Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals.

Refractive-index and linear-birefringence measurements on $\mathrm{K}{\mathrm{Ta}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$, with $x=0.008, 0.012, \mathrm{and} 0.02$, reveal smeared phase transitions into long-range order. In accordance with previous caloric, second-harmonic—generation, dielectric, and x-ray results, the crystal optical data can be explained on the basis of a "cooperative dipole glass" model. This involves local ordering within the finite Curie region of a diffuse phase transition followed by a strain-induced collective reorientation of frozen-in microdomains.

We report the first observation of wetting transitions of hcp ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ crystals grown on a metallic substrate. Crystals grown with the [0001] direction perpendicular to the substrate undergo a continuous wetting transition in the range $0.81<T<\mathrm{}0.85\mathrm{}$ K, whereas crystals grown with the [$10\overline{1}0$] direction perpendicular to the substrate undergo a first-order wetting transition at $T=1.03\mathrm{}$ K. Our results demonstrate that continuous wetting is possible in a system governed by van der Waals forces.

We report the first direct determination of carrier-energy-loss rates in a semiconductor. These measurements provide fundamental insight into carrier-phonon interactions in semiconductors. Unexpectedly large differences are found in the energy-loss rates for electrons and holes in GaAs/AlGaAs quantum wells. This large difference results from an anomalously low electron-energy-loss rate, which we attribute to the presence of nonequilibrium optical phonons rather than the effects of reduced dimensionality or dynamic screening.

An earlier analysis of small one-dimensional rings of normal metal, driven by an external time-dependent magnetic flux, is extended to allow for inelastic scattering. This supplements the Josephson-type behavior of the earlier discussion by dissipative effects. As in the case of strong one-dimensional localization, dc current grows as inelastic scattering is first introduced, but diminishes again with sufficiently intense inelastic scattering.

Numerical studies of the dynamics of finite quantum spin chains are presented which show that quantum systems with few degrees of freedom ($N=7\mathrm{}$) can be described by equilibrium statistical mechanics. The success of the statistical description is seen to depend on the interplay between the initial state, the observable, and the Hamiltonian. This work clarifies the impact of integrability and conservation laws on statistical behavior. The relation to quantum chaos is also discussed.

The joint distribution of energy eigenvalues of a Hamiltonian is derived by means of the usual statistical laws of classical many-body systems. It makes a transition from the Poisson type to the Gaussian type depending on the value of a single parameter characteristic of the Hamiltonian.

A muon-spin-rotation technique has been used to place limits on right-handed currents in ${\mu }^{+}$ decay. The spins of polarized ${\mu }^{+}$ stopped in metal targets were precessed by 70-G or 110-G transverse fields. The muon-spin-rotation signal amplitude produced by high-momentum decay $e^{+}$ emitted near the beam direction implies $\frac{\xi P_{\mu }\delta }{\rho }>0.9955\mathrm{}$ and $M\left(W_2\right)>\mathrm{}370\mathrm{}$ GeV (90% confidence), where $W_2$ is a predominantly right-handed gauge boson. The present result combined with our previous spin-held analysis yields $\frac{\xi P_{\mu }\delta }{\rho }>0.9966\mathrm{}$ and $M\left(W_2\right)>\mathrm{}400\mathrm{}$ GeV.

The observation of a distortion of the $\beta$ spectrum of tritium is reported. This distortion is consistent with the emission of a neutrino of mass about 17.1 keV and a mixing probability of 3%.

We report the first direct measurement of the inclusive decay of $B$ mesons into charged $D^{*}$ mesons. The shape of the momentum spectrum is consistent with the $D^0$ spectrum from $B$ decay. A large fraction of the charm observed in $B$ decay is accounted for by $D^{*}$ mesons.

The triply differential cross sections $\frac{d^3\sigma }{\mathrm{dA}\mathrm{dE}d\theta }$ of binary exit channels in the reactions ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$ + ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$ and ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ + ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ have been measured at energies between 12 and 18.8 MeV/u. Complete relaxation is reached at energy losses up to 650 MeV, corresponding to temperatures in the fragments of up to 5 MeV; the primary mass distributions tend to spread over the full range of mass asymmetries, indicating a loss of the initial target and projectile identity and hence the disappearance of an essential feature of the deep-inelastic process.

The $A$ dependence of the nuclear effect, as recently measured by Arnold et al., has been calculated from the probability of forming $>3q$ clusters, by use of the space-time wave function of valence quarks, and the assumption that nuclei are in hexagonal or cubic dense packing. A fair fit to the $x$ distributions of $\frac{{\sigma }_A}{{\sigma }_{\mathrm{D}}}$ is obtained, if we apply the valence quarks' average Fermi momentum and mass from our previous determinations.

Within a theory which unifies the nuclear collective and single-particle models and which is valid for arbitrary vibrational amplitudes, the average positiveness of the electric quadrupole moments follows from two requirements: (1) The nucleons have to stay within the range of nuclear forces for a strongly deformed nucleus; (2) the wave functions on the average do not show any preference for prolate or oblate shape.

It is shown that a good representation of recent experimental results on the relative production of multiply charged ionic states by intense laser pulses of various wavelengths may be obtained on the basis of Hartree's independent-electron shell model of the atom.

We have observed the $1S-2S$ transition in atomic hydrogen gas with a resolution of 5 parts in ${10}^9$ by Doppler-free two-photon spectroscopy, using multimilliwatt cw radiation near 243 nm generated by sum-frequency mixing. The experiment opens the way to substantial increases in the accuracy of the Rydberg constant, the electron/proton mass ratio, and the Lamb shift of the hydrogen $1S$ ground state.

The first observation of the subradiance phenomenon is reported. As previously predicted, destructive interatomic interference may prevent cooperative deexcitation of a collection of atoms from being complete. In the case of a $j=\frac{3}{2}\to j^{\prime }=\frac{1}{2}$ transition, for example, half the atoms remain excited when the initial atomic state is a full statistical mixture with equal weights of all Zeeman sublevels of the upper level. The experiment has been realized with gallium atoms. The results agree with semiclassical calculations.

Recent experimental data show that the spin-rotation coupling for alkali-metal-noble-gas van der Waals molecules is mainly due to the spin-orbit interaction of the alkali-metal valence electron within the core of the noble gas. We present a simple formula which accounts very well for the systematic variation of the spin-rotation interaction with different noble-gas or alkali-metal atoms.

By use of two-dimensional approximations for the equations that describe a high-gain free-electron laser (FEL) amplifier, and the properties of optical fibers, it is shown that the coherent interaction between the light and the electron beam in an FEL can optically guide the light. In the exponential-gain regime, the FEL performance in the presence of strong diffraction can be simply described by a cubic equation for the complex gain and the dispersion relation for an optical fiber. The phenomenon of optical guiding is illustrated with two-dimensional numerical simulations. The phenomenon has applications to short-wavelength FEL's, to directing of intense light, and to bending of x rays.

The zero-temperature transition pressure from molecular to monatomic nitrogen is predicted to be less than about 1 Mbar, and should be observable in diamond-anvil cells. This estimate is obtained from total-energy calculations in which compressed molecular nitrogen is found to dissociate into a simple cubic monatomic structure, slight distortions of which are common to other group-V elements. The calculated 35% volume change raises the possibility of a large barrier to dissociation and extensive regions of metastability.

A systematic study of the lattice dynamics of several (2×2) overlayers on Ni(100) leads to the conclusion that the reconstruction of the nickel surface with a carbon overlayer is of the softphonon type. The reconstruction is driven by the freezing of the $S_1$ nickel surface phonon.