The structure factor of supercooled water was determined at the small-angle x-ray scattering facilities at the Daresbury Synchrotron for 0.50 ${\mathrm{nm}}^{\mathrm{-}1}$${\mathrm{nm}}^{\mathrm{-}1}$ and for 247 K<T<274 K. In this temperature range the scattered intensity decreases with decreasing temperature. Interpreting our results in terms of an additional Ornstein-Zernike–like contribution at q=0, we report an upper limit to the width of the associated Lorentzian. This width is at variance with earlier results. Our results support the separation of thermodynamic quantities into a normal and an anomalous part.

Space-resolved measurements of x-ray line emission from He-like ions of magnesium plasma, produced by a 20-J, 5-ns Nd:glass laser, expanding in a buffer gas are presented. In the expansion region, the intensity ratio α of the resonance line ($1^1$${\mathrm{S}}_0$ –$2^1$${\mathrm{P}}_1$) to the intercombination line ($1^1$$S_0$–$2^3$$P_1$) is observed to be smaller than (1/3, which is the lower limiting value predicted by existing models. This anomalously small value of α is explained in terms of a preferential population of higher levels (n≥3) through three-body recombination and charge-exchange processes. Small values of the ratio observed here could be used to study the roles of different atomic processes in achieving population inversion in excited states of highly charged ions.

The dc conductivity of hydrated Li-DNA and Na-DNA powdered samples has been detected in a composite capacitor without electrode contacts. In all slightly hydrated samples the conductivity increases exponentially along with the water content with the same exponential factor, confirming initial suggestions of an intrinsic semiconductivity. Above about 25% water content, capillary condensation produces an additional relaxation effect.

Multiphoton ionization of strontium in the wavelength range of Å558-590 nm is studied via the fluorescence from excited ${\mathrm{Sr}}^{+}\left(5p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2},\frac{1}{2}}^{}{}_{}{}^{}\right)$ ions. The fine-structure sublevels in the singly charged ion are fully resolved and are found to be nonstatistically populated. Four- and five- (or more) photon processes are observed to populate ${\mathrm{Sr}}^{+}\left(5p{}_{}{}^2{}_{}{}^{}P\right)$ in the intensity regime of $I\sim 1-50$ GW/${\mathrm{cm}}^2$, but no emission is detected from higher-lying states of ${\mathrm{Sr}}^{+}$.

We study the evolution of interfaces for a generalization of a diffusion-limited aggregation process in two dimensions, where the walkers are launched from any unoccupied site and have a lifetime $\tau$. For $\tau =1\mathrm{}$, the model reduces to variant $B$ of the Eden model. However, Eden model $B$ does not respond to noise reduction in the same way as the other forms of the Eden model ($A \mathrm{and} C$). As $\tau$ increases, stable cellular patterns emerge that resemble the patterns in directional solidification.

A perturbative method, using Lie transforms, is given for calculating the Hannay angle for slow, cyclic evolutions of a classical system, taking into account the finite rate of change of the Hamiltonian. The method is applied to the generalized harmonic oscillator. The classical Aharonov-Anandan angle is also calculated. The interpretational ambiguity in the definitions of geometrical angles is discussed.

The ionization of antiprotonic noble-gas atoms due to formation and Auger deexcitation was investigated. The experiment was performed at low target pressures (p≤50 hPa) in order to prevent electron refilling from neighboring atoms. For this reason, the cyclotron trap was used. This device is designed to stop as many particles as possible in a small stop volume at even low gas pressure. The degree of ionization was determined in antiprotonic neon, argon, and krypton by means of measuring yields of circular radiative transitions at relatively high principal quantum numbers. X rays were observed between 1.6 and 21.0 keV using a Si(Li) detector. Complete ionization was found in all three cases.

Using a formulation of quantum electrodynamics that is not second quantized, but rather based on self-fields, we compute the anomalous magnetic moment of the electron to first order in the fine-structure constant α. In the nonrelativistic (NR) case and in the dipole approximation, our result is $a_e$≡(g-2)/2=(4Λ/3m)(α/2π), where Λ is a positive photon energy cutoff and m the electron mass. A reasonable choice of cutoff, Λ/m=(3/4, yields the correct sign and magnitude for g-2 namely, $a_e$=+α/2π. In our formulation the sign of $a_3$ is correctly positive, independent of cutoff, and the demand that $a_e$=+α/2π implies a unique value for Λ. This is in contradistinction to previous NR calculations of $a_e$ that employ electromagnetic vacuum fluctuations instead of self-fields; in the vacuum fluctuation case the sign of $a_e$ is cutoff dependent and the equation $a_e$=α/2π does not have a unique solution in Λ.

We investigate the domain of validity of previously proposed analytical wave functions for atomic quantum-defect theory. This is done by considering the fine-structure splitting of alkali-metal and singly ionized alkaline-earth atoms. The Landé formula is found to be naturally incorporated. A supersymmetric-type integer is necessary for finite results. Calculated splittings correctly reproduce the principal features of experimental values for alkali-like atoms.

It is well known that the nonlinear Schrödinger equation is the generic envelope description of nonlinear wave trains in the small-amplitude limit. V. E. Zakharov and E. A. Kuznetsov [Physica 18D, 455 (1986)] have shown that for many systems integrable via inverse-scattering techniques it is possible, through the use of multiscale techniques, to derive the nonlinear Schrödinger Lax pair from the Lax pair of the system that is being modulated. It will be shown that this technique of ‘‘multiscale averaging’’ can be applied to the sine-Gordon theory to obtain not only the nonlinear Schrödinger Lax pair from the sine-Gordon Lax pair, but also the nonlinear Schrödinger spectral data, conservation laws, Θ-function solutions, and reality constraint from their sine-Gordon counterparts. This allows a physical interpretation of the mathematical elements of the nonlinear Schrödinger envelope in terms of the physical characteristics of the sine-Gordon system that is being modulated.

The electron affinity of small clusters of rare-gas atoms like ${\mathrm{Ar}}_{\mathrm{n}}$, ${\mathrm{Kr}}_{\mathrm{n}}$, and ${\mathrm{Xe}}_{\mathrm{n}}$ is calculated using a dielectric continuum model. Thus, we obtain for ${\mathrm{Xe}}_{\mathrm{n}}$ a positive electron affinity for approximately n>10. For small clusters (n≃12) the excess electron is only weakly bound in a diffuse orbital. For larger clusters (n>25) we conclude that ${\mathrm{Xe}}_{\mathrm{n}}^{\mathrm{-}}$ is very stable. The clusters ${\mathrm{Kr}}_{\mathrm{n}}^{\mathrm{-}}$ and ${\mathrm{Ar}}_{\mathrm{n}}^{\mathrm{-}}$ are strongly stable only when they are much larger (n>100).

Computed lifetimes for the 3p ${}_{}{}^2{}_{}{}^{}P_{1/2}^{}{}_{}{}^{}$, P3$p^2$$P_{3/2}$, 3d ${}_{}{}^2{}_{}{}^{}D_{3/2}^{}{}_{}{}^{}$, and 3$d^2$$D_{5/2}$ levels in the sodium isoelectronic sequence are presented for Z=11–54, 74, 79, 90, and 92. These calculations agree well with recent high-precision lifetime measurements, and conflict with some ab initio estimates which indicated lifetimes shorter than those experimentally observed. Our calculations involve the inclusion of experimental energy-level data and the use of a realistic model potential, which utilizes comprehensive spectroscopic measurements that have recently become available. We also demonstrate that the core-polarization effects are significant and must be included to obtain agreement with experiment. As part of the study, we used the existing data base and Dirac-Fock methods to determine new values for ionization potentials in this sequence.

Doubly excited ridge states of atoms are states in which two electrons are comparably excited. We introduce an approach which consistently uses wave functions of the pair of electrons rather than products of single-electron functions. Sequences of ridge states out to very high excitation are computed compactly and easily and are organized as Rydberg series converging onto the double-ionization limit. The relevant angular and radial correlations in such states are built in analytically, so that the numerical work with a single variational parameter needs to handle only a final radial equation. The work is set within the context of the Wannier theory for pair escape near threshold, and the numerical eigenvalues for the autoionizing ridge states are compared with other theoretical calculations where available.

We propose a method of solution of the many-body Schrödinger equation that involves an expansion of the wave function in terms of a finite, time-dependent basis of a relevant subspace. The equations of motion for the expansion coefficients generalize previous proposals of approximate dynamics. The method is illustrated in the case of an N-particle system with an SU(2) Hamiltonian, and it is shown that it improves the approximation that disregards off-diagonal elements of the dynamical matrices.

Phase shifts, differential, total, and momentum-transfer cross sections are calculated using a pseudopotential approach for the elastic scattering of electrons by argon atoms in the impact energy range 0–20 eV. In the calculations, we use a pseudopotential for the electron-argon system based on a norm-conserving ionic pseudopotential derived by Bachelet et al. [Phys. Rev. B 26, 4199 (1982)] and screened by an exchange-correlation model potential suitably chosen for electron-scattering studies. The results are found to be in excellent agreement with existing experimental and theoretical data, proving the usefulness of this approach.

In the effective-potential description for low-energy scattering involving a spinless complex (a body with internal structure), the nonadiabatic corrections are sometimes disguised in momentum-dependent terms. These are distinct from energy-dependent corrections. A general procedure is given here by which all the momentum-dependent corrections can be converted into nonadiabatic corrections in truly local form. Circumstances under which an expansion of the effective potential, in terms of the adiabatic term plus nonadiabatic and energy-dependent corrections is allowed and forbidden, are discussed. An example for the latter is in the case of near degeneracy in the spectrum of the complex or in the extrapolation of the effective potential to short-distance behavior. This indicates that certain claims of ‘‘saturation effect’’ at short distances in low-energy electron-atom scattering are invalid.

The theory of spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ relaxation due to hyperfine coupling in ${}_{}{}^3{}_{}{}^{}$${\mathrm{He}}^{+}$ is presented. Comparison is made between the theory and recent experiments which utilized spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ as a charged-particle-beam target. At high He pressures the relaxation rate decreases with pressure. In addition, charge-transfer processes with foreign gases suppress ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ depolarization by reducing the ${}_{}{}^3{}_{}{}^{}$${\mathrm{He}}^{+}$ concentration.

Electron-energy-loss spectra have been measured for the acetylene molecule (X ${}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$) in the angular range of 1°–7°. The incident energy was 1 keV, the excitation energy range was 0–50 eV, and the energy resolution was 0.6 eV (full width at half maximum). Special attention has been given to the broad and intense structures appearing at 13.3 and 15.7 eV, which have been recently studied by photoionization techniques. The generalized oscillator strengths for these transitions have been determined and extrapolated to the optical oscillator strengths. Their values are 0.21±0.03 and 0.54±0.08, respectively. A new electronic transition has also been observed at 18.7 eV.

The process of two-electron capture has been studied for the collision systems ${\mathrm{Ar}}^{+5\mathrm{}}$+${\mathrm{D}}_2$ at 500 and 1000 eV (v=0.022 and 0.032 a.u.). The total laboratory energies of the deuterons emitted in the Coulomb explosion of the target following double capture were measured in coincidence with the charge-changed projectiles detected at selected scattering angles. Using a computer simulation of the kinematics of these reactions, we were able to deduce the effective two-electron binding energies for ${\mathrm{D}}_2$ as a function of collision energy. The results indicate that at these low energies the Franck-Condon principle is not applicable.

The existence of metastable states of negatively charged nitrogen ions ${\mathrm{N}}^{\mathrm{-}}$ has been confirmed following dissociation of (0.5–1.0)-MeV ${\mathrm{N}}_2$, NO, and ${\mathrm{NO}}^{\mathrm{-}}$ molecular projectiles in an argon gas target, by two different experimental procedures. The most intense reaction is the dissociation of ${\mathrm{NO}}^{\mathrm{-}}$ after electron capture, ${\mathrm{NO}}^{\mathrm{-}}$→${\mathrm{N}}^{\mathrm{-}}$+${\mathrm{O}}^{\mathrm{-}}$, which has a cross section of the order of ${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^2$. This cross section is much larger than that of the reaction ${\mathrm{N}}_2$→N+${\mathrm{N}}^{\mathrm{-}}$ and comparable with that of ${\mathrm{O}}_2^{\mathrm{-}}$→${\mathrm{O}}^{\mathrm{-}}$+${\mathrm{O}}^{\mathrm{-}}$. A lower limit of 1.6 μsec was obtained for the mean lifetime of the ${\mathrm{N}}^{\mathrm{-}}$ ions.