Multiple zeros in the dipole matrix elements for photoionizing transitions in excited atomic $d$ states are predicted over a broad range of $Z$ for both $l\to l+1\mathrm{}$ and $l\to l-1\mathrm{}$ channels. The $3d$ (which is nodeless), $4d$, and $5d$ excited states show $d\to p$ zeros while the $5d$ also shows the two zeros in the $d\to f$ channel. The variation of the location of these minima with $Z$ is discussed and analyzed in terms of quantum defect theory and some implications are pointed out.

Spontaneous radiation from a magnetic wiggler or undulator as used in a free-electron laser is shown to be in a squeezed state for a low-density electron beam. It is pointed out that the problem is formally analogous to radiation from a Josephson junction.

Spectra of the $S_0\to S_2$ electronic transition of rhodamine dyes adsorbed at submonolayer coverages on a fused silica substrate have been obtained by resonant second-harmonic generation. Polarization dependences of the second-harmonic signal have been studied and are used in the construction of a model for the orientation of the adsorbates. A value for the molecular second-order nonlinear polarizability at resonance is deduced.

The stability of one-dimensional, solitary vortex structures in the electron phase space (electron holes) is investigated. A linear eigenvalue problem is derived in the fluid limit and solved exactly, assuming that the normal mode is well represented by the lowest eigenstate of a properly chosen field operator. A new dispersion relation is obtained which exhibits purely growing solutions in two dimensions but only marginally stable solutions in one dimension. This explains the numerically well-known fact that vortex structures disappear in going from one to two dimensions.

It is shown that, in the electrostatic approximation, the collapse in a plasma without external pump has a two-dimensional character.

An investigation has been carried out of the radiation emitted by 10-GeV positrons planar channeled in perfect single crystals of silicon. The radiation is found to be narrowly peaked in direction and energy as compared with normal bremsstrahlung radiation, consistent with theoretical predictions. Strong evidence is found for structure not resolved in previous experiments, particularly at angles near the critical angle for channeling where the radiation exhibits a marked periodic structure.

Measurements are presented which show that below the superfluid transition, the generation of first-sound subharmonics in the low-megahertz range quantatively follows the Feigenbaum universal convergence. In addition, by using ion-trapping techniques the physical nature of the onset of the first bifurcation sequence is identified as the threshold for the generation of quantum vortex line, not the threshold for the production of macroscopic classical turbulence, i.e., acoustic cavitation.

It is shown that transitions between the sublevels of a $J=2\mathrm{}$ collective mode can be induced by a radio-frequency field when a splitting is created by a static field. The effects of a cw and a pulsed rf field are investigated and the observation of these transitions through the modification of sound propagation is discussed.

Time-resolved optical reflectivity measurements were used to investigate phase changes which occur in silicon during cw-laser heating. It is shown that amorphous silicon does not melt at a temperature significantly lower than the melting point of crystalline silicon (1693 K), contrary to recent reports which suggested a 300- to 500-K melting-point depression.

The ac susceptibility of thin $\mathrm{Pd}\mathrm{Fe}$ films has been measured before and after low-temperature irradiation with inert noble gases by using a newly developed susceptometer. The disorder introduced by the irradiation causes a reduction in both the Curie temperature by a factor of 2 and the Curie constant by a factor of about 7. From this result it is calculated that the low-temperature irradiation reduces the susceptibility of the Pd host by more than an order of magnitude, thus explaining the recently observed superconductivity in irradiated Pd.

In Heisenberg spin-glasses, the orientation of a state is specified by a reference spin triad ($\hat{n}, \hat{p}, \hat{q}$), and its anisotropy is specified by a rotation $\overrightarrow{\psi }$ of this state away from its equilibrium orientation. Rotation of ($\hat{n}, \hat{p}, \hat{q}$) by $-\overrightarrow{\psi }$ defines an anisotropy triad ($\hat{N}, \hat{P}, \hat{Q}$), whose dynamics is derived. This dynamics is exclusively dissipative, consistent with torque and ESR measurements. Analogy to ordinary glasses suggests a Vogel-Fulcher law for the associated relaxation time.

The loss of nuclear alignment and the precession of the ${\frac{5}{2}}^{-}$ state of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ nuclei ($H_{\mathrm{ext}}=1.3\mathrm{}$ T), recoil implanted into Pb at various temperatures, have been measured by the $\alpha -\gamma$ time differential perturbed angular correlation technique. It was deduced that the implanted ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ ion is in a +6 state with its $5f$ electron in a localized state and that the loss of nuclear alignment is caused by a pure paramagnetic relaxation with an average atomic correlation time of 2.3×${10}^{-13\mathrm{}}$ s. Lead was found to offer a quadrupole-perturbation-free environment for the Np implants.

Chemisorption of Ge on GaAs(110) at 320 °C simultaneously introduces acceptor- and donor-type surface states with the same density. The acceptor levels were found to be linked to $E_{\mathrm{cs}}$. Under the assumption of the same behavior for the donor levels, the band bending versus coverage curve was calculated for samples at 20 °C, and very good agreement was found with the experimental data of Spicer et al. for metal overlayers on GaAs(110). It is concluded that chemisorption on GaAs(110) surfaces introduces a single type of defect.

The $4f$ spin dynamics of extremely dilute Nd, Pr, and Ce ions in liquid metals are microscopically observed by the time-differential perturbed $\gamma$-ray distribution method. The $4f$ spin rates are dominated by strong mixing exchange interaction in all systems investigated, which is, e.g., reflected by a Kondo-like behavior for Nd systems. The pressure-driven valence transition in dilute $\mathrm{La}\mathrm{Pr}$ alloys is probably of the $4f^2\to 4f^1$ type. One has to expect that $4f$ instabilities might occur for all light rare-earth ions in appropriate systems.

It is demonstrated that the total mass of a nontrivial isolated physical system is positive even after part of its mass has been lost because of gravitational radiation. This method is an extension of the one that the authors used in the proff of the classical positive-mass conjecture.

The positive-energy conjecture is proved at null infinity. That is, an isolated system in general relativity can never radiate away more energy than is given by its total Arnowitt-Deser-Misner energy.

A new measurement of the elastic scattering of 250-GeV/c negative pions by electrons provides form-factor results from $0.0368<\mathrm{}{q}^2<0.0940\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. These measurements determine the mean square pion radius to be $〈{r_{\pi }}^2〉=0.439\mathrm{}\pm 0.030$ ${\mathrm{fm}}^2$ or ${〈{r_{\pi }}^2〉}^{\frac{1}{2}}=0.663\mathrm{}\pm 0.023$ fm. Comparisons are made with previous elastic-scattering experiments as well as with results obtained from electroproduction experiments, $e^{+}{e}^{-}$ annihilation experiments, and phenomenological analyses.

The spontaneous breakdown of chiral symmetry is analyzed dynamically via bound-state Bethe-Salpeter equations. While in general spontaneous mass generation is linked to a massless $0^{-}$ pion and no specific constraint on a massive $0^{+}$ meson, for the particular theory of asymptotically free QCD it is shown that a $0^{+}$ $\sigma$ meson should exist with mass $m_{\sigma }\approx 600 \mathrm{to} 700$ MeV.

Extremely large precession angles (0.24 rad) have been observed when Coulomb-excited, 2-MeV/A actinide recoils decelerate in ferromagnetic Gd. In both ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ the $g$ factors increase above spin $I=18\mathrm{}\hslash$. This is the first direct evidence for rotation alignment of $i_{\frac{13}{2}}$ protons in the actinides. In ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ there is also evidence for alignment of $j_{\frac{15}{2}}$ neutrons below $I=16\mathrm{}\hslash$.

The intrinsic state of the Nilsson model is analyzed in terms of nucleon pairs coupled to spins $0^{+}$, $2^{+}$, $4^{+}$, $6^{+}$,.... It is shown that $0^{+}$ and $2^{+}$ pairs dominate the Cooper pair which constitutes the intrinsic state with large quadrupole deformation.