Momentum distributions and spectroscopic factors are obtained in a high-resolution electron-momentum spectroscopy study of krypton at 1000 eV. The shapes and relative magnitudes of the momentum profiles are in good agreement with the results of calculations made within the distorted-wave impulse approximation (DWIA) framework. The DWIA describes the relative magnitudes of the 4p and 4s manifolds as well as giving a good representation of the shapes of the respective 4p and 4s cross sections. Results for the momentum profiles belonging to excited ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ manifolds are also presented. Spectroscopic factors for transitions belonging to the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ manifolds are assigned up to a binding energy of 42 eV. The spectroscopic factor for the lowest 4s transition is 0.51±0.01, whereas that for the ground-state 4p transition is 0.98±0.01. Comparisons of the present binding energies and spectroscopic factors are made against the results of several many-body calculations and photoelectron spectroscopy results. In addition, we outline a procedure, utilizing the experimental 4p and 4s manifold cross sections, that provides information on possible initial-state configuration-interaction effects in krypton. © 1996 The American Physical Society.

In calculating the elastic differential cross section for electron scattering from linear nonpolar molecules, one must include elements of the transition matrix corresponding to a large number of partial waves in order to obtain convergence at small angles. We present an analytic correction to the widely used expression for the differential cross section in terms of body-frame T-matrix elements calculated in the fixed-nuclear-orientation approximation. This correction incorporates contributions from all high-order partial waves via the Born approximation. It efficiently produces accurate differential cross sections and shows rapid convergence even at small angles. We illustrate the importance and accuracy of this procedure for low-energy (less than 10 eV) collisions of electrons with ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$. © 1996 The American Physical Society.

A theoretical model has been developed to study the direct Auger deexcitation process between two-electron states. The model is based on linear response theory. Therefore the excitations induced in the surface may be described with the surface response function. This model has been applied to calculate the Auger deexcitation rates when an excited He atom is moving parallel to an aluminum surface. More precisely, the distance and velocity dependence of the transition from the singlet states 2${\mathrm{}}^1$S and 2${\mathrm{}}^1$P to the ground state is presented. I finally compare these results with the ones obtained by using a one-electron description for the He atom. The differences between both models vanish when effective charges ${\mathit{Z}}_{\mathit{eff}}$=1.69 a.u. and ${\mathit{Z}}_{\mathit{eff}}$=1.57 a.u. are used for the 1s and the 2p states, respectively, in the one-electron model. © 1996 The American Physical Society.

Final-charge-state distributions of argon ions, scattered grazingly from a smooth highly oriented pyrolytic graphite surface, have been measured as a function of initial charge state (q=4–17) and impact velocity (v=0.15–0.62 a.u.) The final-charge-state distribution changes strongly with the impact velocity, but is almost independent of the initial charge state. The neutralization during grazing-angle scattering is compared to the charge-state equilibration experienced by ions passing through a solid (carbon foil), and these two processes seem to have common properties. A K x-ray spectrum from the K-shell vacancy decay of 51-KeV ${\mathrm{Ar}}^{17+}$ projectiles was obtained as a function of the angle between the ion beam and the surface. Measurements of x-ray spectra in coincidence with grazingly scattered ions are reported. A simple model for argon neutralization near and below the surface is proposed. The model assumes a direct side feeding into the Ar M shell followed by Auger and radiative L- and K-shell filling. © 1996 The American Physical Society.

In this work the dynamical behavior of the center of mass of a two-level atom in sequences of multiple optical standing waves is explored. In a standing wave the atomic momentum distribution fans out. But with a suitably designed sequence of standing waves it is possible to refocus the momentum-space wave packet and bring the momentum distribution to the initial value, which may be regarded as an atom optical analog to spin or photon echo. If the total interaction time is fixed, the refocusing performance gets better as the number of standing waves in a sequence increases. Also the atomic wave packet in position space evolves as if in free space. © 1996 The American Physical Society.

The Bogoliubov approximation is used to study the ground state and low-lying excited states of a dilute gas of N atomic bosons held in an isotropic harmonic potential characterized by frequency ω and oscillator length ${\mathit{d}}_0$=√ħ/mω. By assumption, the self-consistent condensate has a macroscopic occupation number ${\mathit{N}}_0$≫1, with N-${\mathit{N}}_0$≪${\mathit{N}}_0$. A linearized hydrodynamic description yields operator forms of the particle-conservation law and Bernoulli’s theorem, expressed in terms of the small density fluctuation operator ρ^′ and velocity potential operator Φ^′, along with the condensate density ${\mathit{n}}_0$ and velocity ${\mathbf{v}}_0$. For positive scattering length a and large stationary condensate (${\mathit{N}}_0$≫${\mathit{d}}_0$/a and ${\mathbf{v}}_0$=0), the spherical condensate has a well-defined radius ${\mathit{R}}_0$≫${\mathit{d}}_0$, and the low-lying excited states are irrotational compressional waves localized near the surface. Approximate variational energies ${\mathit{E}}_{0\mathit{l}}$ of the lowest radial modes (n=0) for successive values of orbital angular momentum l form a rotational band given by ${\mathit{E}}_{0\mathit{l}}$≊${\mathit{E}}_{00}$+1/2${\mathrm{ħ}}^2$l(l+1)/${\mathrm{mR}}_0^2$, with radial zero-point energy ${\mathit{E}}_{00}$∝ħω(${\mathit{R}}_0$/${\mathit{d}}_0$${)}^{2/3}$=(${\mathrm{ħ}}^2$ m${\mathrm{\omega }}^4$${\mathit{R}}_0^2$${)}^{1/3}$. © 1996 The American Physical Society.

We analyze elementary properties of exciton and polariton lasers — devices that generate coherent optical and matter waves using final-state stimulation of exciton-phonon scattering. First we discuss the relation between the conditions for the onset of equilibrium and nonequilibrium excitonic condensates. Provided that the thermal de Broglie wavelength ${\lambda }_{\mathit{T}}$ exceeds the exciton Bohr radius ${\mathit{a}}_{\mathit{B}}$, an exciton laser operates without electronic population inversion. In contrast to previous proposals, this is a different type of laser without inversion which utilizes many-body coherences. When the excitonic character of the polariton branch vanishes, a polariton laser becomes indistinguishable from a photon laser. © 1996 The American Physical Society.

We propose an experimental scheme to probe the relative global phase of two condensates and thus demonstrate the spontaneous symmetry breaking of global gauge invariance in Bose-Einstein condensation. A positive measurement would justify the standard view of the condensate wave function as a coherent state when there are interparticle interactions. © 1996 The American Physical Society.

We point out that the local density approximation (LDA) of Oliva is an adaptation of the Thomas-Fermi method, and is a good approximation when ε=ħω/kT≪1. For the case of scattering length a≳0, the LDA leads to a quantitative result, Eq. (14′), easily checked by experiments. Critical remarks are made about the physics of the many-body problem in terms of the scattering length a. © 1996 The American Physical Society.

The propagation of a two-level atomic wave packet across a laser beam is considered. For small incident kinetic energies of the atom, the wave nature of the center-of-mass motion becomes apparent, giving reflections and refractions in the case of a square profile of the laser beam. When the incident momentum is low enough, we show that one of the dressed states cannot propagate inside the laser beam, giving rise to an evanescent atomic wave packet. Also a stopped excited component is found when the incident momentum equals in modulus—not in direction—the one-photon recoil. © 1996 The American Physical Society.

A nonperturbative method is used to study the two-color multiphoton ionization of atoms by solving the Schrödinger equation in momentum space, which facilitates the extraction of the rapidly varying part of the wave function. Our method generalizes the problem for cases where an arbitrary number of ionization photons are absorbed, and also takes into account the threshold shifts due to both the high- and low-frequency fields. The photoelectron energy spectra of the hydrogen atoms are calculated and the dependence on the laser field strengths and frequencies is discussed. © 1996 The American Physical Society.

We analyze the fluorescence and absorption spectra of a two-level atom driven by a bichromatic field with frequencies ${\mathrm{\omega }}_1$ and ${\mathrm{\omega }}_2$, separated by ${\mathrm{\omega }}_2$-${\mathrm{\omega }}_1$=2δ, and Rabi frequencies (at resonance) 2${\mathrm{\Omega }}_1$ and 2${\mathrm{\Omega }}_2$ such that their ratio α=${\mathrm{\Omega }}_2$/${\mathrm{\Omega }}_1$<1. We focus on the case of ${\mathrm{\omega }}_1$ close to the atomic frequency ${\mathrm{\omega }}_0$ and ${\mathrm{\omega }}_2$ near the Rabi sideband frequency ${\mathrm{\omega }}_1$+2${\mathrm{\Omega }}_1$; the detunings are denoted by ${\mathrm{\Delta }}_1$=${\mathrm{\omega }}_0$-${\mathrm{\omega }}_1$ and ${\mathrm{\Delta }}_2$=${\mathrm{\omega }}_1$+2${\mathrm{\Omega }}_1$-${\mathrm{\omega }}_2$. We find that the spectra depend critically on the detuning ${\mathrm{\Delta }}_2$: For large ${\mathrm{\Delta }}_2$, the fluorescence spectrum consists of the well known Mollow triplet, centered at ${\mathrm{\omega }}_1$; for smaller (but nonzero) ${\mathrm{\Delta }}_2$, the spectrum is composed of a triplet at ${\mathrm{\omega }}_1$ together with doublets near the sideband frequencies ${\mathrm{\omega }}_1$±2${\mathrm{\Omega }}_1$. However, when ${\mathrm{\Delta }}_2$=0 (and α≪1), the spectrum consists of a doublet centered at ${\mathrm{\omega }}_1$ and triplets at ${\mathrm{\omega }}_1$±2${\mathrm{\Omega }}_1$: there is then no fluorescence at ${\mathrm{\omega }}_1$. As α increases, additional triplet structures appear in the spectrum at frequencies ${\mathrm{\omega }}_1$±2n${\mathrm{\Omega }}_1$ with intensities proportional to ${\mathrm{\alpha }}^{2\left(\mathit{n}\mathrm{-}1\right)}$, n≳1, and a line reappears at ${\mathrm{\omega }}_1$, with intensity proportional to ${\mathrm{\alpha }}^4$. The absorption by the system of a weak probe beam is also strongly dependent on the detuning, and the spectrum is composed of emission-dispersion-absorption features located near ${\mathrm{\omega }}_1$ and ${\mathrm{\omega }}_1$±2n${\mathrm{\Omega }}_1$. An analysis in the dressed-atom picture is presented which explains the physical origin of all these features, in both fluorescence and absorption. © 1996 The American Physical Society.

We present the generalization of the Landau-Zener model for a constant coupling of a finite duration. The exact evolution matrix is expressed in terms of sums of by-products of parabolic cylinder functions estimated at the turn-on time and at the turn-off time of the coupling. Various approximations in terms of simpler functions are derived and applied to several physically distinct cases. They allow us to study the dependence of the transition probability on the interaction parameters: coupling strength, coupling duration, and detuning slope. Furthermore, the analytic approximations reveal the effects of the finite coupling duration as well as those caused by adding a constant detuning shift, absence of a level crossing, turn-on time or turn-off time near the crossing (‘‘half crossing’’), turn-on time and turn-off time near the crossing (‘‘nonsubstantial crossing’’). The results are used to obtain analytic approximations to the time evolution in the original Landau-Zener model. Furthermore, following related studies on other models, we define the Landau-Zener class of models that, along with the finite Landau-Zener model presented in this work, contains an infinite number of members that give the same transition probability. Comparison of this class to the Allen-Eberly class shows that the two classes contain members with the same coupling but different detuning chirps as well as members with the same chirp but different couplings. The former case reveals chirp effects while the latter demonstrates shape effects. © 1996 The American Physical Society.

It is demonstrated that both standard and anomalous light-induced drift (LID) profiles as a function of laser frequency can be analytically simulated using the description of LID in terms of transport mean paths. This description allows one to directly link arbitrary LID frequency profiles to derivatives of the unsaturated absorption profile. In order to test this approach, LID experiments have been performed on gaseous ethylene ${(}^{13}$${\mathrm{C}}^{12}$${\mathrm{CH}}_4$) for the rovibrational transition (7,1,6) →${\nu }_7$(8,0,8) in a background of Kr, Xe, ${\mathrm{SF}}_6$, ${\mathrm{CO}}_2$, or ${\mathrm{CH}}_3$Cl. Although the observed LID frequency profiles show a large variation, they are well simulated using this approach. © 1996 The American Physical Society.

By applying the stationary wave functions for an electron interacting with a circularly polarized standing-wave photon field, and a nonperturbative scattering theory, we obtain a theoretical description of the Kapitza-Dirac effect in strong radiation fields. The theory predicts that large momentum transfers can occur, when two propagating light waves have the same angular momentum and opposite helicity, as observed by Bucksbaum, Schumacher, and Bashkansky as a half process in multiphoton ionization. The theory and possible experiments to detect the full Kapitza-Dirac effect in electron scattering from a strong radiation field are discussed. The theory also predicts that with the participation of ponderomotive energy, the incident electron beam, after interacting with a standing photon wave, may have symmetric reflection, asymmetric reflection, penetration, and refraction. Transition rate formulas of these processes are presented. © 1996 The American Physical Society.

We report the investigation of nonresonant multiphoton double ionization of Ga and As utilizing a double-pulse technique. Singly and doubly charged Ga and As ions are formed by a 17.5 ps laser pulse at 266 nm (4.67 eV) in the ${10}^{10}$-${10}^{12}$ W/${\mathrm{cm}}^2$ intensity range. A second pulse is used to probe the ionization generated by the first pulse. Our experiments give clear evidence that the double ionization of Ga and As starts with a simultaneous excitation of two electrons from the ground state of the neutral atoms. The double-pulse ionization is measured at different laser powers and compared with fits of rate equations based on the simultaneous excitation of two electrons. The important role of the pulse duration for the double-ionization process is demonstrated by simulating the temporal evolution of the atom and ion population. © 1996 The American Physical Society.

We have studied the isotopic variation of the vibrational-branching ratio (VBR) and the photoelectron angular distribution (PEAD) as a function of laser intensity in (1+1′)-photon resonance-enhanced multiphoton ionization (REMPI) of HD and ${\mathrm{D}}_2$ molecules via B${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathit{u}}$ (v=4; j=1,2) levels. We have found that with the increase in laser intensity, the non-Franck-Condon nature of the VBR and the deviation of the total as well as the vibrationally resolved PEAD from that obtained in the lower-intensity regime become much more prominent for ${\mathrm{D}}_2$ molecules than those for HD molecules. We have considered the effect of the interference of different ionization channels (resonant as well as near-resonant ionization via nearby rovibrational levels) and the effect of Raman-like two-photon coupling between these intermediate vibrational levels via a continuum on this REMPI process. The difference between the vibrational wave functions and the difference between the spacing of rovibrational energy levels in these two isotopes of ${\mathrm{H}}_2$ molecules lead to different strengths of ionization and the Raman-like two-photon coupling. Hence the isotopic variation in the VBR and the PEAD for these two isotopes shows up as a result of different degrees of interference of parallel ionization channels and different strengths of two-photon coupling via a continuum, both of which become important with an increase in laser intensity. We have also found that the intensity variation of the VBR and the PEAD for both molecules depends on the choice of different rotational levels as resonances. This feature has also been found previously in the REMPI of ${\mathrm{H}}_2$ molecules [Jainab Khatun, S. Sanyal, and Krishna Rai Dastidar, Phys. Rev. A 49, 4765 (1994); Jainab Khatun and Krishna Rai Dastidar, ibid. 52, 2971 (1995), and references therein; J. W. J. Verschuur and H. B. van Linden van den Heuvell, Chem. Phys. 129, 1 (1989)]. © 1996 The American Physical Society.

We report anomalous light-induced drift of potassium vapor in mixtures of Ne with another rare gas. The drift velocity versus laser frequency curve shows a strong deviation from the dispersion curve predicted by the standard theory of light-induced drift based on velocity-independent collision rates. The observed drift velocity versus frequency curves have three zero-crossings. For a qualitative explanation of the results, a strong collision model with velocity-dependent collision rates is used. The partial pressures that cancel normal light-induced drift also provide accurate ratios of cross section for potassium collisions with the various buffer gases. The observation of anomalous light-induced drift in a simple atomic system offers the opportunity to relate the results to potential curves. © 1996 The American Physical Society.

The evolution of a ‘‘dark’’ wave packet in Ba is probed using a bound-state interferometric method. The wave packet is created by photoionizing the 5d7d component of a perturbed Rydberg state having predominantly 6snd character, and thereby redistributing population to neighboring states. Information on which Rydberg-Rydberg Raman transitions dominate the transfer of population to each state is obtained. A closed-form expression for the observed strong-field interferograms is developed for the limit of very short pulses. © 1996 The American Physical Society.

We study collisions between dressed ground-state atoms in a full coupled-channel approach. In this way we examine collisions in the microwave trap, in the cryogenic hydrogen maser, and in the context of rf-induced evaporative cooling. The results for dressed H atoms confirm earlier results obtained within the so-called degenerate internal states approximation. © 1996 The American Physical Society.