X-ray scattering experiments are described which probe the effect of water adsorption and desorption on the lamellar, or layer ordering, in thin substrate-deposited multilayers of the phospholipid dipalmitoylphosphatidylcholine. The analysis of the lamellar or (00l) line shapes on the basis of a model of one-dimensional displacement disorder in a multilayer of finite size, resulting from the presence of two components, reveals the formation of distinct states of hydration during water uptake and release. Transitions between pure states occur via disordered intermediate states that contain at least two pure states, arranged in a variety of sequences. The commonly employed picture of a continuous ‘‘swelling’’ process does not adequately account for our observations. Instead, we propose that water adsorption be viewed as an intercalation process. Its similarities with graphite intercalation and layering transitions encountered in the context of wetting phenomena are discussed in some detail.

A molecular theory of the flexoelectric (FE) effect in nematic liquids is developed. Expressions are derived for both the dipole and quadrupole contributions to the FE coefficients. The dipole contribution is expressed in terms of direct correlation functions of the isotropic liquid having a number density equal to that of the ordered medium. The quadrupole contribution is, however, found to be independent of the direct correlation functions and depends only on the anisotropic part of the molecular quadrupole tensor. The two contributions are found to be nearly equal in magnitude. While the dipole contribution is found to be proportional to both P${¯}_2$ and P${¯}_2^2$, where P${¯}_2$ is the order parameter of the nematic phase, the quadrupole contribution is proportional to P${¯}_2$. The transverse component of the molecular dipole moment contributes significantly more than the longitudinal component. The difference between the two FE coefficients is primarily due to the transverse dipole. It is shown that with increased knowledge of the molecular parameters, such as electric multipole moments, asymmetry in molecular shape, length-width ratio, etc., one can calculate accurate values of the FE coefficients of any given system.

We consider the effects of impurities on the velocity selection of a needle crystal, in the small thermal and solutal Peclet numbers. As in the case of the growth in a pure melt, steady solutions for needle crystals are found only if surface tension is anisotropic. When anisotropy is small, the variation of ${\sigma }^{\mathrm{*}}$ with impurity concentration is found, in agreement with results given by the heuristic model of Lipton et al. [Metall. Trans. A 18, 341 (1987)].

From the explicit solution of a two-level problem previously obtained, it is proved that the single-particle kinetic energy density functional $t_s$[ρ] is the density ρ times a function solely of the ratio ρ’/ρ, with ρ’ the first derivative of ρ. For an n-level system with a harmonic-oscillator potential, the form of $t_s$[ρ] is explicitly obtained. Again $t_s$[ρ] involves only ρ and ρ’; no higher derivatives appear.

Momentum-space two-body perturbation wave functions are applied to an accurate evaluation of the nonadditive three-body interaction among well-separated three hydrogen atoms.

A recent experiment reported a two-order-of-magnitude enhancement of the cross section for double atomic-field bremsstrahlung when compared to various theoretical formulations. In this paper we propose to explain the observed coincidence rate in terms of the coherent emission of two thick-target bremsstrahlung photons from the Mylar vacuum window by electrons elastically scattered in the target. The photon energy dependence of the coincidence rate predicted by this thick-target double bremsstrahlung (TTDB) model is estimated and compared to the previous experimental results for a gold target. Fairly good agreement is obtained with both the order of magnitude of the coincidence rate and its photon energy dependence, even though a crude approximation is used to estimate the TTDB distribution from the better known thick-target single bremsstrahlung spectrum.

An adaptive Monte Carlo algorithm is applied, as a test case, to the evaluation of multiphoton ionization matrix elements for hydrogen. This study indicates that a Monte Carlo approach may be useful in treating high-order multiphoton ionization of alkali-metal atoms, and possibly other atoms, by weak fields.

In this Brief Report we derive analytically the spectral structure of supermodes in the long electron bunch limit.

The superposition of two squeezed vacuum states is analyzed by studying the photon-number probability distribution and the quadrature-phase-eigenstate marginal distributions. Interference fringes in the distributions are observed for some superposition states. The nonlinear oscillator generates a particular superposition of two squeezed vacuum states and the properties of this superposition state are discussed and contrasted with the other superposition states.

By making use of two external pumps here it is shown that a given medium, with high-order optical susceptibility tensor, is a good candidate to get enhanced squeezing, by considerably reducing the interaction length.

We propose a photodetection scheme, based on an interference between emission and absorption processes. Such a scheme, under specific circumstances, leads to sub-Poissonian statistics of the photocurrent and a reduction of quantum noise.

The polarization potential acting between an excess electron and a molecule in a nonpolar fluid is screened by the presence of neighboring molecules. The screening function calculated using the semicontinuum dielectric model of Onsager goes to zero at the density specified implicitly by ε=(1+2$u_1$)/(1-$u_1$), where ε is the dielectric constant and $u_1$ is the ratio of the molecular electric dipole polarizability to the radius of the Onsager cavity. We explain this phenomenon in terms of a polarization charge density induced on the surface of the cavity.

We present a linear-stability analysis for the kink solutions of a two-component nonlinear scalar model in (1+1) dimensions. The study follows the traditional approaches which directly treat the normal-mode problem.

We use the connected-moment expansion, recently developed by Cioslowski [Phys. Rev. Lett. 58, 83 (1987)], to investigate the ground-state energy of the single-impurity Anderson model. It is found that the moment expansion obtained for this Hamiltonian is not a Stieltjes series and thus does not provide a useful method for estimating ground-state energies.

A simple analysis and a numerical calculation show that the ability to retrieve information from the Hopfield network is enhanced when thermal noise is introduced. It is also shown that the best efficiency of the neural network is achieved in the range of temperatures $T_1$$T_2$, where $T_1$ corresponds to the noise necessary to eliminate spurious memories and $T_2$ to the temperature at which the memorized states are no longer stable.

In a recent paper Boya, Kmiecik, and Bohm [Phys. Rev. A 37, 3567 (1988)] discuss the one-dimensional hydrogen atom and mention what one should take if one wants to form parity eigenfunctions. In this Comment we show that such even and odd superpositions cannot represent physical states due to a superselection rule that leads to spontaneous breaking of the symmetry of inversion through the origin.

The energy-gain spectra and angular distributions for single-electron capture in collisions between ${\mathrm{Ar}}^{4+}$ and Ar have been measured with a new technique for six laboratory energies in the range 33–200 eV. The spectra show that the cross section is dominated by capture to the 4p state of ${\mathrm{Ar}}^{3+}$, while there is no evidence of capture to the 4s state. For energies below 68 eV, forward scattering is suppressed and strong angular oscillations within the acceptance of ±7° indicate interferences between different paths on the 4p potential curves.

We have used the Schwinger multichannel formulation [Takatsuka and McKoy, Phys. Rev. A 24, 2473 (1981)] to carry out ab initio studies of the effects of polarization on the cross sections for elastic scattering of electrons by the polyatomic molecule ${\mathrm{CH}}_4$ for collision energies from 7.5 down to 0.02 eV. Although the closed-channel expansion through which polarization effects are included in this approach is not fully converged, the resulting cross sections do show a Ramsauer-Townsend minimum between 0.1 and 0.2 eV, about 0.3 eV below where it is seen experimentally. Our static-exchange cross sections show no Ramsauer-Townsend minimum. At the higher energy of 7.5 eV, where polarization effects are less significant, the calculated differential cross sections are in excellent agreement with the measured values. Differential cross sections are also shown at 3.0 eV.

Equilibrium mean charges q¯ of ions passing through a carbon foil were observed to oscillate vs projectile atomic number Z at ion energies 0.55, 1, and 2 MeV/u. The maxima of the oscillation take place for closed-shell ions having the electrons in mean number Z-q¯=10 and 28.

The decay of the photon-excited 2$p^5$3$s^2$3$p^6$3d4$s^2$ core resonances of atomic Ca has been studied by electron spectroscopy and by multiconfiguration calculations of the autoionization rates. Strong correlation effects give rise to complicated line structures that are mostly due to the autoionization of the resonances into the 3$p^4$3d4$s^2$εl, 3$p^4$3d4$p^2$εl, and 3$p^4$3$d^3$εl continua. The breakdown of the spectator model, which is caused by the collapse of the 3d orbital, manifests itself in the dramatic differences between the spectra excited via the two resonances split by the spin-orbit and 2p-3d interactions.