Thermal He scattering is proposed as a method to investigate two-dimensional gas-solid phase transitions down to very low coverages (∼0.1%). The two-dimensional Xe gas-solid phase transition on a clean and defect-free Pt(111) surface is studied. The heat of vaporization of the two-dimensional solid phase into the two-dimensional ideal gas phase is 1.1±0.1 kcal/mole.

Intramolecular scattering resonances in near-edge x-ray-absorption fine-structure spectra of chemisorbed molecules are used to determine the molecular orientation, bonding, and bond length. In particular, we demonstrate that the position of the $\sigma$ shape resonance relative to the Fermi level is directly correlated with the intramolecular bond length of the chemisorbed molecule. By comparison with the known molecular distances in the gas phase, changes induced by the chemisorption bond are revealed.

The first sizable single crystals have been prepared of plutonium compounds. The field and temperature dependence of magnetization of single-crystal PuSb is highly evocative of that of CeSb or CeBi, and PuSb gives anisotropy and magnetic structural behavior as expected from hybridization-mediated interionic interactions for moderately delocalized $f$-electron systems.

The ground-state energy of excitonic molecules is evaluated as a function of the ratio of electron and hole masses, $\sigma$, with use of the Green's-function Monte Carlo method. For all $\sigma$, the Green's-function Monte Carlo energies are significantly lower than the variational estimates and in favorable agreement with experiments. In excitonic rydbergs, the binding energy of the positronium molecule ($\sigma =1\mathrm{}$) is predicted to be - 0.06 and for $\sigma \ll 1$, the Green's-function Monte Carlo energies agree with the "exact" limiting behavior, $E=-2.346+0.764\mathrm{}\sqrt{\sigma }$.

The gravitational field produced by an infinite static cylinder with a net spin polarization along its symmetry axis is considered, in the context of the Einstein-Cartan theory of gravitation. An exact solution is obtained, which shows that the cylinder's spin angular momentum gives rise to magneticlike components in the gravitational field outside.

A new computational approach to time-dependent transition amplitudes in multistate quantum systems is developed. The method, which avoids explicit computation of matrix eigenvectors, is based upon the recursive generation of residues of Green's functions. The residues are computed from the eigenvalues of several tridiagonal matrices. Numerical results are presented for multiphoton excitation of overtones in a system with over 3000 states.

The theory is developed for the second moment of the first-passage-time variable of statistical flows with memory (non-Markovian dynamics) and applied to a model of free non-Markovian Brownian motion. More importantly, a novel quantity, called memory index $\sigma$, is introduced, giving a quantitative characterization of the influence on non-Markovian behavior. Thus, a theoretical or experimental evaluation of $\sigma$ allows one to assess the quality of widely used approximative (Markovian) Fokker-Planck-type descriptions of the dynamics.

Unitary representations of the infinite-parameter group $\mathrm{Diff}\left(R^3\right)$ are presented which describe particles with spin as well as tightly bound composite particles. These results support the idea that $\mathrm{Diff}\left(R^3\right)$ can serve as a "universal group" for quantum theory.

The spin and statistics of solitons in the (2+1)- and (3+1)-dimensional nonlinear $\sigma$ models is considered. For the (2+1)-dimensional case, there is the possibility of fractional spin and exotic statistics; for 3+1 dimensions the usual spin-statistics relation is demonstrated. The linking-number interpretation of the Hopf invariant and the use of suspension considerably simplify the analysis.

A search for single electrons from the decay of singly produced scalar electrons has been made at the PEP storage ring at SLAC. No events of this type have been found in 123 ${\mathrm{pb}}^{-1\mathrm{}}$ of data, resulting in a cross-section limit of less than 2.4×${10}^{-2\mathrm{}}$ pb within the detector acceptance, and a 95%-confidence-level lower limit on the scalar-electron mass of 22.2 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^2$.

A new method is suggested to bosonize the path integral of lattice field theories with fermion fields. The procedure is illustrated in the case of the Schwinger model.

A narrow peak structure has been observed in positron spectra from U+Cm collisions at bombarding energies near the Coulomb barrier whose origin cannot be associated with established dynamic mechanisms of positron production involving Rutherford trajectories only. The peak's energy is centered at 316±10 keV. Its width of ∼ 80 keV, dominated by Doppler broadening, implies that the emitting system exists for longer than ∼ ${10}^{-20\mathrm{}}$ sec.

The yrast sequence in ${}_{}{}^{168}{}_{}{}^{}\mathrm{Hf}$ has been extended to $I^{\pi }={34}^{+}$ and two negative-parity side-bands have been observed to $I^{\pi }={33}^{-} \mathrm{and} {30}^{-}$. At high rotational frequencies all three bands have little spin alignment and have moments of inertia which remain constant as a function of frequency. Possible evidence for the vanishing of pairing correlations is discussed.

Isospin mixing in light nuclei is examined through isospin-forbidden particle decay channels. From data on ($\overrightarrow{\mathrm{p}}$,$p$) isospin-forbidden resonances, isospin-mixing amplitudes are extracted. They show definitively a periodicity of 8 in $A$ for $8<~A<~40$. These results are combined with neutron- and alpha-decay data in an isospin analysis that estimates isovector and isotensor amplitudes and nucleon isobar contributions to isospin mixing.

Polarization measurements of nonresonant radiation produced by impact excitation of mercury atoms by polarized electrons are reported. In particular, the circular polarization of the transition $7{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 6{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ (404.7 nm) is presented as a function of electron energy. At excitation threshold, the polarization predicted by theory has been confirmed while above threshold the polarization is strongly affected by resonances. Connections with the role of the atomic interactions in the excitation process and with electron polarimetry are discussed.

Absolute cross sections for the formation of positronium in positron collisions with He, Ar, and ${\mathrm{H}}_2$ have been measured for incident positron energies up to 76.3 eV. The results are markedly different, in magnitude and energy dependence, from those of Charlton et al. The measurements in He and ${\mathrm{H}}_2$ agree well with distorted-wave and charge-exchange calculations.

For the ground-state hydrogen atom in parallel electric and magnetic fields the coefficients in the double perturbation series are calculated to large order and certain asymptotic recurrences are stated. With use of only the real coefficients, the complex energy eigenvalue is obtained as a function of both field strengths by rational approximant methods. This represents the first application of two-variable approximants to the calculation of eigenvalues for a physical system.

The effect of modulating the temperature difference between the plates at the onset of convection in a doubly diffusive system has been studied. Both Stern and Veronis configurations show stabilization and destabilization depending on the modulation frequency, in contrast with the behavior of single-component systems. The stability effects are particularly pronounced in the Veronis configuration because of parametric resonance.

Diffusion coefficients for soot particles were measured in a flame with use of a dynamic-light-scattering technique. Experimental measurements for particle Knudsen numbers in the range from 10 to 75 are in good agreement with the results of kinetictheory calculations.

New measurements of the heat capacity of a sample of helium 3 mm high are reported which extend to within 5×${10}^{-8\mathrm{}}$ deg of the lambda transition at the vapor pressure. From an analysis of the results allowing for the effect of gravity, the values -0.0127± 0.0026 ($2\sigma$) for the exponent $\alpha (={\alpha }^{\prime })$ and 1.058±0.004 for the leading singularity ratio $\frac{A}{A^{\prime }}$ are obtained. These values are in closer agreement with the theoretical predictions than those reported previously.