We investigate the electronic absorption induced by a complex potential in one-dimensional quantum mechanics. By solving the Schrödinger equation for spatially localized and extended complex potentials, we derive the electronic absorption coefficient α. Neither a single nor a double Dirac-δ potential can represent a totally absorbing potential (α=1) for any choice of the complex amplitude of the potential. Maximum absorption coefficients for these potential types are 0.5 and 2(√2-1)≃0.828, respectively. On the contrary, wall and well potentials do account for total absorption at a particular limit of the potential parameters. © 1996 The American Physical Society.

Simultaneous excitation of several valence electrons in atoms gives rise to a dense spectrum of compound autoionizing states (AIS). These states are almost chaotic superpositions of large numbers of many-electron basis states built of single-electron orbitals. The mean level spacing D between such states is very small (e.g., D<0.01 eV for the numerical example of ${\mathit{J}}^{\mathrm{\pi }}$=$4^{\mathrm{-}}$ states of Ce just above the ionization threshold). The autoionization widths of these states estimated by perturbations, γ=2π|W${\mathrm{|}}^2$, where W is the Coulomb matrix element coupling the AIS to the continuum, are also small, but comparable with D in magnitude: γ∼D. Hence the nonperturbative interaction of AIS with each other via the continuum is very essential. It suppresses greatly the widths of the autoionizing resonances (Γ≃${\mathit{D}}^2$/3γ≪D), and leads to the emergence of a ‘‘collective’’ doorway state which accumulates a large share of the total width. This state is in essence a modified single-particle continuum decoupled from the resonances due to its large width. Narrow compound AIS should be a common feature of atomic spectra at energies sufficient for excitation of several electrons above the ground-state configuration. The narrow resonances can be observed as peaks in the photoabsorption, or, in electron-ion scattering, as Fano-type profiles on the background provided by the wide doorway-state resonance. It is also shown that the statistics of electromagnetic and autoionization amplitudes involving compound states are close to Gaussian. © 1996 The American Physical Society.

Partial photoionization cross sections ${\mathrm{\sigma }}_{\mathit{n}}$ and photoelectron angular distributions ${\mathrm{\beta }}_{\mathit{n}}$ were measured for all possible final ionic states ${\mathrm{He}}^{+}$(n) in the region of the double excitations N(K,T${)}^{\mathit{A}}$ up to the N=5 threshold. At a photon energy bandpass of 12 meV below the thresholds N=3,4, and 5, this level of differentiation offers the most critical assessment of the dynamics of the two-electron excitations to date. The experimental data are very well described by the most advanced theoretical calculations. Weaker double-excitation series with K=N-4 are clearly visible in the ${\mathrm{\beta }}_{\mathit{n}}$ data, and even previously unobserved extremely weak series members with A=-1 can be discerned, showing the high sensitivity of the angular resolved measurements. The shapes of the resonance-induced variations of ${\mathrm{\sigma }}_{\mathit{n}}$ or ${\mathrm{\beta }}_{\mathit{n}}$ in the double excitations below a given threshold N change radically depending on the final ionic state n but display striking similarities when comparing the satellite states with n=N-1 and n=N-2 below each threshold N. These systematic patterns may indicate a general rule for the underlying two-electron dynamics. © 1996 The American Physical Society.

Differential rotational cross sections as a function of Δj and the scattering angle are calculated using the two-dimensional (2D) close-coupled equations and a comparison with the 2D classical trajectory method is presented. A full potential, i.e., including repulsive and attractive forces, is used and the analysis of rainbow structure is discussed in detail. The rainbow positions have been identified by following the classical trajectory and it has been found that the rainbows can be attributed mainly to the attractive part of the interaction potential. The 2D ellipsoid model [Phys. Rev. A 22, 2617 (1980)] is also applied to analyze the rainbow trajectory and did not predict the same number of rainbows obtained via 2D classical trajectory. In addition, it showed a large deviation for the angular region of the first impact. This model, as expected, was able to explain a larger Δj transition when attractive forces were included. © 1996 The American Physical Society.

The photoionization cross sections for ${\mathrm{Ar}}^{+}$ through ${\mathrm{Ar}}^{4+}$, produced by the Auger decay of a 2p hole in argon, have been measured between 242 eV and 253 eV by the use of synchrotron radiation. The high resolution of the monochromator has allowed a detailed study of the postcollision interactions that occur in this spectral region. The concept of photoelectron recapture by ${\mathrm{Ar}}^{2+}$ to produce the ${\mathrm{Ar}}^{+}$ continuum is studied. The relative values of the quantum-mechanical calculations of the photoelectron recapture probability are shown to be in excellent agreement with the present data. The magnitude and shape of the ${\mathrm{Ar}}^{2+}$ continuum has been explained on the basis that about 67% of the recaptured photoelectrons produce excited states of ${\mathrm{Ar}}^{+}$ which subsequently reemit the electrons by autoionization. © 1996 The American Physical Society.

We apply a recently formulated close-coupling technique [Robicheaux et al., Phys. Rev. A 52, 1319 (1995)], which includes radiation damping via an optical potential, to the calculation of dielectronic recombination cross sections of ${\mathrm{Ar}}^{15+}$. Specifically, we compare the results from both Wigner-Eisenbud and eigenchannel R-matrix calculations to perturbative methods. For this system, no evidence for interfering resonance phenomena is found, and excellent agreement is obtained among all methods. This indicates that the present close-coupling treatment of the dielectronic recombination is highly accurate and can therefore be used to investigate systems where lower-order perturbative methods are inappropriate, such as where interference effects between neighboring resonances and between direct and resonant recombination processes are expected. © 1996 The American Physical Society.

Cross sections have been measured for the production of ${\mathrm{He}}^{+}$ (np)${\mathrm{}}^2$${\mathit{P}}^{\mathit{o}}$ states, n=2,3,4, by proton impact on helium over a projectile velocity range of 1.42–2.45 a.u. (50 ⩽E⩽150 keV). Cross sections were determined by measuring the extreme ultraviolet photons emitted from excited ${\mathrm{He}}^{+}$ ions. The data indicate a lower energy than expected for the maximum cross section. A comparison of the present results in terms of projectile energy dependance with the cross sections for excitation to He (1snp)${\mathrm{}}^1$${\mathit{P}}^{\mathit{o}}$, ionization, and total electron capture suggests the primary mechanism for the production of excited ${\mathrm{He}}^{+}$ at low energies is transfer excitation, with ionization excitation being the dominant mechanism at higher energies. © 1996 The American Physical Society.

The resonant Auger electron spectra of the HCl and DCl molecules have been measured at the first core-to-Rydberg resonances. The low binding energy region of the spectra has been decomposed into transitions to different bound states and their vibrational progressions. Several transitions have been assigned using the strict spectator model. The total ion yield spectrum of HCl in the energy region of the core-to-Rydberg resonances has been recorded and its assignment refined with the help of the Auger electron and chlorine 2p photoelectron spectra. © 1996 The American Physical Society.

We report the measurement of nondipolar asymmetry parameters for the angular distributions of argon 1s, krypton 2s, and krypton 2p photoemission within 2–3 keV above their respective thresholds. Pronounced asymmetries with respect to the direction of photon propagation are present even at low electron kinetic energies. The energy dependence of the asymmetry is Z and subshell specific and causes the direction of preferred electron emission to change sign at least once in each of the cases studied. Towards higher energies the asymmetry increases and the photoelectron emission pattern is forward skewed for all three cases. The measured asymmetry parameters are in good agreement with recent predictions from nonrelativistic calculations which include the interference between electric-dipole and electric-quadrupole transition amplitudes in the photoabsorption process. © 1996 The American Physical Society.

Angle-resolved Auger electron spectra of ${\mathrm{N}}_2$ have been measured with good statistics at photon energies corresponding to the π* resonance and the σ* shape resonance, below and above the N 1s threshold, respectively. Angular anisotropy is observed in both cases, but disappears as expected far above threshold. Satellite Auger transitions also show some angular anisotropy close to the N 1s threshold. This is attributed to the creation and decay of conjugate shakeup initial states, which have non-ground-state symmetry. © 1996 The American Physical Society.

A time-dependent close-coupling method is combined with a time-independent distorted-wave method to calculate the electron ionization cross section for hydrogen. A second-order differencing of the time propagator for the close-coupled equations is found to be very efficient. Low partial-wave close-coupling results are added to high partial-wave distorted-wave results to yield total ionization cross sections in excellent agreement with experiment between 30- and 50-eV incident electron energy. The distorted-wave method found most suitable for the high partial waves is based on a mixture of ${\mathit{V}}^{\mathit{N}}$ and ${\mathit{V}}^{\mathit{N}\mathrm{-}1}$ scattering potentials. © 1996 The American Physical Society.

Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$. Data are presented for the production of ${\mathrm{H}}_2^{\mathrm{}+}$ and ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_2$; the production of ${\mathrm{N}}_2^{\mathrm{}+}$, ${\mathrm{N}}^{+}$+${\mathrm{N}}_2^{\mathrm{}2+}$, and ${\mathrm{N}}^{2+}$ from ${\mathrm{N}}_2$; and the production of ${\mathrm{O}}_2^{\mathrm{}+}$, ${\mathrm{O}}^{+}$+${\mathrm{O}}_2^{\mathrm{}2+}$, and ${\mathrm{O}}^{2+}$ from ${\mathrm{O}}_2$. The product ions are mass analyzed with a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all energetic fragment ions are collected. The overall uncertainty in the absolute cross-section values for singly charged parent ions is ±3.5% and is marginally higher for fragment ions. Previous cross-section measurements are compared to the present results. © 1996 The American Physical Society.

We have measured the double differential cross sections (DDCS) (${\mathit{d}}^2$σ/d${\mathrm{\varepsilon }}_{\mathit{ed}}$${\mathrm{\Omega }}_{\mathit{e}}$) of low-energy electron emission in the ionization of ${\mathrm{H}}_2$ bombarded by bare carbon ions of energy 30 MeV. The energy and angular distributions of the electron DDCS have been obtained for 12 different emission angles and for electron energies varying between 0.1 and 300 eV. We have also deduced the single differential and total ionization cross section from the measured DDCS. The data have been compared with the predictions of first Born approximations and the CDW-EIS (continuum distorted wave–eikonal initial state) model. The CDW-EIS model provides an excellent agreement with the data. © 1996 The American Physical Society.

The single- and double-electron detachment processes have been studied for 85 keV ${\mathrm{H}}^{\mathrm{-}}$ on He collisions measuring the energy spectra of the electrons emitted in forward direction. In the spectrum belonging to the single-electron loss (SEL) the nonresonant part (cusp) has been resolved from the resonant part [lines from the (2s2p${)}^1$${\mathit{P}}^{\mathit{o}}$ shape resonance of ${\mathrm{H}}^{\mathrm{-}}$]. The ratio of the integrated yield of the double-electron loss (DEL) to that of SEL was found to be 0.36±0.02. The yield of the cusp in the SEL spectrum was found to be surprisingly small, only (70±20)% of the yield of the cusp in the DEL spectrum. The formation of the cusp in SEL is interpreted as a result of dipolar interaction between the electron and the outgoing ${\mathrm{H}}^0$ atom. © 1996 The American Physical Society.

We present an experiment for the investigation of the interaction of spin-polarized Na atoms with a dielectric surface. A laser beam, which is resonant with an atomic transition, probes the spin polarization near the surface. It is reflected at the surface and the change of its polarization is measured. Our analysis takes into account that the spin polarization is reduced near the surface because of surface-induced spin relaxation. The theory shows that strong wall relaxation leads to a clear modification of the shape and strength of the optical line. The dependence of the line shape on the angle of incidence immediately below the critical angle of total internal reflection provides a sensitive tool for the quantitative determination of the depolarization probability ɛ per wall collision. Measurements performed at a Pyrex glass surface provide a clear experimental confirmation of the theory and yield a value of ɛ≊0.5. The case of weak wall relaxation is exemplified by experimental results from a silicone-coated glass surface, which are equally well reproduced by the theory. The method offers the prospect of using surface-induced spin relaxation for the study of surface properties and atom-surface interactions for various surfaces. © 1996 The American Physical Society.

We present a theoretical study of the index of refraction of the light noble gases He and Ne for the propagation of sodium matter waves. Our approach is based on the well known multiple scattering theory inside a medium, which leads to an approximation for the index of refraction that depends only on the single forward scattering amplitude and the density distribution functions for the atoms of the medium. The forward scattering amplitude between a sodium atom and a noble gas atom is calculated from molecular potential curves using the eikonal approximation. An explicit phase space average is performed over the noble gas atom distribution in the medium. Our results show very good agreement with the recent interferometry measurements of Schmiedmayer et al. [Phys. Rev. Lett. 74, 1043 (1995)]. Our calculations provide an interpretation of the experimental data, and may shed some light on possibilities for obtaining improved interatomic potentials through atomic interferometry measurements. © 1996 The American Physical Society.

We investigate the prospects of atomic interference using samples of Bose condensed atoms. First we show the ability of two independent Bose condensates to create an interference pattern. This holds even if both condensates are described by Fock states. Thus, the existence of an experimental signature for a broken gauge symmetry, seen in a single run of the experiment, is not necessarily reflected by a broken symmetry on the level of the quantum mechanical state vector. Based on these results, we simulate numerically a recent experiment with two independent Bose condensates [K. B. Davis et al., Phys. Rev. Lett. 75, 3969 (1995)]. The existence of interference fringes is predicted based on the nonlinear Schrödinger equation. Finally we study theoretically the influence of finite temperatures on the visibility of the interference in a double pinhole configuration. © 1996 The American Physical Society.

A theoretical evaluation of inelastic transition amplitudes corresponding to collisions between low-energy electrons (up to 5 eV) and small metallic clusters is presented. The target is excited into a particle-hole state and the cross section is obtained in the Born approximation. The formalism is applied to the ${\mathrm{Na}}_8$ cluster. Form factors for the direct and exchange-correlation terms of the residual interaction are shown as well as angle-integrated cross sections as a function of the energy of the incident electron. These cross sections present resonances associated with quasibound states in the outgoing and/or incoming channels at incident energies related to the ${\mathrm{Na}}_8$ single-particle states through the Q values of the transitions. The results also show the importance of the inclusion of the residual exchange-correlation contribution. © 1996 The American Physical Society.

The interactions of the ${\mathrm{Na}}_2$ dimer with a ${\mathrm{C}}_{60}$ molecule are examined by considering endohedral and exohedral complexes. Five different sites for the adsorbate are investigated: fivefold, threefold, long bond, short bond, and on top. Two orientations of the dimer with respect to the symmetry axes of the fullerene, parallel and perpendicular, were considered. The ground-state electronic configuration for both the endohedral ${\mathrm{C}}_{60}$${\mathrm{Na}}_2$ and the exohedral ${\mathrm{C}}_{60}$${\mathrm{Na}}_2$ complexes is found to be a triplet. The largest adsorption energy was found for the on-top (T) adsorption in the endohedral ${\mathrm{C}}_{60}$${\mathrm{Na}}_2$ complex. All the endohedral complexes were found to be stable against fragmentation into the three important dissociation channels, including the dissociation into a Na atom and a ${\mathrm{C}}_{60}$Na fragment. The relative stabilities of the different complexes were related to their structural parameters. For exohedral ${\mathrm{C}}_{60}$${\mathrm{Na}}_2$, the triplet electronic configuration destabilizes and the singlet state becomes dominant for adsorbate distances larger than 6 Å. The highest occupied molecular orbital–lowest unoccupied molecular orbital gap in both the endohedral and exohedral complexes was found to be approximately half of its value in the bare fullerene. Trends in the electronic energy levels of the two types of complexes are discussed in detail. © 1996 The American Physical Society.

The role played by a buffer gas in the ground-state superposition created in coherent-population-trapping experiments is investigated theoretically. The density matrix equations are solved on the basis of the coupled-noncoupled states. The velocity-changing collisions are examined in the limit of strong collisions. The relaxation rate for transfer of the ground-state superposition between atoms with different velocities is derived from a fit of available experimental results. © 1996 The American Physical Society.