We have made systematic measurements of scattered projectile charge-state distributions for multicharged He, N, Ne, Na, and Ar ions grazingly incident on a Au(110) single-crystal target. For projectiles whose neutral binding energies lie below the valence band (N, Ar, He, and Ne), observed 1+ charge fractions were small at low velocities and steeply increased above well-defined threshold velocities. However, for Na projectiles with neutral binding energy above the Fermi level, a large (about 84%) 1+ charge fraction was found. The velocity dependence of the Na neutral fraction shows a ‘‘kinematic resonance’’ due to the virtual population of electronic states above the Fermi level in the projectile rest frame. The measured sample temperature dependence of the scattered 1+ charge fraction for ${\mathrm{Ne}}^{9+}$ projectiles incident along the [110] surface channeling direction reveals a significant decrease in projectile neutralization once the (2×1)-(1×1) phase transition temperature of the Au(110) surface at 650 K has been reached. © 1996 The American Physical Society.

We report a successful experimental test of an interferometer for cold neutrons using multilayer mirrors. Interference fringes that are analogous to the Brewster’s fringes in classical optics have been clearly observed using the cold neutron beam with a wavelength of 12.6 Å and a bandwidth of 3.5% at full width at half maximum. The observed interference fringes demonstrate the coherence of the reflection off the multilayer neutron mirror and confirm the feasibility of cold neutron interferometry using multilayer mirrors. © 1996 The American Physical Society.

The spatial and momentum distributions of the uncondensed cloud associated with a confined Bose-condensed gas are found to have a universal form which differs from the classical Maxwell-Boltzmann distribution. This universality is a direct consequence of the condensation which stabilizes the density of the cloud on the threshold of condensation. The deviation of the distributions from the Maxwell-Boltzmann distribution is a macroscopic signature of the quantum statistics. © 1996 The American Physical Society.

Bose-Einstein condensation (BEC) of an ideal gas is investigated for a finite number of particles. In three dimensions, we find a transition temperature which is lower than in the thermodynamic limit. Lowering the dimension increases the transition temperature and is therefore favorable for BEC. This is in contrast to the standard result obtained in the thermodynamic limit which states that BEC is not possible in, e.g., a one-dimensional (1D) harmonic potential. As a result, 1D atom traps, such as radially tightly confining magnetic traps or optical dipole traps, are promising for studying BEC. © 1996 The American Physical Society.

The properties and stability of a trapped Bose-Einstein condensate are strongly influenced by attractive interactions between the particles. We describe the spatial distribution, stability, and collisional loss rates for a weakly interacting gas in the mean-field limit. We show how the condensate contracts and becomes unstable as the number of condensate atoms increases. We further show how the number of atoms is limited by the collisional loss rates associated with the contraction of the condensate; this loss is in addition to the particle ejection decay indentified by Kagan et al. © 1996 The American Physical Society.

A theoretical investigation of the electronic excited states of ${\mathrm{Na}}_3$ is presented by employing the multiple-scattering model. All the excited states are treated in a unified manner in the framework of quantum defect theory. For the relatively low excited states, our calculated transition energies are in good agreement with the experimental data and could help to assign the spectra bands. For the high excited states, we present the quantum defects for all Rydberg series, which call for further experimental verifications. © 1996 The American Physical Society.

It has recently been suggested that an exactly solvable problem characterized by a new quantum number may underlie the electronic shell structure observed in the mass spectra of medium-sized sodium clusters. We investigate whether the conjectured quantum number 3n+l bears a similarity to the quantum numbers n+l and 2n+l, which characterize the hydrogen problem and the isotropic harmonic oscillator in three dimensions. © 1996 The American Physical Society.

We study the mean-field dynamics of atoms in a velocity-selective coherent population trapping (VSCPT) configuration, under the influence of resonant dipole-dipole interactions. By elimination of the transverse electromagnetic field we derive a Heisenberg master equation for an arbitrary atomic operator. A closed equation of motion for the one-particle density matrix can be given by employing a time-dependent Hartree-Fock (TDHF) factorization of the two-particle density matrix, thereby neglecting two-particle correlations, i.e., collisional effects. In the case of bosons, the mean-field evolution mainly shows a quantum-statistical enhancement of the spontaneous emission rates, sensitive to the final-state occupation. The description is specialized to treat atoms with a J=1→${\mathit{J}}^{\prime }$=1 transition in a one-dimensional ${\mathrm{\sigma }}^{+}$-${\mathrm{\sigma }}^{\mathrm{-}}$ VSCPT configuration. The TDHF equations are integrated numerically. It is shown that for a density at which the gas is optically dense, the number of atoms, trapped in the dark state, is increased and its momentum width is decreased. © 1996 The American Physical Society.

The ionization of hydrogen Rydberg atoms by circularly polarized microwaves is studied quantum mechanically in a model two-dimensional atom. We apply a combination of a transformation to the coordinate frame rotating with the field, with complex rotation approach and representation of the atomic subspace in a Sturmian-type basis. The diagonalization of resulting matrices allows us to treat exactly the ionization of atoms initially prepared in highly excited Rydberg states of principal quantum number ${\mathit{n}}_0$≊60. Similarities and differences between ionization by circularly and linearly polarized microwaves are discussed with a particular emphasis on the high-frequency regime and on the localization phenomenon. The dependence of the ionization character on the initial state (circular, elliptical, or low angular momentum state) as well as on the helicity of the polarization is discussed in detail. It is shown that, in the high-frequency chaotic regime, close encounters with the nucleus do not play a major role in the ionization process. © 1996 The American Physical Society.

An alternative perturbative formulation is introduced to reduce the task of designing optimal controls of molecular motion. The method provides multiple optimal control fields as eigenvectors corresponding to the positive eigenvalues of a matrix {${\mathit{A}}_{\mathit{mn}}$}. Analytical equations for the matrix elements ${\mathit{A}}_{\mathit{mn}}$ are derived. Numerical examples are given for simple three- and four-state molecular systems. The method is shown to be computationally simple and efficient in providing good quality control results. © 1996 The American Physical Society.

It is argued that the ponderomotive potential does not alter atomic energy differences, notwithstanding discussions over the years on intense laser interactions with atomic systems. Alternative ionization energies that pertain to such interactions are formulated, helping to remove the confusion that has plagued these discussions. © 1996 The American Physical Society.

We discuss, via a numerical calculation, the main properties of atomic photoelectron spectra, as they would be obtained by using a radiation pulse containing N+1 frequencies associated with a ‘‘Dirac comb’’ of N higher harmonics together with the laser which has been used to generate them. We address more precisely the physically relevant situation in which the harmonics have much weaker intensities than the one of the laser. In such (N+1)-color photoionization processes, the atom can simultaneously absorb harmonic uv photons and exchange, i.e., absorb and/or emit, (via stimulated emission) laser ir photons. We have simulated the photoelectron spectra by numerically solving the time-dependent Schrödinger equation for a three-dimensional hydrogen atom in the presence of the radiation pulse. Our results show that, everything else being kept fixed, the magnitudes of the photoelectron peaks are strongly dependent on the difference of phase between successive harmonics. This strong dependence results from interference effects taking place between competing quantum paths leading to a given final state. An interesting feature is that these interferences involve transitions in the continuum states of the atom and do not depend on resonances in the discrete spectrum. Another interesting outcome of our study is to show that such effects should be observable with currently developed harmonic sources. © 1996 The American Physical Society.

We consider wave-packet spreading and ionization of a one-dimensional model hydrogen atom interacting with a strong high-frequency laser field in the stabilization domain. We demonstrate the effect of the pulse shape on the dynamics in phase space and visualize our results with both the Wigner and the Q function phase-space quasiprobability representation. The Wehrl entropy is introduced and utilized as a convenient one-parameter measure of wave-packet spreading, phase-space localization, and ionization. © 1996 The American Physical Society.

A classical model is developed to study molecular ionization in strong laser fields. The model is compared with a one-dimensional quantum-mechanical model for the case of ${\mathrm{H}}_2^{\mathrm{}+}$. The intensity threshold for ionization of ${\mathrm{H}}_2^{\mathrm{}+}$ is studied as a function of laser polarization, wavelength, pulse duration, and internuclear separation. A range of internuclear separations is found where the ionization rate is enhanced over that at the equilibrium separation, in agreement with the quantum-mechanical model. The classical model is then extended to multielectron diatomic molecules. A regime is found where simultaneous two-electron ionization occurs. An intensity range is suggested for looking for enhanced ionization effects in dissociating diatomic molecules. © 1996 The American Physical Society.

We describe a quasiclassical approach to correct electron motion following tunneling ionization in intense laser fields, for the presence of the Coulomb potential. The technique is applied to model the dependence of high-harmonic generation on the ellipticity of the incident laser field. We present experimental data on the harmonics with photon energies close to the ionization potential of an atom, identify their source, and explain their unusual dependence on the ellipticity of the incident laser field. Our calculations are in good agreement with experimental data. © 1996 The American Physical Society.

The collision-induced stimulated processes in the ${\mathit{S}}_{1/2}$-${\mathit{P}}_{1/2,3/2}$ system are studied. Propagation of an intense resonant pump pulse and probe pulses in the medium consisting of the ${\mathit{S}}_{1/2}$-${\mathit{P}}_{1/2,3/2}$ atoms and a buffer gas is considered. Attenuation and amplification of the probe pulses are investigated. The quasistationary regime is analyzed analytically. The appropriate set of the Maxwell-Bloch equations is solved numerically. © 1996 The American Physical Society.

We consider a single-mode dye laser with colored pump noise and white quantum noise. An analytical approach is applied to study the statistics of the intensity fluctuations for arbitrary correlation times of the pump noise. Full numerical solutions are compared with our approximation and previous approximations schemes. The present approximation scheme compares favorably over wide regimes in parameter space with the numerical data. © 1996 The American Physical Society.

We extend the master-equation treatment of the single-mode laser [Gardiner and Eschmann, Phys Rev. A 51, 4982 (1995)] to include two subthreshold side modes. We calculate spectra for a wide range of decay constant ratios and confirm the predictions found by Marin et al. [Phys. Rev. Lett. 75, 4606 (1995)]. © 1996 The American Physical Society.

Amplification in a V system is studied both in the framework of the density matrix formalism and using the S matrix in the usual Hilbert space of the dressed-state vector. We show that it is possible to achieve amplification without inversion both in the bare- and in the dressed-state basis. In this case there is an asymmetry between amplification and absorption because of the occurrence of interferences between Feynman diagrams that can reduce the absorption. The comparison with the two-level atom shows a close analogy with the gain observed about the central resonance of the Mollow transmission spectrum. © 1996 The American Physical Society.

We consider a dual recycled interferometer for detecting gravitational waves. Squeezed light is injected into the interferometer through the output port. We analyze the sensitivity of the detector for two cases of interest, namely, the broadband and the narrow-band modes of operation. The improvement in sensitivity due to the squeezed light is investigated in such a setup. © 1996 The American Physical Society.