Inverted populations of practical interest in the x-ray region are obtainable from selective inner-shell vacancy production in intermediate-energy ion-atom collisions. A high-current ion beam swept at nearly the speed of light along the length of an extended foil provides an active region in synchronization with a resonant x-ray pulse traveling parallel to the foil surface. Significant amplification may be realized with presently available ion sources.

We report the generation of 1773-, 1520-, and 1182-Å radiation by frequency tripling and summing in a phase-matched mixture of Cd and Ar. For the third-harmonic process, $5320\to 1773$ Å phase matching occurs for Cd and Ar atoms in the ratio 1: 25, and ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}\cong 2\times {10}^{-34\mathrm{}}$ esu/atom. The energy conversion efficiency to 1773 Å was about ${10}^{-4\mathrm{}}$, yielding a peak picosecond power of 7 kW.

A quadratic term, characterized by the coefficient ${\alpha }_1$, in the phonon spectrum $\epsilon \mathrm{}\left(k\right)=c\hslash k(1-{\alpha }_{1}k-{\alpha }_2{k}^2-\cdots )$ is shown to be inconsistent with data on the low-temperature specific heat of liquid He II. Consistency obtains if ${\alpha }_1$ is set identically equal to zero.

The phase diagram of a charged Bose gas is drawn. We discuss the domains of existence of the solid, fluid, and superfluid phases. It is predicted that superdense helium can be superfluid at densities higher than ${10}^6$ ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3$.

It is shown both by theory and by computer experiments that a plasma subjected to a long-wavelength electric field oscillating near the lower hybrid frequency causes parametric instabilities which lead to an anomalous heating of both ions and electrons. Because the instability threshold is much lower than that for the corresponding electron plasma-wave problem, the saturation of the electric field can be much higher.

We have measured the temperature (1.2 to 80 K) and pressure (3 to 50 kbar) dependences of the electrical resistivity of the exchange-enhanced $\alpha$ phase of cerium. The exchange-enhancement factor decreases with increasing pressure, enabling us to determine the spin-fluctuation resistivity ${\rho }_{\mathrm{sf}}\mathrm{}(T,P)\mathrm{}$ by subtracting out the phonon resistivity obtained at high pressures where the enhancement becomes relatively smaller, A strong initial $T^2$ dependence of ${\rho }_{\mathrm{sf}}$ was observed. These results can be interpreted within the framework of the Kaiser-Doniach theory.

The quadratic term in the hyperfine pressure shift, recently observed for alkalies and muonium in noble gas atmospheres, is interpreted in terms of the interaction between three atoms. Approximate calculations show that short-range contributions predominate, even for heavy atoms. The results of the theoretical estimate are in reasonable agreement with the experimentally observed values in view of the approximations used.

The first observations of unusually energetic x rays from fast heavy-ion collisions in which the projectile ion captures a free or weakly bound target electron are reported. Energy calculations based on this model of radiative electron capture are in reasonably good agreement with the observed x-ray data. The phenomenon has been observed for 10- to 140-MeV sulfur, chlorine, and bromine ions incident on various targets from boron to uranium.

The $K$-shell fluorescence yield ${\omega }_K$ for metallic lithium was determined from intensity measurements of the $K$ emission band excited in fluorescence with the synchrotron radiation of the 7.5-GeV DESY electron synchrotron. The value obtained for lithium is ${\omega }_K=(1.06\mathrm{}\pm 0.53)\times {10}^{-4\mathrm{}}$. Recently determined values of ${\omega }_K$ for Be, B, and C are tabulated. The value of ${\omega }_K$ for C previously published by the author was incorrect; the corrected value is ${\omega }_K=(13.0\mathrm{}\pm 3.9)\times {10}^{-4\mathrm{}}$.

Angular distributions of electrons emitted by autodetachment of the first excited ${}_{}{}^1{}_{}{}^{}S$ state of ${\mathrm{H}}^{-}$ produced in collisions of ${\mathrm{H}}^{-}$ on He exhibit an asymmetric shape peaked in the forward direction.

A limiting intensity is shown to exist for light propagation in transparent liquids and solids. In pure bulk materials it is determined by avalanche ionization over a wide range of pulse durations, wavelengths, and band gaps. The ionization rate per unit time is deduced from the thickness dependence of the dc breakdown. The negative real part of the index of refraction of the carriers stabilizes the size of self-focused filaments.

Mössbauer measurements of the 77.34-keV transition in ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ are reported for thin absorbers of AuCN in $p$-azoxyanisole, AuCN dispersed in an inert powder, and KAu${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_2$ similarly prepared. The average linewidth is 1.54 ± 0.05 mm/sec. This result is only 80% of that obtained from lifetime measurements, but the latter are in good agreement with results in absorbers of gold metal obtained elsewhere and corroborated here.

Experimental measurements of the collisionless damping of a finite-amplitude electron wave confirm recent self-consistent numerical solutions of the nonlinear Vlasov equation.

Critical exponents for a $d$-dimensional system with an isotropic $n$-component order parameter and long-range attractive interactions decaying as $\frac{1}{r^{d+\sigma }} (\sigma >0)$ are derived, using the renormalization group approach, as power series in $\epsilon =2\mathrm{}\sigma -d>\mathrm{}0\mathrm{}$ ($\sigma \ne 2$, fixed) or $\Delta \sigma =\sigma -\frac{1}{2}d>\mathrm{}0\mathrm{}$ ($d$ fixed) and, separately, to order $\frac{1}{n}$ for all $d$ and $\sigma \ne 2$. For $\epsilon <0\mathrm{}$ the exponents have fixed ("classical") values; when $\epsilon =\Delta \sigma =0\mathrm{}$ fractional powers of $\ln \left(\frac{\Delta T}{T_c}\right)$ appear; when $\sigma >2\mathrm{}$ the exponents assume their short-range values.

Magnetic measurements have been made on a sample of ${\mathrm{He}}^3$ in a Pomeranchuk cell. Below about 2.7 mK, the NMR line apparently associated with the liquid portion of the sample shifts continuously to higher frequencies during cooling. Below about 2 mK the frequency shift vanishes, and the magnitude of the liquid absorption drops abruptly to approximately ½ its previous value. These measurements are related to the pressure phenomena reported by Osheroff, Richardson, and Lee.

We report measurements on superconducting tunnel junctions illuminated with optical radiation, which confirm the model of nonequilibrium superconductors proposed by Owen and Scalapino. Measurements of the chemical potential ${\mu }^{*}$ for quasiparticle excitations and of the temperature dependence of the quasiparticle recombination time are presented.

A magnetic system with intraplanar and interplanar interaction strengths $J$ and $\mathrm{RJ}$ is is treated. Rigorous relations are established concerning the first few derivatives with respect to $R$ of the susceptibility $\chi \mathrm{}\left(R\right)\mathrm{}$. Considering $\chi \mathrm{}\left(R\right)=b_0+b_{1}R+b_2{R}^2+\cdots$, we find $b_1$ and the order of magnitude of $b_2$. Hence we can predict when the system "crosses over" from $d$-dimensional to $d$-dimensional behavior (e.g., for quasi—two-dimensional systems, $d=2\mathrm{}$, $\overline{d}=3\mathrm{}$, while for quasi—one-dimensional systems, $d=1\mathrm{}$, $\overline{d}=3\mathrm{}$). These results also support scaling with respect to the anisotropy parameter $R$.

In ${\mathrm{V}}_3$Si a noticeable reduction of the martensitic transformation temperature was found in high magnetic fields. The effect is in good agreement with recent predictions of Dieterich and Fulde, whose calculations are based on the linear chain model of Labbé and Friedel, thus giving strong confirmation of the latter also. Lifetime effects are also briefly discussed.

The Mg $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ absorption edge ($h\nu =50\mathrm{}$ eV) has been studied as a function of composition in amorphous Mg-Bi and Mg-Sb alloys using a synchrotron source and a high-resolution grazing-incidence spectrometer. Exciton lines occur and are most prominent for low-conductivity material, e.g., corresponding to ${\mathrm{Mg}}_3$${\mathrm{Bi}}_2$. Screening of the electron-hole interaction takes place as the conductivity increases, but a remnant enhancement remains at threshold even in pure Mg metal.

The Hall effect of the high ionic conductor Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$ was studied from 0 to 40°C. By comparison with the results on photoexcited carriers, it is determined that the origins of Hall signals in R${\mathrm{Ag}}_4$${\mathrm{I}}_5$ are the mobile silver ions. The value of Hall mobility is about 0.05 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ at room temperature, which is about 30 times as large as that of the ions in NaCl at 780°C previously obtained by Read and Katz.