Experimental angular distributions are reported for electron capture by protons of 2.82, 5.42, and 7.40 MeV from He. A clear peak in $\frac{d\sigma }{d\theta }$ appears near the Thomas angle of 0.47 mrad for the higher two bombarding energies, supporting the widely held belief that the double-scattering mechanism plays an important role for high-velocity electron capture.

Resonant three-photon ionization of Ba to a structureless continuum via $6snd$ Rydberg states was performed in the range $19<~n<~30$. It is shown that state mixing in the Rydberg states strongly affects the photoion and photoelectron yields as well as the angular distributions of photoelectrons. The experimental results are explained on the basis of a three-channel quantum-defect theory for the perturbed Rydberg series.

Emission at the Na $D$ lines has been observed from the intersection of crossed molecular beams of K and NaCl irradiated by a cw dye laser in the wavelength range 600 to 700 nm. The signal is approximately independent of wavelength in this region but decreases near 700 nm. This three-beam signal is attributed to ${\mathrm{Na}}^{*}$ formed by photoexcitation of the NaKCl reaction complex. The effective two-body cross section is circa ${10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$ in a laser field of ∼3000 W/${\mathrm{cm}}^2$.

High levels of current and energy fluctuation appeared on an intense electron beam traversing a smooth drift tube in which two or more wall perturbations were inserted. The frequency of the fluctuations depended strongly on the geometry of the drift region. Theory and particle simulation showed that wall perturbations drastically altered the motion of the beam causing strong coherent current modulation.

A discrete path-integral representation for the partition function of a two-level tunneling system coupled to acoustic phonons is derived. This representation allows calculation of properties in the whole coupling range. As a function of the coupling there is an abrupt (ground-state) transition from the weak-coupling regime to the self-trapped state. From a moment analysis of the time-dependent spin-correlation function it follows that there is loss of phase coherence if the coupling exceeds the critical coupling. In this regime the mean effective tunneling rate vanishes linearly with temperature.

A phenomenological mean-field model of globules for a three-component microemulsion is presented and typical phase diagrams are obtained. Two types of two-phase coexistence are found as well as a three-phase equilibrium. The topology of the phase diagram is determined by the ratio of the critical globule radius to the natural bending radius and by the radius dependence of the attractive interactions.

A rod-to-coil conformational transition is demonstrated for polydiacetylene, 4-butoxycarbonylmethylurethane, in solution. With use of quasielastic light scattering, a coillike structure with relatively small hydrodynamic radius is indicated for the polymer in a good solvent, CH${\mathrm{Cl}}_3$. As the proportion of nonsolvent (hexane) is increased, the hydrodynamic radius increases abruptly to a value consistent with the molecule being a rigid rod.

Photoinduced absorption spectra of trans-${\left(\mathrm{CH}\right)}_x$ and -${\left(\mathrm{CD}\right)}_x$ between 15 and 8000 ${\mathrm{cm}}^{-1\mathrm{}}$ are presented. The data demonstrate that the photoexcitations result in charged solitons which are pinned. The isotope shifts observed in trans-${\left(\mathrm{CD}\right)}_x$ provide new information about the internal modes of the excitation. Saturation of the signal at low levels is discussed in terms of a photoassisted decay channel.

The continuum electron-phonon system is studied for ${\mathrm{}\left(\mathrm{AB}\right)\mathrm{}}_x$-type polymers with an average charge of $1+\hat{\rho }$ per site. For $\hat{\rho }=0\mathrm{}$ spontaneous bond dimerization and soliton excitations are possible if $\alpha <{\Delta }_0$, where $\alpha$ is the intrinsic gap and ${\Delta }_0$ is the spontaneous dimerization gap when $\alpha =0\mathrm{}$. Recently observed neutral-to-ionic transitions in mixedstack organic compounds are reinterpreted as the onset of such bond dimerization. For $\hat{\rho }\ne 0$ spontaneous order and a soliton lattice are found even if $\alpha >~{\Delta }_0$. The electronic gap then has a maximum at a finite $\hat{\rho }$.

An order parameter for spin-glasses is defined in a clear physical way: It is a function on the interval 0-1 and it is related to the probability distribution of the overlap of the magnetization in different states of the system. It is shown to coincide with the order parameter introduced by use of the broken replica-symmetry approach.

Surface electron-energy-loss fine structure above the oxygen K edge and the nickel $M_{23}$ edge has been investigated for several oxygen coverages on a Ni(100) system. In agreement with recent results, it is found that at low coverage the oxygen chemisorbs in the hollow position (0.85 ± 0.05 Å above the nickel plane). On increasing the coverage, islands of nickel oxide are formed. This technique for determining the local geometry of adsorbates is especially useful for light surface species like carbon, oxygen, and nitrogen.

Magnetotransport of two-dimensional electrons and holes was studied in magnetic fields up to 300 kG and temperatures down to 0.5 K. In addition to previously reported structures at Landau-level filling factors $\nu =\frac{1}{3} \mathrm{and} \frac{2}{3}$, new structures were resolved at $\nu =\frac{4}{3}, \frac{5}{3}, \frac{2}{5}, \frac{3}{5}, \frac{4}{5}, \mathrm{and} \frac{2}{7}$. The results suggest that fractional quantization of the Hall effect exists in multiple series, each based on the inverse of an odd integer.

The six-quark system has been studied in a nonrelativistic quark model which incorporates some of the features expected from chromodynamics. With use of a large basis space which includes color, spin, and orbital excitations, and a Hamiltonian completely determined by previous studies of baryon structure, a number of the features of low-energy nuclear physics are derived. When supplemented with a reasonable model for one-pion exchange, the residual color interactions give a good account of the properties of the deuteron.

Cross sections for the reactions ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(\gamma , n_0+n_2)$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(\gamma , p_0)$ have been measured for photon energies in the range 60-120 MeV. Comparison is made between the data and simple calculations based on the modified quasideuteron and quasifree knockout models, which suggests that the former dominates.

The energy levels of the nuclei ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tm}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Er}$ have been studied up to high spins ($\frac{61}{2}$ and $\frac{53}{2}$, respectively) and rotational frequencies (480 and 417 keV, respectively) by inbeam spectroscopic methods. On the basis of blocking arguments, the results show that the second backbending in the yrast band of even-even nuclei near $N=90\mathrm{}$ is due to $\pi h_{\frac{11}{2}}$ protons.

Positron and $\delta$-ray spectra have been measured in coincidence with quasielastic scattered particles and fission fragments from the bombardment of Pd, U, and Cm targets with U beams of energies between 5.9 and 8.4 MeV/u. For collisions leading to a fission reaction, the atomic positron and $\delta$-ray spectra fall off more steeply at high energies than expected from calculations based on pure Rutherford trajectories. A quantitative analysis of this effect is in accord with a nuclear contact time of about ${10}^{-21\mathrm{}}$ s.

Absolute cross sections for electron capture into different $\mathrm{nl}$ states for ${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{6+}$, ${\mathrm{O}}^{6+}$, ${\mathrm{Ne}}^{6+}$-He and ${\mathrm{N}}^{6+}$, ${\mathrm{O}}^{6+}$-${\mathrm{H}}_2$ collisions in the 0.15-0.5 a.u. velocity range have been measured. These cross sections strongly depend on the collision velocity, in contrast with the total capture cross sections and the cross sections for capture into different $n$ states. Similarities in ${\sigma }_{\mathrm{nl}}\mathrm{}\left(v\right)\mathrm{}$ dependences for different ions with the same ionic charge are found which are attributed to the hydrogenlike structure of the energy levels of these ions.

This Letter reports the first optogalvanic detection of molecular ions (${\mathrm{N}}_2^{+}$, C${\mathrm{O}}^{+}$). The present results indicate a new optogalvanic mechanism involving direct alteration of ion mobility by laser excitation, due to a difference in charge-exchange collision rates for excited- vs ground-state ions. The technique provides a sensitive probe of ions in the cathode dark space. Ion kinetic energies are obtained from Doppler shifts and internal energy distributions from rotational line intensities.

The vibrational spectrum of hydrogen chemisorbed on Si(111)7×7 is studied with high-resolution infrared spectroscopy. A unique chemisorption site characterized by a dangling bond perpendicular to the surface is identified for the first time and assigned to a silicon atom at the bottom of a deep hole formed by long-range reconstruction.

Phonon-assisted resonances with surface bound states are shown to give important effects in time-of-flight spectra of He scattered from a LiF (100) surface. The comparison with theory indicates that inelastic resonances produce sharp distortions in the response of the surface phonon spectrum. At low temperature, theory predicts that resonances can locally amplify the phonon response by an order of magnitude.