The Brueckner-Goldstone many-body perturbation theory is utilized to study the hyperfine interaction in atomic phosphorus. By this method, a correlation contribution to the hfs constant of a positive sign and an appropriate magnitude is obtained, which predominates over the negative core-polarization effect to produce substantial agreement with experiment.

An exact formula is obtained for $C_{\mathrm{HH}}$, the van der Waals constant which appears in $V_{\mathrm{HH}}=-\frac{C_{\mathrm{HH}}}{R^6}$, the London potential for two hydrogen atoms. Numerical evaluation gives $C_{\mathrm{HH}}=6.4990297\mathrm{}\pm 0.0000004$.

Zero or near-zero minima of the generalized oscillator strength occur frequently, and their positions are related to the nodes of the radial wave functions for the states involved. Some general implications of the minima are discussed, and, as an example, experimental and theoretical results for a transition in Xe are presented.

We report direct experimental evidence of the coexistence of two distinct optical double-resonance processes resulting in the blue (0.41-μ) fluorescence in Cd${\mathrm{F}}_2$:1%${\mathrm{Er}}^{3+}$. Our experiments contradict the conclusions of Feofilov and Ovsyankin who claim that the cooperative process properly describes long-wavelength activated visible fluorescence.

Self-induced transparency effects are examined in an absorbing medium having degenerate optical transitions. Undistorted propagating light pulses exist under restrictive conditions for $j\rightleftharpoons j$ transitions ($j>\mathrm{}0\mathrm{}$) and for 0 ⇌ 1 and $\frac{1}{2}\rightleftharpoons \frac{3}{2}$. Pulses which propagate without energy loss for any transition ${j\rightleftharpoons j}^{\prime }$ also include zero-degree pulses defined as $\int \mathcal{E}\mathrm{}(z,t)dt=0\mathrm{}$.

The thermal conductivity of helium I was measured along its vapor pressure curve over the range $3\times {10}^{-6\mathrm{}}°\mathrm{K}<T-T_{\lambda }<1\mathrm{}°\mathrm{K}$ using the parallel-plate technique. The exponent in a power-law variation of thermal diffusivity with $T-T_{\lambda }$ was found to depend on the plate spacing. These results could imply the existence of a relatively temperature-independent characteristic length in helium I near $T_{\lambda }$ approximately ${10}^4$ to ${10}^5$ times larger than the coherence length.

The thermal conductivity of HeI was measured at saturated vapor pressure from ${10}^{-7\mathrm{}}$ °K to 5 × ${10}^{-3\mathrm{}}$ °K above $T_{\lambda }$. The results agree in detail with dynamic scaling predictions and yield a critical exponent of 0.334 ± 0.005. The prediction is $\frac{1}{3}$.

We have derived the quantum kinetic equation appropriate to the Darwin Hamiltonian by a method identical to that given earlier in deriving the classical kinetic equation, with a redefinition of operators. It is conjectured that the transverse interaction term may lead, as in the classical case, to a long-ranged effective interaction.

With Éliashberg's equations for the two temperature Green's functions of a superconductor, the effect of real phonons on the transition temperature $T_c$ is discussed. The equation derived for $\frac{T_c}{T_{c0\mathrm{}}}$ describes a temperature-depdendent pair-breaking mechanism that depends on both the real and the imaginary part of the electron-phonon self-energy ($T_{c0\mathrm{}}=\mathrm{transition}\mathrm{temperature}\mathrm{in}\mathrm{the}\mathrm{absence}\mathrm{of}\mathrm{real}\mathrm{phonons}$).

A minimum in the superconducting flux-flow resistivity has been found in a ${\mathrm{Pb}}_{60}$${\mathrm{Tl}}_{40}$ alloy and compared with a recent theory by J. R. Clem based on dissipation associated with temperature gradients across a moving fluxoid. The minimum disappears but the thermal dissipation remains significant in a ${\mathrm{Pb}}_{95}$${\mathrm{Tl}}_{05}$ sample of longer electron mean free path.

Optical absorption of a cleaved surface of germanium shows a band at energies smaller than the gap, that is removed when the surface is oxidized. The band is assumed to be due to optical excitation of a surface state practically coincident with the top of the valence band to a set of states 0.16 eV below the bottom of the conduction band.

Optical refrigeration has been observed by subjecting single crystals of yttrium aluminum garnet and Nd-doped yttrium aluminum garnet to intense 1.064-μ laser radiation. In addition to this first observation of optical refrigeration, the present experiments yield a very accurate value for the fluorescent quantum efficiency of Nd-doped yttrium aluminum garnet.

NMR measurements in solid ${\mathrm{H}}_2$ at low ortho concentrations between 0.4 and 4.2°K show that the equilibrium distribution of ortho molecules in the lattice is temperature dependent and is approached approximately exponentially in time at a given temperature. It is suggested that the rearrangement process occurs by means of ortho-para interchange.

The theory of Raman scattering of light by optical phonons in metals is presented. We develop a detailed theory of the inelastic scattering of light from a metallic surface and discuss the mechanisms that contribute to the Raman effect.

The magnetostriction of Pd:Ni alloys shows that the volume derivative of the localized exchange enhancement is large and positive in contrast to the corresponding parameter in Pd. The low-temperature thermal expansion has a logarithmic singularity near the critical concentration and shows an anomaly in its temperature dependence like that predicted theoretically.

We have investigated the role of spin-independent and spin-dependent impurity scattering in itinerant Fermi systems which have a large susceptibility enhancement and find that at very low temperatures the smearing of the Fermi surface due to spin-independent scattering leads to a stronger temperature dependence of the specific heat than for pure systems, but that at still lower temperatures spin-dependent scattering changes the temperature dependence to the usual power-series expansion.

Measurements on the low-temperature electrical resistivity of dilute nickel-based alloys give strong evidence that spin-↑ and spin-↓ electrons carry current in parallel, providing important implications for the interpretation of transport properties of pure as well as alloy ferromagnets.

Theory and experiment are presented for the ultrasonic attenuation of the rare-earth metals. In the helical spin state of the metals, the sound energy transferred to the spin system is dissipated to the lattice by the spin rotation around the $c$ axis, arising from the spin-lattice relaxation mechanism, which may explain the large increases of the attenuation observed on Dy and Ho single crystals below their Néel temperatures.

We present theoretical and experimental results for electron excitation of particle-hole states in ${\mathrm{C}}^{12}$.

The photoneutron cross sections for ${\mathrm{Ni}}^{58}$, ${\mathrm{Ni}}^{60}$, and natural nickel from threshold up to 25 MeV have been measured. The integrated ($\gamma ,n$) cross section for ${\mathrm{Ni}}^{60}$ is 2.6 times as large as the value for ${\mathrm{Ni}}^{58}$, and the two cross-section curves exhibit markedly different structure over the giant-dipole resonance region. The results suggest that it is important to include the shell effects explicitly in the theoretical treatment of the giant-dipole states in the medium nuclei such as nickel isotopes.