Direct evidence of covalency effects on spin densities of the ${\mathrm{Cu}}^{2+}$-${\mathrm{F}}^{-}$ antibonding orbital in ferromagnetic ${\mathrm{K}}_2$Cu${\mathrm{F}}_4$ has been observed by means of critical scattering of neutrons. The form factor observed was in good agreement with the one calculated based on linear combination of atomic orbital wave functions using covalency parameters determined by NMR.

The interface density profile of the two-dimensional Ising ferromagnet is investigated for all temperatures below the critical point. The width of the interface diverges in the thermodynamic limit.

Evidence for a bound ${\Delta }_{33}\mathrm{}\left(1236\right)-{\Delta }_{33}\mathrm{}\left(1236\right)\mathrm{}$ component of the deuteron is sought by using ${\pi }^{+}$, ${\pi }^{-}$, and $K^{+}$ mesons as probes of nuclear structure. Events are observed which might contain $\Delta \mathrm{}\left(1236\right)\mathrm{}$ "spectators" in the breakup of a deuteron in a $\Delta \Delta$ state. However, other mechanisms may contribute to the signal, and we cannot unambiguously ascribe our events to a $\Delta \Delta$ contribution in the deuteron. With certain assumptions we obtain an upper limit of ∼0.7% for the $\Delta \Delta$ component of the deuteron.

Excitation functions for the $0^{+}$, $2^{+}$, and $4^{+}$ states of ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{184}{}_{}{}^{}\mathrm{W}$, and ${}_{}{}^{186}{}_{}{}^{}\mathrm{W}$ were measured at 140° and 173.5° with incident $\alpha$-particle energies between 12.5 and 19 MeV. Strong destructive nuclear-Coulomb interference effects were observed. A coupled-channels code was developed and the deformation parameters ${{\beta }_2}^N$ and ${{\beta }_4}^N$ of the optical potential were determined and are compared with those of the charge distribution, ${{\beta }_2}^c$ and ${{\beta }_4}^c$.

We show that Dirac's proposed scalar-tensor theory of gravitation and electromagnetism can be rewritten in such a way that the scalar field decouples from the rest of the world. The symmetry breaking implicit in the formalism is shown to be unobservable.

Single-particle distributions in transverse and longitudinal momenta have been studied in $\mathrm{pp}$ interactions at 300 GeV/c. By comparing these distributions for different multiplicities of produced particles and with published low-energy data, we put forth the hypothesis of a new type of scaling: The shape of distributions in $p_T$ and $p_L$ are independent of multiplicity and incident energy.

We present rigorous bounds on the process $\mathrm{pN}\to l^{+}{l}^{-}X$ derived in the context of quark-parton models. As constraints we use the positivity of the parton probability functions and data on deep inelastic leptoproduction. Generally, our upper bounds fall below the Brookhaven National Laboratory data on muon-pair production. This result forces us to conclude that the Brookhaven data do not represent the scaling limit for this process, and/or that the usual ideas of the (colored) quark-parton model are not all correct.

We have measured the fraction and lifetime of the ${{\left(\mu {}_{}{}^4{}_{}{}^{}\mathrm{He}\right)}_{2S}}^{+}$ systems formed by stopping negative muons in gaseous helium at 50 atm. An estimate of the external Auger effect, which contributes to depopulate the ${{\left(\mu \mathrm{He}\right)}_{2S}}^{+}$ level, is given. First experimental evidence of the two-photon decay of such a level is presented.

Pressure-induced rotational-vibrational $U$ transitions ($\Delta J=4\mathrm{}$) of hydrogen gas have been observed for the first time at 195 K for densities in the range 180-300 amagat. The spectrum is interpreted in terms of a hexadecapolar induction mechanism, and from the experimental integrated absorption coefficient of the $U\left(1\right)\mathrm{}$ line, rough values are obtained for the hydrogen-molecule hexadecapolar moment and its derivative.

A new method for the experimental study of multiphoton processes uses production of highly excited atoms in keV electron-transfer collisions. Such atoms can be ionized over a wide range of frequencies of an external electromagnetic field, permitting studies of multiphoton-ionization processes for regimes not yet achieved in laser experiments. Results for microwave ionization of hydrogen atoms with principal quantum numbers in the band $63<~n<~69$ are discussed in terms of the available theory.

An inelastic resonance scattering of monochromatic Cu $K\alpha$ x rays incident on various targets is observed when an absorption edge of the target is just above the energy of the incident x rays. This frequency-dependent and angular-independent inelastic scattering is interpreted with the x-ray scattering theory of anomalous dispersion. Conservation-of-intensity arguments allow a comparison of the observed inelastic intensity with the real part of the anomalous dispersion corrections to the coherent atomic scattering factors for x rays.

The $\alpha$ spectra emitted during $K$ and $L$ ionization following $\alpha$ decay of ${}_{}{}^{210}{}_{}{}^{}\mathrm{Po}$ are investigated in a coincidence experiment with x rays. We present the first observation of $\alpha$ continuous spectra emitted by radioactive samples. The probabilities of this phenomenon and the shape of the $\alpha$ spectra are discussed in the scope of an $\alpha$ collision with zero impact parameter.

A tunable narrow-frequency laser is used to produce an isotopically selected reaction product of bromine. Natural ${\mathrm{Br}}_2$ (${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$/${}_{}{}^{81}{}_{}{}^{}\mathrm{Br}$=1) is photopredissociated by selective excitation into the ${}_{}{}^3{}_{}{}^{}\mathrm{II}_{0^{+}\mathrm{u}}^{}{}_{}{}^{}$ state. Bromine atoms enriched in one isotope react with HI before scrambling occurs to produce 80-85% ${\mathrm{H}}^{81}$Br. The method should yield an efficient technique for isotope separation. Spectroscopic requirements and the optimum reaction conditions are discussed. The relative amount of nonselective product from excitation of the ${\mathrm{Br}}_2$ continuum is determined.

The effects of degeneracy on self-induced transparency (SIT) have been studied using optical pulses gated from a cw dye laser and four different degenerate systems in Na. Delays, nonlinear transmission, peaking, and breakup were altered only slightly by degeneracy. This unexpected insensitivity to degeneracy of SIT features is in agreement with the McCall-Hahn pulse-area, pulse-energy description, computer simulations, and a simple physical explanation of general applicability.

By a molecular-dynamics simulation of a three-dimensional system of 4000 particles, we show that the long-time tail of the velocity autocorrelation function has the form ${\alpha }_0{t}^{-\frac{3}{2}}$, where $t$ is the time and ${\alpha }_0$ a coefficient depending on the kinematic viscosity and on the diffusion coefficient.

The spatial positions of discrete quantized vortex lines in rotating superfluid helium have been directly visualized by a photographic technique. The positions of the lines in the apparatus do not form a regular array.

We give an explanation of the data of Richards, Pope, and Widom in terms of the incoherent tunneling of strongly interacting particles.

The behavior of the beam-plasma instability in a nonuniform plasma depends only on the dimensionless quantity $\lambda \sim {k_i}^{-2\mathrm{}}\frac{\partial k}{\partial x}$, where $k_i$ is the spatial growth rate in the homogeneous plasma. If $\left|\lambda \right|>~1$ the instability is quenched.

X-ray measurements of the shift in lattice parameter $\frac{\Delta a}{a}$ versus pressure up to 120 kbar indicate that ${\mathrm{V}}_3$Si is not following a normal cubic dependence for a stable lattice. Hydrostatic pressures in excess of 15 kbar have produced a lattice distortion which may be representative of a cubic to tetragonal transformation.

Measurements of magnetic breakdown in dilute alloys of zinc show that the breakdown field between the second-zone monster and the third-zone needles increases linearly with the Dingle temperature with the same scaling factor for all alloys measured. A WKB calculation yields a general expression for breakdown in the presence of scattering which is in good agreement with experimental observations.