A solid Kr film partially covered with ${\mathrm{O}}_2$ molecules is charged by a 0- to 10-eV electron beam. The electron energy dependence of the trapping cross section indicates that surface charging in the 0-2-eV range is due to stabilization via the ${}_{}{}^2{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}{{\mathrm{O}}_2}^{-}$ resonance, whereas in the 4-10-eV region, it is due to ${\mathrm{O}}^{-}\left({}_{}{}^2{}_{}{}^{}P\right)$ formed by dissociation of the intermediate ${}_{}{}^2{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$ state of ${\mathrm{O}}_2^{-}$.

We have observed a radially localized electron wave packet in a direct pump-probe experiment with 6-ps laser pulses. Use is made of the fact that both excitation and ionization take place predominantly close to the atomic core. A wave packet is created by coherently exciting Rydberg states of rubidium atoms around $n=41\mathrm{}$ with a pump pulse, and detected through ionization with a delayed probe pulse. The wave packet is observed to return to the core twice before the spreading becomes too large.

Using a time- and space-resolved coherent anti-Stokes Raman-scattering technique we study the dephasing of a polariton state that crosses and strongly couples with a two-phonon band. Polariton damping is found to be dominated by a direct and phonon-assisted disintegration of the polariton into the two-phonon continuum.

We present a method for the study of coherences between states of different excitation energies. This method is based on the occurrence of interferences between scattered and ejected electrons resulting from the electron impact excitation of autoionizing states. Preliminary measurements are presented which for the first time show such interference effects.

The dynamic behavior of chemical patterns induced by the coupling of chemical waves and hydrodynamic flows in the Belousov-Zhabotinskii reaction was investigated by 2D spectrophotometry and 2D velocimetry. Periodically varying deformations of chemical-wave profiles were observed in a reactive layer with a liquid/gas interface. They are due to an oscillatory hydrodynamic flow detected close to the layer surface.

A single electron orbiting in a Penning trap may be used to demonstrate that radiation appears in the vacuum in an accelerated reference frame. As the electron accelerates centripetally in the trap's magnetic field, the vacuum radiation excites its motion along the trap axis, which is in turn measurable with present electronic techniques. A realizable example of such a detector is presented.

Simulations of the growth of planar invasion percolation clusters exhibit novel dynamic scaling. The probability to invade a site at a distance $r$ from a reference site at a time $t$ after that site was invaded behaves as $N(r,t)=r^{-1\mathrm{}}f\left(\frac{r^D}{t}\right)$, where $D$ is the fractal dimension of the invaded region. The scaling function has the unusual timing behavior $f\left(u\right)\mathrm{}\sim u^a(u\ll 1)$ and $\sim u^{-b}\left(u\gg 1\right)$, with $a\simeq 1.4$, $b\simeq 0.6$, and growth occurring mainly around $r\sim t^{\frac{1}{D}}$.

Comparison between pressure-volume relations in helium at high pressures indicates that theoretical calculations based on gas-phase pair potentials are not adequate to explain recent x-ray experiments. Regardless of what potential is used, the gas-phase potentials give an equation of state that is much too stiff. It is shown that if the changes in the atomic wave functions induced by the high-pressure crystal environment are taken into account, then agreement between experimental and theoretical pressure-volume curves is greatly improved.

Combined proton-induced x-ray-emission and Rutherford-backscattering channeling measurements on <111> CdTe and ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}\mathrm{Te}$ ($x=0.04,0.1,0.3$) crystals reveal a narrowing of the Cd-sublattice channeling pattern relative to that of Te for $x=0.1\mathrm{}$ which does not exist for the other $x$ materials. This observation is attributed to enhanced vibrations of Cd atoms caused by the bonding changes induced by the Zn.

Spin and orbital magnetism of Fe ions in $\mathrm{sp}$ metals are reduced simultaneously from stable ionic ${\mathrm{Fe}}^{2+}$ to nonmagnetic behavior with increasing lattice pressure. The various data and interpretations of the host and temperature-dependent susceptibilities, of $3d$ spin dynamics, and of crystal fields of Fe ions in $\mathrm{sp}$ metals provide a new critical test of models on moment formation, $d-\mathrm{sp}$ exchanges, and the Kondo effect of $3d$ ions in $\mathrm{sp}$ metals. Fe in $\mathrm{sp}$ metals reflects basic features common to certain $4f$ systems, but with qualitative differences from the magnetism of Fe in noble metals.

The $T=0\mathrm{}$ behavior of the antiferromagnetic, nearest-neighbor, spin-1 chain is studied in a space of interactions which includes exchange anisotropy, bond alternation, and biquadratic exchange. We find that bond alternation can stabilize Néel order; that with sufficient biquadratic coupling the ground state may be spontaneously dimerized and simultaneously critical; and that there is a line of multicritical points where the Néel-ordered phase touches the manifold of isotropic coupling. Predictions of Affleck and Haldane regarding the criticality and ordering in these models are tested.

Measurements of the superconducting-to-normal transition in very-small-diameter In wires are presented. The results are discussed in terms of the currently accepted theory, in which dissipation below $T_c$ is attributed to thermally activated motion of the Ginzburg-Landau order parameter over the free-energy barriers which separate metastable states. This theory is consistent with our results for temperatures within about 0.2 K of $T_c$, but at lower temperatures it fails qualitatively. We suggest that the low-temperature behavior is dominated by quantum-mechanical tunneling through the free-energy barrier, and show that our results are consistent with this picture.

In order to understand some discrepancies in the recent Letter of Griniasty and Fishman, a study has been made of localization by a one-dimensional potential with slowly varying period. Away from the band center, localization is produced by local regions in which the Bragg condition allows backward scattering, and the result agrees with that obtained from perturbation theory. The localization length increases as the band center is approached in a manner that cannot be obtained directly from perturbation theory. It is shown that rounding errors set an upper bound to the localization length that would be determined from numerical studies of such a problem.

We show that a one-dimensional tight-binding electron moving in a slowly varying potential, $V_n=\lambda cos\left(\alpha n^{\nu }\right)$, where $n$ is the site index and $0<\mathrm{}\nu <1\mathrm{}$, has a mobility edge in its spectrum provided that $2\lambda$ is smaller than the total unperturbed bandwidth of the system. We study the nature of the localized and extended eigenstates of this system as a function of $\lambda$ and $\nu$.

The small positive ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ Knight shift in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ ($K_Y\simeq 2\times {10}^{-4\mathrm{}}$) reported by two groups is shown to require that the in-plane conduction band be formed from the $\pi$-bonding $\mathrm{Cu} {3d}_{\mathrm{xy}}$ and $\mathrm{O} 2p{\pi }_x,\left(y\right)\mathrm{}$ orbitals with $\mathrm{O}-2p\pi$-like antibonding states at the Fermi level. The negative $K_Y\simeq {10}^{-4\mathrm{}}$ reported by another group is consistent with either this $\pi$ conduction band or one formed from the $\sigma$-bonding $\mathrm{Cu} {3d}_{x^2-y^2}$ and $\mathrm{O} 2p{\sigma }_x,\left(y\right)\mathrm{}$ orbitals provided again that the Fermi-level states are largely oxygenlike.

We generalize the concept of level spacings to dissipative quantum maps. For periodically kicked tops with damping, we find linear and cubic level repulsions under conditions of classically regular and chaotic motion, respectively. The numerically obtained spacing distribution for the chaotic top appears to be universal: It compares favorably with the spacing distribution of general complex matrices of large dimension, the analytical form of which we also present.

We show that the usual derivations of the Lorentz-Dirac equation are not valid at points where the charge's path is a nonanalytic function of time, and that such points inevitably appear when the applied force is freely alterable in the future, i.e, when one considers questions of causality. This enables us to avoid causality violation in classical radiation reaction, making the usual hedge of quantum effects at small distances quite unnecessary.

A search for bursts of air-shower events from Hercules X-1 at energies above 50 TeV during the calendar period 2 April 1986 to 5 July 1987 yielded two significant bursts, both occurring on UT 24 July 1986. The events during these bursts were pulsed with a period of 1.235 68 s, significantly different from estimates of the contemporaneous x-ray period. The probability that this represents random statistical fluctuations of the background is estimated to be 2 × ${10}^{-5\mathrm{}}$. The muon content of the burst events is anomalous when compared with expectations from $\gamma$-ray showers.

A renormalizable and asymptotically free $\mathrm{O}\mathrm{}\left(N\right)\mathrm{}$ nonlinear $\sigma$ model is defined on a $p$-adic field. The effective potential is computed to the leading order in the $\frac{1}{N}$ expansion and it is shown that the mass term for the $\mathrm{O}\mathrm{}\left(N\right)\mathrm{}$ vector field is generated dynamically and the full $\mathrm{O}\mathrm{}\left(N\right)\mathrm{}$ symmetry is realized linearly. Also a discrete scale invariance of the classical action is shown to be violated because of quantum effects.

The low-energy spin excitations in the flux phase of the (2+1)-dimensional large-$n$ $\mathrm{SU}\mathrm{}\left(n\right)\mathrm{}$Heisenberg antiferromagnet are relativistic fermions coupled to a U(1) gauge field. These excitations are studied in the physical $n=2\mathrm{}$ case with the use of a $\frac{1}{n}$ expansion. A Chern-Simons term is included to remove gauge configurations forbidden in the $n=2\mathrm{}$ theory.