A study of the optical absorption spectrum of GaSe at the energy gap shows that the strength of the exciton line decreases as hydrostatic pressure is increased. It is proposed that this effect comes from increasing interference of the discrete exciton line with a continuum of structure due to indirect interband electronic transitions. The sign and magnitude of the pressure coefficients of the gaps in gallium chalcogenides are found to be consistent with recent band-structure studies.

We have observed shifts in energy of a number of Raman modes of ${\mathrm{Cu}}_2$O as a function of incident photon energy. We explain such energy shifts as due to dispersion in the phonons involved and obtain quantitative agreement between theory and experiment. From our data we have also determined the effective mass of the $1s$ yellow exciton in ${\mathrm{Cu}}_2$O to be $(3.0\mathrm{}\pm 0.2)m_0$.

The interaction of low-energy positive and negative pions with ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ was studied by the detection of $\gamma$ radiation emitted from residual nuclei formed in the reaction. Large cross sections for the removal of $\alpha$ clusters were found for both nuclei and both pion charges. A large asymmetry is observed between the cross sections for the (${\pi }^{-},{\pi }^{-}p$) and (${\pi }^{+},{\pi }^{+}n$) on ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ leading to mirror states in ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{27}{}_{}{}^{}\mathrm{Si}$. The ratio of ${\pi }^{+}$ to ${\pi }^{-}$ cross sections for proton knockout is about a third of the value predicted by a simple impulse approximation.

The differential cross section for ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ has been analyzed with the finite-range distorted-wave Born-approximation computer code MERCURY (which includes recoil effects exactly). The general features of the data were reproduced only by assuming that the $\alpha$ particle was transferred in an excited state. Good fits to the data were found by also including $L$-dependent absorption in the optical potentials.

We have measured the differential cross section for $\overline{p}p\to \overline{n}n$ near 180° in the center of mass for incident $\overline{p}$ momenta between 1 and 3 GeV/c. A backward peak is observed at all energies, whose slope increases with increasing momentum. The $s$ dependence shows some departure from smooth behavior. The magnitude of backward charge-exchange scattering is comparable to backward elastic scattering and thus rules out a diffractive interpretation of the $s$-dependent structure.

The Pauli exclusion principle has been tested for atomic electrons giving a lifetime for such a violation, per iodine atom, of > 2 × ${10}^{27}$ sec. This result is compared to a normal electromagnetic dipole transition, and a limit is set on a possible symmetric wave-function admixture to a $K$-shell electron wave function.

We report measurements of the charged-hadron multiplicity for electrons scattered by neutrons at momentum transfers up to $Q^2=6\mathrm{}$ ${\mathrm{GeV}}^2$, and for s up to 14 ${\mathrm{GeV}}^2$. The average multiplicity shows little or no $Q^2$ dependence at fixed $s$, and except at low $s$ it is consistent with being the same as the multiplicity in electron-proton scattering.

Nuclear interactions at high energy are sensitive to the space-time structure of basic hadronic collisions. A model of this structure is proposed; it provides a parameter-free account of National Accelerator Laboratory and cosmic-ray emulsion data, which shows that the multiplicity of mesons produced in nuclei exceeds that in hydrogen by an energy-independent ratio remarkably close to unity.

Recent observations of Alfvén waves in the interplanetary medium provide an improved upper limit on the photon rest mass. We find a reliable upper limit $\mu <~3.6\times {10}^{-11\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $m_{\mathrm{ph}}<~1.3\times {10}^{-48\mathrm{}}$ g, and a stronger, but less certain upper limit $\mu <3.1\mathrm{}\times {10}^{-12\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $m_{\mathrm{ph}}<1.1\mathrm{}\times {10}^{-49\mathrm{}}$ g. These represent improvements on the heretofore best reliable estimate by 0.5 and 1.5 orders of magnitude, respectively.

Numerous model calculations suggest that hadron elastic, diffractive production, and inclusive amplitudes should obey factorization numerically at a level of accuracy somewhat better than has yet been tested by experiment, but that those amplitudes will not obey the exact factorization in functional form associated with a pure Regge pole.

By use of the Ward identities of current algebra, it is shown in the context of a class of ${\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}_L\otimes \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ gauge models that, after renormalization of strong coupling constants and masses, the divergent part of the corrections of order $\alpha$ to the semileptonic and leptonic amplitudes mediated by the $W$ meson is a universal multiple of the zeroth-order amplitude, independent of the strong interactions. Furthermore this divergent part can be absorbed by a universal renormalization of the $W$ coupling constant $g$.

The technique of two-photon absorption without Doppler broadening has made possible the observation of the Zeeman splitting of a transition between two ${}_{}{}^2{}_{}{}^{}S$ states. Selection rules are discussed. The observed pattern of hyperfine components is narrower in very high fields than in zero field, in agreement with the theory.

Raman-scattering transitions have been observed between the hindered rotational states of the Jahn-Teller-distorted ${}_{}{}^2{}_{}{}^{}E_{\mathrm{g}}^{}{}_{}{}^{}$ ground state of ${\mathrm{Cu}}^{2+}$ in CaO. Impurity-induced scattering from the CaO vibrations was also observed. The results are in good agreement with the cluster model for the low-lying vibronic states.

A new approach was taken to measure the electron thermal conductivity across magnetic surfaces by utilizing localized upper hybrid resonance heating. The electron thermal conductivity coefficient measured in the FM-1 spherator was increased with an increase of electron temperature for $T_e>1\mathrm{}$ eV. The dependence is similar to that of the particle diffusion coefficient. The absolute value was 10-20 times smaller than the Bohm coefficient.

We report measurements of the transverse and longitudinal NMR spectra of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in low magnetic fields which support the axial-state model for the liquid in the $A$ phase. A quite unexpected additional result found in this work is that the width of the shifted transverse resonance line becomes extremely broad at temperatures well below $A$.

We have determined the relative liquid and vapor densities $\frac{{\rho }_L}{{\rho }_c}$ and $\frac{{\rho }_V}{{\rho }_c}$ along the coexistence curve in ${\mathrm{SF}}_6$ using capacitance measurements of the dielectric constant together with an independent determination of the dielectric-constant-density relationship. Near the critical point, the diameter ${\rho }_d=\frac{1}{2}({\rho }_L+{\rho }_V)$ exhibits a successively larger departure from the extrapolated linear dependence on temperature. This feature is characteristic of many liquid-vapor critical-point models, but it has not been previously observed.

The previously termed "anomalous" properties of the various polytypes of $d^1$ Ta${\mathrm{S}}_2$, Ta${\mathrm{Se}}_2$, etc. are attributed to charge-density waves, their periodic structure distortions, and the superlattices they induce.

Spontaneous interconfiguration fluctuations have been detected in TmTe and TmSe by x-ray photoemission. The resulting instantaneous picture of the Tm ions fluctuating between the divalent and trivalent state within the same environment allows a direct measurement of the magnitude of the Coulomb correlation energy, $U_{\mathrm{eff}}$. Spontaneous interconfiguration fluctuations are shown also to affect the binding energies of core levels within the same rare-earth ion, resulting in a chemical shift of 3.5 ± 0.1 eV for the Tm ($5p$) core levels.

In high-resolution ($n,n^{\prime }\gamma$) studies the previously assigned $3^{-}$ octupole states in ${}_{}{}^{114}{}_{}{}^{}\mathrm{Cd}$ and ${}_{}{}^{116}{}_{}{}^{}\mathrm{Cd}$ have not been observed.

In a study of the reaction ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ni}$(${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^6{}_{}{}^{}\mathrm{He}$)${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ at 34.0 MeV, the angular distributions of the three low-lying states have been observed to exhibit not only an $L$ dependence but also $l$ and $j$ dependences at angles forward of 15°. The $l$ and $j$ dependences of the shapes can be understood in terms of the angular-momentum selection rules. Finite-range distorted-wave Born approximation calculations, including recoil exactly, reproduce the data. Relative spectroscopic factors are in good agreement with those from the reaction ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ni}({}_{}{}^3{}_{}{}^{}\mathrm{He},d){}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$.