We derive a new general method for reducing the number of coupled channels in scattering calculations, and we test it for electron–hydrogen-atom scattering in a truncated Hilbert space. The new method yields encouraging results.

A detailed procedure with mostly analytical formulas is given for calculation of the K-shell direct-ionization probabilities in symmetric collisions. This method, based on the Briggs united-atom model, applies in the slow-collision limit. The binding energy is deduced from a priori calculations, with averaging over impact parameters done by iteration. Two important effects are included for the first time and are consequences of the molecular-orbital (electron-promotion) model. One is outer-shell excitation that has a significant effect, in addition to ionization, on the binding energy. The other is the anisotropy of the 2pσ united-atom orbital. After inclusion of the probability of rotational-coupling excitation from the 2pσ molecular orbital, the predictions agree with the experimental K-shell excitation probabilities of Schuch et al. for Ni-Ni collisions at 54.4 MeV. Thus, united- and separated-atom calculations are reconciled within the common framework of the molecular-orbital model and the Born approximation.

The 3p electrons of ${\mathrm{Na}}^{\mathrm{*}}$ are aligned with the use of a single-mode linearly polarized laser. The reaction occurs at subthermal kinetic energy in a single atomic beam. The ion yield decreases by ∼50% when the electric vector of the light is rotated 45° from a direction parallel to the sodium beam. A polarization effect for this reaction has been previously described and our data disagree with those results.

We have extensively investigated quasielastic state-changing collisions (QESCC) of high-Rydberg atoms with rare-gas atoms including Kr and Xe based on the free-electron model. Theoretical results are compared with recent experimental findings for 25≲n≲48. It has been found that they explain almost all existing experimental data on the QESCC for high n available at present consistently and in a unified way. More detailed analyses concerning the higher-order term of the free-electron model show that there is still some discrepancy among the existing experimental data on the QESCC.

l-mixing collisions Li($n^2$D)+X→Li($n^2$L,L>2)+X induced in Rydberg states of lithium (n=4,5,...,13) by rare-gas atoms or Li ground-state atoms are studied in a heat-pipe arrangement. The l-mixing cross sections deduced from the time analysis of the $n^2$D laser-induced fluorescence agree with experimental values obtained with the heavier alkali-metal atoms and with different theoretical models developed for quasielastic Rydberg-electron–target collisions. Quenching of the $n^2$D fluorescence at higher rare-gas pressures is interpreted as inelastic collisional transfer of the L≥1 mixed states to the nonhydrogenic S sublevel. Finally the radiative lifetimes of the $n^2$D states and $n^2$L mixed states with L≥1 are deduced, respectively, from the low-pressure limit and intermediate-pressure study of the $n^2$D fluorescence relaxation rate.

Results of ab initio calculations for the interaction energy of the Mg-${\mathrm{He}}^0$, Mg-${\mathrm{He}}^{+}$, and Mg-${\mathrm{He}}^{2+}$ systems are presented, for a wide range of internuclear distances. The importance of using ab initio methods for the evaluation of accurate pair-wise potentials is discussed. A string dependence of the interaction energy on the charge state of the projectile is demonstrated. This effect, which exhibits marked deviations from the predictions of any parametric theory, is shown to be also crucial in the behavior of corresponding string and planar potentials commonly used in both experimental channeling and surface scattering studies. The role of long-range binary terms on the construction of string and planar potentials is also discussed.

A general purpose projectile-frame expansion fitting method is used to characterize the shape of the electron capture to the continuum (ECC) cusp for bare carbon, oxygen, neon, and argon projectiles, incident on atomic and molecular hydrogen, and helium targets, over the velocity range 6–18 a.u. The method used to fit the ECC cusp shape allows the direct comparison of spectra taken under differing instrumental conditions in an apparatus-independent manner. The results reveal a substantial similarity in the ECC cusp shape for a wide variety of projectile Z, projectile velocity, and target combinations which has not yet been explained by current theories which predict the shape of the ECC cusp.

Alignment and orientation for H(2p) excitation in ${\mathrm{H}}^{+}$-Ar collisions have been measured for incident proton energies of 1.5 and 3 keV, and in the range of scattering angles between 0.5° and 3.5°. From the experimental results the relative population of magnetic substates H(2$p_0$) and H(2$p_{\pm 1}$) is extracted. The observed magnitude of the H(2$p_0$) excitation suggests that, in addition to a 3dσ-3dπ rotational coupling, a 3dσ-4pσ radial coupling is effective.

A distorted-wave model [Phys. Rev. A 27, 1904 (1983); 28, 2180 (1983)] is applied to calculate the formation of positronium in the n=2 states in $e^{+}$ scattering from hydrogen and helium atoms. The incident wave is represented by a polarized-orbital method. The first-Born-approximation results of the 2p-excited-state capture cross section in the case of helium is reported for the first time. The first Born approximation is found to be unsuitable for prediction of the rearrangement processes. The present total (ground- and excited-state) positronium-formation cross sections have been compared with the corresponding observed values of Fornari et al. [Phys. Rev. Lett. 51, 2276 (1983)] and of Charlton et al. [J. Phys. B 16, L465 (1983)].

The effect of overlapping resonances on dielectronic recombination cross sections and rate coefficients is examined using a simple two-channel model. Results obtained by multichannel quantum-defect theory and the isolated-resonance approximation are found to be closely similar, for realistic choices of scattering parameters and of radiative widths; i.e., except just below threshold where resonances strongly overlap.

We describe a new mechanism for generating macroscopic fluxes in polyatomics absorbing infrared radiation. The mechanism is based on the recoil kinematics which accompany the vibrational-translational relaxation of stretch-type modes. A five-level model is analytically solved, and expressions for the resultant flux are given. The excitation of the ${\nu }_3$ asymmetric stretch of ${\mathrm{C}}_2$${\mathrm{H}}_2$ in a Kr buffer gas is discussed, and flux magnitudes due to this mechanism are given.

Total photon cross sections at energies 6.4, 7.1, 8.04, 10.53, 14.414, 17.44, and 22.1 keV are reported in four Pt-Rh alloys of different proportions. Small deviations are observed from the data compilations, based primarily on experimental determination at certain energies. However, agreement is generally good between the theoretical photoelectric cross sections and the photoelectric cross sections which are deduced from the measured totals. Thus the deviations, though small, suggest the need for more accurate experimental compilations perhaps based on certain criteria for selecting the data from the available measurements for each element in the region of absorption edges.

The 3s,3p satellite spectrum of argon has been studied with use of synchrotron radiation in the 40–70-eV energy range. In the region of the 3s Cooper minimum (42.5 eV±0.2), the integral intensity of the satellites is ten times larger than the 3s partial cross section. The angular asymmetry parameters have been measured as a function of the photoelectron energy for the 3s ${}^2$S,3p ${}^2$P final states as well as for the satellites. For the lowest photon energies it appears that the β parameter values of the most intense satellites are strongly dependent on the total angular momentum of the final ionic state.

The energy difference between two hyperfine levels of the ground state, measured with high precision by collinear laser-rf double-resonance spectroscopy, is influenced by the light shift, depending on the intensity and frequency of the laser field. We report on a measurement of the change in rf transition frequency between the F=2 and F=3 levels of the 5d ${\mathrm{}}^2$$D_{3/2}$ hyperfine multiplet of ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}_{}^{+}{}_{}{}^{}$ as a function of laser frequency. The observed results are in good agreement with theoretical calculations.

A generalized model for the magnetic resonance signal corresponding to the 0-0 hyperfine transition in an optically pumped alkali-metal vapor is presented. Under conditions of microwave power broadening, the model predicts ‘‘anomalous’’ light broadening and ‘‘anomalous’’ relaxation narrowing of the magnetic resonance linewidth. Good agreement between theory and experiment was obtained in an experiment that confirmed the prediction of anomalous light broadening. These results show that the 0-0 hyperfine transition cannot be described as a simple two-level system; rather, the alkali-atom’s nuclear spin plays an important role in determining the magnetic resonance linewidth.

Properties of the 1s2$l^n$(2l’${)}^m$ (n+m=3) states of Be-like ions are calculated relativistically for 6≤Z≤26 using multiconfiguration Dirac-Fock method. Generalized Breit interaction and quantum-electrodynamic corrections are taken into account in the calculations of energies. Transition rates are computed in intermediate coupling with configuration interaction. The generalized Breit interaction is included both in the evaluation of mixing coefficients and relativistic Auger matrix elements. The contributions of the radiationless transitions to the decay of the metastable quintet states are found to be very significant. The effects of relativity and configuration interaction on the transition rates are discussed.

We evaluate finite-temperature corrections to the two-photon exchange potential between polarizable systems. We find a complex (additive) correction of the form: Re$V^T$∼1/r, Im$V^T$∼const, for r<1/T; and Re$V^T$∼1/$r^6$, Im$V^T$∼1/$r^6$, for r>1/T. These results are to be compared with the Feinberg-Sucher zero-temperature answer, Re$V^{T=0\mathrm{}}$∼1/$r^7$, Im$V^{T=0\mathrm{}}$≡0.

A theoretical investigation has been carried out on the dependence of the photoelectron angular distributions on laser intensity for the case of two-photon-resonant three-photon ionization of sodium. Good overall agreement is obtained with the experimental anisotropy parameters for ionization via the 4$D_{3/2}$ and 4$D_{5/2}$ intermediate states. The comparison of the theoretical results with the experimental data also clearly shows that the hyperfine splitting of the ground state has to be taken into account at high laser intensities although the low-intensity angular distributions do not depend on the initial ground hyperfine state.

The absolute values of the photoionization cross sections of atomic oxygen have been measured from the ionization threshold to 120 Å. An autoionizing resonance belonging to the 2s2$p^4$(${}^4$P)3p(${}^3$$D^o$, ${}_{}{}^3{}_{}{}^{}S_{}^o{}_{}{}^{}$,${}^3$$P^o$) transition has been observed at 479.43 Å; and another line, at 389.97 Å. However, although the experimental data are in excellent overall agreement with rigorous close-coupling calculations, which include electron correlations in both the initial final states, there are discrepancies near threshold that may be significant.

With use of a narrow-band laser, the free ${\mathrm{NO}}_2$ molecule is prepared in a well-defined excited state under conditions that correspond to an optical-frequency–radiofrequency double-resonance experiment. We observe a resonant increase or decrease (dependent on the divergence of the light beam) of the total fluorescence intensity when the radiofrequency field is in resonance with the Zeeman splitting in the ground or excited state of the laser-induced transition. These resonances differ significantly from well-known optical-rf double-resonance signals and laser saturation effects. The resonances are, however, well described with use of a model which assumes an irreversible evolution in the free ${\mathrm{NO}}_2$ molecule.