The influence of the presence of fractionally charged particles on the ionization potentials of molecules is studied by an ab initio method.

The atomic quadrupole shielding factor $R_{\Theta }$ has been calculated for ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ ground states of Al and In using the Sternheimer uncoupled perturbation numerical approach and Hartree-Fock-Slater wave functions. The calculated values of $R_{\Theta }\left(\mathrm{Al}\right)=0.134\mathrm{}$ and $R_{\Theta } \left(\mathrm{In}\right)=0.098\mathrm{}$ are in good agreement with the available experimental estimates of 0.096±0.053 and 0.114±0.030, respectively.

Variational Hylleraas-type wave functions are calculated for the lowest doublet and quartet $P^{\mathrm{o}}$ states of Li and ${\mathrm{C}}^{3+}$. The (nonrelativistic) energies obtained are, for Li, -7.41008 and -5.36780 a.u., and, for ${\mathrm{C}}^{3+}$, -34.481 89 and -23.969 30 a.u., respectively. For the doublet states, the hyperfine-structure constants are also evaluated with results in good agreement with other theoretical and experimental studies.

A revised formulation of the variation-perturbation method is given for two-particle interactions. Calculations with this technique require an amount of labor comparable to the traditional one, but appear to converge more rapidly. For ground states, the procedure generates a lower bound to the second-order energy, complementing the upper bound supplied by the usual method.

A coupled-channel approach based on the Arthurs and Dalgarno formalism is presented to describe the scattering of a Rydberg atom and a rare-gas atom. The method permits the calculation of cross sections for changes in the electronic orbital angular momentum of the Rydberg atom. The theory explicitly relates the scattering behavior of the Rydberg state to the electron-rare-gas scattering length. Preliminary results for ${\mathrm{Na}}^{*}\mathrm{}\left(\mathrm{nd}\right)\mathrm{}$+He and ${\mathrm{Na}}^{*}\mathrm{}\left(\mathrm{nf}\right)\mathrm{}$+He are in good agreement with experiment.

The Hartree (mean-field) approximation to the description of the scattering of two heavy bound states is studied in a model consisting of bosons with attractive $\delta$-function interactions. The approximation is derived from matrix elements of the Heisenberg equations of motion of the system and requires careful enumeration of all necessary approximations. The arguments made are verified by comparison of exact and approximate scattering amplitudes. The derivation also yields the physical significance of the amplitude which satisfies the Hartree approximation; it is a Fourier sum over amplitudes for different channels, which can in prinicple be recovered individually. The approach is not restricted to the model studied.

Crossed beams of electrons and ${\mathrm{Li}}^{+}$ ions have been used to measure the absolute emission cross section for the process, $e+{\mathrm{Li}}^{+}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)\to e+{\mathrm{Li}}^{+}\left(n{}_{}{}^3{}_{}{}^{}l\right)\to h\nu$ (548.5 nm), from below the threshold at 61.26 to 162 eV. The cross section exhibits the sharp onset at threshold characteristic of positive-ion excitation. The cross section at threshold is deduced to be 2.0 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$, decreasing to 0.2 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 162 eV. Total uncertainties at a 68% confidence level are typically about ±15%. Pronounced structure is observed in the near-threshold region. From about 1.5 times the threshold energy to the highest energies measured, the cross section follows the generally predicted $E^{-3\mathrm{}}$ behavior for ${}_{}{}^1{}_{}{}^{}S$ to ${}_{}{}^3{}_{}{}^{}P$ transitions. Theoretical predictions of the cross sections differ from one another by up to a factor of 2, and none appears to fit the data really well.

The semiclassical theory of electronic excitation of an atom in a strong nonresonant laser field due to a collision with another atom is presented. It is shown that in the presence of asymptotic degeneracy of the excited state the Landau-Zener two-state model is inaccurate, the exact solution of a two-state model is inaccurate, and both differ qualitatively from the accurate solutions of the equations when all (four) states are included. With parameters chosen to model the process $\mathrm{Sr}\left({}_{}{}^1{}_{}{}^{}S\right)+\mathrm{Ar}+\hslash \omega \to \mathrm{Sr}\left({}_{}{}^1{}_{}{}^{}P\right)+\mathrm{Ar}$, cross sections on the order of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ are obtained for $\hslash (\omega -{\omega }_0)\sim 17$ ${\mathrm{cm}}^{-1\mathrm{}}$ over a large range of the Rabi frequency. Depolarization predominates at high field strengths.

The Si $K$ x-ray production cross sections for ${\mathrm{F}}^{q+}\mathrm{}(q=3-9\mathrm{})\mathrm{}$ ion impact in the energy range of 0.4-2.2 MeV/amu are determined in the limit of a vanishingly thin target from the observed target thickness dependence of the x-ray yields. The ionization cross sections are deduced from the measured x-ray production cross sections using an average fluorescence yield ($\tilde{\omega }=1.5\mathrm{}{\omega }_0$, where ${\omega }_0$ is the fluorescence yield for an atom with single $K$ vacancy) determined from high-resolution x-ray spectra and configuration fluorescence yields. For ${\mathrm{F}}^{q+}$ ($q\le 6$) ion impact, the measured ionization cross sections are compared with the plane-wave-Born-approximation prediction with and without the Coulomb deflection and increased-binding-energy effects. The Si $K$ shell to F $K$ shell electron-transfer cross sections are determined from the difference between the ionization cross sections for ${\mathrm{F}}^{9+}$ ions and those for ${\mathrm{F}}^{q+}$ ions ($q\le 6$). The results are compared with the prediction based on the two-state atomic-expansion method and that based on the modified Oppenheimer-Brinkman-Kramers approximation. A comparison is also made between the Si solid-target measurements with those obtained for gaseous Ne and Ar gas targets. The solid-target and gas-target data are consistent at 1.58 MeV/amu. Finally, from a comparison between the measured equilibrium x-ray yields and the calculated x-ray yields using the equilibrium charge distribution of projectiles after passing through foils, it is found that the projectiles have a considerably larger number of $K$-shell vacancies inside solids than after passing through solids at lower-impact energies.

An explanation is provided to the experimental results of Rockett et al. [Phys. Rev. Lett. 40, 649 (1978)] on the anomalous transmission of a ring-shaped laser beam through an overdense plasma. The laser beam heats the electrons of the illuminated region which in turn heat the electrons of the central region through thermal conduction. In order to equalize the pressure everywhere, a density depletion of the spot size of the laser beam is created which paves the way for the beam to be transmitted.

The cross sections for deactivation of the Na $5s-9s$ and $n=5-8\mathrm{}$ manifold ($l\ge 2$) states by ${\mathrm{N}}_2$ have been measured using a laser-induced fluorescence approach. The cross sections are: $6s$, 84(8) ${\mathrm{\AA }}^2$; $7s$, 82(9) ${\mathrm{\AA }}^2$; $8s$, 91(8) ${\mathrm{\AA }}^2$; $9s$, 104(10) ${\mathrm{\AA }}^2$; five manifold, 43(7) ${\mathrm{\AA }}^2$; six manifold, 36(8) ${\mathrm{\AA }}^2$; seven manifold, 32(7) ${\mathrm{\AA }}^2$; and eight manifold, 18(6) ${\mathrm{\AA }}^2$. The $s$-state cross sections are in qualitative agreement with a curve-crossing model. The decrease of the manifold cross sections with $n$ is attributed to the increasing multiplicity of the manifold with $n$.

The population transfer between excited $3p^{5}4p$ argon levels induced by the collisional process. ${\mathrm{Ar}}^{*}{\left(3\mathrm{}{p}^{5}4p\right)}_i+\mathrm{Ar}\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)\rightleftarrows {\mathrm{Ar}}^{*}{\left(3\mathrm{}{p}^{5}4p\right)}_j+\mathrm{Ar}\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\Delta E$ was studied in the afterglow of an argon pulsed discharge, at the pressure range of 0.2-1 Torr. Selective population of one particular argon $3p^{5}4p$ level was achieved by laser excitation from one metastable state by use of a tunable cw dye laser. The populations of the laser-excited level and of the collisional excited levels were determined by intensity measurements of the fluorescence line and of the sensitized fluorescence lines. The time-dependence study of the population of the metastable state, of the laser-excited state, and of the collisional excited states was carried out to ascertain the product channel and to determine the collisional transfer rate coefficients.

Cross sections of the title process are theoretically evaluated to be of order of ${10}^{-14\mathrm{}}$ - ${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$, using the model in which an excited Rydberg electron behaves as if it were "free" and slow, with interaction between the excited electron and a polar molecule playing a decisive role. The $n$ dependence of the cross section shows a sharp peak as the energy defect becomes small.

The angular distributions of ${\mathrm{H}}_2^{+}$ fragments produced in dissociative collisions in a gas target were measured using an iris aperture. The various dissociation channels were identified by measuring the two fragments and by checking their time coincidence. We have analyzed the statistical moments of the angular distributions in the various dissociation channels and the dependence of these moments on the pressure of the gas target. This analysis presents experimental evidence for the dependence of the dissociation cross sections on the vibrational states of ${\mathrm{H}}_2^{+}$, and yields information on the dissociation process.

By analyzing the time-of-flight distribution of metastable helium atoms produced by bombardment of a ground-state atomic beam by low-energy electrons, we have measured the differential cross section for excitation of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state over the energy range 19.9-20.4 eV. The results are compared with those determined from electron energy-loss experiments as well as with various theoretical predictions.

We study the scattering of charged particles in the presence of a homogeneous magnetic field. Using the Green's function formalism, an appropriate transition amplitude for the scattering process is defined, and application is made on scattering by a Coulomb potential in the high-energy approximation. For this case, the transition amplitude is obtained in a closed form; its behavior with the magnetic field intensity and initial translational energy is qualitatively discussed for transitions from the first Landau level. In the ultrastrong-field limit, the total transition probability presents periodic threshold structures with increasing values of the initial translational energy.

The influence of long-range forces is examined in excitations between bound states of simple one- and two-center systems. The photoabsorption of light and electron scattering in the near-forward direction are the two excitation processes explored. The one-center system used is an excited state of helium with a nonpenetrating outer electron; the two-center system used consists of two hydrogen atoms separated by a large distance. I consider only nonrelativistic theory and ignore spin. It is found that the long-range forces exert themselves by means of the polarizabilities and induced electric moments in the part of the system which is not excited. As a result, the leading long-range corrections to the transition amplitudes may be expressed in a simple form depending only on these polarizabilities and one-electron operators between hydrogen wave functions.

Annihilation lifetime spectra have been measured for positrons in ${\mathrm{N}}_2$ gas at 77 K at densities in the range 1.9-3.6 amagat, and in ${\mathrm{N}}_2$-Ne gas mixtures at 77 K having ${\mathrm{N}}_2$ partial densities in the range 0.5-3.7 amagat and with Ne concentrations in the range 0.34-0.98. The annihilation rate of positrons with ${\mathrm{N}}_2$ was found to be independent of the Ne concentration in the gas mixtures. ${\left(Z_{\mathrm{eff}}\right)}_{{\mathrm{N}}_2}$ remains constant at 40.0±0.9 at ${\mathrm{N}}_2$ densities below about 2.3 amagat, but increases to about 108 at 3.7 amagat. Shoulder regions in the lifetime spectra of positrons in ${\mathrm{N}}_2$ and in ${\mathrm{N}}_2$-Ne mixtures have been observed which have a constant width of about 14 nsec amagat at ${\mathrm{N}}_2$ densities below about 2 amagat, but which increase to about 24 nsec amagat at 3.7 amagat of ${\mathrm{N}}_2$. For ${\mathrm{N}}_2$ densities in the range 2-3.7 amagat, the measured shoulder widths correspond to a constant time of about 7 nsec. The slow-positron annihilation rates and the shoulder widths are discussed in terms of the self-trapping of positrons within clusters of ${\mathrm{N}}_2$ molecules.

Transitions among discrete and continuum electronic states of diatomics, induced by atom-atom collisions and leading to ionization, are described by rigorously making the continuous electron energies $\epsilon$ discrete. An expansion in functions of $\epsilon$ leads to a set of coupled-channel scattering equations which may be solved to obtain differential and integral cross sections per unit of electron energies. This procedure is applied to Penning ionization in ${\mathrm{He}}^{*}(1s2s, {}_{}{}^3{}_{}{}^{}S)$ + Ar collisions by choosing a suitable expansion basis and numerically integrating a selected set of coupled equations. Results confirm the Franck-Condon nature of electron emission used in previous theoretical analyses. They were obtained with a parametrized exponential coupling potential between the states of HeAr* and He${\mathrm{Ar}}^{+}$+$e^{-}$. The calculations include total integral cross sections per unit electron energies and the contribution of individual partial waves at a given final energy. These cross sections show prominent features that may be related to the potential of the product heavy particles.

The individual effect of the ground-state configuration interaction and the final-state intershell and interchannel interactions to the photoionization of the $3s$ and $3p$ electrons is studied in the vicinity of their respective Cooper minima. Channels of configurations corresponding to these many-body interactions are included in our study by extending a recently developed theory of atomic transitions. Our calculation has successfully removed the quantitative disagreement between the experimental measurements and earlier theoretical estimates.