It has been recently suggested that a retardation effect could possibly be observed in very low-energy helium-atom scattering. In this paper calculational results are presented which show that the effect arises from small changes in the potential near the bowl rather than in the long-range potential. We indicate that the measurements provide a sensitive measure of the helium interatomic potential.

In a transmission geometry, dispersion terms were observed in the shapes of the Mössbauer resonance lines of pure $E2\mathrm{}$ and mixed $\frac{E2\mathrm{}}{M1\mathrm{}}$ transitions. For the pure $E2\mathrm{}$ transitions studied, the relative magnitude of these terms, which are attributable to interference effects, is typically 1.5% of the depth of the absorption line. We discuss possible consequences of the resulting asymmetric line shape for isomer-shift measurements.

Stimulated emission of vacuum ultraviolet radiation has been observed on two $Q1\mathrm{}$ lines in the Werner band ($C^1{\Pi }_{\mathrm{u}}\to X^1{{\Sigma }_{\mathrm{g}}}^{+}$) of molecular hydrogen. These lines, 1161 and 1230 Å, contain 5 kW peak power and were produced by a traveling-wave discharge system. Using this excitation principle, evidence of gain has been obtained by observations of optimum velocity matching at pulse velocities less than $c$.

Stimulated emission has been observed in the $Q1\mathrm{}$ and some $P3\mathrm{}$ lines of the $v^{\prime }\to v^{\prime \prime }=1\mathrm{}-4,2-5,2-6, \mathrm{and} 3-7$ Werner bands ($C^1{\Pi }_{\mathrm{u}}\to X^1{{\Sigma }_{\mathrm{g}}}^{+}$) of molecular hydrogen near 1200 Å. Light pulses of approximately 1 nsec duration were produced by bombarding hydrogen gas with ∼ 400-keV electrons from a commercial electron-beam generator.

The liquid-solid phase transition of helium at low temperatures is shown to result from the instability of the roton mode. In a simple model of the roton-roton interactions, the instability arises at a finite value of the roton energy as a result of the hybridization of a single roton with a bound roton pair. Thus our model exhibits a discontinuous jump of the soft-mode energy at the phase transition in agreement with neutron scattering experiments.

The temperature dependence of the superfluid density for liquid helium contained in a restricted geometry has been studied using fourth-sound, persistent-current, and oscillating U-tube techniques. Particular emphasis is given to the temperature range below 1 K. At the lowest temperatures the effective superfluid density becomes a linear function of temperature; this behavior is in contrast to bulk helium where a small $T^4$ temperature dependence is expected.

The Tokamak discharge, with its characteristic "enhanced resistivity," can permit singly charged ions to maintain themselves in the applied electric field at energies considerably above the mean thermal energy.

The critical exponents $\gamma$, $\eta$ and the "crossover index" $\varphi$ are computed for generalized classical Heisenberg models with $n$ internal degrees of freedom as an exact expansion in $\epsilon =4-d$ ($d$ is the number of space dimensions). Results are obtained to order ${\epsilon }^2$ for $\gamma$ and to order ${\epsilon }^3$ for $\eta$. The results to this order for the three-dimensional Ising case ($n=\epsilon =1\mathrm{}$) are $\gamma =1.244\mathrm{}$ and $\eta =0.037\mathrm{}$.

The interaction of a conducting pseudoperiodic grating with volume acoustic modes of a piezoelectric crystal has been tentatively explained in terms of an acoustoelectric periodic structure exhibiting exponential and periodic couplings.

We present a new $t$-matrix formulation of the perturbation approach to the theory of low-energy electron diffraction. Good agreement is obtained between theory and experiment for the (100) and (110) clean surfaces of aluminum and nickel. Speed and good convergence make it particularly suited to the analysis of overlayer structures.

We report on the oscillatory behavior of the reduction factors for the electronic orbital angular momentum of Jahn-Teller coupled non-Kramers doublets, which comes from the diagonal matrix elements of the orbital angular momentum. The calculated envelope of the magnetic circular dichroism for the parity-allowed transition $A\to E$ also exhibits an oscillatory behavior, which is given by the nondiagonal matrix elements of the orbital angular momentum.

Solid metallic hydrogen is only metastable at low pressures, but the transition to the molecular form may require surface phenomena. The "evaporation" of pairs of adjacent surface atoms (forming free $\mathrm{H}_2^{}{}_{}{}^{-}$ molecules) is discussed. A monolayer of surface atoms is lost in $e^G$ times a lattice vibration period, where $G\lesssim 20$ at zero pressure and temperature. $G$ increases with pressure and decreases with temperature above $T_c\sim 150$ °K.

A recently presented dynamic model concerning the electronically induced crystallographic transition is solved rigorously with higher-order terms in the distortion operator also taken into account.

The oxidation of strontium has been studied by ultraviolet photoelectron spectroscopy. Minimal oxidation produces a relatively sharp peak (width about 0.5 eV) in the energy distribution curves originating from a state 5.0 eV below the Fermi level; this state is seen to broaden strongly into a band as the ${\mathrm{O}}^{--}$ concentration increases and ${\mathrm{O}}^{--}$-${\mathrm{O}}^{--}$ overlap occurs. The metallic component of the emission appears to originate from a thin layer near the surface and is little affected by the oxidation.

$\alpha$-particle-induced quasifree scattering in deuteron breakup exhibits no difference between kinematically equivalent $\alpha -p$ and $\alpha -n$ channels, in contrast to previously measured proton-induced breakup. This supports the hypothesis that spin and statistics play a dominant role in deuteron breakup reactions and that previously postulated charge and final-state effects cannot explain the proton data.

A state at $E_x=6.350\mathrm{}$ MeV in ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ is shown to have $J^{\pi }={\frac{9}{2}}^{-}$ and is assigned to the rotational band based on the ${\mathrm{½}}^{-}$ state at 2.640 MeV. Its $\gamma$-ray decay lifetime and branching ratios are determined and combined with known information about the proposed $J=\frac{1}{2}$ to ${\frac{7}{2}}^{-}$ members of this band to show that there is an anomalous $E1\mathrm{}$ decay pattern between the members of this band and the members of the ground-state band. This anomalous pattern can be explained using the Nilsson model with band mixing.

We have utilized SU(3) invariance to relate the reactions $K^{-}p\to (\Lambda ,{\Sigma }^0)\varphi$ with ${\pi }^{-}p\to (\Lambda ,{\Sigma }^0)K\left(890\right)\mathrm{}$ in the peripheral region. We find that in all observable distributions, including the baryon polarization and the vector-meson density-matrix elements, the data are in good agreement with the SU(3) predictions. These observations lead us to conclude that potential sources for the symmetry breaking, such as external-mass—dependent factors or absorptive effects, do not significantly affect the simple SU(3) picture.

We propose a linear sum rule for inclusive-type discontinuities and prove its equivalence to the basic discontinuity equation recently derived by Cahill and Stapp. This, in turn, is believed to be equivalent to unitarity. The possibility of formulating an "inclusive bootstrap" is pointed out.

Single-charge-transfer channels in the systems ${\mathrm{He}}^{++}$-He, -Ne, -Ar are observed for ${\mathrm{He}}^{++}$ energies 0.2-1.0 keV and ${\mathrm{He}}^{+(*)\mathrm{}}$ exit angles 0°-15°. (${\mathrm{He}}^{+(*)\mathrm{}}$ denotes either the ground state or an excited state.) The ground-state channels have cross sections below the present ${10}^{-20\mathrm{}}$-${\mathrm{cm}}^2$ detection limit. Outgoing channels are found to be favored when their separated ions can originate with the same molecular symmetry as the incoming channel.

The theory of ion escape from vortices in helium is re-examined with the introduction of non-Markoffian features of the ion-vortex interaction. After several physically motivated approximations the present theory takes the form of earlier theories except that the hydrodynamic potential is replaced by an effective potential depending on the ion mobility. A conflict with experiment is thereby resolved, and one can proceed to use escape data to investigate the vortex core structure.