We have studied the nonlinear optical absorption in silicon at 1.06 μm using picosecond pulses. By measuring the time dependence of the induced absorption we are able to separate the effects of indirect simultaneous two-photon absorption and stepwise absorption processes. Values of the two-photon absorption coefficient at 1.06 μm were measured to be 1.9 cm/GW (20 K) and 1.5 cm/GW (100 K).

The optical absorption spectrum near an $M_3$ critical point of a model doped semiconductor or metal is calculated as a function of screening wave vector, using a realistic effective-mass model and a statically screened final-state interaction, but neglecting exchange. Wavelength-modulation line shapes are derived by direct (analytic) differentiation, and it is shown that screening enhances the absorption spectrum and its derivatives near an $M_3$ threshold. Experiments to detect $M_3$ critical points should concentrate on materials with heavy band masses near the conduction-band minimum and light (negative) masses at the band maximum.

We present dispersion curves for phonons along $\Gamma \mathrm{KM}$ in Be. It is shown that the ordering of the branches corresponding to the phonon modes polarized in the basal plane at the symmetry point $K$ provides a simple and direct qualitative criterion for the presence of tensor forces. The existing pseudopotential calculations of the dynamics are inherently incapable of explaining these results. Generalizations required in the theory are mentioned.

Electron capture by neutral shallow donors is shown to be the dominant capture mechanism in uncompensated silicon at low temperatures. The mechanism is strongly dependent on spin polarization, which permits its separation from other competing mechanisms and the evaluation of its capture cross section. We also determine the negative donorion thermal ionization energy and lifetime.

We present a mechanism capable of explaining surface reconstruction which is based on the fact that the presence of the surface can change the value of the dynamic effective charge on the atoms near the surface. We perform a lattice dynamical calculation and find that the long-range interaction between these induced charges can lower the frequency of the surface phonon to the point where the surface region becomes unstable and reconstruction must occur.

With photon energies within 1.5 eV above the threshold, photoelectric yields, as measured from Cu (100) and (111) with obliquely incident light, exhibit strong deviations from the $Y\propto {(\hslash \omega -\varphi )}^2$ dependence found at normal incidence. A preliminary energy analysis shows the distribution of the emitted electrons to shift towards the high-energy edge when the angle of incidence changes from 0° to 60°. The results are interpreted in terms of a surface photoelectric effect.

We report spectral and intensity-dependent measurements of the two-photon photoconductivity in ZnS crystals using two different frequency radiations. Photocurrents produced by each beam separately were determined and subtracted. We found that the two-photon photoconductivity usually varies linearly with the intensity of one source when that of the other source is held constant. The spectral measurement shows a rise of the two-photon photoconductivity at the crystal energy gap (3.82 eV), and an interesting forbidden one-photon but allowed two-photon transition at 4.57 eV.

Energy distribution spectra have been measured for photoelectrons emitted normal to three tungsten single-crystal faces, namely (100), (110), and (111), for photon energies between 7.7 and 11.7 eV. The spectra show marked differences for the different faces, in agreement with the assumption that the electrons originate from states along different symmetry lines in the Brillouin zone. Structure in the spectra is interpreted in terms of emission due to direct optical interband transitions and surface photoelectric emission.

The measured strong space-charge electret equilibrium polarization in high-purity Ca${\mathrm{F}}_2$ follows the form predicted for stored charge in a Schottky barrier. The electret decay obeys simple bilinear dynamics with a thermally activated rate constant. The formation of the electret as well as its decay is directly governed by the sample's bulk ionic conductivity.

A procedure is presented for constructing a solution to the inverse problem subject to the constraint that the wave functions be prescribed at a finite number of energies.

The Griffiths-Hurst-Sherman inequalities and the Lee-Yang zero theorem in the theory of Ising ferromagnets are shown to hold in a two-dimensional self-coupled Bose quantume field theory with interaction: $a{\varphi }^4+b{\varphi }^2-\mu \varphi$:. Applications include the continuity of the infinite-volume "magnetization," $〈\varphi \mathrm{}\left(0\right)\mathrm{}〉$, away from $\mu =0\mathrm{}$. Our results should carry over to three or four dimensions once it is known how to control the ultraviolet divergences in these theories.

An Ansatz is proposed for the asymptotic field of a charged particle which gives the known power-law behavior ${(p^2-m^2+i\epsilon )}^{-1-\beta }$, $\beta \ne 0$, for the electron propagator near the mass shell. This allows the electron-wave-function renormalization to be related to the normalization of the asymptotic field. A reduction formula is presented, expressing the $S$ matrix in terms of Green's functions which are evaluated according to a precise on-mass-shell renormalization prescription.

We have measured the cosmic-ray muon charge ratio, $\frac{{\mu }^{+}}{{\mu }^{-}}$, from 10 GeV to 1.5 TeV with a sea-level magnetic spectrometer at eight different zenith angles between 0° and 90°. The data are consistent with a constant charge ratio between 50 GeV and 1.5 TeV, in agreement with the predictions of the scaling hypothesis. Assuming the charge ratio to be constant, we obtain its value as 1.261 ± 0.009.

We have experimentally determined the absolute cross sections for the photoionization of He($2{}_{}{}^1{}_{}{}^{}S$) and He($2{}_{}{}^3{}_{}{}^{}S$) metastable atoms from threshold to 2400 Å. The results are compared with recent theoretical calculations.

Highly excited $S$, $D$, and $F$ levels in cesium and rubidium have been populated utilizing a two-step optical excitation method. In the first step an rf lamp is used to transfer atoms into the first $P$ states, and the subsequent excitation is performed with the tunable radiation of a cw dye laser. We give results for the hyperfine structure of several $D$ and $F$ states in ${\mathrm{Cs}}^{133}$ and ${\mathrm{Rb}}^{87}$, obtained in level-crossing and optical double resonance experiments.

High-resolution measurements of the ${\mathrm{He}}^4$-${\mathrm{He}}^4$ total scattering cross section are presented for reduced collision energies between 0.2 and 200 meV. Clear evidence for the Ramsauer-Townsend effect is observed and thirteen backward glory extrema are resolved. From these extrema we derive the energy dependence of the $s$ phase shift. Applying the semiclassical inversion method proposed by Miller we then compute the repulsive potential up to 0.14 eV.

The 90° electron spectra from oxygen-ion single collisions with Ar have been measured at energies of 17 to 41 MeV and charge states of 3+ to 8+. A broad peak in the spectra is observed and identified as electrons lost from the incident ion. A simple model of the electron-loss process is described which supports the identification.

We have observed the generation of high-energy electrons by longitudinal electrostatic waves in a beam-plasma system. We find a direct correlation between the measured field and particle spectra.

Measurements of attenuation and velocity of sound in the collisionless regime have been made in normal and extraordinary phases of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at several pressures. The attenuation has a frequency-independent component with a sharp maximum just below $T_c$ and a frequency-independent component with a BCS-like temperature dependence. The velocity, frequency dependent very near $T_c$, becomes frequency independent at lower $T$ and may be approaching the velocity of first sound.

Minute deviations from symmetry in the plasma current flow pattern are sufficient to break up magnetic surfaces and braid the lines of force in a toroidal plasma under magnetic confinement. Magnetic braiding can be expected to enhance current penetration, electron heat transport, and particle loss rates; sudden onset of braiding may be accompanied by voltage spikes.