We report results of an electron-beam-dump search for neutral particles with masses in the range 1 to 15 MeV and lifetimes τ between ${10}^{\mathrm{-}14}$ and ${10}^{\mathrm{-}10}$ s. No evidence was found for such an object. We fule out the existence of any 1.8-MeV pseudoscalar boson with τ>8.2×${10}^{\mathrm{-}15}$ s and an absorption cross section in matter less than 1 mb per nucleon, and exclude τ>1×${10}^{\mathrm{-}14}$ s were its cross section to equal 50 mb per nucleon. In conjunction with measurements of the electron’s anomalous magnetic moment, this experiment shows that the narrow positron peaks observed in heavy-ion collisions at the Gessellschaft fu¨r Schwerionenforschung are not due to an elementary pseudoscalar.

We propose a laboratory experiment to produce and detect a light neutral pseudoscalar particle that couples to two photons. The pseudoscalar would be produced by a (real) photon from a laser beam interacting with a second (virtual) photon from a static magnetic field; it would be detected after it reconverts to a real photon in a duplicate magnetic field. The bounds on the coupling constant that could be obtained from a null result in such an experiment compete favorably with astrophysical limits and would substantially improve those from direct measurements.

The S-matrix pole structure of the ${\mathrm{J}}^{\mathrm{\pi }}$${=(3/2\mathrm{}}^{+}$ resonance in ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ is obtained from R-matrix parameters for the system. Two poles are found on different unphysical Riemann sheets. One is a conventional resonance that is primarily responsible for structure in the n-α total cross section. The other is a ‘‘shadow’’ pole that contributes to the large values of the cross section for the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(d,n${)\mathrm{}}^4$He at low energies. It is the first experimental evidence for the existence of shadow poles in nuclear and particle physics. The two-pole structure of the resonance accounts for different peak positions and widths in the amplitudes for the n-α total cross section and for the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(d,n) reaction cross section.

A kinematically complete experiment on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(π,dN)N absorption in flight reveals an absorption mechanism in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ with only one spectator nucleon. This adds to the evidence that modes beyond the two-nucleon mechanism contribute to the pion absorption process.

The nuclear magnetic moments of ${}_{}{}^{213}{}_{}{}^{}\mathrm{Ra}$ and ${}_{}{}^{225}{}_{}{}^{}\mathrm{Ra}$ have been measured at ISOLDE (isotope separator at the CERN synchrocyclotron) by observation of the Larmor precession of optically pumped atoms in a fast beam. The results ${\mu }_I$(${}^{213}$Ra)=0.6133(18)${\mu }_N$ and ${\mu }_1$${(\mathrm{}}^{225}$Ra)= -0.7338(15)${\mu }_N$ provide an accurate test of ab initio and semiempirical calculations from optical hyperfine structures in Ra i and Ra ii. .AE

We study the influence of a strong resonant driving field on the spectral properties of a single, cavity-confined, two-level atom. Under conditions of atom-cavity resonance, the lines of the Mollow resonance fluorescence triplet are found to narrow with increasing driving field strength, indicating a dynamical decoupling of the atom from the vacuum field.

We present results showing narrow resonant behavior in a collisionless four-wave mixing interaction in atomic sodium. In a two-level system the excited state radiatively decays to the ground state at rate ${\gamma }_{\mathrm{sp}}$, and the two states decay to the reservoir at ${\gamma }_1$ and ${\gamma }_2$. If ${\gamma }_1$ and ${\gamma }_2$ equal the transit time, the resonance width is determined by the total decay rate out of state 2. In the presence of additional excited-state decay to another state, a narrow resonance with width ${\gamma }_1$ is oberved as a result of the effect of the additional excited-state decay on the magnitude of the ground-state probability amplitude.

Histograms of the normalized values of the helicity and the smallest principal component of the rate of strain are found for one realization of a ${128}^3$ isotropic simulation of forced turbulence. In regions of high dissipation the strain has a structure indicating sheets. Peak-to-valley ratios in the helicity histograms are less than in other numerical flows and this ratio decreases further when the helical large scales are filtered or active regions characterized by the strain are sampled.

The ${\mathrm{H}}^{\mathrm{-}}$ formation process has been investigated in a magnetically filtered multicusp source operated at a modest plasma density. By use of a mixture of xenon and hydrogen gas, it is found that the production of ${\mathrm{H}}^{\mathrm{-}}$ ions in the tandem discharge geometry is consistent with a model of dissociative attachment of low-energy electrons to vibrationally excited hydrogen molecules.

Neutral atoms photodesorbed by monochromatic synchrotron radiation between 7 and 30 eV from monolayers and multilayers of Ar and Kr on Ru(001) have been detected by positionization and lock-in mass spectrometry. From multilayers, primary processes leading to desorption are surface and bulk excitons of various orders, ionic excitations, and two-step processes (photoelectron-induced excitons). Ar monolayers show surface excitons besides ionic excitations. The acting mechanisms are briefly discussed.

We propose a new approach to describe the many-body effects in the lattice dynamics of metals. We represent here the electron charge density in each unit cell by an expansion over the multipole moments, where the coupling of the fluctuating multipoles to ion displacements gives rise to the observed many-body effects. This model reproduces the observed surface-phonon dispersion curves and explains also their anomalous features.

We present rigorous results on a phase in antiferromagnets in one dimension and more, which we call a valence-bond solid. The ground state is simply constructed out of valence bonds, is nondegenerate, and breaks no symmetries. There is an energy gap and an exponentially decaying correlation function. Physical applications are mentioned.

We give results from an accurate determination of the partition function of the three-dimensional Ising model on lattices of size up to ${10}^3$. We compute the two complex zeros of Z closest to the real temperature axis. From a finite-size scaling analysis of these, we get the estimates ν=0.6295(10) for the correlation-length exponent and φ=52.2(7) for the angle between the first two zeros and the real u=$e^{\mathrm{-}4\beta }$ axis.

We have studied electron tunneling in ultralow-capacitance silver particles sandwiched between artificial tunnel barriers. We have observed steps in the current-voltage characteristics of these systems with voltage widths of e/C (C being the capacitance of the particles), and associated current rises of e/RC. This represents the first observationof the so-called Coulomb staircase, expected for the charging of particles with extremely small capacitance by a discrete number of electrons.

Electron attachment to water clusters was explored by the quantum path-integral molecular-dynamics method, demonstrating that the energetically favored localization mode involves a surface state of the excess electron. The cluster size dependence, the energetics, and the charge distribution of these novel electron-cluster surface states are explored.

Shubnikov–de Haas and persistent-photoconductivity measurements are used to study the mobility, the free-electron density (n), and the occupancy of the DX center in heavily doped n-GaAs [Si,Sn] as a function of doping level and hydrostatic pressure. The results show that the DX center produces a resonant donor level between the Γ and L conduction-band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration-mode measurements indicates that the DX level can be identified with ${\mathrm{Si}}_{\mathrm{Ga}}$.

The quasiparticle band structures of Na and Li are calculated by evaluation of the electron self-energy to first order in the dynamically screened Coulomb interaction. For Na, our results explain quantitatively the large bandwidth reduction observed in recent angle-resolved photoemission experiments. The inclusion of exchange-correlation effects in the dielectric screening and the requirement that the energy dependence of the electron Green’s function be treated self-consistently are found to be important in our obtaining the correct bandwidths.

The electronic states and dynamical properties of dilute liquid K-KCl solutions are studied by simultaneous integration of the coupled time-dependent Schrödinger equation for the excess electrons and classical equations of motion for the ions. Excess electrons are found to spend most of their time in localized F-center states, whose size and shape can fluctuate significantly. Transport is mostly due to jumps between localized states, with a jumping rate ≃3×${10}^{12}$ ${\mathrm{s}}^{\mathrm{-}1}$. The optical excitation spectrum and the electron diffusion coefficient are calculated and found to be in agreement with the experimental data.

Quantum statistics of an ideal gas of Bose particles predicts a phase boundary for Bose-Einstein condensation. The expected relation for the saturated-gas density, n=${\mathrm{CT}}^{3/2}$, is observed for an exciton gas in ${\mathrm{Cu}}_2$O by time resolution of the energy spectrum.

The effect of electron correlation on bond alternation of polymers is clarified by calculation of the electron correlation function. Instead of the extended Hubbard model, we start from the full Coulomb interaction with any strength and range. In contrast to the other theories, our results disclose the following: (1) When the interaction range is short the bond alternation monotonically decreases with increasing strength. (2) Extension of the interaction range will increase bond alternation. (3) If the electron-lattice coupling is strong, bond alternation is suppressed by electron repulsion; but if it is weak the bond alternation is mainly caused by repulsion.