The ${\mathrm{HeH}}^{2+}$ molecular ion decays by an electronic dipole transition from its bound first excited state (2pσ) to the repulsive ground state (1sσ). We have calculated the mean lifetimes of the vibrational states of a few ${\mathrm{HeH}}^{2+}$ isotopes and found a large isotopic effect, in particular for highly excited vibrational states, i.e., states with the same v have different decay rates. The measured decay curves of ${}_{}{}^4{}_{}{}^{}\mathrm{HeH}_{}^{2+}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}\mathrm{HeD}_{}^{2+}{}_{}{}^{}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{HeD}_{}^{2+}{}_{}{}^{}$ ( i.e., the number of ${\mathrm{HeH}}^{2+}$ molecular ions as a function of their flight time from the target cell where they were formed), in contrast, are similar to each other. The lack of a measurable isotopic effect is related to the ${\mathrm{HeH}}^{2+}$ creation mechanism. In the charge-stripping collisions, a distribution of vibrational states is populated by vertical transitions and is thus centered around roughly the same vibrational energy and not around the same quantum number v. The mean lifetimes of the different isotopes as a function of their energy are surprisingly similar to each other, therefore washing out the isotopic effect. © 1996 The American Physical Society.

Azimuthal asymmetry of the differential cross sections between two polarized states has been measured in elastic electron scattering by laser-excited sodium atoms prepared in the 3${\mathrm{}}^2$${\mathit{P}}_{3/2}$,F=3 (${\mathit{M}}_{\mathit{F}}$=+3 or ${\mathit{M}}_{\mathit{F}}$=-3) polarized state. These results are an observation of the orbital effect in the elastic electron collision due to ${\mathit{M}}_{\mathit{L}}$ atomic state perpendicular to the scattering plane. Data are reported for electron energies in the range of 1–10 eV, comparable with the kinetic energy of the atomic valence electron. At polar scattering angle θ=135° we found the asymmetry to be more significant when the incident electron energy is lower. Differential cross sections of these excited, polarized states were obtained at 135° as a function of electron energy as well. The experimental results are compared with R-matrix-based close coupling, and convergent-close-coupling calculations. © 1996 The American Physical Society.

Measurements are presented of the variation in energy of spectator Auger transitions as they evolve to normal Auger transitions, for Rydberg levels based on the ${\mathrm{Ca}}^{+}$3${\mathit{p}}^5$4${\mathit{s}}^{22}$${\mathit{P}}_{1/2,3/2}$ and ${\mathrm{Ca}}^{+}$3${\mathit{p}}^5$3d4${\mathit{s}}^2$${\mathit{P}}_{1/2,3/2}$ ion cores. It is shown that a simple model can explain the variation. From angular distribution measurements information is obtained on the photoelectron outgoing wave symmetries and theoretical predictions of the behavior of the ${\mathrm{\beta }}_{\mathit{A}}$ parameter verified. © 1996 The American Physical Society.

We study the fractal scattering patterns of collinear collisions between an electron and a helium ion or a hydrogen atom. We have found that the collisional time plotted against initial energy or initial phase consists of a bi-infinite sequence of cusp-shaped regular intervals interlaced by chaotic bands and repeated enlargements of the chaotic bands show similar patterns. These patterns resemble the previous ones obtained in the retrograde region of the coplanar scattering system, however, the dynamical origin of the self-similar patterns is different and can be understood in terms of various combinations of motions perpendicular and parallel to the Wannier ridge and binary collisions. In particular, we have found that the trajectories near the cusp tips of regular intervals are strongly influenced by a set of triple-collision orbits, trajectories originate and end at the triple-collision point. Using the code and winding number for these orbits, we can organize the fractal scattering patterns into a tree structure. Furthermore, using an ensemble of trajectories with uniformly selected initial phases, we calculate the transition probabilities of excited electronic states from a certain initial state of the hydrogenlike ion or atom using the quasiclassical trajectory method. These transition probabilities illustrate that chaotic regions on the average correspond to higher electronic excitation than that corresponding to the regular regions. © 1996 The American Physical Society.

Measurement of absolute, total cross sections for single- and double-electron detachment of ${\mathrm{H}}^{\mathrm{-}}$ projectiles in sulfur hexafluoride, methane, and carbon oxide has been made. The relative energy range under investigation was from 1 to 5 keV. The resulting cross sections for the single-electron loss of ${\mathrm{H}}^{\mathrm{-}}$ in ${\mathrm{SF}}_6$ exhibit a flat behavior, whereas for methane and carbon monoxide the single detachment cross section increases with the energy. A free-collision model for electron loss was applied, and very good agreement was found for ${\mathrm{SF}}_6$, while for the other targets there is a departure of the trend of the experimental data. A comparison with the results from other research laboratories is given. The measured double-electron detachment cross sections are roughly one order of magnitude smaller than the single detachment cross sections. © 1996 The American Physical Society.

The linear polarization of resonance vacuum-ultraviolet (VUV) radiation emitted by electron-impact excited Ar, Kr, and Xe atoms has been measured in the energy range from threshold to 480 eV using two sets of apparatus. The first apparatus measured the polarization function in the near-threshold region using an energy selected electron beam with a resolution of 160 meV. This enabled the effects of negative-ion resonances on the polarization function to be examined in detail. Predictions of the effects of negative-ion resonances on the observed polarizations have been made using a generalized Baranger-Gerjuoy theory. Excitation via electron exchange is shown to be a significant process very close to threshold. The second apparatus used an electron beam with ∼1 eV resolution to probe the broad features of the VUV polarization functions over the energy range from threshold to 480 eV. Measurements are compared with other published data. © 1996 The American Physical Society.

The quantum deformation created by the stereographic mapping from ${\mathit{S}}_2$ to C is studied. It is shown that the resulting algebra is locally isomorphic to su(2) and is an unconventional deformation of which the undeformed limit is a contraction onto the harmonic oscillator algebra. The deformation parameter is given naturally by the central invariant of the embedding su(2). The deformed algebra is identified as a member of a larger class of quartic q oscillators. We next study the deformations in the corresponding Jordan-Schwinger representation of two independent deformed oscillators and solve for the deforming transformation. The invertibility of this transformation guarantees an implicit coproduct law which is also discussed. Finally we discuss the analogy between Poincaré’s geometric interpretation of the quantum Stokes parameters of polarization and the stereographic projection as an important physical application of the latter. © 1996 The American Physical Society.

Photodissociation of quasibound levels in the i${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{g}}$ state of molecular hydrogen is studied using fast-beam photofragment spectroscopy. The molecules dissociate either via molecular fluorescence to a repulsive state or by barrier tunneling followed by atomic fluorescence. We measured the branching between the two pathways and the anisotropy of the photofragment distribution. The branching ratios were calculated from the potential-energy curve (see the theoretical treatment given in the following paper [Phys. Rev. A 54, 531 (1996)]). The observed and calculated branching ratios are found to be in good agreement. Small discrepancies can be accounted for by the electronic mixing with neighboring states. The photofragment anisotropy is a diagnostic tool for the dissociation continuum and the lifetime of the excited state. The experimentally found anisotropy parameters corroborate the electronic mixing and also show that the fine and hyperfine structure are resolvable only for the lowest three rotational levels examined. © 1996 The American Physical Society.

A quantum-mechanical treatment describing photofragmentation of a homonuclear molecule is presented. The treatment is developed to describe results on photofragmentation in molecular hydrogen [preceding paper, Phys. Rev. A 54, 522 (1996)] in which photoexcitation takes place to an excited state that can dissociate using different channels, all of which are accompanied by fluorescence. The kinetic energy of the photofragments, which is directly related to the fluorescence spectrum, and their anisotropy were measured. The theoretical treatment incorporates electronic-state mixing due to nonadiabatic effects in the excited state. Fluorescence may occur as molecular fluorescence, that is, before dissociation, or atomic fluorescence, after dissociation has taken place. Both are treated simultaneously and the branching is determined. A remarkable result is the occurrence of interference when molecular fluorescence is degenerate with atomic fluorescence. It is shown that in case of nonadiabatic effects the anisotropy of the photofragments can depend on the decay channel; the actual anisotropy does not represent the anisotropy of the ensemble of excited molecules directly. © 1996 The American Physical Society.

A representation of three particle wave functions well adapted to computations of low-energy fragmentation states of systems interacting electrostatically is derived. A basis called an angle-Sturmian basis, is introduced. Exact wave functions are represented by sums over the angle-Sturmian functions and integrals over the index of Bessel functions. Equations for the coefficients of the Sturmian functions are derived. Solutions of these equations are given in the approximation that one Sturmian is employed. Integral representations of the approximate three-particle wave functions are obtained. Evaluation of the integral for large hyper-radius R gives the hidden-crossing theory, familiar from representations of ion-atom interactions at low energy. It is shown that ionization components emerge simply only for complex values of R. Such components conform to Wannier’s threshold law. © 1996 The American Physical Society.

The two-dimensional R-matrix propagator of Le Dourneuf et al. [J. Phys. B 23, L559 (1990)] is generalized to arbitrary angular momenta and applied to the study of electron-hydrogen collisions at energies up to the n=5 threshold. Results are presented for phase shifts, resonance positions and widths, as well as cross sections for partial waves of total angular momenta L≤3. The stability and efficiency of the method are established. A comparison is made below the n=3 threshold with results from other theories. Agreement is very good, in particular with the results of a finite-difference numerical integration of the Schrödinger equation by Wang and Callaway [Phys. Rev. A 50, 2327 (1994)]. Partial cross sections for ${}_{}{}^1{}_{}{}^{}$S between the n=3 and 5 thresholds illustrate the potential of the method for studying excitation into high Rydberg states. © 1996 The American Physical Society.

According to Dirac’s and Bergmann’s physical ideas, we derive the expression of the finite Dirac contact transformation, propose an extended Dirac conjecture, extend Dirac’s original consistency conditions, and obtain the correct definition of physical observables as well as more universal gauge conditions in general singular Lagrangian systems. The difficulties in Cawley’s first and second counterexamples of Dirac’s conjecture are overcome. Our results are applicable to Hamiltonization of systems with Hessian variable rank and systems with the proper subalgebra of the minimum evolution closed Poisson bracket of first-class constraints, and so provide a correct tool for quantization of these systems. © 1996 The American Physical Society.

A quasiclassical-trajectory Monte Carlo (QTMC-EB) model is proposed to extend the classical-trajectory Monte Carlo (CTMC) method to targets having more than one electron. Quasiclassical stability is achieved via constraining potentials that enforce lower energy bounds on the one-electron dynamics. Cross sections for all possible electronic rearrangements (single and double electron transfer, single and double ionization, and transfer ionization) in ${\mathrm{H}}^{+}$+H, ${\mathrm{H}}^{+}$+He, ${\mathrm{He}}^{2+}$+He, and ${\mathrm{Li}}^{3+}$+He collisions are calculated with this model and with the previously proposed model (QTMC-KW) of Kirschbaum and Wilets [Phys. Rev. A 21, 834 (1980)]. The results are compared with accurate experimental data. The regime of validity for the two-electron targets is found to be similar to that of the usual CTMC model for one-electron targets. © 1996 The American Physical Society.

The electron-impact single ionization of low-charged Mo ions is studied both experimentally and theoretically. The absolute cross sections for ${\mathrm{Mo}}^{\mathit{q}+}$ ions in charge states q=1–8 are measured using a crossed-beam technique. Distorted-wave calculations are made including both direct ionization and excitation-autoionization contributions. For ions in charge states q=6–8, the measured cross sections are in good agreement with calculations involving only the lowest-energy metastable configuration. For ions in charge states q=3–8, the good agreement between experiment and theory confirms the dominant role played by excitation-autoionization contributions to the total ionization cross section. © 1996 The American Physical Society.

We study the behavior of ionization in intermediate-energy collisions of antiprotons with atomic hydrogen by direct solution of the time-dependent Schrödinger equation represented on a three-dimensional Cartesian lattice. Total cross sections for these processes are computed over the collision energy range of 0.2 to 500 keV from knowledge of the asymptotic state probabilities as a function of impact parameter. The computed ionization cross sections are in good agreement with results from recent experiments conducted at CERN [Phys. Rev. Lett. 74, 4627 (1995)]. In the energy range from 0.2 to 30 keV, for which measurements are not available, our calculations are in qualitative agreement with other results based on classical-trajectory and coupled-channel methods, confirming the predicted significant difference from the analog proton-impact ionization process. This contrast with proton-hydrogen collisions is also explored qualitatively by employing a model two-dimensional space in which lattice solutions are less computationally intensive. © 1996 The American Physical Society.

The anisotropy of the resonant Auger decay of the photoexcited Kr 3${\mathit{d}}_{3/2,5/2}^{\mathrm{-}1}$5p states has been studied by comparing the new high-resolution experimental results with the multiconfiguration Dirac-Fock calculations. The measurements were made with very high photon and electron energy resolutions in the Auger resonant Raman mode giving ‘‘subnatural’’ linewidths of ∼ 35 meV, which has enabled us to resolve the fine structures in detail. Comparison with high-quality experimental results allows us to confirm the roles of exchange interaction as well as initial- and final-state interactions in the theoretical description of the angular anisotropy of the resonant Auger transitions. © 1996 The American Physical Society.

We have performed a consistency test of recent data for the ratios of cross sections of double to single ionization of helium by photons and by fast charged particles. It is based on a relationship that connects the dipole and nondipole components of cross sections by charged particle impact to photoionization and Compton scattering, respectively. We find the high energy ratio ${\mathit{R}}_{\mathit{Z}}$ for charged particles to be a sensitive probe of the photoionization data near threshold. Using recent experimental data for photoionization, the asymptotic ratio ${\mathit{R}}_{\mathit{Z}}$ is found to range from 0.240% to 0.295%, as compared with direct experimental measurement of 0.26%. © 1996 The American Physical Society.

The ${\mathit{L}}_{\gamma 1,5}$, ${\mathit{L}}_{\gamma 2,3,\left(6\right)\mathrm{}}$, ${\mathit{L}}_{\gamma 4}$, and Lα x-ray fluorescence (XRF) cross sections for 11 elements with 71≤Z≤83 have been measured at 22.6 keV incident photon energy. The measured values have been interpreted in terms of theoretical ${\mathit{L}}_{\mathit{i}}$ (i=1,2,3) subshell photoelectric cross sections, fluorescence and Coster-Kronig yields, and radiative transition probabilities. A fairly good agreement is found between the measured and theoretical cross sections for the Lα and ${\mathit{L}}_{\gamma 1,5}$ x rays. In the case of the ${\mathit{L}}_{\gamma 2,3\left(6\right)\mathrm{}}$ and ${\mathit{L}}_{\gamma 4}$ XRF cross sections, the measured values are in agreement with the theoretical ones for elements Z=71–74 and are higher by up to 55% for elements with Z≥75. The ${\mathit{L}}_1$ subshell fluorescence yields (${\mathrm{\omega }}_1$), deduced from these cross sections, are found to vary smoothly with atomic number rather than exhibiting a big jump at Z=75 due to the onset of intense ${\mathit{L}}_1$-${\mathit{L}}_3$${\mathit{M}}_5$ Coster-Kronig transition as predicted by the relativistic Dirac-Hartree-Slater model. © 1996 The American Physical Society.

Dielectronic recombination of He-like ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$ ions channeled along the 〈110〉 axis of a thin Si crystal has been studied by the measurement of the charge-state distribution and x-ray production. The results of the charge-state measurements confirm that the probability of resonance capture is proportional to the valence electron density sampled by the ion. The energy-loss distributions of channeled ions are also in good agreement with theoretical estimates. © 1996 The American Physical Society.

The energy loss of an ion moving parallel to a solid surface is studied within the dielectric function formalism. Based on the specular-reflection model, the surface response function can be expressed in terms of the bulk dielectric function, and the effect of the exchange-correlation interaction of electrons in the solid is taken into account by the local-field correction dielectric theory. For low-velocity ions, semianalytical expressions of the energy loss are obtained. In the high-velocity limit, the contributions of the surface plasmon to the energy loss are studied using the hydrodynamic approximation (HA) and the plasmon pole approximation (PLA) to the bulk dielectric function. It is found that at low velocity, the exchange-correlation interaction of electrons enhances the magnitude of the energy loss, but at high velocity, its effect is less important, and both the HA and the PLA provide an adequate description for surface-plasmon description. © 1996 The American Physical Society.