scattering on a single pair of states, the $2_g$, $2_u$ pair correlating to the ${\mathrm{Ar}}^{*}\left({}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)+\mathrm{Ar}\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ asymptote; an explanation for this choice is presented. Analysis of the structure in terms of a theory developed for ion-atom symmetric charge transfer yields the potential difference between the $2_g$ and $2_u$ potential curves. A time-of-flight pulsing technique has been used to identify two structureless components of the differential cross section, photons from radiative states populated by inelastic transitions, and slow metastables formed through excitation transfer to undeflected target particles.

The energy straggling ${\Omega }^2$ of 0.5-, 1.0-, 1.5-, and 2.0-MeV $\alpha$ particles in thin vapor-deposited films of Ti, Cr, Co, Cu, and Ag onto a thick substrate has been measured to ±6% by an elastic scattering technique using a magnetic spectrometer. The measurements reveal a linear ${\Omega }^2$ versus target thickness dependence and a mild increase with energy for a fixed target thickness. When ${\Omega }^2$ is plotted as a function of $Z_2$, a decrease in energy straggling with increasing atomic number is observed, although it is considerably higher than predicted by the Bonderup-Hvelplund modification of the Lindhard statistical approach.

The $R$-matrix method for electron-molecule scattering developed by one of us (B.I.S.) is applied to the elastic scattering of electrons from ${\mathrm{F}}_2$ in the static-exchange approximation. Calculations are presented using both ${\mathrm{F}}_2$ and ${\mathrm{F}}_2^{-}$ core orbitals to construct the electron-molecule interaction potential. The total and differential elastic scattering cross sections are quite different for the two molecular fields and can be interpreted in terms of the presence or absence of an ${\mathrm{F}}_2^{-}$ bound negative ion at the equilibrium internuclear separation in ${\mathrm{F}}_2$. Since there is little experimental data on ${\mathrm{F}}_2^{-}$ or $e+{\mathrm{F}}_2$ collisions, the calculations coupled with some detailed electron-beam experiments would be of great value in furthering our understanding of the chemistry of ${\mathrm{F}}_2^{-}$.

Energy straggling has been calculated for protons and helium ions in each of the elements, by using the Hartree-Fock-Slater charge distribution for the target atom with Bonderup and Hvelplund's formulation. The results reveal a $Z_2$ structure in energy straggling, and afford a ready explanation of some earlier measurements.

The preferential population of magnetic quantum levels of atomic states produced in beam-foil collisions is explained using a density-matrix formulation. The density matrix for the beam in the interior of the foil is evolved in time to obtain the density matrix after emergence from the foil. The dependence upon tilt angle of the foil relative to the beam is calculated. The Stokes parameters of the light emitted by decay of the atomic state are given in terms of the density matrix. Formulas which are valid for arbitrary angular momentum of the atomic state are derived. Comparison with experiment shows good agreement with data.

Spin-exchange relaxation rates are measured, as a function of the atomic hydrogen density, in the storage bulb of a hydrogen maser of special design. The values of the hydrogen-hydrogen spin-exchange cross section in the temperature range 77-363 K result. These experimental values are in very good agreement with the result of a theoretical calculation by Allison. For ambient temperature, the experimental value is $\sigma =23.1\mathrm{}\pm 2.8$ ${\mathrm{\AA }}^2$ (the theoretical value is 23.5 ± 2.4 ${\mathrm{\AA }}^2$) and it varies slowly with temperature.

Electron-impact energy-loss spectra of KI were studied experimentally in the 15° to 130° angular range at impact energies of 6.7, 15.7, and 60 eV. The spectra reveal a number of excitation features which have not been detected previously and indicate that KI is a strong photon absorber in the vacuum-uv region. From the spectra, differential and integral electronically elastic and inelastic cross sections have been obtained by normalizing the experimental data to theoretical results at low scattering angles. Rotational excitation cross sections corresponding to $\Delta j=0,\pm 1$ have been calculated using a dipole-plus-repulsive-core interaction potential and the distorted-wave approximation. For purpose of comparison, the rotational ($\Delta j=\pm 1$) excitation cross sections have also been calculated in the Born point-dipole approximation.

Inelastic collisional processes of the ${\mathrm{Ne}}^{+}$ + Ne system have been examined by studying the emission of spectral lines in the range between 360 and 7200 ÅA at impact energies from 30 eV to 7 keV. Thresholds and emission cross sections for lines which originate from several excited configurations have been determined for both NeI ($2p^{5}3s,2\mathrm{}{p}^{5}3p$) and NeII ($2p^{4}3s,2\mathrm{}{p}^{4}3p$). The production of NeI lines is primarily the result of single-electron excitation, and the most strongly excited configuration is $2p^{5}3s$. The threshold for this reaction, 23.5 ± 1 eV, is in excellent agreement with theoretical calculations. The apparent dominance of excitation of the $2p^{5}3s$ configuration differs from the results observed by measuring the energy loss of scattered ions. It is suggested that this difference may be due to disparities in the cross sections for direct excitation and for charge exchange into excited states. Excitation of NeII seems to occur in two-electron processes with simultaneous ionization of the target atoms. Low-intensity lines of NeIII are also observed.

We report measurements of lateral multiple scattering of 15-60-keV ions with atomic number from 6 to 82 on xenon targets of reduced thickness $0.01<\mathrm{}\tau <0.17\mathrm{}$. Significant deviations from the scaling properties inherent in current multiple-scattering theory are observed. The widths of the multiple-scattering distributions are not strictly proportional to the reciprocal ion energy, and they oscillate heavily with ion atomic number. These oscillations are similar to those found in electronic stopping power.

The stabilization method of Taylor has been applied to calculations on the ${}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ resonance in C${\mathrm{O}}^{-}$ at 10.04 eV. For this Feshbach resonance, an electron affinity of 0.22 eV has been obtained (experimental value: 0.35 eV). An ab initio calculation using this wave function and the "golden-rule" formula gives a width of 71 meV, compared with the experimental value of 45 meV. It is shown that the partial width increases as the energy of the electron decreases. In the II channels the main contribution comes from the $l=1\mathrm{}$ partial wave of the continuum electron. Some predictions are made for the widths of observed Feshbach resonances in N${\mathrm{O}}^{-}$.

It is shown that in the central-field approximation, a relativistic calculation reproduces the inverted fine structures experimentally observed in the series of excited sodium $\mathrm{nd}\left(3\mathrm{}\le n\le 16\right)$ and cesium $\mathrm{nf}\left(4\mathrm{}\le n\le 12\right)$ terms. Theoretical results are in good agreement with measured values. In view of the simplicity of the method used, it can easily be applied to all the states of a given Rydberg series. This work brings to light the fact that a complete relativistic treatment (which introduces automatically the relativistic contributions on the large components of the wave functions) is equivalent to a multiconfigurational expansion of nonrelativistic wave functions taking into account relativistic corrections (in the low-$Z$ Pauli limit).

The problem of the interaction between a two-level atom and a quantum electromagnetic field is treated without the use of perturbation theory, without introduction of classical fields or factorization conditions for the states, and without assumptions about loss of memory. The calculation is carried out in the Heisenberg picture, without mode decomposition, and the conclusions all refer to physically measurable quantities, such as the fluorescence detected in the far field of the atom. It is shown that in a coherent field of constant amplitude the system always settles down to a quasistationary state, and that the stationarity is a manifestation of the quantum fluctuations. A solution for the growth of the fluorescent light intensity is presented that holds for any coherent exciting field. The two-time correlation function and the spectral density of the fluorescence are calculated, and are found to agree in the long-time limit with earlier results of Mollow. The two-time intensity correlation function of the field is derived, which corresponds to measurable photoelectric pair correlations, and it is found that this reflects several quantum features of the field. It is shown that quantum fluctuations are manifest more explicitly in two-time correlations in the steady state than in transient effects, like spontaneous emission in the vacuum. The measurement of such correlations therefore presents an opportunity for further experimental tests of quantum electrodynamics.

We have investigated the intensity and pressure dependence of the fluorescence spectrum of OH in the presence of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$O molecules. molecules. Saturation of the absorption transition was observed at low pressures, and the corresponding fluorescence signal was found to vary as the square root of the exciting intensity. This observed dependence agreed with the predicted dependence which took into account the presence of laser modes in the spectrum of the exciting radiation. With full laser power incident, a saturation parameter as high as 3 × ${10}^5$ was observed. The fluorescence spectrum was found to peak at 3145 Å and at 3090 Å, with the relative peak intensities dependent upon gas pressures and upon the particular rotational electronic transition used for excitation. It is concluded that vibrational relaxation of the electronically excited OH due to water vapor in the system plays a dominant role in determining the observed flurescence spectrum.

The wavelengths of eight x-ray lines of the transuranium elements Np, Pu, and Am have been determined. The existence of the $M{\alpha }_2$ line of actinides, reported by some experimenters but not observed in this work, is discussed. Present and previously reported values of the wavelengths of these emission lines are compared with those of ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}$, ${}_{91}{}^{}{}_{}{}^{}\mathrm{Pa}$, ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}$ by means of Moseley diagrams. Wavelength values for seven previously unreported x-ray lines are given. In addition, inaccuracies in several previously reported wavelength values were discovered and more accurate values presented.

Multiconfiguration relativistic Dirac-Fock calculations of $\mathrm{KLL}$ Auger energies are reported for neon, argon, uranium, and americium. These selected examples demonstrate the feasibility of this method to calculate completely ab initio, in intermediate coupling, Auger transition energies with a precision of a few eV. Comparison of our results with semiempirical ones shows that this ab initio method performs at least as well as those which rely partially on experimental results.

We report high-precision measurements of fine-structure intervals in Rydberg states of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ as follows: $8{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-8\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ and $9{}_{}{}^3{}_{}{}^{}D_{2,3}^{}{}_{}{}^{}-9\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ near 21 GHz and $n{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ for $n=16, 17, \mathrm{and} 18$ near 23, 19, and 16 GHz, respectively. A microwave-optical-resonance method and electron-bombardment excitation were used. We state and discuss formulas for predicting as yet unobserved members of $n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-n{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ and $n{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ series. We give an accurate result for the $9{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-9\mathrm{}{}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$ interval obtained indirectly from our work. New results on singlet-triplet mixing coefficients in $F$ states and on coefficients in a Ritz series expansion of quantum defects are also given.

Absolute measurements have been made of total cross sections for the photodissociation of $\mathrm{Ar}_2^{}{}_{}{}^{+}$, $\mathrm{Kr}_2^{}{}_{}{}^{+}$, and $\mathrm{Xe}_2^{}{}_{}{}^{+}$ over the photon wavelength range 565-695 nm. A drift-tube mass-spectrometer apparatus was used with a tunable dye laser for these measurements. Calculations have been made of the bound-free Franck-Condon factors between the relevant ground and dissociating states of $\mathrm{Ar}_2^{}{}_{}{}^{+}$ and a comparison is made to the photodissociation results. It is shown that the photodissociation measurements provide a sensitive test of ionatom potential curves and that the absolute value of the transition dipole matrix element can be obtained from a match to the experimental data. In addition, we have obtained photodissociation cross sections for Ar$\mathrm{N}_2^{}{}_{}{}^{+}$, Kr$\mathrm{N}_2^{}{}_{}{}^{+}$, and Kr${\mathrm{N}}^{+}$.

A spectroscopic method is developed in which the frequency of microwave radiation is added to or subtracted from that of infrared radiation by using two-photon processes in molecules. By utilizing many lines of the C${\mathrm{O}}_2$ and ${\mathrm{N}}_2$O lasers, this method enables us to perform systematic high-resolution spectroscopy in the 10-μm region. Both straightforward two-photon absorption spectroscopy and two-photon Lamb-dip spectroscopy can be used. We describe the theory of this method and its application to the observation of the ${\nu }_2$ band of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}_3$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}_3$.