The inverse scattering theory has been used to define effective local electron-methane interaction potentials. Those interactions have been obtained first by inversion of the total cross-section data as a function of energy. The WKB approximation has been used in that determination. The same approximation has been used to effect (fixed energy) inversion of differential cross-section data from electron-methane scattering for energies of 200, 300, 500, and 700 eV. The resulting interactions, which we have assumed to be purely real, are very similar and show little energy dependence. They are in good agreement with a theoretically derived potential for this system at those radii for which the inversion potentials are determined sensitively by the scattering data. In addition, the generalized unitarity theorem has been used to specify the scattering amplitude from the 700-eV data. The S function given by Legendre integration of that amplitude has been inverted to specify another purely real potential for the collision. The result is very similar to those found by the other analyses. © 1996 The American Physical Society.

Eigenchannel R-matrix calculations (including effects of long-range multipole interactions beyond the reaction volume) for ${\mathrm{Li}}^{\mathrm{-}}$ photodetachment partial cross sections from the vicinity of the Li 3s threshold to the Li 6s threshold (3.8 eV ≤ħω≤ 5.65 eV) are presented. Excellent agreement with the relative total cross section measurements of U. Berzinsh et al. [Phys. Rev. Lett. 74, 4795 (1995)] in the vicinity of the Li 3s and Li 3p thresholds is found. The calculated resonance structures are analyzed in detail. In particular, the energy region between the Li 4s and Li 5p thresholds (for which there are as yet no experimental measurements) is shown to have types of doubly excited resonances which are prominent only because of the nonhydrogenic ${\mathrm{Li}}^{+}$ core; such types are weak or absent in higher-energy regions as well as in ${\mathrm{H}}^{\mathrm{-}}$ photodetachment spectra. © 1996 The American Physical Society.

Thermalization and degradation of subexcitation electrons with initial energies of 1, 3, and 5 eV in ${\mathrm{H}}_2$ gas at a number density of 2.69 × ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and a temperature of 300 K are studied through a numerical solution of the Boltzmann equation. The collision term in the Boltzmann equation includes contributions from elastic collisions as well as inelastic collisions, viz., collisions leading to rotational and vibrational excitation or deexcitation. The time evolution of the electron energy distribution function and of the cumulative degradation spectrum, as well as thermalization times, are calculated. Time-dependent yields or collision numbers for rotationally and vibrationally inelastic processes are calculated by using the cumulative degradation spectrum. We discuss extensively the present result for thermalization times in comparison with experimental data and other theoretical results. © 1996 The American Physical Society.

Absolute reduced absorption coefficients for the Hg resonance line at 253.7 nm broadened by Ar were determined between 390 and 430 K in the spectral range from 20 to 1000 ${\mathrm{cm}}^{\mathrm{-}1}$ on the red wing and from 20 to 400 ${\mathrm{cm}}^{\mathrm{-}1}$ on the blue wing. The resultant reduced absorption coefficients are in fair agreement with those obtained by Petzold and Behmenburg [Z. Naturtorsch. Teil A 33, 1461 (1978)]. The observed A${\mathrm{}}^3$$0^{+}$←X${\mathrm{}}^1$$0^{+}$ spectrum in the spectral range from 80 to 800 ${\mathrm{cm}}^{\mathrm{-}1}$ on the red wing agrees remarkably well both in shape and magnitude with the quasistatic line shape calculated using the potential-energy curves of the HgAr van der Waals molecule given by Fuke, Saito, and Kaya [J. Chem. Phys. 81, 2591 (1984)], and Yamanouchi et al. [J. Chem. Phys. 88, 205 (1988)]. The blue-wing spectrum is interpreted as the B${\mathrm{}}^3$1←X${\mathrm{}}^1$$0^{+}$ free-free transition of HgAr by a simulation of the spectrum using uniform semiclassical treatment for the free-free Franck-Condon factor. The source of the satellites on the blue wing is attributed to the phase-interference effect arising from a stationary phase-shift difference between the B- and X-state translational wave functions. The stationary phase-shift difference arises owing to the existence of a maximum in the difference potential between the B and X states. The repulsive branches of the potential-energy curves of HgAr for the X and B states have been revised to give excellent agreement between the observed and calculated spectra, both in shape and magnitude. © 1996 The American Physical Society.

We present energy-gain distributions for net one- and two-electron transfer in slow ${\mathrm{Ar}}^{\mathit{q}+}$-${\mathrm{C}}_{60}$ collisions. The ΔE-spectra for the former peak at Δ${\mathit{E}}_1$=12.9±0.4 (nl=7s and 7p), 15.5±1.0 (n=10), 15.1±0.5 (n=11), and 13.8±0.4 eV (n=12) for q=8, 13, 14, and 15, respectively. The total reaction cross sections are ${\mathrm{\sigma }}_{\mathrm{tot}}$= 4.6±1.4 (q=8), 10.1±2.8 (q=13), 7.1±2.0 (q=14), and 10.0±3.1 (q=15) in units of ${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$. These results are discussed within a simple qualitative model in which the transient localization of the positive charge on the ${\mathrm{C}}_{60}^{+}$ ion just after transfer of one electron is a free parameter. © 1996 The American Physical Society.

We present classical simulations for the energy gain, ΔE, due to the image acceleration of very highly charged ions (Q≫10) by metal surfaces. We show that for large Q the simulated values ΔE, based on the classical over-barrier model, fall below the ‘‘staircase’’ model but still exceed the lower bound for classically allowed transitions. The results are in reasonable agreement with recent experimental data. Effects of the nonzero velocity of the projectile parallel to the surface appear to be insignificant for the energy gain. © 1996 The American Physical Society.

Energy spectra are measured in the (1–2)-keV range of secondary electrons induced by bombardment of Si(100) and Al(110) with a 1.5-MeV ${\mathrm{He}}^{+}$ beam. The ion beam is either aligned with a major crystallographic direction or incident along a random direction. The shape of the experimental secondary electron spectra are successfully compared with that of spectra calculated with an efficient Monte Carlo model for electron-transport simulation. In addition, the effective layer thickness L for secondary electron generation under channeling incidence conditions is determined. It is found that ${\mathit{L}}_{\mathit{A}}$ for KLL Auger electron generation is equal to the surface peak area in the spectra of the backscattered ions. This similarity is a consequence of the small values of the backscattering collision diameter and the adiabatic radius for K-shell ionization, as compared to the atomic vibration amplitude. In contrast, ${\mathit{L}}_{\mathit{B}}$ for the generation of electrons by direct Coulomb ionization is much larger than that for Auger emission. The large value for ${\mathit{L}}_{\mathit{B}}$—an indication of a reduced channeling effect—is attributed to the relatively large contribution from the moderately localized L shell to the measured spectra. © 1996 The American Physical Society.

We study the binary-electron production due to single-electron excitation by the direct screened Coulomb interaction between the projectile and the surface. We analyze the energy spectra of electrons emitted during the interaction of ions with surfaces under grazing incidence conditions. For high ion energies (≳100 keV/amu), the binary-electron peak is shifted to energies lower than those expected in ion-gas collisions. The calculations of the electron energy distributions for different projectile charges, surfaces with different Fermi energies and work functions, and several electron observation angles are in qualitative agreement with the experimental data. For lower ion energies (30–100 keV), the present model gives a better description at large observation angles. This indicates that the ‘‘shifted convoy’’ structure, which dominates the electron emission spectrum around the direction of the specular reflection, cannot be ascribed to a binary-collision mechanism. © 1996 The American Physical Society.

The spatial coherence function in all three orthogonal directions has been measured by means of perfect crystal neutron interferometry. It has been demonstrated that the coherence function is given by the Fourier transform of the related momentum distribution, which in turn is determined by the collimation and monochromatization of the beam incident upon and traversing the interferometer. Thus, a measurement of the coherence function can replace a measurement of the momentum distribution in certain cases. Retrieval of the coherence can be accomplished by phase echo and/or postselection methods. A complete retrieval of the coherence is impossible in principle due to unavoidable loss factors.

We study properties of the ground state of a bosonic gas of atoms in a tight trap. The atoms interact via model molecular potentials that may have either positive or negative scattering lengths. We discuss the possibility of Bose-Einstein condensation in such a system. We show that when the size of the trap becomes comparable to the correlation length of the condensate, stable Bose-Einstein condensation might occur independent of the sign of the scattering length. © 1996 The American Physical Society.

Using the knowledge of two-body collision properties that has recently become available, we estimate the three-body recombination rate for doubly-spin-polarized ultracold gas samples of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ on the basis of the Jastrow approximation. We find that only recombination leading to the formation of the highest two-body bound states is important. The rate for the highest bound level with zero angular momentum is found to increase strongly with the absolute value of the two-body scattering length. © 1996 The American Physical Society.

This paper reports the measurements of the K-Ar electron-spin-relaxation cross section ${\mathrm{\sigma }}_{\mathrm{SR}}$ and the diffusion constant, ${\mathit{D}}_0$ for K in Ar. The spin-relaxation cross section is ${\mathrm{\sigma }}_{\mathrm{SR}}$=6.3×${10}^{\mathrm{-}22}$±0.6×${10}^{\mathrm{-}22}$ ${\mathrm{cm}}^2$ and the diffusion constant at one atmosphere and T=390 K is ${\mathit{D}}_0$=0.31±0.03 ${\mathrm{cm}}^2$/s. The measurement of the cross section is carried out in a high magnetic field, which offers a number of simplifications for relaxation-time measurements. © 1996 The American Physical Society.

Radiative renormalization is used to obtain an exact steady-state solution of a four-level system coupled by three arbitrarily strong, near-resonant, monochromatic fields. The four-wave mixing and absorption terms are given for a weak fourth field. The results demonstrate the effects of optically induced energy-level mixing and shifting, and population redistribution. © 1996 The American Physical Society.

We construct minimum-uncertainty solutions of the three-dimensional Schrödinger equation with a Coulomb potential. These wave packets are localized in radial and angular coordinates and are squeezed states in three dimensions. They move on elliptical Keplerian trajectories and are appropriate for the description of the corresponding Rydberg wave packets, the production of which is the focus of current experimental effort. We extend our analysis to incorporate the effects of quantum defects in alkali-metal atoms, which are used in experiments. © 1996 The American Physical Society.

We have observed long-lived dark states in velocity-selective coherent population trapping (VSCPT) in a one-dimensional lin-angle-lin configuration with metastable He atoms using the $J=1\mathrm{}\Rightarrow 1$ transition at $\lambda =1.083\mathrm{}$ μm. These states are characterized by two peaks in the atomic momentum distribution at $\pm Q\hslash k(Q=\mathrm{positive}\mathrm{integer})$, where $k$ is the magnitude of the optical wave vector. VSCPT states previously observed are characterized by $Q=1\mathrm{}$, but we have observed long-lived states with $Q=2\mathrm{}$. Optical Bloch equation calculations show that these high-velocity dark states appear at higher $Q$ for greater light intensity.

Collective laser cooling of two ions moving in a trapping potential is studied theoretically in the limit when the distance between the ions is smaller or comparable to the wavelength of the exciting laser light. A set of rate equations for the population of the trap levels is derived in the Lamb-Dicke limit, and for low laser intensities describing the effects of dipole-dipole interaction and super- and subradiance on the cooling dynamics. © 1996 The American Physical Society.

Population changes among the hyperfine levels of the sodium D1 transition, created by a weakly saturating narrow-band pump, are detected by a counterpropagating probe in a 10-cm-long cell containing sodium vapor without any buffer gas. All Lamb dips and cross over resonances with linewidths approaching natural widths are clearly resolved. We have applied a five composite level model to interpret the observed probe absorption. The rate equations were solved analytically. The model predicts specific relationships between the frequency integrated probe absorption coefficient of crossover and Lamb dip resonances. We find excellent agreement in the case of crossover resonances that involve a single ground-state hyperfine level. The agreement is not so good for crossover resonances for which both ground hyperfine levels participate. © 1996 The American Physical Society.

We present a many-body theory of a driven and damped trapped gas of interacting bosons, and demonstrate that one of the trap levels can become coherently populated, thereby leading to a coherent atomic-beam generator, or ‘‘laser for atoms.’’ The specific system we consider consists of a sample of bosonic atoms interacting via the near-resonant dipole-dipole interaction. The transverse center-of-mass motion of the atoms is confined by a two-dimensional potential well created by an array of cooling laser beams, while their longitudinal motion is quantized by a Fabry-Pérot for atoms. Under appropriate conditions, the dipole-dipole selection rules lead to the simplification that only two quantized levels of atomic motion need to be considered explicitly, the other levels being treated as reservoirs. One of the two levels is the ‘‘pump level,’’ while the other is the one where atomic coherence builds up (the ‘‘lasing’’ level). The master equation describing the dynamics of these levels can be solved numerically, and its solution exhibits a ‘‘threshold behavior’’ with a transition from super-Poissonian to Poissonian atom statistics in the ‘‘lasing mode.’’ © 1996 The American Physical Society.

We present an optical implementation of a twisted Landau-Zener model for which local aspects of the geometric phase strongly influence the transition amplitude. The transition amplitude is influenced by an interference effect caused by the interplay between geometric and dynamic phases. © 1996 The American Physical Society.

A dressed-state rate-equation model is used to describe the Autler-Townes effect that is produced by a resonant 100% amplitude-modulated field driving a two-level resonance. The purpose of using this model is to give an intuitive interpretation of the rather complex absorption spectra that are produced when a weak probe field couples this strongly driven transition to a third atomic state. We obtain analytic expressions for the positions, strengths, and widths of the components of the absorption spectra, which are in excellent agreement with experimentally obtained spectra presented in the accompanying paper [S. Papademetriou, following paper, Phys. Rev. A 53, 997 (1996)]. © 1996 The American Physical Society.