The cross sections for electron removal from highly excited ($n=9-24\mathrm{}$) hydrogen atoms in fast collisions with multiply charged ($q=1-5\mathrm{}$) N, O, and Ar ions were investigated in an ion-atom crossed-beams experiment. The ion-atom collisions occurred inside a deflector where a moderate electrostatic field of up to 1.8 kV/cm was applied. The range of collision velocity ($v_c$) investigated is $v_c=1.0\mathrm{}{v}_1-2.0\mathrm{}{v}_1$, where $v_1=2.2\mathrm{}\times {10}^8$ cm/s is the Bohr velocity. The electron-removal cross section was found to be independent of ion species for a given $q$ and $v_c$, to increase as $q^2$ for a given $v_c$, and to decrease as $v_c^{-2\mathrm{}}$ for a given $q$. These $q$ and $v_c$ dependences of the experimental cross section are in accord with classical Coulomb ionization theories. The experimental $n$ dependence of the cross section differs significantly from the theoretically predicted dependence, but the difference can be accounted for if we assume the presence of the external electric field in the collision volume reduces the ionization energy.

With the use of the generalized-oscillator-strength formulation of the plane-wave Born approximation, the contribution of individual subshell ionization and excitation to the proton stopping power is calculated for ${\mathrm{Al}}^{n+}(0\mathrm{}\le n\le 11$) for proton energies between 0.1 and 100 MeV. The neutral-A1 results are in excellent agreement with experiment. The subshell-peak stopping power due to ionization is in excellent agreement with scaled neutral-atom values. The optical oscillator strength was used to calculate the Bethe meanexcitation energy $I$ for all the A1 ions. Equating the explicitly calculated stopping power at 100 MeV to the Bethe formula led to an alternative $I$ value. While the two $I$ values agreed for some ions, for others they disagreed by as much as 50%. However, even in such cases, a change of less than 10% in the stopping power at 100 MeV would produce agreement between the two methods of calculating $I$.

The Bloch-Nordsieck treatment of infrared radiation in scattering theory, based on the use of coherent states of the radiation field, is generalized, within the context of a nonrelativistic model of the scattering, to include the case where the potential has a long-range Coulomb tail. The procedure followed here is based on a construction of modified asymptotic states which properly account for the motion of the projectile under the combined influence of the Coulomb tail and the low-frequency modes of the radiation field. The differential cross section for scattering accompanied by the emission of an arbitrary number of unobservable photons (those with frequencies below some cutoff value ${\omega }_c$) reduces, in lowest approximation, to the field-free cross section in agreement with the Bloch-Nordsieck sum rule for scattering in a short-range potential. In a more accurate approximation, corrections are obtained which differ from those derived previously for the short-range case through the appearance of terms depending logarithmically on ${\omega }_c$. A low-frequency theorem for single-photon bremsstrahlung in a Coulomb potential is derived as a special case of the formalism developed here.

Formulation and exact numerical calculation of differential and total $1s-1s$ electron-capture cross sections with the use of the second Born approximation with the free-wave Green's function are presented. At energies above 15 MeV there appears in the differential cross section a peak characteristic of a second Born or two-step process. For total cross sections, our calculations show that below 7 MeV the second-order cross section is larger than the first-order cross section, indicating a breakdown of second-order Born approximation with the free-wave Green's function. Results using the peaking approximation of Drisko converge to our exact second Born results only at energies well above 30 MeV. According to our exact second-Born calculations, observation of the angular distribution above 15 MeV should reveal structure related to at least one two-step mechanism for electron capture.

Electron-photon angular correlations have been measured for excitation of the $2P$ states of hydrogen at an incident energy of 35 eV. The data presented relate to electron-scattering angles from ${\theta }_e=8\mathrm{}°$ to ${\theta }_e=120\mathrm{}°$ and yield values for the parameters $\lambda$ and $R$. The experimental results are compared with several theories. In general, none of the theories is found to be adequate at this energy.

The influence of an applied electric field on state changing in collisions between laser-excited $\mathrm{Xe}\mathrm{}\left(\mathrm{nf}\right)\mathrm{}$ atoms and xenon target gas is described. The presence of the field results in marked changes in both the rate constant for collisional state changing and the final-state distribution. The changes can be generally understood in terms of energy-transfer arguments based on the "essentially-free" electron model, although the degree of spatial overlap between the wave functions associated with the initial and final states may also be important.

Relativistic differential and integrated cross sections for excitations of the Li-like ions have been calculated in the first Born approximation. The relative importance of various relativistic effects in the cross sections for the inelastic scattering of fast charged particles with atoms is discussed. Fano's small-angle approximation of the relativistic (transverse) interaction is generalized to include $E1\mathrm{}$-forbidden transitions as well as allowed ones. It is shown that the transverse interaction for $\Delta l\ge 2$ excitations increase integrated cross sections as incident energy is increased, similar to the dipole-allowed transitions though with smaller magnitudes.

Angular distributions of electrons elastically scattered from ${\mathrm{O}}_2$ have been measured by a crossed-beam method. The energy and angular range measured were from 2.0 to 200 eV and from -96° to + 156°, respectively. Total elastic and momentum-transfer cross sections have been obtained and compared with the previous measurements. It is found that there are discrepancies among the measurements in the total elastic cross sections and a serious discrepancy with previous measurements in the momentum-transfer cross sections.

Recoil-ion-production cross sections in rare gases are measured for 1.4-MeV/amu ${\mathrm{U}}^{44+}$ ions and are calculated for 1-5-MeV/amu projectiles in charge state 44 + with the use of the classical-trajectory Monte Carlo method and the independent-electron model. The calculations reasonably predict the net-charge-production cross sections, which exceed ${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^2$ for the heavy rare gases; the calculations are found to underestimate the cross sections for low-charge-state recoil ions and to overestimate the cross sections for high-charge-state recoil ions. The experimental cross sections for production of highly charged recoil ions are found to be large, e.g., greater than 1×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for production of ${\mathrm{Ne}}^{8+}$, ${\mathrm{Ar}}^{10+}$, ${\mathrm{Kr}}^{12+}$, and ${\mathrm{Xe}}^{18+}$.

Treatment of the continuum spectrum of a scattering system by a discrete set of pseudostates generated by the diagonalization of the Hamiltonian in square-integrable basis functions results in spurious resonances. An energy-averaging procedure is formulated using the Stieltjes mean values for the spectral density. The connection between this $L^2$ basis approach to scattering problems and the stabilization method is discussed, and an improved form of the latter is derived.

The conventional treatment of electron-ion collisions at low energies by the coupled-channel method (CCM) neglects the radiative channels. The effect of radiation is then usually included afterwards perturbatively. For heavy ionic targets, however, the radiative coupling is often much stronger than the electron-electron correlations, so that the CCM should be modified to incorporate the radiative channels correctly. We present a modified formulation in which both the radiative and Auger channels are treated on an equal footing. At medium energies pseudostates are generated with the inclusion of radiative coupling, and an averaging over the pseudoresonances is carried out using the Stieltjes mean spectral functions. The resulting formalism should be useful in the evaluation of dielectronic recombination and Auger ionization amplitudes.

A modified Glauber approximation has been employed to investigate the elastic scattering of electrons by the hydrogen molecules in the intermediate-energy range (50-1000 eV). The exchange effects are included through the Glauber-Ochkur approximation. In general the effect of multiple scattering is found to be small and the differential cross sections agree very well with the absolute experimental data for $E\ge 100$ eV. The total cross sections for electron as well as positron scattering are also obtained via the optical theorem. The values are in good agreement with the available experimental data.

The $s$-, $p$-, and $d$-wave phase shifts have been calculated for elastic scattering of electrons by hydrogenlike ions ${\mathrm{He}}^{+}$, ${\mathrm{Li}}^{2+}$, ${\mathrm{Be}}^{3+}$, and ${\mathrm{B}}^{4+}$ with the use the polarized-orbital method of Callaway et al. The present method takes account of exchange polarization explicitly following Sloan. Our $s$-wave triplet phase shifts are in very good agreement with those of the variational method. In the case of $e^{-}$-${\mathrm{He}}^{+}$ scattering, our singlet $s$-wave phase shifts differ by about 7.5% whereas for $e^{-}$-${\mathrm{B}}^{4+}$ scattering, agreement is very good.

The photoionization of C${\mathrm{O}}_2$ has been studied using accurate frozen-core Hartree-Fock final-state wave functions. The Hartree-Fock continuum equations were solved using the iterative Schwinger variational method. We present differential and total cross sections for photoionization leading to the $X{}_{}{}^2{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$, $A{}_{}{}^2{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$, $B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, and $C{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$, states of C${\mathrm{O}}_2^{+}$ as well as for oxygen and carbon $K$-shell photoionization. The present cross sections are compared to experimental data and are found to be in generally good agreement. The theoretical cross sections exhibit features due to a narrow shape resonance in those channels where the continuum wave functions have ${\sigma }_u$ symmetry. The relation between these results and experimental cross sections is discussed. The present fixed-nuclei results have also been compared to published theoretical results obtained using the Stieltjes-Tchebycheff moment theory approach and the continuum multiple-scattering method.

A time-dependent, wave-packet description of atomic collisions in the presence of laser radiation is extracted from the more conventional time-independent, stationary-state description. This approach resolves certain difficulties of interpretation in the time-independent approach which arise in the case of asymptotic near resonance. In the two-state model investigated, the approach predicts the existence of three spherically scattered waves in this asymptotically near-resonant case.

The well depths of the $A{}_{}{}^2{}_{}{}^{}\Pi$ potentials for Li-He and Li-Ne molecules have been determined by the observation of the intensity of the red wing of the Li $D$ lines, perturbed by He and Ne, as a function of temperature and with the use of the quasistatic theory of line broadening. Significant enhancement of the fluorescence signal was achieved by the use of cw dye-laser excitation and by the operation of the fluorescence cell at low temperatures. For Li-He, we obtained $D_e=850\mathrm{}\left(100\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$; for Li-Ne, we obtained $D_e=225\mathrm{}\left(30\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$.

Quenching and excitation-transfer processes involving the $3{}_{}{}^2{}_{}{}^{}P$ and $3{}_{}{}^2{}_{}{}^{}D$ states of the Li atom are studied in a temperature-controlled heat-pipe oven with the use of two-step laser excitation of the $4{}_{}{}^2{}_{}{}^{}S$ state followed by radiative cascade towards the $3{}_{}{}^2{}_{}{}^{}P$ state. Numerical identification of the collision-induced $3{}_{}{}^2{}_{}{}^{}D\to 2{}_{}{}^2{}_{}{}^{}P$ fluorescence with a three-level model allows the determination of radiative coefficients and collisional rate coefficients. $4{}_{}{}^2{}_{}{}^{}S$, $3{}_{}{}^2{}_{}{}^{}D$, and $3{}_{}{}^2{}_{}{}^{}P$ radiative lifetimes are found to be in agreement with accepted values. Quenching and excitation transfer strongly depend on the colliding atoms. Rate coefficients for the $3{}_{}{}^2{}_{}{}^{}D\to 3{}_{}{}^2{}_{}{}^{}P$ excitation processes induced by Li, He, and Ne ground-state atoms are measured in the ratio 1.05: 0.23: 0.02 expressed in ${10}^{-9\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{cm}}^3$.

Properties of the $1s2p^2$ states of Li-like ions are calculated relativistically for $6\le Z\le 30$, with particular attention to the effects of the Breit interaction on multiplet splitting and the radiationless transitions through which these states decay. The full Breit interaction (magnetic and retardation terms) is included in calculating fine structure. Transition rates are computed in relativistic intermediate coupling with configuration interaction. Both the spin-orbit and magnetic interactions are incorporated in the calculations. Results are compared with earlier calculations and with experiment, and the physics of observed relativistic and configuration-interaction effects is discussed.

The effect of electric fields of strengths up to 30 kV/cm on the absorption spectra of helium near the ionization limit has been investigated in detail with the use of synchrotron radiation from the SURF-II storage ring. In contrast to previous work on the heavier noble gases, the results show a strong dependence on the polarization of the source. A simple theoretical model is developed to explain the results. The relationship between the work reported here and previous theoretical and experimental studies is discussed.

A new exact result is presented on the lossless propagation of simultaneous different-wavelength optical pulses through an atomic medium with general $N$ energy levels in an extended $\Lambda$ or $V$ configuration.