[en] The quantum-mechanical virial theorem for scattering of a particle by a central force is expressed in terms of Wigner's definition of time delay, tau = 2h deta/dE. By using semiclassical approximations, the quantum-mechanical virial is transformed into the classical virial and tau becomes exactly the classical delay time

[en] The authors have measured the absolute total cross section for low-energy electrons scattered by sodium atoms in the 3 ²P/sub 3/2/, m/sub J/ = +- 3/2 state using a modified atomic-beam-recoil technique. A single-mode tunable dye laser is used to excite these states selectively, and their populations are determined by using the atomic recoil in resonant photon interactions to separate excited from nonexcited beams spatially. Total cross sections have been measured at seven electron energies between 0.84 and 6.0 eV

[en] A novel, classical many-body model, previously introduced for nuclear collisions, has been extended to atomic and molecular structure, with the goal of providing a framework for atomic collisions. In addition to the usual kinetic and Coulomb potential terms, a momentum-dependent two-body potential acts between electron pairs of identical spin in order to approximate the Pauli constraint r/sub i/jp/sub i/j> or =hxip, where xi is a dimensionless parameter here set equal to 2.767. A similar potential is introduced to simulate the Heisenberg constraint, r/sub i/Np/sub i/N=h, where N refers to each nucleus. Because of these constraints, the atomic and molecular ground-state configurations are stable. The hydrogen ground state is given exactly. Calculations in H^-, He, Li, Ne, and Ar reproduce total ground-state energies to better than 15%; this is considerably better than the Thomas-Fermi model, in which the errors are approximately 28% for neon and 23% for argon. The resulting electrostatic potential is in general intermediate between Thomas-Fermi and Hartree-Fock calculations. H₂⁺ and H₂ molecules are overbound; in contrast, the Thomas-Fermi model does not bind neutral molecules

[en] The effect of elastic collisions of U⁺ with U, U⁺, and U²⁺ in a U⁺ plasma has been investigated. The required potential-energy curves were calculated by means of the electron-gas (Gordon-Kim-Rae) statistical method. An effective polarization potential was added to the ion-neutral potential curve. Collisional phase shifts were evaluated by the semiclassical JWKB method using a very efficient algorithm. Modifications necessary to take into account the long-range shielded Coulomb potential in ion-ion collisions are described. Momentum-transfer and viscosity cross sections, without charge transfer, are presented and their dependences on the long- and short-range potentials are examined

[en] The calculation of complex resonance eigenvalues for electron-atom and electron-molecule scattering by discrete-basis-set methods yields total resonance widths directly. The authors show that, as a consequence of the separable nature of the S matrix near a resonance, discrete-basis-set methods may also be used to compute partial widths for resonances in a multichannel problem. The procedure is applied to a model three-channel problem to demonstrate its numerical effectiveness

[en] The authors obtain differential cross sections for He²⁺ impact on He(1s²) using a molecular treatment of the collision and the impact-parameter eikonal approximation. Small angles and low energies for which elastic and two-electron resonance transfer reactions occur are considered. The results are compared with the experimental data of Chen et al. Total cross sections are also given and compared with other theoretical and experimental results

[en] Accurate values for the density shift and broadening of five isolated H/sub α/ fine-structure transitions have been measured. Doppler-free polarization and double-quantum saturation spectroscopy with a cw dye laser are used in a He dc discharge with 1% H₂. The negative shifts and the huge broadening values are correctly predicted by an impact-approximation theory taking into account the large cross sections for fine-structure transfer (quenching) collisions and only Van der Waals interaction between H* and He. Consequences of a comparison with the positive shift of the unresolved H/sub α/ line and of the alkali-metal D lines in He buffer gas are discussed. For pressures below 0.1 Torr hfs decoupling collisions give rise to a positive shift of single H/sub α/ fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance

[en] The authors investigate the form of a standing wave in a cold inhomogeneous electron plasma, taking into account ''relativistic effects.'' The wave amplitude is greatly modified near the critical surface for high intensities

[en] A theoretical formalism is given for the calculation of electron pair production in a point-Coulomb potential using the method developed by Sud, Wright, and Onley for expressing the radial integrals over the product of Dirac-Coulomb functions in terms of a matrix generalization of the GAMMA function. A partial differential equation in lepton energy satisfied by the GAMMA function is obtained. This partial differential equation can be integrated numerically to obtain the positron spectrum for high-energy photons

[en] A modified Glauber method proposed recently for electron-atom scattering is extended to the analysis of the 1-2 excitation of atomic hydrogen by electron impact. Exchange effects are not neglected, but included in the calculation through the exact eikonal exchange formulas. The results are compared with experimental data acquired by absolute measurements. Good agreement with experimental data is found. The differential cross sections calculated with the conventional Glauber amplitude, but without neglecting the exact eikonal exchange effects, are also obtained and shown for comparison