Optical quenching of photoluminescence by radiation with photon energy less than the band-gap energy has been observed in glassy ${\mathrm{As}}_2$${\mathrm{Se}}_3$ and ${\mathrm{As}}_2$${\mathrm{Se}}_3$·${\mathrm{As}}_2$${\mathrm{Te}}_3$. The quenching spectra provide evidence of optical absorption by a narrow band of localized states in the gap which are involved in the radiative recombination process and which are attributable to defects or impurities rather than to tails in the density of states extending into the gap.

The method of tight binding has been applied in a calculation of the imaginary part of the dielectric function, ${\epsilon }_2$, for a LiF crystal. The large variations of the transition matrix elements over portions of the Brillouin zone are found to have very pronounced effects on ${\epsilon }_2$. Our results suggest a possible reinterpretation of the experimental 13- and 62-eV optical structure as band-to-band, instead of excitonic, transitions.

The Green's-function method has been used to study the changes in electronic structure of fcc $d$-band metals on going to ultrathin films. Varying the boundary where the electronic wave functions vanish allows an accurate treatment both for a $d$-band metal film embedded in a simple metal and for a $d$-band metal film in vacuum. Calculations for a copper monolayer predict striking changes from bulk behavior, in agreement with photoemission experiments.

Detailed measurements of the conductivity in controlled-occupancy energy-band tails of isotropic bulk material at low temperatutes (<20 K) are found to be accurately described by the relation $log\sigma \sim T^{\frac{-1\mathrm{}}{2}}$. The only theory that predicts such a relation applies to one-dimensional conduction, not three. It is suggested that these results may reflect the filamentary character of electron transport in random potentials.

Interband Landau transitions have been investigated magneto-optically as a function of crystal orientation at the $E_1$ edge of InSb. The energy spacings of the levels are largely independent of crystal orientation for a series of samples with surface normals in a [110] plane. This result is at variance with theories for bulk interband transitions at an anisotropic band edge. A hypothesis involving a bound surface resonance is suggested to explain our measurements.

Nuclear-acoustic-resonance dispersion has been observed for the first time in a metal. Absorption and dispersion signals in aluminum were investigated as a function of frequency, sound-wave velocity, and temperature. The line shapes of both absorption and dispersion signals show temperature-dependent asymmetries which are in striking agreement with theory.

Magneto-optic Kerr-effect spectroscopy of Ni, Co, and Fe is used to study $d$-band widths and electron spin polarization (ESP). The $d$-band widths determined on the basis of magneto-optic Kerr-effect spectra are wider than those established by photoemission, and the sign of ESP of electrons near the Fermi level in Ni is found to be negative, as predicted by band theory. We discuss the relationship between this work and other recent studies of ESP.

A report of the first direct measurement of the drift velocity of excess electrons is given as a function of applied field in amorphous Si${\mathrm{O}}_2$ using a transit-time technique. The drift mobility is found to be 20 ± 3 ${\mathrm{cm}}^2$/V sec at 300 K, the highest known for a solid amorphous material, and it decreases with increasing temperature. These results can be explained by a model involving LO phonon scattering at high temperatures and boundary scattering at lower temperatures.

An experiment performed at the Brookhaven National Laboratory alternating-gradient synchrotron has yielded six events above negligible background which satisfy criteria for the decay $K_L^{}{}_{}{}^0\to {\mu }^{+}{\mu }^{-}$. The $K_L^{}{}_{}{}^0$ flux, measured by means of the decay $K_L^{}{}_{}{}^0\to {\pi }^{+}{\pi }^{-}$, leads to a value for the branching ratio $\frac{\Gamma (K_L^{}{}_{}{}^0\to {\mu }^{+}{\mu }^{-})}{\Gamma (K_L\to \mathrm{all})}=11\mathrm{}\times {10}^{-9\mathrm{}}$.

The equilibrium thermodynamic conditions obeyed by a pion condensed system are given. These include the statement that the average spatial currents of conserved quantities must always vanish in the state of lowest free energy, even for ${\pi }^{-}$ condensation into a running-wave mode. The physical significance of pion-condensation thresholds is also discussed.

A theoretical interpretation is given of delayed spontaneous emission of light pulses recently observed in HF gas. It is shown that the large delays can be accounted for by inhomogeneous broadening, which can quench the cooperative emission of radiation to the benefit of incoherent decay.

The penetration of a magnetic pulse into a field-free plasma is calculated by a numerical integration of two-fluid equations which include self-consistently the effects of a time- and space-evolving instability-driven turbulence. This theory explains many of the results of a recently published experiment by Bengtson et al. which measured this effect, and explains the discrepancy between the results of that experiment and the predictions of previously existing models of turbulent resistivity.

The effect of a strong magnetic field on the tetragonal phase of ${\mathrm{V}}_2$Si is examined theoretically using the linear-chain model of Labbé and Friedel. A careful examination of the free energy of this system shows that, for any temperature below 17.4°K, some applied magnetic field will produce a discontinuous structural transformation from the state with $\frac{c}{a}>1\mathrm{}$ to that with $\frac{c}{a}<1\mathrm{}$. At $T=17.4\mathrm{}$°K, the needed field is $H\cong 275$ kOe.

The correlation energy of an electron-hole liquid has been calculated, taking multiple scattering into account, in three cases: (i) isotropic electron-hole liquid, (ii) Si under a large [100] strain, and (iii) Ge under a large [111] strain. We predict that both Si and Ge under a large strain would exhibit a metallic state relative to free excitons. In each case, enhancement factors have also been calculated.

We have studied the critical behavior of the electron-paramagnetic-resonance linewidth in the planar $\mathrm{XY}$ antiferromagnet Ni${\mathrm{Cl}}_2$; it is found that the linewidth diverges like $\xi \sim {(T-T_N)}^{-0.7\mathrm{}}$ rather than ${\xi }^{\frac{5}{2}}$ predicted by the current random-phase-approximation theory.

We have found an indication of systematic discrepancies in the tabulated mass values in the iron region. We have measured the $Q$ value for ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}(p,\alpha ){}_{}{}^{55}{}_{}{}^{}\mathrm{Co}$ and found it to be - 1.3410 ± 0.0029 MeV. This differs by 17.3 keV from the value of - 1.3583 ± 0.0045 MeV calculated from the 1971 mass tables.

Using a relativistic form for the relation between the nucleon-nucleon scattering amplitude and the scattering operator, a single-scattering nucleon-nucleus optical potential is found which gives a precise fit with no adjustable parameters to the small-angle scattering of protons on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at 1-GeV incident lab energy. The method then allows a determination of the ratio of real to imaginary nucleon-nucleon forward-scattering amplitudes at other energies such as 600 MeV.

We present experimental fusion barriers for ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ions on ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$. These and published data for Ar and Kr ion-induced reactions are analyzed in terms of a simple classical formula for barrier heights. A prescription based on equivalent uniform charge radii from electron scattering is shown to reproduce all results to within the experimental uncertainty.