The laser-induced desorption of CO molecules from a Cu(111) surface is time resolved with 100-fsec resolution. We find the desorption reaction is completed in ≤325 fsec after the 100-fsec, 2-eV photon energy pump pulse. The only mechanism consistent with this abrupt, efficient (σ∼${10}^{18}$ ${\mathrm{cm}}^2$) desorption is a novel one: The transient hot substrate electrons (${\mathit{T}}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n},\mathrm{m}\mathrm{a}\mathrm{x}}$∼3600 K, while ${\mathit{T}}_{\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{e},\mathrm{m}\mathrm{a}\mathrm{x}}$∼160 K) drive ImultipleP electronic excitation-deexcitation cycles of the CO-Cu complex within the vibrational relaxation time. This vibrationally pumps the molecules into desorbing.

Various interface structures formed between Si(111) and a thick Ag overlayer are investigated by grazing-incidence x-ray diffraction. The (7×7) reconstruction of Si(111) is preserved under a room-temperature deposited Ag film. Upon annealing to 250 °C the interface becomes (1×1). This is contrasted by the (√3 × √3 )R30° structure formed by annealing a thin Ag film on Si(111). By depositing a thick Ag film on this (√3 × √3 )R30° Ag/Si(111) surface at room temperature, the (√3 × √3 )R30° reconstruction is suppressed.

An analog of the ${\mathrm{C}}_{60}$ buckyball structure is investigated, in which seven-fold rings play the same role as the five-fold rings of ${\mathrm{C}}_{60}$. This periodic ${\mathrm{C}}_{168}$ structure is topologically ideal in the same sense as ${\mathrm{C}}_{60}$: Every atom is in a topologically equivalent local environment, participating in one seven-fold and two six-fold rings. The excess energy per atom to create this structure from an ideal graphitic sheet is predicted to be substantially less than for ${\mathrm{C}}_{60}$. If this material could be synthesized, we predict that it would have elastic constants comparable to those of Si, but with a density nearly half that of graphite.

Ultrafast four-wave mixing experiments using chirped-pulse excitation of a dye molecule in solution are reported and analyzed. Novel time-delayed signals are observed which are assigned to coherent Stokes-Raman scattering involving the excited state. Numerical calculations of four-wave mixing with linearly chirped pulses based on a stochastic dynamical model show that the signal near zero delay time is extremely sensitive to the presence or absence of slow fluctuations in the solute-solvent interaction.

Raman scattering experiments performed on n-type doped GaAs samples as a function of hydrostatic pressure reveal the existence, at low temperature and beyond some critical value of the pressure, of bound phonons associated with electrons trapped at an effective-mass-like level. At higher temperatures, this level is depopulated to the benefit of a deep state which has all the characteristics of the DX center. These results indicate a shallow-deep bistable character of n-type impurities in GaAs under high pressure.

Normal-emission spectra from 1T-${\mathrm{TiS}}_2$ have been calculated within the one-step model with explicit consideration of the dielectric response to the incident p-polarized photon field, because calculations within the dipole approximation failed to explain relative intensities of experimental energy distribution curves. Following the hydrodynamic model we added to the photon vector potential a longitudinal plasmon-induced plane wave. We show that these corrections generally are of distinct importance, and, in the present case, lead to an essentially better agreement between theoretical and experimental spectra.

We propose that superconductivity in ${\mathit{A}}_3$${\mathrm{C}}_{60}$ (A=K, Rb) with ${\mathit{T}}_{\mathit{c}}$≥30 K results from a favorable combination of high phonon frequencies and the existence of two different energy scales optimizing the coupling constant λ=NV. Calculations show that electron scattering V is dominated by particular on-ball Jahn-Teller-type modes on the scale of the large on-ball π-hopping energy, while the density of states N is controlled by the weak interball hopping energy. This factorization has several observed experimental consequences. Crucial differences to intercalated graphite explain the much smaller ${\mathit{T}}_{\mathit{c}}$ values in the graphite compounds.

We have used iodine intercalation to alter the interplane interaction and the anisotropic resistivity tensor of the oxide superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$. Real-space transmission electron microscopy images confirm that iodine is epitaxially intercalated between the Bi-O bilayers. In the normal state above ${\mathit{T}}_{\mathit{c}}$, the metallic ${\mathrm{CuO}}_2$ plane sheet resistance is unaffected by intercalation, while the out-of-plane conduction is dramatically changed from semiconductorlike to metalliclike. This result is inconsistent with some of the theoretical predictions based on holon-spinon scattering.

The component of the magnetic susceptibility periodic in 1/B, the de Haas–van Alphen effect, has been investigated using a 100-T flux compression system. It is concluded from these measurements that ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.97}$ exhibits a Fermi surface with at least three separate extremal frequencies for the c axis parallel to the applied field. Orbital masses, renormalization λ’s, and scattering rates are estimated from an amplitude analysis.

The phase diagram of the 2D t-J model is investigated using high-temperature expansions. Series for the Helmholtz free energy, the inverse compressibility, the chemical potential, and the uniform spin susceptibility through tenth order are calculated and analyzed. A region of phase separation is found at T=0 for J/t lying above a line extending from J/t=3.8 at zero filling to J/t=1.2 at half filling. For very small J/t near half filling where the Nagaoka effect is possible, we find a region of divergent uniform magnetic susceptibility at T=0.

A novel type of simulation has been designed to study the dynamics of long DNA fragments, containing millions of base pairs, as they migrate during gel electrophoresis. It shows that in a continuous field, large molecules assume a branched configuration—a behavior qualitatively different from that previously described for smaller fragments. Simulations in crossed fields reveal a new kind of reorientation mechanism which explains the well-known but puzzling fact that the power of separation increases abruptly as the angle between fields is raised above 90° and remains approximately constant for a wide range of obtuse settings.

A new adiabatic theory permits the understanding of one-dimensional dynamics of particles interacting with a large-amplitude wave packet for bounce time short compared with the transit time. This theory differs from previous ones in that the Hamiltonian varies slowly not with the time, but with the coordinate. The resulting adiabatic invariant is not equal, even in lowest order, to the usual action. This theory predicts the basic features of the interaction observed in previous numerical studies.

Delay times relative to a beat envelope are deduced from a path-integral solution of the telegrapher equation, analytically continued to imaginary time when the square of the effective velocity becomes negative. The results are approximately described by a semiclassical model translated in frequency from the nominal to an effective cutoff, thus improving the agreement with experimental results.

Gott has shown that a spacetime with two infinite parallel cosmic strings passing each other with sufficient velocity contains closed timelike curves. We discuss an attempt to build such a time machine. Using the energy-momentum conservation laws in the equivalent (2+1)-dimensional theory, we explicitly construct the spacetime representing the decay of one gravitating particle into two. We find that there is never enough mass in an open universe to build the time machine from the products of decays of stationary particles. More generally, the Gott time machine cannot exist in any open (2+1)-dimensional universe for which the total momentum is timelike.

We reexamine the causal properties of geometries generated by parallel, moving cosmic strings, particularly our statement that closed timelike curves are forbidden there. Contrary to a recent claim, such acausal behavior cannot be realized by physical, timelike, sources.

Recent experiments indicate that a 17-keV neutrino mixes with ${\nu }_{\mathit{e}}$ with 1% probability. We discuss the constraints from neutrinoless double β decay, cosmology, and astrophysics that any model possessing a 17-keV neutrino must obey. Several recently proposed models are shown to be incompatible with these constraints.

We present the results of a search for neutrino oscillations at the Brookhaven National Laboratory. The experiment searched for the appearance of ${\nu }_{\mathit{e}}$ (ν${\mathrm{¯}}_{\mathit{e}}$), 1 km from the source of a wide-band ${\nu }_{\mathrm{\mu }}$ (ν${\mathrm{¯}}_{\mathrm{\mu }}$) beam. The experiment used a total of 3×${10}^{19}$ protons on target from the Alternating Gradient Synchrotron. The data collecting was split evenly between positive and negative horn polarities, corresponding to neutrino and antineutrino beams. No excess of ${\nu }_{\mathit{e}}$ or ν${\mathrm{¯}}_{\mathit{e}}$ over the expected backgorund was detected. The 90%-confidence-level limits obtained are Δ${\mathit{m}}^2$≤0.075 ${\mathrm{eV}}^2$ for maximal mixing, and ${\sin }^2$2θ≤0.003 for large Δ${\mathit{m}}^2$.

We present results of an experiment in which approximately 500 events of the decay ${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ were observed. We have measured the branching ratio for ${\mathit{K}}_{\mathrm{\pi }\mathit{e}\mathit{e}}^{+}$ to be (2.75±0.23±0.13)×${10}^{\mathrm{-}7}$ assuming a vector interaction with a form factor of λ=0.105±0.035±0.015. We have also found the branching ratio times decay probability for ${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$${\mathit{X}}^0$, ${\mathit{X}}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ to be less than 1.5×${10}^{\mathrm{-}8}$ (99% C.L.) over an ${\mathit{X}}^0$ invariant mass range from 150 to 340 MeV/${\mathit{c}}^2$.

The Mark III Collaboration has measured the J/ψ leptonic branching fractions using the process ψ(2S)→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$J/ψ, J/ψ→${\mathit{l}}^{+}$${\mathit{l}}^{\mathrm{-}}$. The results are B(J/ψ→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)=(5.92±0.15±0.20)% and B(J/ψ→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$)=(5.90±0.15±0.19)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the J/ψ is (5.91±0.11±0.20)%. This result is used to obtain the strong coupling constant ${\mathrm{\alpha }}_{\mathit{s}}$ and the QCD scale factor ${\mathrm{\Lambda }}_{\mathrm{MS}}$ (MS¯ denotes the modified minimal-subtraction scheme).

Strong correlations between baryon stopping in the projectile rapidity hemisphere and target excitation have been found in the light-ion-induced reactions at the BNL Alternating Gradient Synchrotron (AGS) (E814 group). Results in the framework of the relativistic molecular dynamics approach (RQMD) describe recent E814 data quite well. We discuss the RQMD results together with proton and pion data from the E802 group near midrapidity. They have raised the question of whether partial transparency could be seen in these experiments. The RQMD results indicate strong transverse baryon flow in central Si+Au collisions after the projectile has been stopped in the target.