Electron-impact detachment of ${\mathrm{H}}^{\mathrm{-}}$ was studied with the TARN II storage ring (at the Institute for Nuclear Study, University of Tokyo) and the associated high-resolution electron beam. The relative cross sections were measured for the relative energies between electron and ion from 0 to 60 eV in search of the ${\mathrm{H}}^{2\mathrm{-}}$ resonances that were reported earlier. No evidence was found for the existence of these resonance states. © 1996 The American Physical Society.

Full quantum calculations are carried out for the collisional processes involving H atoms in their ground electronic state and electronically excited lithium atoms, Li(1${\mathit{s}}^2$2p). The channels that are being considered are those leading to the formation of bound rotovibrational states of LiH(X${\mathrm{}}^1$${\mathrm{\Sigma }}^{+}$). The effects of both the specific features of the involved electronic potential-energy curves and of the transition moments coupling bound and continuum states during the process are studied in relation to the low-energy behavior of the corresponding cross sections. The role of the multitude of open channel resonances on the final rate constants is also analyzed and discussed over a broad range of bath temperatures. © 1996 The American Physical Society.

In this paper we obtain the closed classical orbits of the detached electron from ${\mathrm{H}}^{\mathrm{-}}$ in electric and magnetic fields with any orientation that lies in three-dimensional space. Using closed-orbit theory, we calculated the photodetachment cross section of ${\mathrm{H}}^{\mathrm{-}}$, which shows oscillations. These oscillations are correlated with three-dimensional closed orbits. Finally, we discuss the similarities and differences between the case of α=π/2 and α≠π/2 (α is the angle between the electric and magnetic fields). © 1996 The American Physical Society.

The lowest-order rescattering contribution (triangle amplitude) in three-body models of exchange reactions with charged particles contains the off-shell two-body T matrix describing the intermediate-state Coulomb scattering of charged subsystems. General properties of the exact exchange triangle amplitude, when the incoming and outgoing particles are on the energy shell, are derived. This includes the analytic behavior, i.e., the positions and characters of its leading singularities, in the cosϑ plane, where ϑ is the scattering angle, in the vicinity of the forward- and backward-scattering directions. Since for computational reasons the Coulomb T matrix is usually replaced by the Coulomb potential, the effects of such an approximation on the analytic properties are investigated. The theoretically established behavior of the exact and the approximate exchange triangle amplitudes is then illustrated by numerical calculations, for both atomic and nuclear reactions, for energies below and above the corresponding three-body dissociation thresholds, for elastic and inelastic exchange. © 1996 The American Physical Society.

Energy-analyzed secondary electrons from ${\mathrm{H}}^{+}$+Ar collisions at eight proton energies from 20 to 120 keV were detected in coincidence with recoil ions of charge states 1–3. The charge states were distinguished by time-of-flight measurements and the electron energies were analyzed by a newly designed electrostatic analyzer of the low-pass high-pass filter type which has a large (∼1.45π steradian) solid angle acceptance. For each of nine electron energies from 10 to 160 eV, the ionizing events associated with each residual ion charge state were counted. The electron energy dependence of the charge-state ratios and the average charge determined from the experimental data were in satisfactory agreement with calculations made with a simple statistical model based on the assumption of independent interactions between the projectile and the ejected electrons. In this model the single-electron ionization probability is assumed to be given by P(b)=P(0)exp(-b/${\mathit{r}}_0$) where ${\mathit{r}}_0$ is a characteristic length and b the impact parameter. Values of ${\mathit{r}}_0$ amd P(0) were determined by fitting the experimental data with this model. The charge-state ratios averaged over ejected electron energies agree with the data of DuBois et al. [Phys. Rev. A 29, 70 (1984)]. © 1996 The American Physical Society.

It is shown in the present study that configuration interaction between the 2${\mathit{p}}^5$3l3${\mathit{l}}^{\prime }$${\mathit{nl}}^{\mathrm{″}}$ resonances formed by electrons incident on Na-like ions leads to a reduction in the calculated dielectronic recombination cross section by as much as a factor of 2. This explains the previous discrepancies between independent theoretical calculations and experimental measurements of dielectronic recombination [Linkemann et al., Nucl. Instrum. Methods Phys. Res. Sect. B 98, 154 (1995)] and resonant transfer plus excitation [Bernstein et al., Phys. Rev. A 40, 4085 (1989)]. New calculations are performed for ${\mathrm{Fe}}^{15+}$, ${\mathrm{Se}}^{23+}$, and ${\mathrm{Nb}}^{30+}$ using the independent-processes, isolated-resonance, distorted-wave method, and comparison with experiment is made where available. © 1996 The American Physical Society.

We have experimentally obtained the state multipoles up to rank two for J-resolved ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{7/2,5/2}$ states of ${\mathrm{Ar}}^{+}$ for the two-electron process ${\mathrm{He}}^{+}$+Ar→He(1${\mathit{s}}^2$)+${\mathrm{Ar}}^{+}$(3${\mathit{p}}^4$${[}^1$D]4p${\mathrm{}}^2$${\mathit{F}}_{7/2,5/2}^{\mathit{o}}$) by measuring the Stokes parameters of the fluorescence light from (${\mathrm{Ar}}^{+}$)* in coincidence with the scattered He. By expanding the state multipoles of each J state in terms of total orbital and total spin multipole moments, we have determined the contributions to the experimentally determined state multipoles of each J state due to total orbital and total spin angular momenta. When the process is spin dependent, it is possible to extract magnetic octupole moments of the excited state due to the orbital charge circulation. © 1996 The American Physical Society.

We present semiempirical scaling laws for different electronic rearrangement features in slow collisions with ions of high charge q. The absolute cross section for removing exactly r electrons from the target is found to be well described by the scaling (sc) relation ${}_{}{}^{\mathit{sc}}{}_{}{}^{}\mathrm{\sigma }_{\mathit{q}}^{\mathit{r}}{}_{}{}^{}$=(2.7×${10}^{\mathrm{-}13}$)qr/[${\mathit{I}}_1^2$${\mathit{I}}^2$ dr${\sum }_{\mathit{j}=1\mathrm{}}^{\mathit{j}=\mathit{N}}$(j/${\mathit{I}}_{\mathit{j}}^2$)] ${\mathrm{cm}}^2$, where ${\mathit{I}}_{\mathit{j}}$ is the jth target ionization potential in eV, and N is the number of outer-shell electrons. This expression and the related total recoil-ion charge-state fractions, ${}_{}{}^{\mathit{sc}}{}_{}{}^{}$${\mathit{f}}_{\mathit{q}}^{\mathit{r}}$${=}^{\mathit{sc}}$${\mathrm{\sigma }}_{\mathit{q}}^{\mathit{r}}$/${\sum }_{\mathit{r}}^{\mathit{sc}}$σ ${}_{\mathit{q}}{}^{\mathit{r}}{}_{}{}^{}$, compare favorably with recent experimental results ${(}^{\mathit{ex}}$${\mathrm{\sigma }}_{\mathit{q},\mathit{q}\mathrm{-}\mathit{p}}^{\mathit{r}}$) for slow ${\mathrm{Xe}}^{\mathit{q}+}$ ions (15⩽q⩽43) colliding with He, Ar, and Xe. We discuss the possibility of establishing scaling laws for phenomenological cross sections ${\mathrm{\sigma }}_{\mathit{q},\mathit{q}\mathrm{-}\mathit{p}}$, where only the number of electrons retained by the projectile p is specified. © 1996 The American Physical Society.

We present some experiments which demonstrate definitively that the filling of the L shell of argon hollow atoms formed below a surface proceeds, like those formed above a surface, through a cascade of eight spontaneous transitions. This result means that the hollow atoms’ clock property may be used in both cases to study continuously the approach and penetration of an ion onto a surface as it is experimentally shown. © 1996 The American Physical Society.

We present a description for potential-energy-induced sputtering of protons by slow, highly charged ions in front of insulator surfaces, based on the classical over-barrier model. Proton emission results from the localized Coulomb explosion after breaking the covalent C-H bond in hydrocarbons at the surface. The model describes the very steep dependence on the projectile charge (∼${\mathit{Q}}^4$–${\mathit{Q}}^6$) for moderately high charge states (Q≲10), in agreement with the experiment. It furthermore predicts the saturation of the Q dependence for even higher charge states. © 1996 The American Physical Society.

The energy loss of large molecular-hydrogen clusters incident on aluminum and amorphous carbon targets is analyzed as a function of the cluster size and velocity, using a dielectric formalism to describe the electronic interactions between the projectile and the target. The dependence of the energy loss with the cluster size predicts a marked different behavior for aluminum or amorphous carbon. The energy loss of ${\mathrm{H}}_2$ clusters in aluminum indicates that for each cluster velocity there is a ‘‘resonant’’ cluster size for which the intermolecular contribution to the energy loss is maximum. On the other hand, the intermolecular contribution to the energy loss of ${\mathrm{H}}_2$ clusters in amorphous carbon saturates with the cluster size. The origin of this behavior lies in the low-energy plasmon present in the energy loss function of amorphous carbon. We also discuss the consequences of a proper description of the dielectric properties of the target. © 1996 The American Physical Society.

We investigated the transport of heavy-ion-induced electrons in solids by both experiment and numerical simulation. We measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d≊3 μg/${\mathrm{cm}}^2$–50 mg/${\mathrm{cm}}^2$) bombarded with swift, highly charged ${\mathrm{Cu}}^{\mathit{q}+}$ (q=25–28 and ${\mathit{E}}_{\mathit{P}}$=9.6 MeV/u) and ${\mathrm{Ni}}^{\mathit{q}+}$ (q=26, 28 and ${\mathit{E}}_{\mathit{P}}$=74 MeV/u) ions. We obtained the transport lengths of high-energy (E≳100 eV) electrons and diffusion lengths of slow electrons (E≲100 eV) and deduced a mean energy of the ejected electrons (≊1 keV at 10 MeV/u and ≊8 keV at 74 MeV/u). The high-energy electrons represent a fraction of 15–20 % of the total electron yields at 9.6 MeV/u, but up to 35% at 74 MeV/u. We show that backscattering of fast, forward-emitted electrons towards the beam entrance surface cannot be neglected in fast-ion-induced electron emission. The experimental results are used as a benchmark for the improvement of our numerical simulation of the primary stage of the ion-matter interaction. © 1996 The American Physical Society.

We report the measurements of electron-impact vibrational excitation of amorphous methanol condensed on a polycrystalline Pt substrate over incident energies of 1–20 eV at a large off-specular angle. The spectra obtained show that vibrations within solid methanol in its ground state are rather sensitive to the variation of impact energy and there is tangible evidence of the formation of at least three compound temporary-negative-ion (TNI) states. The low-energy feature lying between 3 and 6 eV is assigned tentatively to a ${}_{}{}^2{}_{}{}^{}$A′ valence-shape resonance with a ${}_{}{}^2{}_{}{}^{}\mathit{N}$,(8a′σ*${)}^1$] configuration, whose parent state is the ground state ${}_{}{}^1{}_{}{}^{}$A′ of the molecule. The origin of the second broad structure centered around 7.0 eV in the excitation functions may be threefold: a Feshbach-type resonance ${}_{}{}^2{}_{}{}^{}$A″ of the electronic configuration ${}_{}{}^2{}_{}{}^{}[$(2a″${)}^1$,(3sa′${)}^2$] (about 7 eV); a nearby shape resonance ${}_{}{}^2{}_{}{}^{}$A′ of ${}_{}{}^2{}_{}{}^{}[$(N,(9a′σ*${)}^1$] (about 8 eV); and probably also a minor contribution from another ${}_{}{}^2{}_{}{}^{}[$(7a′${)}^1$,(3sa′${)}^2$] Feshbach resonance ${}_{}{}^2{}_{}{}^{}$A′ (about 9 eV). The third broad bandlike resonant feature, extending from 11 up to 17 eV, may consist of overlapping TNI states, dominated by the ${}_{}{}^2{}_{}{}^{}[$(6a′${)}^1$,(3sa′${)}^2$] Feshbach resonance ${}_{}{}^2{}_{}{}^{}$A′. Plausible decay channels from their configurations are discussed. © 1996 The American Physical Society.

Electron emission distributions, triply differential in emission energy and angles, are presented for collisions of singly through quadruply charged C ions impinging on a Si(100) surface, with ion energies between 3.6 and 6.0 MeV and grazing angles of incidence between 0.1° and 1.0°, respectively. The electron emission spectra measured under these conditions confirm previous reports of a shift of the convoy electron peak toward higher energy due to surface wake or image acceleration. Detailed angular distributions demonstrate a shift of convoy emission towards larger angles with respect to the surface plane, which is further evidence of the acceleration of emitted electrons by the surface wake potential. In addition, we observe the peak energy shift to depend upon the incident ion charge state for the higher projectile velocity (∼4.5 a.u.). This dependence may be understood as a consequence of incomplete projectile charge equilibration at the time of convoy electron production. The charge-state dependence is not observed at the lower collision velocity (∼3.5 a.u.). © 1996 The American Physical Society.

We investigate the response of a Bose-Einstein condensate of trapped, neutral atoms to weak and strong sinusoidal perturbation of the trapping potential both by solving the Bogoliubov equations and by direct integration of the time-dependent, driven Ginzburg-Pitaevskii-Gross equation. We find that the distortion of the condensate is maximal when the frequency of the perturbation equals one of the mode positions of the condensate’s excitation spectrum. On resonance, the condensate exhibits a strong nonlinear response that can be used as a clear signature of the mode frequency in an experiment where the trap potential is weakly perturbed. For strong driving, we find evidence for an array of nonlinear effects such as harmonic generation and frequency mixing. These phenomena are the matter-wave analogs of conventional nonlinear optics and should be straightforward to study in evaporatively cooled samples of alkali-metal atoms. © 1996 The American Physical Society.

We present an analysis of Bose-Einstein condensation for a system of noninteracting spin-0 particles in a harmonic-oscillator confining potential trap. We discuss why a confined system of particles differs both qualitatively and quantitatively from an identical system which is not confined. One crucial difference is that a confined system is not characterized by a critical temperature in the same way as an unconfined system such as the free boson gas. We present the results of both a numerical and analytic analysis of the problem of Bose-Einstein condensation in a general anisotropic harmonic-oscillator confining potential. © 1996 The American Physical Society.

We quantize the recent hydrodynamic analysis of Stringari for the low-energy collective modes of a trapped Bose gas at T=0. This is based on the time-dependent Gross-Pitaevskii equation, but omits the kinetic energy of the density fluctuations. We diagonalize the hydrodynamic Hamiltonian in terms of the normal modes associated with the amplitude and phase of the inhomogeneous Bose order parameter. These normal modes provide a convenient basis for calculating observable quantities. As applications, we calculate the depletion of the condensate at T=0 as well as the inelastic light-scattering cross section S(q,ω) from low-energy condensate fluctuations. The latter involves a sum over all normal modes, with a weight proportional to the square of the q Fourier component of the density fluctuation associated with a given mode. Finally, we show how the Thomas-Fermi hydrodynamic description can be derived starting from the coupled Bogoliubov equations. © 1996 The American Physical Society.

Through a suitable expansion of the Gross-Pitaevskii equation near the classical turning point, we obtain an explicit solution for the order parameter at the boundary of a trapped Bose gas interacting with repulsive forces. The kinetic energy of the system, in terms of the classical radius R and of the harmonic oscillator length ${\mathit{a}}_{\mathrm{HO}}$, follows the law ${\mathit{E}}_{\mathrm{kin}}$/N∝${\mathit{R}}^{\mathrm{-}2}$[ln(R/${\mathit{a}}_{\mathrm{HO}}$)+ const], approaching, for large R, the results obtained by solving numerically the Gross-Pitaevskii equation. The occurrence of a Josephson-type current in the presence of a double trap potential is finally discussed. © 1996 The American Physical Society.

We report experimental results on two-color multiphoton ionization and four-wave-mixing production under conditions where one form of laser-induced transparency (LIT) occurs. Under the conditions of LIT, multiphoton-ionization line shapes obtained by tuning one laser through a two-photon resonance exhibit a pronounced Autler-Townes splitting at low concentrations, when a second laser couples the two-photon state to a third level. In this concentration region we observed a four-wave-mixing field that increased quadratically with concentration. As the concentration is increased the Autler-Townes splitting starts to decrease, and disappears completely as a critical concentration is reached. Simultaneously, the four-wave-mixing field intensity starts to level off and finally becomes concentration independent. These observations are explained in terms of odd-photon destructive interference between different excitation pathways. In rubidium, we demonstrate that above concentrations of n≃5×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}3}$ the four-wave-mixing field is concentration independent and the multiphoton-ionization line shape shows no Autler-Townes splitting, and the width of the line shape is determined by the laser bandwidth. The results presented here demonstrate that destructive interference significantly limits the high-efficiency and high-intensity nonlinear optical generation promised in early studies on LIT, at least for the multimode laser system used in the present experiment. © 1996 The American Physical Society.

The response of a diatomic or symmetric-top molecule in a nonlinear optical process through its third-order susceptibility is investigated in terms of the internuclear axis orientation and the plane of molecular rotation. The effect of the spatial arrangement of the molecular system in a Raman-type or two-photon absorption-type intermediate resonance is studied for laser pulses having a time-duration short compared to a simple molecular rotation. We observe strong steric effects on the internuclear axis angular distribution depending drastically on the resonant rotational structure and on the incident polarizations. © 1996 The American Physical Society.