We have measured ultracold ionizing collision rates for three bosonic ${(}^{132}\mathrm{Xe},{}^{134}\mathrm{Xe},$ and ${}^{136}\mathrm{Xe})$ and two fermionic ${(}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe})$ isotopes of xenon in the $6s[3/2{]}_2$ metastable state, for both spin-polarized and unpolarized samples. For unpolarized samples at temperatures above the p-wave centrifugal barrier $\left(\sim 39\hspace{0.5em}\mu \mathrm{K}\right),$ we find that collision rates for all isotopes are identical. Quantum-statistical effects forbid s-wave collisions for spin-polarized fermions, giving rise to significant differences between bosonic and fermionic isotopes below the p-wave barrier. We present a technique for measuring collision rates at temperatures below $1\hspace{0.5em}\mu \mathrm{K},$ and find that the ratio of polarized to unpolarized collision rates for fermions decreases by a factor of 2 at low temperatures, while the ratio for bosons increases by $50\%.$ We find no evidence of an overall reduction in the collision rate for spin-polarized samples, as has been observed in metastable helium. These results are explained using a simple theoretical model of transmission and quantum reflection off long-range interatomic potentials.

A complete description of the multipolar Casimir-Polder effects between two alkali-metal atoms in the ground state is presented. We give both general and particular expressions for the electric-electric, electric-magnetic, magnetic-magnetic, diamagnetic-electric, and diamagnetic-magnetic two-photon multipole processes, many of which have not been discussed previously in literature, to our knowledge. The asymptotic behavior of the Casimir-Polder potential is studied in terms of the $K_7,\hspace{0.5em}{K}_9,$ and $K_{11}$ long-range coefficients. We have established the range of validity of the Casimir-Polder potential by analyzing the first-order correction to the energy given by the matrix element of the contact interaction term between the atomic charge distributions. Numerical values for the Casimir-Polder long-range interaction potentials for both homonuclear and heteronuclear alkali-metal dimers are presented. Comparisons with previous analytical and numerical results are included.

The absolute cross sections of all the possible dissociation channels following the electron-impact multiple ionization of CO have been measured. The cross sections of the channels corresponding to the production of ion pairs are measured by coincidence techniques. Those of the channels associated with neutral fragments are obtained indirectly. At high excitation energies, the total cross sections of different stages of ionization show an exponential decrease as the ionization stage increases. The cross sections of different channels reveal the internal dissociation dynamics of the dissociation following the multiple ionization of the molecules. The appearance potentials corresponding to different dissociation channels are also measured, which show that the ground states of triply and quadruply ionized CO are nearly Coulomb-like because the Coulomb interactions inside the molecules are stronger.

It is still a question of interest why the distorted-wave Born approximation theory predicts well the structure of the angular distributions particularly at low energies, in good agreement with the coupled-channel wave theory. The phases of the coupled-channel waves are found to be crucially important in determining the structures of the angular distributions. The phases of coupled-channel wave functions are shown to be close to the phase of the entrance channel in the classically forbidden region at low energies, thus allowing agreement between the two theories.

The completeness and convergence of the pseudostate representation of continuum states are studied for ionization of atomic hydrogen by proton impact within the framework of the two-center atomic-orbital close-coupling method. Three types of two-center expansions are compared: Pseudocontinuum states are used (a) both on the target and on the projectile, (b) only on the target, and (c) only on the projectile. Satisfactorily consistent ionization cross sections are obtained for the three expansions at intermediate energies, though the latter two show much slower convergence. This implies that pseudocontinuum states on a center can complement the states on another center to some extent even though they are localized. Expansion (c) fails to converge at high energies within a realistic CPU time, while both treatments (b) and (c) reveal a deficiency arising from the lack of symmetry at low energies below a few keV. The converged theoretical ionization cross sections are larger than the available experimental data by 20% at a collision energy of 50 keV.

We studied the ${\mathrm{H}}^{-}$ production from the dissociation of fast ${\mathrm{HeH}}^{+}$ projectiles, which occurs only through the ${\mathrm{He}}^{2+}+{\mathrm{H}}^{-}$ dissociative channel as electron capture is negligible at large velocities. We measured the lateral profiles of the ${\mathrm{H}}^{-}$ ions produced in collisions of ${\mathrm{HeH}}^{+}$ with $\mathrm{He}$ and Ar atoms at velocities of 2.8, 4.0, and 4.9 a.u. and extracted from them an ${\mathrm{H}}^{-}$ kinetic energy distribution in the projectile center-of-mass frame. These results, together with a published internuclear distance distribution measured with a similar ion source, allowed us to obtain the potential energy curve associated to this dissociative channel. This energy curve crosses several excited states of the ${\mathrm{HeH}}^{2+}$ and ${\mathrm{HeH}}^{+}$ ion dissociating into ${\mathrm{He}}^{2+}$ plus H and ${\mathrm{He}}^{+}$ plus H, respectively, thereby suggesting the existence of an inhibition in the ${\mathrm{H}}^{-}$ formation process through transition to these ${\mathrm{HeH}}^{2+}$ and the ${\mathrm{HeH}}^{+}$ states. The presence of this inhibition was corroborated by measuring the ${\mathrm{H}}^{-}$ total cross section for $\mathrm{He},$ Ne, and Ar targets and $v$=4 a.u.

Electron-capture cross sections in collisions of ${\mathrm{N}}^{4+}$ ions with atomic hydrogen is studied using the close-coupling two-center atomic orbital expansion method by treating the collision system in a quasi-two-electron model. Total electron-capture cross sections to the dominant individual singlet and triplet excited states are evaluated over the energy range of 50–20 000 eV/amu. The results are compared to existing experimental data and to theoretical calculations based on the molecular orbital expansion method. It is found that while the general overall agreement is satisfactory, the large experimental uncertainty cannot be used to discriminate the different theories. For the present energy range our results are closer to the semiclassical close coupling calculations based on the molecular orbital expansion method of Shimakura et al. [Phys. Rev. A 45, 267 (1993)] than the quantum calculations of Zygelman et al. [Phys. Rev. A 56, 457 (1997)].

The interaction between atoms behaves as $-\alpha {/r}^n$ at large distances and, owing to the large reduced mass μ of the collision pair, allows a semiclassical treatment within the potential well. As a result, the low-energy scattering is governed by two large parameters: the asymptotic parameter $\gamma =\sqrt{2\mu \alpha }/ħ\gg a_0^{\left(n-2\right)/2}$ ($a_0$ is the Bohr radius) and the semiclassical zero-energy phase $\Phi \gg 1.$ In our previous work [Phys. Rev. A 48, 546 (1993)] we obtained an analytical expression for the scattering length $a,$ which showed that it has 75% preference for positive values for $n=6,$ characteristic of collisions between ground-state neutral atoms. In this paper we calculate the effective range and show that it is a function of $a,$ $r_e{=F}_n-G_n{/a+H}_n{/a}^2,$ where $F_n,$ $G_n,$ and $H_n$ depend only on γ. Thus, we know the $s$-phase shift at low momenta $k\ll {\gamma }^{-2/\left(n-2\right)}$ from the expansion $k\cot {\delta }_0\simeq -1/a+\frac{1}{2}{r}_e{k}^2.$ At $k\gg {\gamma }^{-2/\left(n-2\right)}$ the phase shift is obtained semiclassically as ${\delta }_0=\Phi +\pi /4\mathrm{}-I_n{\gamma }^{2/n}{k}^{\left(n-2\right)/n},$ where $I_n=[n/(n-2\left)\right]\Gamma \left(\right(n-1)/n)\Gamma \left(\mathrm{}\right(n+2\mathrm{})/2n)/\sqrt{\pi }.$ Therefore, γ and Φ determine the $s$-wave atomic scattering in a wide range of momenta, as well as the positions of upper bound states of the diatomic molecule.

Total cross sections for electron ${(e}^{-})$ and positron ${(e}^{+})$ scattering from ${\mathrm{C}}_3{\mathrm{H}}_8$ and ${\mathrm{C}}_3{\mathrm{F}}_8$ have been measured from 0.8 to 600 eV and 0.7 to 600 eV, respectively. We have also investigated differential elastic cross sections by electron impact from 2.0 to 200 eV, and compared them with the present theoretical results. For $e^{-}$ scattering from ${\mathrm{C}}_3{\mathrm{H}}_8,$ the cross sections are found to be larger by a factor of 2 than those of $e^{+}$ scattering below 20–30 eV. They show a large peak at 8 eV due to a shape resonance and a shoulderlike structure in the region of 20–40 eV. For $e^{-}$ scattering from ${\mathrm{C}}_3{\mathrm{F}}_8,$ the cross sections are again larger by at least a factor of 2 than those of $e^{+}$ scattering below 50 eV, and they have two peaks at 4 and 8 eV, followed by a broad peak in the region of 20–40 eV. Some small structures overlie the broad hump. Both $e^{-}$ and $e^{+}$ impact cross sections for ${\mathrm{C}}_3{\mathrm{H}}_8$ and ${\mathrm{C}}_3{\mathrm{F}}_8$ quickly approach each other beyond 200 eV. From the differential cross section study, we have been able to provide more detailed information on shape resonances, and also we have carried out some analysis of resonances in vibrational excitation results. In general, the total and integrated elastic cross sections are in good qualitative and quantitative agreement.

The recently developed R-matrix method with pseudostates (in a 19-state close-coupling approximation) has been applied in the calculation of integral cross sections for electron-impact excitation of neutral Mg. A configuration interaction representation with frozen core is used for the outer electrons. Results are presented for transitions from both the ${(3s}^2{)}^{1}S$ ground state and the ${\mathrm{}\left(3s3p\right)\mathrm{}}^{3}P$ metastable state to the 13 lowest-excited target states (with principal quantum numbers $n=3,4,5),$ as well as elastic cross sections. Incident electron energies range from threshold to 30 eV. Interpretations are given, with emphasis on comparison of absolute values of the theoretical and measured cross sections and their energy dependence in the near-threshold region.

The deexcitation of a $1s$ core hole in argon via parallel Auger cascade branches leads to the emission of several superimposed $L_{23}-MM$ vacancy satellite spectra. These spectra arise from the presence of various configurations of additional “spectator” holes left by preceding Auger transitions. Such spectator vacancies (henceforth enclosed by brackets) cause characteristic energy shifts and a more complicated multiplet splitting. The most intense lines of the superimposed spectra have been successfully assigned previously. Here we present an extensive refinement and augmentation of that work. The experimental analysis obtained for the two particular $L_{23}-MM$ satellite spectra arising from the principal ${K-L}_{23}{L}_{23}$ deexcitation branch is compared to multiconfiguration Dirac-Fock calculations of energies and intensities. Excellent agreement is found, enabling an almost complete assignment of the recognizable lines. Furthermore, by comparison with the electron impact excited $L_{23}-MM$ spectrum a number of other lines are attributed to the normal $L_{23}-MM$ and the $L_{23}\left[M\right]-MM\left[M\right]$ satellite spectrum, which here originate, respectively, from concomitant single primary $L_{23}$ and $L_1$ ionization. We believe this to be the first comprehensive analysis of a compound Auger cascade spectrum.

Nonrelativistic time-independent quantum scattering theory for collisions with rearrangements is investigated for standing-wave solutions, principal-value Green’s operators, and related reaction operators. The solutions corresponding to different arrangement configurations are obtained directly from the properties of the noninvertible Green’s operators, without having recourse to arguments based on unitarity. By casting multichannel quantum defect theory (MQDT) as a particular case of standing-wave scattering, a formalism for multiarrangement MQDT is presented and methods for calculating the wave functions and reaction matrices are proposed. The formalism is illustrated in connection with current work on ionization and dissociation in ${\mathrm{H}}_2.$

Based on closed orbit theory, we derive a simple analytic formula for the photodetachment cross section of ${\mathrm{H}}^{-}$ in a gradient electric field. It is found that the detachment spectrum displays a sawtooth structure, in contrast to the smooth oscillation for the same but in a homogeneous electric field.

The scattering of orthopositronium (Ps) by hydrogen atoms has been investigated in a five-state coupled-channel model allowing for $\mathrm{Ps}\mathrm{}\left(1s\right)\mathrm{}\mathrm{H}\mathrm{}(2s,2p)\mathrm{}$ and $\mathrm{Ps}\mathrm{}(2s,2p)\mathrm{}\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ excitations using a recently proposed electron-exchange model potential. The higher $(n>~3)$ excitations and ionization of the Ps atom are calculated using the first Born approximation. Calculations are reported of scattering lengths, phase shifts, elastic, Ps and H excitation, and total cross sections. Remarkable correlations are observed between the S-wave Ps-H binding energy and the singlet scattering length, effective range, and resonance energy obtained in various model calculations. These correlations suggest that if a Ps-H dynamical model yields the correct result for one of these four observables, it is expected to lead to the correct result for the other three. The present model, which is constructed so as to reproduce the Ps-H resonance at 4.01 eV, automatically yields a Ps-H bound state at $-1.05$ eV that compares well with the accurate value of $-1.067$ eV. The model leads to a singlet scattering length of ${3.72a}_0$ and effective range of ${1.67a}_0,$ whereas the correlations suggest the precise values of ${3.50a}_0$ and ${1.65a}_0$ for these observables, respectively.

By studying the branching ratios in the Auger decay of many triply excited three-electron atomic systems, we try to discover the physical reason behind the relative magnitude of the partial widths. It appears that Auger decay is most likely to happen when the two interacting electrons are in close proximity and the overlap of their charge distribution is significant in coordinate space. It is also pointed out that when an ${\mathrm{nl}}_1{\mathrm{nl}}_2{n}^{\prime }{l}_3{(n}^{\prime }>n>~2)$ state decays into an $n^{″}{l}_1{n}^{\prime }{l}_3(n>\mathrm{}{n}^{″})+\epsilon l_2$ channel, the $n^{\prime }{l}_3$ orbital of the initial state is more diffused than that of the two-electron target. Therefore, some $n^{″}{l}_1{\mathrm{ml}}_3(m>\mathrm{}{n}^{\prime })+\epsilon l_2$ channels will also contribute significantly to the Auger width of this state. The argument presented in this work is also applied to two- and four-electron systems. Explicit examples are presented.

We have measured radiative lifetimes and collisional deactivation cross sections of the $5d6p$ excited states of Ba atoms in He buffer gas. Barium atoms are produced by laser ablation of a solid Ba sample, whereby populating the $6s5d{}^3{D}_J$ metastable states about $0.1–1\%$ of the total population. The $5d6p{}^{1,3}{P}^o,{\hspace{0.5em}}^{1,3}{D}^o,$ and ${}^{1,3}F°$ states, most of which are not accessible from the ground state by single-photon transition, are excited from the metastable states by a laser, and radiative lifetimes as well as collisional deactivation cross sections are determined from the decay rates of the laser-induced fluorescence. Radiative lifetimes of the $5d6p{}^3{P}_J^o,{\hspace{0.5em}}^3{D}_J^o,$ and ${}^3{F}_J^o$ states do not depend on J, indicating the validity of LS coupling scheme. Collisional deactivation cross sections by He are in the range of $5–20\hspace{0.5em}{\mathrm{\AA }}^2$ for the $5d6p{}^3{D}_J^o,{\hspace{0.5em}}^1{D}_2^o,$ and ${}^3{F}_{3,4}^o$ states, whereas those for the $5d6p{}^3{P}_J^o,{\hspace{0.5em}}^1{P}_1^o,{\hspace{0.5em}}^3{F}_2^o,$ and ${}^1{F}_3^o$ states are less than $1.5\hspace{0.5em}{\mathrm{\AA }}^2$ in the He pressure range of 60–400 Pa. Among $5d6p$ manifolds, the ${5d}^2{}^3{P}_2$ state is populated by the collisional deactivation from the laser-excited $5d6p{}^3{D}_1^o$ state, leading to the sensitized fluorescence from ${5d}^2{}^3{P}_2.$ Radiative lifetime and collisional deactivation cross section of the ${5d}^2{}^3{P}_2$ state are also determined from the decay rates of the sensitized fluorescence.

We report on some unusual magnetic-resonance phenomena of optically pumped Rb vapor in high-pressure gas cells. When Rb-Rb spin exchange is the fastest spin-relaxation rate, we observe a considerable narrowing of the magnetic-resonance linewidths with increasing pump-laser power. The experimentally measured Rb magnetic-resonance linewidths are in excellent agreement with a theoretical model, which includes the processes of Rb-He and Rb-Xe spin destruction, Rb-Rb spin exchange, and Rb optical pumping.

A straightforward numerical technique, based on the Gross-Pitaevskii equation, is used to generate a self-consistent description of thermally excited states of a dilute boson gas. The process of evaporative cooling is then modeled by following the time evolution of the system using the same equation. It is shown that the subsequent rethermalization of the thermally excited state produces a cooler coherent condensate. Other results presented show that trapping vortex states with the ground state may be possible in a two-dimensional experimental environment.

The parametric oscillation of a single trapped electron is studied and used to measure enhanced spontaneous emission. Hysteresis in this motion provides a 1-bit memory to store information about excitations made with the electron “in the dark.” The time dependence and stability criteria for the parametric excitation are examined. The cyclotron motions for one and two electrons are also studied.

We consider potential scattering of electrons in a powerful laser field. The laser-dressed electron is described by the solution of the Klein-Gordon equation in a powerful plane-wave radiation field. The scattering process is treated in the first Born approximation. It is shown that the differential cross sections of induced and inverse bremsstrahlung are asymmetric with respect to the direction of propagation of the radiation field. This asymmetry has its origin in the laser-induced drift motion of the electrons and should, by present day experimental standards, become an observable effect. Numerical calculations also show the existence of broad dark angular windows, i.e., scattering angles for which the nonlinear cross sections are marginal. The positions of such windows depend on the laser field intensity as well as on the nonlinearity of the laser-induced scattering process.