We report experimental rate coefficients for the energy-pooling collisions Cs(6${\mathit{P}}_{1/2}$)+Cs(6${\mathit{P}}_{1/2}$)→Cs(6${\mathit{S}}_{1/2}$)+Cs(${\mathit{nl}}_{\mathit{J}\prime }$) and Cs(6${\mathit{P}}_{3/2}$)+Cs(6${\mathit{P}}_{3/2}$)→Cs(6${\mathit{S}}_{1/2}$)+Cs(${\mathit{nl}}_{\mathit{J}\prime }$) where ${\mathit{nl}}_{\mathit{J}\prime }$=7${\mathit{P}}_{1/2}$, 7${\mathit{P}}_{3/2}$, 6${\mathit{D}}_{3/2}$, 6${\mathit{D}}_{5/2}$, 8${\mathit{S}}_{1/2}$, 4${\mathit{F}}_{5/2}$, or 4${\mathit{F}}_{7/2}$. Atoms were excited to either the 6${\mathit{P}}_{1/2}$ or 6${\mathit{P}}_{3/2}$ state using a single-mode Ti:sapphire laser. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode ring dye laser beam, tuned to either the 6${\mathit{P}}_{1/2}$→8${\mathit{S}}_{1/2}$ or 6${\mathit{P}}_{3/2}$→7${\mathit{D}}_{3/2,5/2}$ transitions, which could be translated parallel to the pump beam. Transmission factors, which describe the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed, were calculated for all relevant transitions. Effective lifetimes of levels populated by energy-pooling collisions are modified by radiation trapping, and these factors were calculated using the Molisch theory. These calculated quantities have been combined with the measured excited-atom densities and fluorescence ratios to yield absolute energy-pooling rate coefficients. It was found that the rate for production, in all cases, is greatest for 6D, but that 1/2-1/2 collisions are significantly more efficient than 3/2-3/2 collisions for populating 7P. It was also found that 7${\mathit{P}}_{1/2}$ is populated two to three times more efficiently than 7${\mathit{P}}_{3/2}$ in 1/2-1/2 collisions, but that the 7P fine-structure levels are approximately equally populated in 3/2-3/2 collisions. © 1996 The American Physical Society.

Recoil-ion charge-state-resolved doubly differential cross sections for ejecting electrons at ∼55° with respect to the incident beam direction in collisions between 1 MeV/u ${\mathrm{C}}^{5+}$ projectiles and Ar and He targets have been measured. Electrons with kinetic energies between 100 and 1250 eV have been detected. A prominent feature in the electron energy distributions is the binary-encounter peak. Experimental results are compared with binary-encounter electron production cross sections obtained using the impulse approximation and with theoretical predictions from many-body classical trajectory Monte Carlo calculations. An enhancement in the fraction of electrons detected with singly charged He recoil ions and a corresponding decrease in the fraction detected in coincidence with doubly charged He recoil ions as a function of the electron energy have been observed near the binary-encounter electron energy. This structure has been predicted by recent many-body classical trajectory Monte Carlo calculations. © 1996 The American Physical Society.

A technique for electron spectroscopy which yields full two-dimensional momentum distributions for continuum electrons has been used to study ejected electrons from single ionization of He by ${\mathrm{C}}^{6+}$ and proton projectiles at low velocities. Projectile velocities of 1.63, 1.38, and 1.16 a.u. for ${\mathrm{C}}^{6+}$ and 2.39, 1.71, 1.15, .85, and 0.63 a.u. for protons were used. All spectra show much broader distributions along the beam than transverse to the beam. For the case of proton bombardment, the spectra are strongly influenced by both target and projectile potentials, maximizing near the velocity of the saddle in the potential between the two receding ion cores for the lowest projectile velocities. For ${\mathrm{C}}^{6+}$ projectiles, the spectra appear to be dominated by the projectile potential and the center of the distribution is strongly shifted toward the projectile velocity. Theoretical results from the continuum-distorted-wave–eikonal-initial-state and classical-trajectory–Monte Carlo methods are in rather good agreement with the proton data but do not agree well with the ${\mathrm{C}}^{6+}$ data. © 1996 The American Physical Society.

We have performed K-shell radiative electron capture (K-REC) measurements with bare 60.1-MeV/u incident krypton ions, both in channeling conditions and for random orientation of a 37-μm silicon crystal. The sampled electron densities are quite different in each case, which has an influence both on the shape and on the amplitude of the K-REC photon peak. We have developed simulations of the K-REC photon lines: for this we have determined the impact parameter distribution at statistical equilibrium for various beam incidence conditions (direction and angular spread) using the continuum potential model for channeled ions. Multiple scattering effects were included. The K-REC photon peak was calculated within the nonrelativistic dipole approximation, K-REC being assumed to be a purely local process. Solid state electron densities were used, and impact parameter dependent electron momentum distributions (Compton profiles) were calculated for 2s and 2p silicon electrons. A remarkable agreement is found between the spectra measured with very high statistics, and the calculated ones, which leads to the following results: (i) The dependence of the K-REC yield on the beam incidence angle is obtained separately for silicon core and valence electrons, which was never observed before. We find that the core electron contribution to REC is still significant for axial alignment, whereas it is generally neglected in the literature. (ii) Electron Compton profiles are found to vary significantly with impact parameter. (iii) The free electron gas model represents a fair approximation for the description of valence electron Compton profiles. (iv) The K-REC cross section is measured with an absolute accuracy better than 20%, and found to be close to the value calculated within the nonrelativistic dipole approximation. © 1996 The American Physical Society.

We develop an approximate integral for the amplitudes of electron transfer in ion-atom collisions. The resulting model pertains to collisions involving low-n excited states and relative ion-atom velocities near the average speed of the target valence electron. Three points are essential to the derivation: (1) the atomic states are treated as a dielectric taking into account the long-range interaction, (2) the process is viewed in momentum space quantifying the idea of velocity matching, and (3) a semianalytic expression is derived using the Born approximation. From the resulting integral propensity rules are derived telling which final states are favored based on the initial target state and parameters of the collision. © 1996 The American Physical Society.

We have investigated the resonant collision process 26s+24d→26p+25p in K, which occurs when the 26s and 24d states are tuned by a static electric field so that the intervals 26s-26p and 24d-25p are equal. Resonant collision line shapes are observed and analyzed for three collision velocity distributions: a thermal beam, a small band of velocities in a laser-cooled beam, and two well-separated velocity groups in a laser-cooled beam. Collisions between atoms with a broad distribution of relative velocities produce cusp line shapes. In contrast, collisions occurring between atoms in two well-defined velocity groups produce approximately Lorentzian line shapes. © 1996 The American Physical Society.

Absolute total cross sections for electron-impact single ionization of ${\mathrm{Ne}}^{\mathit{q}+}$ (q=2,4–6) ions have been measured using a crossed-beams technique from below the ground-state ionization threshold to 800 eV with typical total uncertainties near the peak of the cross sections ranging from 9% for ${\mathrm{Ne}}^{2+}$ to 13% for ${\mathrm{Ne}}^{6+}$. Details of the apparatus and experimental procedures used in this study are presented along with a discussion of the experimental uncertainties. The measured cross sections for all four ions are dominated by direct ionization and are in excellent agreement with the Lotz semiempirical formula. Ionization rate coefficients and fitting parameters calculated from the measured cross sections are also reported. © 1996 The American Physical Society.

Total cross-section calculations are reported for the electron detachment from ${\mathrm{H}}^{\mathrm{-}}$ in collision with multiply charged ions by using a simple two-state model. In the collision energy range where this model is expected to be realistic, results are in fair agreement with both very recent experimental and theoretical results. © 1996 The American Physical Society.

We have calculated the asymmetry parameter and the momentum distribution of the recoiling nucleus when a helium atom undergoes double ionization by one (linearly polarized) photon with energy 85, 99, or 159 eV. An empirical fit, based on representing the dynamical part of the doubly differential cross section through a single-parameter Gaussian model, is surprisingly accurate over this energy range. © 1996 The American Physical Society.

High-resolution translational energy-gain spectra for single-electron capture by ${\mathrm{N}}^{3+}$ ions from ${\mathrm{H}}_2$, He, Ne, and Ar have been measured experimentally at laboratory impact energies of 6, 9, 12, and 15 keV. For ${\mathrm{N}}^{3+}$-He and Ne collisions, transfer excitation into the 2s2${\mathit{p}}^2$ state of ${\mathrm{N}}^{2+}$ is significantly populated, while in ${\mathrm{N}}^{3+}$-${\mathrm{H}}_2$ collisions, transfer excitation into the 2s2${\mathit{p}}^{22}$P state dominates at low energies. In ${\mathrm{N}}^{3+}$-Ar collisions, pure single-electron capture into 3s is selectively populated. In all the collision systems studied here, contributions from processes commencing with a long-lived metastable state of ${\mathrm{N}}^{3+}$(2s${(}^2$S)2${\mathit{p}}^3$P) are detected. The translational energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the single-crossing Landau-Zener model and the extended version of the classical over-the-barrier model. Total cross sections for single-electron capture for ${\mathrm{N}}^{3+}$ ions colliding with He and ${\mathrm{H}}_2$ are also measured and compared with available measurements and theoretical calculations. © 1996 The American Physical Society.

Damped oscillations of the optogalvanic signals are observed in an inductively coupled, electrodeless radiofrequency glow discharge illuminated by a cw diode laser tuned to a resonant frequency of the argon 1s-2p transitions. Spontaneous oscillations are also observed in the same discharge. Moreover, temporal oscillations of the optical fluorescence emission, the free-electron density, and the density of metastable atoms are measured at the same time. From these results, the physical mechanism for the damped and the spontaneous oscillations is suggested to be associated with the stepwise ionizations of the metastable levels due to collisions with slow electrons. An oscillation frequency of about 8 kHz in this electrodeless discharge cell is also explained by the diffusion frequency of the ions and the ionization frequency of the metastable states. © 1996 The American Physical Society.

In theories of photoelectron angular distributions at soft-x-ray energies from 1 to 10 keV, first-order corrections to the dipole approximation give rise to two nondipolar asymmetry parameters in addition to the well-known dipolar asymmetry parameter β. The nondipolar parameters characterize the forward-backward asymmetry with respect to the propagation vector of the photon beam. Experimentally, the measurement of the nondipolar effect has been hampered because of the complications resulting from the three-parameter angular distribution. In this paper, we suggest various experimental approaches to the measurement of the three asymmetry parameters using partially linearly polarized x rays, corresponding to measurements on synchrotron radiation beamlines. We describe methods to extract the asymmetry parameters using such approaches. © 1996 The American Physical Society.

The differential cross section of the dissociative ionization of ${\mathrm{H}}_2^{+}$ by fast (2-keV) electron impact is determined theoretically using a two-center continuum wave function for the slow (50-eV) ejected electron satisfying the correct boundary conditions. The variation of the sevenfold differential cross section with the scattering angle for fixed molecular alignment shows diffraction patterns, which differ from those obtained by the multicenter atomic model of Messiah. The effect of the molecular alignment is studied for small, intermediate, and large scattering angles. This reveals preferential directions for the internuclear axis. © 1996 The American Physical Society.

In this work the three-body Coulomb continuum problem is reformulated in a set of parabolic-hyperspherical coordinates. It is shown how this procedure yields additional information on the angular and radial behavior of correlated three-body Coulomb continuum wave functions. For two electrons receding from a nucleus it is shown that three-body coupling is essential in describing the dependence of the fragmentation dynamics on the ratio of the electronic distances from the nucleus whereas the angular distribution of these electrons is less sensitive to this coupling. The spin asymmetry in the total cross section for electron-impact ionization of atomic hydrogen is analyzed and found to be sensitive to the radial correlation of the outgoing electrons. © 1996 The American Physical Society.

We present extensive studies of basic one-electron matrix elements involved in ion-metal surface scattering theory. Our method allows for the convenient generation of matrix elements for high principal quantum numbers of hydrogenic projectile states and thus provides basic building blocks for the ab initio description of the interaction of highly charged ions with surfaces. The matrix elements related to the energy shifts can be evaluated for an arbitrary one-dimensional potential therefore allowing for flexibility in the choice of model-perturbation potentials. The transfer matrix elements between hydrogenic levels and conduction-band states have been formerly evaluated for the special case of eigenfunctions to the step-function potential. We extend these calculations to wave functions generated from an arbitrary one-dimensional surface potential and show applications to various surface potentials and hydrogenic levels. © 1996 The American Physical Society.

We calculate the spontaneous and stimulated nonresonant combination scattering cross sections from a finite-size trapped alkali-metal Bose-Einstein condensate (BEC). The probabilities for scattered radiation at frequencies which differ from the incident frequency by elementary vibrational-rotational-spin excitations of the BEC are thereby determined. © 1996 The American Physical Society.

An analytic theory of resonantly enhanced two-photon excitation of N states with laser light of an arbitrary field is developed. Closed-form expressions for the time evolution of the system are derived. The formulation is used to explore adiabatic passage to a level embedded amongst N other levels. ‘‘Counterintuitive’’ pulse sequence is shown to lead to complete population transfer under adiabatic conditions. The reverse case of stimulated emission from N initial states is also studied. Complete population transfer to the ground state from an initial state satisfying the two-photon resonance condition is shown to be possible. © 1996 The American Physical Society.

We consider the reflection of cold atoms from a temporally modulated evanescent wave, with laser intensity noise, including stochastic surface adsorption. The stochastic surface adsorption is explicitly modeled by means of quantum trajectories while the effect of noise is modeled using the method of stochastic Hamiltonians. The results show that noise destroys quantum features such as interference and splitting, which is especially rapid for semiclassical states. For small noise, modulation can still produce a splitting of the atomic beam, with added dispersion resulting from heating of atoms. In order to distinguish the quantum features, a classical analysis is also presented. © 1996 The American Physical Society.

We calculate photoelectron spectra for a two-electron atom under the influence of two external driving fields, using an essential states formalism. We focus on the regime of coherence transfer, in which electron-electron correlation transfers field-induced photocoherence from one electron to the other. In the case studied here, two laser fields are resonant with coupled atomic transitions, in the manner familiar from three-level dark-state spectroscopy. Dynamical two-electron effects are monitored via the photoelectron energy spectrum. We show that the distribution of the photoelectron energies can be singly, doubly, or triply peaked depending on the relative laser intensities. The electron spectra are independent of the turn-on sequence of the fields. © 1996 The American Physical Society.

The quantum theory of high-order harmonic generation by a low-frequency laser field is generalized to the case of a bichromatic elliptically polarized laser field. The quasiclassical cutoff law is analyzed. Numerical results for a linearly polarized bichromatic laser field are presented and analyzed for different laser field frequencies, intensities, and relative phases. Harmonic intensity is many orders of magnitude higher in the bichromatic case than in the monochromatic one, but the cutoff is shifted toward the lower harmonics. The plateau height can be controlled by changing the relative phase of the fields. A qualitative agreement with the recent experiments is shown. © 1996 The American Physical Society.