Lifetime measurements of the theoretically predicted metastable state ${}_{}{}^4{}_{}{}^{}\mathrm{S}$ of the ${\mathrm{Ar}}^{\mathrm{-}}$ ion favor the existence of only one state with a mean lifetime of 260±25 nsec. The ${\mathrm{Ar}}^{\mathrm{-}}$ was produced from a 600-keV ${\mathrm{Ar}}^0$ beam by a single electron capture from an Ar gas target. In similar collisions between ${\mathrm{Ne}}^0$ and Ar no (metastable) ${\mathrm{Ne}}^{\mathrm{-}}$ ions were detected.

Collision experiments employing a preparation and/or an analysis of spin polarization provide a powerful tool to study individual interaction mechanisms. For atoms or ions with nonvanishing nuclear moments the hyperfine interaction before and after the collision may affect the spin polarization and any collision-induced orientation and alignment, thereby substantially reducing experimentally observable effects. This disturbance can be avoided by applying an external magnetic field, as is demonstrated here for the inelastic collision process ${\mathrm{He}}^{+}$+Na(↑) →${\mathrm{He}}^{+}$+${\mathrm{Na}}^{\mathrm{*}}$(3p). Easily attainable magnetic field strengths are sufficient to decouple electronic and nuclear spins whereby the figure of merit for both, orientation and alignment, is increased by more than one order of magnitude.

We present the results of a simple model calculation that highlights the effect of the finite (i.e., nonzero) angular resolution of electrostatic energy analyzers on the measurement of polarization correlation parameters in electron-photon coincidence experiments. It is demonstrated that measurements can yield drastically reduced ‘‘apparent’’ parameters in cases where the coincidence parameters vary rapidly as a function of electron scattering angle. For example, a measurement of the $P_1$ parameter for Ar${(}^1$P) excited by 100-eV electrons which are scattered in the forward direction and detected by an analyzer with an angular resolution (full width at half maximum) of 3° will yield an apparent $P_1$ value of 0.83 compared to the nominal value of 1.00. Since the measurement of $P_1$ in the forward direction reveals important information about the relative contributions of direct excitation and exchange excitation, it is imperative that an experiment can distinguish a $P_1$ value truly smaller than unity from an ‘‘apparent’’ value which is reduced merely by instrumental effects.

The theory of resonance effects in electron-atom scattering in a chaotic, nonresonant laser field with a bandwidth is reexamined. It is pointed out that, even though the laser is nonresonant with any of the atomic frequencies, use of the nonresonant dressed atomic states is not permissible for this purpose except for very weak fields. An ansatz for estimating the cross sections for these processes is presented, which is valid for higher field strengths.

Using a computer-simulation model, we have investigated ion dynamic effects on the spectral line shape of the neutral helium line 4471 Å. For electron densities of arc plasma conditions ${(10\mathrm{}}^{15}$ ${\mathrm{cm}}^{\mathrm{-}3}$≤${\mathrm{n}}_{\mathrm{e}}$≤${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$), our simulation profiles are compared with experimental profiles. We discuss our results in comparison with calculations based on the static ion approximation, and with results including ion-dynamic effects treated by the model microfield method or a generalized collision operator approach.

An excimer-pumped dye laser has been used to populate the 4p levels of Ca ii in a fast ion beam. The observed decay of the fluorescence intensity gives lifetimes of 7.07±0.07 and 6.87±0.06 ns for the J=(1/2 and J=(3/2 levels, respectively. These results represent a significant improvement in precision compared with our previously reported values. The lower uncertainties are the results of several improvements in our experimental procedure. The lifetimes obtained are in good agreement with previously published results. However, they differ significantly (about 9%) from the available theoretical estimates. The values for the lifetimes at the level of precision achieved in this experiment clearly indicate that previously published theoretical calculations do not adequately account for the transition probabilities from the ${}_{}{}^2{}_{}{}^{}\mathrm{P}$ levels of singly ionized calcium.

Moderately upscaled versions of existing free-electron lasers would provide optimal conditions for the detection of scattering of light by light.

The polarization vector is computed in second order for a coherent state and it is shown that the phase of the coherent state determines a significant contribution to the optical rectification value.

A direct calculation of causality behavior in Coulomb-gauge solutions to the Maxwell equations is given, with an arbitrary time-dependent source.

We consider the parametric excitation of quasistatic electromagnetic fields in an electron plasma situated in an external oscillating electric field. In contrast to previous studies, we turn our attention to the case where all waves are transverse. A new instability mechanism is found.

Numerical simulations of mercury injection using three-dimensional resistor networks demonstrate that the electrical resistance decreases in stepwise fashion with increasing occupation probability near the resistance percolation threshold. The power-law nature of the resistance-step distribution and the value of the exponent are consistent with results on porous materials obtained during mercury injection in the presence of a gravitational gradient. These studies include the effects of gravity and a distribution of pore sizes. The results provide a direct connection between the configuration of the injected mercury and the geometry of a percolation cluster in both the standard and the graded percolation problems.

The mean-field critical exponents, τ=(5/2, σ=1/2, γ=β=1, etc., for the self-organized critical state are derived viewing the self-organized critical state as a critical branching process.

In the modeling of damped quantum systems by coupling to a heat bath, it has been usual to assume the initial factorization of the density matrix into system and bath components. For realistic couplings, this does not represent a physically reasonable state. One of the simplest such models is that of phase damping, and an alternative characterization of its initial states can be obtained exactly and without ambiguity. The consequences of these new states are particularly interesting when the system is considered as a primitive model of number measurement: The local system cannot, with a finite excitation energy, be put into a superposition of number states. This is a stronger result than from previous treatment of this system, which only showed that such superpositions would decay to mixtures. Similar conclusions can be drawn for other quantum nondemolition measurements.

Large-cluster growth by diffusive processes has been made practical by a diffusion-enhancement technique introduced by Meakin [Phys. Rev. A 27, 604 (1985); 27, 1495 (1985)]. This technique has been questioned on the grounds that a variable diffusivity will change the flux of walkers at the surface of the cluster given any particular boundary conditions. We demonstrate that, carried out properly, diffusion enhancement does not modify the flux. Thus, enhanced diffusion yields the same growth equations that nonenhanced diffusion does.

The effective potential in low-energy two-body scattering obtained by the ‘‘self-energy method’’ is identical to the effective potential commonly defined as the Fourier transform of the scattering transition amplitude (the T matrix) with respect to the momentum transfer. Because of the convenience that may occur in the exchange of the order of integration, nonadiabatic recoil corrections may be disguised as momentum-dependent corrections. These momentum-dependent corrections are distinct from energy-dependent corrections. In certain forms, they may violate either T invariance or Hermiticity. Their origin is attributable to a mathematical artifact and there exists a systematic method of converting the momentum-dependent correction terms into truly local form. When recoil effect is important, these nonadiabatic corrections cannot be ignored even at threshold energy when all energy-dependent corrections vanish. In certain cases, such as in the extrapolation of the effective potential to short-distance behavior, each of the nonadiabatic corrections may be comparable to the leading term. On resummation, these may greatly alter the behavior of the effective potential indicated by the leading term alone. The previous analysis of J. R. Manson and R. H. Ritchie [Phys. Rev. A 35, 5249 (1987)] on the saturation effect due to recoil in the scattering of Coulombic systems is based on neglect of these nonadiabatic corrections disguised as momentum-dependent corrections while extrapolating the effective potential to short-distance behavior. The present Comment points out the flaw in such a procedure.

For the model of a two-dimensional Coulomb gas made up of finite-size ions fixed on the sites of a lattice, and of classical mobile electrons, the dielectric-plasma (or Kosterlitz-Thouless) transition takes place at a value of the coupling constant which is Γ=4 in the low-density limit, as for the standard model in which both the ions and the electrons are mobile.

Exact differential ionization probabilities have been calculated for inelastic ion-atom collisions in one dimension with interaction potentials of zero range. The ionization spectrum confirms the existence of the so-called $\frac{\upsilon }{2}$ electrons; i.e., a hump in the spectrum at a velocity close to half of the velocity of the projectile at intermediate collision energies while the customary electron-capture-to-continuum (ECC) and electron-loss-to-continuum (ELC) peaks are missing. The enhancement near $\frac{\upsilon }{2}$ disappears at high energies with the binary-encounter peak being the predominant feature, while shoulder structures develop near the target and projectile velocities which resemble but do not coincide with ECC and ELC peaks.

In a recent paper Allen, Anderson, and Lin [Phys. Rev. A 37, 349 (1988)] reported significant differences (that varied with impact energy), in the excitation of the $\mathrm{Na} 3p$ and $3d$ levels when the excitation was caused by ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{-}$ impact. I report theoretical calculations, based on a distorted-wave-type approximation, that reproduce at least the relative cross sections for the two impact cases, and I analyze quantitatively the origin of the above differences.

Interference between overlapping Rydberg- and valence-level autoionizing states can have dramatic effects on photoionization dynamics. Our calculations demonstrate this type of interference in NO, where we predict that the autoionizing structure is of a mixed Rydberg-valence character and dominates the cross sections, obscuring the underlying shape resonance. Such studies have important implications for understanding autoionization mechanisms in molecular systems as well as other areas of physics.

Squeezed states of light are generated by the process of second-harmonic conversion within an optical cavity resonant at both fundamental and harmonic frequencies. Observations of squeezing are made by analyzing the spectral density of photocurrent fluctuations produced by the total field reflected from the nonlinear cavity. Reductions in photocurrent noise of 13% relative to the coherent-state or shot-noise level are achieved for frequency offsets near 4 MHz.