The phenomenon of antibunching of photoelectric counts has been observed in resonance fluorescence experiments in which sodium atoms are continuously excited by a dye-laser beam. It is pointed out that, unlike photoelectric bunching, which can be given a semiclassical interpretation, antibunching is understandable only in terms of a quantized electromagnetic field. The measurement also provides rather direct evidence for an atom undergoing a quantum jump.

O${\mathrm{H}}^{-}$ ions were injected into a "Pelletron"-type tandem electrostatic accelerator and dissociated in the high-voltage terminal by passage through gas and foil strippers. The dissociated protons were further accelerated and their energy spectrum was analyzed in a 90° magnet. The ions stripped in foils exhibit the effects of a Coulomb explosion and a wake field. The stripping in gas appears to be of a different character. An explanation of this difference is proposed, based on the assumption of a strictly sequential removal of electrons from the ions.

Coherent states are completely parametrized in terms of multipole moments and their time derivatives. For hydrogen atoms excited by electron impact, our calculations show that these parameters provide new information concerning the final-state interactions of the hydrogen atom and the projectile electron. These large final-state-interaction effects make first-order perturbation calculations inappropriate. We present calculations which properly incorporate these dynamical effects. Experiments which determine these parameters are discussed.

A nonlinear theory of the collective processes in runaway tokamak discharges is presented. The conditions under which runaway electrons can be trapped in toroidal field ripples, leading to liner damage, are derived and compared with the experimental evidence.

Results are reported from a numerical-simulation study on the propagation of a rotating proton pulse and the formation and trapping of the resulting strong proton ring. It is observed that the self-magnetic field has a very pronounced effect on the dynamics of such systems, and is responsible for a somersaulting and energy spread of the beam.

Low-energy-electron diffraction $I-V$ spectra obtained from the clean, cooled $\mathrm{W}\mathrm{}\left\{001\right\}(\sqrt{2}\times \sqrt{2})R45\mathrm{}°$ surface structure at < 190 K display $2mm$ point-group symmetry, revealing a preferential orienting of equivalent domains. A further novel effect, that the half-order beam spectra ($\pm \frac{h}{2}, \pm \frac{h}{2}$) are identical in relative intensity to the ($\pm \frac{h}{2}, \mp \frac{h}{2}$) spectra but different in absolute intensity by a constant factor, uniquely establishes the space-group symmetry of the domains as $p2\mathrm{mg}$. A structure for a single-layer surface rearrangement is proposed.

We have determined the coordination of arsenic impurities in amorphous silicon-hydrogen alloys prepared by plasma decomposition of silane-arsine mixtures. From analysis of the extended x-ray absorption fine structure (EXAFS) on the arsenic $K$-shell absorption, we find at low arsenic concentrations that an average of two arsenic atoms in ten are fourfold coordinated with silicon. This is the first direct evidence for the occurrence of substitutional doping in an amorphous semiconductor.

${}_{}{}^4{}_{}{}^{}\mathrm{He}$ channeling measurements of minimum yields in single-crystal ${\mathrm{V}}_3$Si have been compared with those calculated with the Karlsruhe phonon density of states. Atomic displacements versus temperature and defect level have been so obtained. In situ radiation-damaged crystals show ∼ ${10}^{-1\mathrm{}}$-Å displacements and significant phonon stiffening. The observations explain the large changes in electrical resistivity in the defect state, and suggest that both thermal and static (defect) displacements diminish the electron-phonon/defect interactions, and to equivalent degrees.

We find that certain cyano Schiff bases and cyanobiphenyls with alkyl or alkoxy end groups exhibiting a bilayer smectic-$A$ phase also exhibit a reentrant nematic phase at high pressure. A tentative model of the bilayer smectic phase held in place via the attractive interaction of the alkyl or alkoxy chains accounts for most of our experimental results.

Impact ionization in semiconductors is shown to be strongly affected by temperaturetuning the band structure. In $〈100〉$ GaAs between 25 and 150°C the hole ionization rate $\beta$ is greater than the electron rate $\alpha$, whereas above this temperature $\alpha$ becomes greater than $\beta$. This effect is attributed to the sudden lowering of the electron threshold ionization energy close to 200°C and is shown to be evidence for the existence of the pseudogap between the ${\Gamma }_6-X_6$ and ${\Gamma }_7-X_7$ conduction bands.

An unusual temperature dependence of the quadrupole splitting of ${\mathrm{Fe}}^{2+}$ in $1T$-${\mathrm{Fe}}_{0.1}$${\mathrm{Ta}}_{0.9}$${\mathrm{S}}_2$ can be understood in terms of a magnetic modulation of the lattice electric-field gradient. This modulation is produced by a strictive interaction between the local lattice environment and the continuous low-spin-high-spin transition of ${\mathrm{Fe}}^{2+}$. The onset of the charge-density-wave instability can also be observed.

Computer modeling techniques are used to obtain information about the spin dynamics of model spin-glass Hamiltonians in the zero-temperature limit. The density of states and dynamic structure factor are computed numerically for the Edwards-Anderson and Mattis models with Heisenberg interactions. We find no evidence for spin-wave modes in the former; the latter shows spin-wave behavior characteristic of a disordered $\mathrm{XY}$ model.

The low-frequency fluctuations resulting from the nonlinear coupling of high-frequency fluctuations is shown to possess a $\frac{1}{f}$ noise spectrum.

The parity-nonconserving electron-nucleon interaction induced by neutral currents should give rise to a (thermodynamically stable) electric-dipole-moment density directed along the characteristic rotation axis $\hat{\omega }$ of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$; if observed, this would constitute the first example of parity nonconservation on a macroscopic scale. Although the magnitude of the effect depends critically on a poorly known chemical parameter, it quite probably lies within the experimentally detectable range.

We have studied the spin-parity structure of the $3\pi$ system produced opposite a proton or ${\Delta }^{++}$ in ${\pi }^{+}p$ interactions at 15 GeV/c. Our results suggest that the broad enhancement at 1.1 GeV, traditionally associated with the $A_1$, does not have the properties usually associated with a resonant state. We obtain similar results for the $A_3$ and $A_4$ enhancements.

Multifragment reactions are directly observed in heavy-ion-induced reactions using mica track detectors in $2\pi$ geometry. Sequential fission is found to be the major reaction channel in the reaction 800-MeV ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$+U; however, binary decay can still be identified. A small contribution of reactions with four fragments is observed for the first time in ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$- and ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$-induced reactions with uranium.

Eighteen $M1\mathrm{}$ transitions to the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ground state have been identified and their radiative widths measured in a study of the reaction ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}(n, \gamma )$ combined with the results from recent neutron transmission and elastic scattering measurements. In the excitation region between 7.37 and 8.23 MeV, ∼50% of the expected total $M1\mathrm{}$ strength in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ has been located.

A determination of the ratio of $g$ factors of the electron bound in the hydrogen atom to that of the free electron has been made. For the first time mass-independent terms in the theory are confirmed through the ${\alpha }^3$ radiative correction. Our result is $\frac{g_J({}_{}{}^1{}_{}{}^{}H, 1{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{})}{g_s\mathrm{}\left(e\right)\mathrm{}}=1-17.709\mathrm{}\left(13\right)\mathrm{}\times {10}^{-6\mathrm{}}$.

A laser excitation and ionization process is used to measure Cs-Ar interaction forces at long range. With energy densities of 1 J/${\mathrm{cm}}^2$, nonlinear excitation persists as far out as 70 Å at one atmosphere of Ar. This method provides extreme sensitivity (even single absorption events can be measured) which allows absolute measurements on the very far wing where absorption or fluorescence becomes vanishingly small.

The effects of a broadband (stochastic) pump on the parametric decay instability is studied experimentally. The growth rate decreases with pump bandwidth, $\Delta \omega$, as if only the fraction of the pump power within the resonance width is available to drive the instability, in agreement with theory. The saturated wave amplitude and suprathermal-electron production well above threshold are only slightly decreased (for bandwidths presently available, $\frac{\Delta \omega }{\omega }\lesssim 3\times {10}^{-3\mathrm{}}$) in agreement with simulation calculations.