This paper provides the first experimental test of the Bloch equations for an optical transition in a low-temperature impurity-ion solid, ${\mathrm{Pr}}^{3+}$: La${\mathrm{F}}_3$, where the condition $T_1\gg T_2$ applies. Intensity-dependent optical free-induction-decay measurements are performed by use of an ultrastable cw dye laser (linewidth, 300 Hz) and reveal an $\exp (-\frac{2t}{T_2})$ decay law at low Rabi frequency ($\chi$), as predicted by the Bloch equations, and an $\exp (-\chi t)$ decay at high $\chi$, which violates the Bloch prediction but agrees with Redfield's characterization of NMR saturation in solids.

The role of multiplicative colored noise on the photon statistics of the dye-laser output is investigated. This model explains consistently the recent experimental results by Short, Mandel, and Roy.

A full multiple-scattering calculation is reported for large-angle electron-energy-loss spectroscopy of H/W(001) with the inclusion of a surface image potential. The form of the image potential was determined by comparison with experimental data of elastic fine structures on W(001) and confirmed on W(110). With the inclusion of the image potential, good agreement is obtained with experimental data for the polar dependence of the electron-energy-loss cross section. Also, a breakdown of a pseudoselection rule for large-angle electron-energy-loss spectroscopy is illustrated.

EPR experiments with uniaxial stress on the Ga vacancy $V_{\mathrm{Ga}}$ in GaP reveal that its spin is $\frac{3}{2}$. The spectrum splits linearly with stress into three components with a coefficient $C_{11}=34\mathrm{}\times {10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ ${(\mathrm{d}\mathrm{y}\mathrm{n}/{\mathrm{cm}}^2)}^{-1\mathrm{}}$ at 20 K. The high spin demonstrates the importance of the exchange interaction for this vacancy.

A new, semiempirical model of metals and impurities (embedded atom method) makes possible a static treatment of the brittle fracture of a transition metal in the presence of hydrogen. Results indicate that hydrogen can reduce the fracture stress in nickel.

A simple empirical relation connects the critical temperature of the order-disorder transition $T_c$ to the dipolar reorientation rate ${\tau }^{-1\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ of C${\mathrm{N}}^{-}$ molecules by the concept of a critical reorientation rate ${{\tau }_c}^{-1\mathrm{}}\left(T_c\right)$. This relation is derived from experimental data on four pure alkali cyanides, the dipole-diluted ${\left(\mathrm{KBr}\right)}_{1-x}:{\left(\mathrm{KCN}\right)}_x$ system, and the mixed alkali cyanides ${\left(\mathrm{KCN}\right)}_{1-x}:{\left(\mathrm{NaCN}\right)}_x$ and ${\left(\mathrm{RbCN}\right)}_{1-x}:{\left(\mathrm{KCN}\right)}_x$. The observed disappearance of the order-disorder transition at a certain $x$ value occurs in all mixed systems, when the spread of relaxation rates exceeds a particular value.

The structure, concentration, and chemical activity of atomic vacancies at the TiC(001) surface have been directly analyzed by specialized low-energy ion-scattering spectroscopy. It has been found that carbon vacancies are formed at the surface under a certain condition, and they capture oxygen atoms into the vacancy holes exhibiting a very high activity.

Spatial fluctuations in spin-glasses are studied by an expansion to quadratic order around the dynamic mean-field theory. Below $T_c$, a spatial decay of the "spin-glass susceptibility" ${\left[{\chi }^2\right(\overrightarrow{\mathrm{R}}, \omega \left)\right]}_{\mathrm{av}}$ is characterized at infinitesimal frequencies by a correlation length $\xi \left(\omega \right)$ which diverges as the inverse of the time-dependent Edwards-Anderson order parameter $q\left(\frac{1}{\omega }\right)$ in the limit $\omega \to 0$. The Fourier transform of equal-time correlations ${\left[{〈S\left(\overrightarrow{0}\right)S\left(\overrightarrow{\mathrm{R}}\right)〉}^2\right]}_{\mathrm{av}}$ diverges as $k^{-4\mathrm{}}$, signaling a breakdown of mean-field theory at low $T$ below $d=4\mathrm{}$.

Experiments involving competitive and cooperative adsorption of pyridine, ${\mathrm{Cl}}^{-}$, SC${\mathrm{N}}^{-}$, and Tl, and voltage induced resonance shifting show that the largest surface-enhanced Raman scattering intensity comes from complexes of these species adsorbed on defects which cover less than 3% of the Ag surface. The resonance is attributed to charge transfer excitation from Ag to pyridine and from SC${\mathrm{N}}^{-}$ to Ag which is communicated to all vibrations of the complex.

Two possible techniques for reducing the external pressure required to induce the insulator-metal transition in solid hydrogen are described. One uses impurities to lower the energy of the metallic phase relative to that of the insulating phase. The other utilizes a negative pressure induced in the insulating phase by electron-hole pairs, created either with laser irradiation or pulsed synchrotron sources.

Measurements have been made of cw nonlinear far-infrared magnetoabsorption in pure, partially compensated $n$-GaAs. A new optical observation of the magnetoimpurity resonance has been demonstrated. Analysis in terms of a three-level model gives effective lifetimes of ∼0.5 μs "off resonance" and 30 ns "on resonance," implying the possibility of laser action in the former case.

A new type of spectrum was obtained from an inhomogeneously broadened ESR line under multiple modulation when one of the modulation frequencies was swept. Each spectral line appeared to be equivalent to the individual spin packet of the ESR line.

It has been assumed that the optical properties of small spheres can be understood by means of a Drude dielectric function that incorporates a boundary scattering rate $\frac{1}{{\tau }_s}\simeq \frac{v_{\mathrm{F}}}{R}$, where $v_{\mathrm{F}}$ is the Fermi velocity and $R$ is the sphere radius. An effective scattering rate $\frac{1}{{\tau }_s}=f\frac{v_{\mathrm{F}}}{R}$ is calculated and $f$ is evaluated as a function of photon frequency. The largest contribution to $f$ is due to the electron density profile of the sphere rather than the boundary scattering that is reduced an order of magnitude by electron screening.

Random anisotropy in ferromagnets causes small reorientations of the spins, so that they are not precisely collincar. As a consequence, an anisotropy torque develops for rotations about any axis. This requires the introduction of both a new macroscopic spin variable (to represent the macroscopic orientation about the magnetization) and an anisotropy triad. The macroscopic dynamics, including the dissipative motion of the anisotropy triad, is obtained, and a number of applications are considered. Most striking is the prediction of a longitudinal spin resonance mode.

An exact diagrammatic analysis of transport of electronic excitations among chromophores distributed randomly on a lattice is presented. An approximate solution is obtained that allows calculation of transport properties for transfer rates of any range for any lattice type and any concentration. Simple cubic lattice results are presented for Förster transfer and nearest-neighbor transfer. The latter has a percolation threshold that is described in a qualitatively correct fashion by our approximation.

The feasibility of using Monte Carlo methods in finite fermion systems has been investigated. The basic tool is a (recently suggested) functional integral representation of $Z=\mathrm{tr}\left[\exp \right(-\beta II\left)\right]$ using real coherent states of Slater determinants. Conditions for using the discretized ($N$-point lattice) version as a starting point for Monte Carlo integrations are studied. The limit of large $\beta$ is achieved only with large $\surd N$. For a two-fermion system numerical calculations of the ground-state energy agree farily well with the exact value.

It is observed that a color-neutral scalar phonon mode of velocity $\frac{c}{\sqrt{3}}$ should exist in the QCD plasma. The energy density of the phonons is $\frac{3\sqrt{3}}{16}$ that of noninteracting massless gluons and compensates to a considerable extent for the suppression of gluon modes due to collective plasma effects.

An inconsistency is found involving heavy quarkonia energy levels in the usual single-channel potential picture. Coupling to the nearest hadronic channel resolves the difficulty.

It is shown that nucleon collisions in a heavy nucleus may satisfy the conditions required for the inhibition of proton decay in the framework of the quantum mechanical theory of the decay of unstable systems.

The reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({\pi }^{+}, 2p)$ was studied at 165 and 245 MeV over a broad kinematic range by coincidence measurement of the outgoing protons. The $p-p$ angular correlations and proton energy spectra allow an identification of the "quasideuteron" component of the reaction. The angular distribution of this component has a shape similar to that of the ${\pi }^{+}d\to \mathrm{pp}$ reaction. It is found that one-step quasideuteron absorption accounts for at most 25% of the total absorption cross section.