Previous investigators have reported the existence, at certain values of the drift velocity, of discontinuities in the mobility of positive and negative ions in superfluid helium. A different experimental technique gives results which seem to indicate that such discontinuities are probably spurious.

Low-frequency sound-velocity measurements in the range 1.5-50 kHz have been made along several isochores near the critical point of ${\mathrm{He}}^4$ using a resonance technique. The adiabatic compressibility along the critical isochore diverges as ${(T-T_c)}^{-\alpha }$ with $\alpha$ larger than zero. The data are consistent with $\alpha \simeq \frac{1}{8}$. The temperature dependence of the measured dispersion agrees with a recent calculation by Kawasaki.

The emission spectrum of electron-bombarded superfluid helium has been measured as a function of wavelength between 550 and 2500 Å. The spectrum is characterized by a very intense asymmetric band of continuous emission peaking at approximately 800 Å with half-width of roughly 150 Å, along with a series of less intense bands between the wavelengths of 620 and 710 Å. This continuum is due to the radiative dissociation of neutral ${\mathrm{He}}_2$ molecules in the reaction $A{{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}\to X{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$.

The spectral distribution of luminescence excited in liquid helium by fast electrons was measured in the region between 500 and 3000 Å. The spectrum shows transitions of the ${\mathrm{He}}_2$ molecule and is sensitive to the presence of impurities such as ${\mathrm{N}}_2$.

The motion of a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ particle through a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ lattice leads to an excitation (a mass fluctuation wave) which strongly couples to the phonons and provides a simple explanation for the NMR data in solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures.

The specific heat of two spheres and of one cylinder of compacted powdered cerous magnesium nitrate has been measured from $T^{*}=3.5\mathrm{} \mathrm{to} 20$ mK. Above 6.5 mK a correction of +0.3 mK must be added to convert $T^{*}\left(\mathrm{sphere}\right)$ to the thermodynamic temperature $T$, and +1.1 mK to convert $T^{*}\left(\mathrm{cylinder}\right)$ to $T$. Below 6.5 mK, $T^{*}\left(\mathrm{sphere}\mathrm{of}\mathrm{powder}\right)+0.3=T^{*}\left(\mathrm{cylinder}\mathrm{of}\mathrm{powder}\right)+1.1\mathrm{} \mathrm{mK}=T^{*}(\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{l}\mathrm{e}-\mathrm{c}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e})+\ominus \left(\mathrm{Weiss}\mathrm{constant}\right)$. The specific heat of cerous magnesium nitrate is given by $\frac{C}{\mathrm{nR}}=\frac{6.4}{T^2}-\frac{12}{T^3}$.

Splitting of the conduction ESR spectrum in InSb ($N=1\mathrm{}\times {10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$) has been observed at 105 GHz and at 1.6°K. Two resonance lines have been obtained instead of the previously reported single resonance line. Experimental $g$ factors and their shift associated with hot-electron effects fit well to the theoretical $g$ values calculated for $n=0\mathrm{}$ and $n=1\mathrm{}$ Landau levels.

Localized orbitals for magnetic fields are constructed in the nearly free-electron model. The method of construction is such as to allow these orbitals to approach the ordinary Wannier functions as the magnetic field is reduced. These functions should provide a good method of examining the range of validity of the effective Hamiltonian for magnetic fields.

Pulsed electron-electron double resonance techniques have been applied to the ĊH fragment in an irradiated single crystal of malonic acid. Damped Torrey oscillations about the pumping field and also about the observing field have been observed, and the spin-lattice and cross-relaxation times have been measured directly. Display of the transient signal intensity at a constant delay as a function of the pump frequency (i.e., relaxation spectroscopy) yielded very narrow EPR hyperfine lines.

The reaction ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$(${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$*)${}_{}{}^6{}_{}{}^{}\mathrm{Li}$*, where ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$* denotes the $T=1\mathrm{}$, $J^{\pi }=0^{+}$ state at 3.56-MeV excitation, was experimentally studied. The angular distributions of this reaction and the elastic scattering were measured at 32.0-MeV bombarding energy. The differential cross section was measured at 90° in the center-of-mass system, for bombarding energies between 28.0 and 33.0 MeV.

The elastic differential scattering cross section of protons on deuterons was measured for a series of incident proton momenta from 1.7 to 6.4 BeV/c. The observed scattering was compared with that predicted by the Glauber multiple-diffraction theory, out to momentum transfers of 1.0 BeV/c. Within experimental error (∼10%), the experiments and theoretical predictions are in agreement.

Renormalized perturbation theory in the anharmonic oscillator diverges.

Proton-proton elastic scattering has been measured at 15.2, 20.0, and 29.7 BeV/c. The data at 20.0 BeV/c confirm the presence of a break at $-t\cong 1.2$ ${(\mathrm{B}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. The data at 29.7 BeV/c show essentially the same behavior. The cross section is still falling with increasing energy in this $-t$ range.

The ratio of ${\pi }^{-}$ to ${\pi }^{+}$ off deuterium was measured as a function of incident photon energy from 600 to 1700 MeV in the forward direction. The ratio shows a broad dip around a center-of-mass energy of 1700 MeV, resulting presumably from the collective effect of several isospin-½ resonances in this energy region. Such a change in the ratio is reflected in the rapid variation of the isoscalar photoproduction amplitude since we found the isovector photoproduction amplitude to be a relatively smooth function decreasing slowly with increasing incident photon energy.

Self-consistency conditions, derived for the vector and axial-vector mesons on the basis of current algebra and the soft-pion technique, imply that, if the $A_1$ is an axial-vector meson, its width due to the $\pi \rho$ decay should be much wider than the present world average. The broad enhancement $K_A\mathrm{}(1200-1350)\mathrm{}$, if it includes an axial-vector meson, should consist of a broad resonance that decays into $\pi K^{*}\mathrm{}\left(890\right)\mathrm{}$. Also, the decay $A_1\to \pi +\sigma$, where $\sigma$ is an isoscalar $S$-wave $\pi \pi$ resonance, should be small compared with its $\pi \rho$ decay, even if the $\sigma$ appears at 700 MeV.

Explanation is given for observed sharp forward peaks in distributions of transverse momentum for hadrons produced in deeply inelastic hadron collisions. Small $Q$-value decay of peripherally produced nucleon resonances is the basic mechanism responsible for peaks in both $\pi$ and proton spectra. Implications of this result are discussed.

Parametric frequency conversion of resonance radiation in an optically pumped atomic vapor has been observed for the first time. Conversion efficiencies of ${10}^{-3\mathrm{}}$ have already been observed in ${\mathrm{Rb}}^{87}$ vapor, and considerably higher conversion efficiencies should be attainable.

The molecular-beam magnetic resonance method has been used to measure directly the anisotropy of the static electric polarizability of the deuterium molecule. This is the first measurement of this quantity for any molecule using the molecular-beam resonance method.

It is shown that the neutron-electron interaction is orders of magnitude too small to allow neutrons to be bound to electron-excess color centers in alkali-halide crystals, as was indicated experimentally by Grant and Cobble.

We have calculated and measured the effective velocity distribution for metastable hydrogen atoms produced in a typical beam apparatus. The distribution is characterized by $U^n\exp (-U^2)$, where $n\simeq 4$ rather than $n=2\mathrm{}$. We discuss modifications of velocity-dependent corrections to beams measurements of the Lamb shift in the first excited state of atomic hydrogen.