It is well known that ionized atoms cannot be both very negative and stable. The maximum negative ionization is only one or two electrons, even for the largest atoms. The reason for this phenomenon is examined critically and it is shown that electrostatic considerations and the uncertainty principle cannot account for it. The exclusion principle plays a crucial role. This is shown by proving that when Fermi statistics is ignored, then the degree of negative ionization is at least of order $z$, the nuclear charge, when $z$ is large.

A one-dimensional linear analysis is presented of the startup of a free-electron-laser oscillator. The model treats electron-beam pulses of arbitrary shape and many significant three-dimensional effects are included heuristically. A closed equation is derived for the ensemble-averaged electromagnetic energy density matrix which contains the spontaneous emission as a source, and which represents the small gain per pass and losses via coefficient matrices. Numerical solutions are compared with the Stanford experimental results.

Finite—ion-Larmor-radius modification of lower-hybrid waves in the ACT-1 toroidal plasma at frequencies $\omega \simeq \mathrm{}(4-8)\mathrm{}{\omega }_{\mathrm{ci}}$, close to the lower-hybrid resonance layer, is observed experimentally. Probe and C${\mathrm{O}}_2$ laser scans of the wave amplitude show patterns at frequencies above each ion cyclotron harmonic. Modeling with a hot-ion, electrostatic-ray-tracing code and Fourier reconstruction of the wave form provide evidence for linear mode conversion to hot-plasma waves.

Three-dimensional evolution of current-driven ion-acoustic instability in a magnetic field is studied numerically in the quasilinear framework. The electron-cyclotron resonance (anomalous Doppler effect) is shown to be responsible for the observed rapid cross-field heating of electrons. The formation of a high-energy ion tail, previously studied in a one-dimensional model, remains effective both parallel and perpendicular to the electron current and governs the saturation of the instability, subsequent decay, and anomalous $k$ spectrum dominated by cross-field modes.

The particle confinement time in field-reversed configurations has been experimentally investigated in the FRX-$C$ device. The measured confinement times of 70 to 190 μs are consistent with $\frac{R^2}{{\rho }_{i0\mathrm{}}}$ scaling and are in good agreement with theoretical predictions of lower-hybrid-drift-induced particle transport.

This paper reports the results of an x-ray diffuse scattering study of the melting transition of monolayer xenon on the surface of single crystals of exfoliated graphite. It is found that the two-dimensional xenon solid melts into an orientationally ordered liquid (or hexatic) phase. The temperature dependence of the orientational correlations suggests that the hexatic phase exists as a consequence of the continuous melting process, not the substrate.

Photoemission and resistivity measurements of granular Pd samples are reported which exhibit a metal-insulator transition with decreasing metal fraction. As the net metallic concentration is reduced the resistivity passes through $\sqrt{T}$, $\ln T$, $\exp \{+{\left(\frac{A}{T}\right)}^{\frac{1}{4}}\}$, and insulating $\exp \{+{\left(\frac{B}{T}\right)}^{\frac{1}{2}}\}$ temperature dependences while the photoemission results show a continuous reduction in the Pd local density of states within 1 eV of the Fermi level. In the insulating regime a gap is observed in the photoemission spectrum.

Experimental evidence is presented for solid-state effects on the $4d$ photoionization around the Cooper minimum for a few transition metals: Ag, Pd, Mo, and Zr. The $4d$ parital cross section appears to be very sensitive to the $d-d$ interaction; thus, the antibonding $d$ states have a more atomiclike cross section than the bonding $d$ states.

This Letter reports on changes in magnetization when Cd${\mathrm{Cr}}_2$${\mathrm{Se}}_4$ is subject to optical pumping. The Faraday rotation of a second beam was used as a monitor. Circularly polarized pumping was found an order of magnitude more effective than unpolarized pumping. Models using magnetic coupling mechanics of first and second order in the exchange energy, respectively, account quite well for the observations.

In polycrystalline samples, NMR "powder spectra" are broad and much structural information is lost as a result of the orientational disorder. In this Letter Fourier-transform NMR in zero magnetic field is described. With no preferred direction in space, all crystallites contribute equivalently and resolved dipolar splittings can be interpreted directly in terms of internuclear distances. This opens the possiblity of molecular structure determination without the need for single crystals or oriented samples.

It is shown that a time-varying magnetic field through the superconducting loop in a rf SQUID can cause transitions between the fluxoid states. Experimental observation of these transitions should constitute evidence for the quantum mechanical character of the macroscopic variable, the total flux through the loop. In the case that dissipation is characterized by linear damping in the quasiclassical limit, the photon absorption process depends critically on the ratio of the resistance of the weak link to the fundamental unit of resistance $\frac{h}{e^2}$.

Measurements on 2790-at.-ppm $\mathrm{Cu}\mathrm{Mn}$ between 0.3 and 30 K and in fields to 75 kOe show a small, field-dependent specific-heat anomaly that is, apart from magnitude, in qualitative agreement with recent theoretical predictions for the spin-glass transition. The results support the approximate validity of mean-field theory for real spin-glasses, and, indirectly, the reality of the predicted phase transition. The field dependence of the entropy is compared with the Parisi-Toulouse projection hypothesis.

Reflected broadband (multichannel) coherent anti-Stokes Raman scattering was used to measure the ring-breathing-mode vibrational frequencies of neat liquid benzene, neat liquid deuterated benzene (benzene-$d_6$), and mixtures of these under shock compression to pressures up to 1.53 GPa. The pressure-induced frequency shifts determined for neat benzene agree with those obtained previously by use of backward stimulated Raman scattering. Evidence for nearest-neighbor effects on the vibrational frequency shifts is presented.

The equilibrium polymerization of chains and rings can be viewed as a bicritical point in a suitably augmented parameter space. The polymerization of chains in equilibrium with rings is described by the cross-over exponent, $\phi$, for quadratic anisotropy in the $\mathrm{O}\mathrm{}\left(n\right)\mathrm{}$ vector model with $n\to 1$. While the total mass fraction of polymerized material is described by the Ising exponent $\alpha$, the mass fraction of chains separately (and therefore also of rings) is described by the exponent ${\alpha }_c=\phi -1+\alpha$.

It is shown numerically that the stability intervals for limit cycles of the circle map form a complete devil's staircase at the onset of chaos. The complementary set to the stability intervals is a Cantor set of fractal dimension $D=0.87\mathrm{}$. This exponent is found to be universal for a large class of functions.

The results of a search for fractional charges on 24 steel spheres with a total mass of 720 μg are reported. No fractional charges, spurious or otherwise, greater than $0.15e$ were found.

In this application of Dirac phenomenology a relativistic impulse approximation is used to describe proton-nucleus elastic scattering at intermediate energies. The results demonstrate the superiority of this relativistic treatment over the nonrelativistic impulse approximation, especially with regard to spin observables. The method has important implications with respect to the extraction of neutron distributions.

A global study of the mass, energy, and angular distributions of all products formed in collisions of 180-MeV protons with ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ is reported. These data are compared with calculations based on intranuclear-cascade-plus-evaporation, preequilibrium hybrid, and semiempirical models. It is found that there is evidence for enhanced energy deposition in nucleon-nucleus collisions relative to predictions of intranuclear cascade calculations. In contrast, preequilibrium calculations produce stronger energy damping, more consistent with observed data.

A novel approach to the determination of signs of quadrupole deformations of high-spin isomers is presented. Recoiling ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$(${10}^{+}$) isomers, populated by the reaction ${}_{}{}^{45}{}_{}{}^{}\mathrm{Sc}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, p2n)$,, are polarized by passage through an array of tilted carbon foils and stop in a Cd single crystal. The observation of a quadrupole precession serves to measure both the induced nuclear polarization and the sign of the quadrupole moment. A positive sign of $Q\left({}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}\right({10}^{+}\left)\right)$ has been deduced.

In chemical reactions between species $A$ and $B$, which interact within a background gas at high density $N$ via a non-Coulomb attraction, it is shown that the key sequence of physical mechanisms for transport is diffusion at asymptotic $\mathrm{}(A-B)\mathrm{}$ separations $R$, diffusional drift at intermediate $R$, and pure diffusion at the reaction sink. Reduction in gas density $N$ causes the onset at the sink of pure inward drift. For pure Coulomb species in a gas, the corresponding sequence is asymptotic drift, diffusional drift, and pure diffusion at high $N$, and diffusional drift at lower $N$.