We have recently discovered that positrons form long-lived resonances with large molecules. We present a study of the positive ions that are produced when the positrons annihilate with an electron in such a resonance for the case of alkane molecules (${\mathrm{C}}_{\mathrm{n}}$${\mathrm{H}}_{2\mathrm{n}+2\mathrm{}}$). This method of ion production is qualitatively different than those conventionally used and may be useful in the chemical analysis of unknown molecular species by mass spectrometry.

This Rapid Communication reports fully quantum-mechanical calculations of inelastic-scattering cross sections for ${\mathrm{Li}}_2^{\mathrm{*}}$(${\mathrm{v}}_{\mathrm{i}}$,$j_i$) +Ne→${\mathrm{Li}}_2^{\mathrm{*}}$($v_f$,$j_f$)+Ne with 40≤$j_i$≤100. The calculations are performed within the coupled-states approximation. They reveal large cross sections for vibrational relaxation to remarkably narrow final-rotational-state distributions. The theoretical cross sections reproduce the Δj=-4Δv correlation observed experimentally. Comparison of the quantum-mechanical results to classical trajectory calculations confirms the accuracy of the latter procedure.

Recent measurements have shown that the angular distribution of Auger electrons observed in near threshold resonant excitation with energy resolution adequate to partially resolve the multiplet structure corresponding to final core states exhibited a marked change in the angular distributions as a function of final electron energy. A method of calculating these angular distributions is proposed that extends the formalism developed for treating angular distributions in photoeffect via a separation into parity-favored and -unfavored components. As an example of the use of the method, calculations are presented for the 2p-4s resonant Auger process in argon and compared with recent experiments.

Time-resolved uranium spectra emitted from a relatively low-density ($n_e$∼${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$), high-temperature ($T_e$∼1 keV) plasma confined in the Texas experimental tokamak (TEXT) have been recorded in the 50–100 Å range. It is found that narrow emission bands originating from transitions within charge states having 5$p^6$5$d^k$ and 5$s^2$5$p^k$ ground configurations, U xv to U xxxi, dominate these spectra. The identification of the transition arrays is based on the comparison with both ab initio intermediate-coupling relativistic level structure computations including configuration-interaction effects for the simple cases and, for complex charge states, with the predictions of the unresolved transition array model.

Transition-frequency measurements in triatomic hydrogen allow a unique identification of Rydberg series reported in previous work. From spectroscopic combination differences we derive an experimental value for the symmetric-stretch frequency of the metastable ${\mathrm{H}}_3$ 2$p^2$ $A_2^{\mathcal{’}\mathcal{’}}$ state as well as for ${\mathrm{P}}_3^{+}$. .AE

A precision measurement has been made of the lifetime of the $2^3$$S_1$ state of ${\mathrm{Xe}}^{52+}$ by beam-foil time-of-flight spectroscopy. The result is τ${(2\mathrm{}}^3$$S_1$)=2.554(76)×${10}^{\mathrm{-}12}$ sec, in agreement with theortical calculations. This measurement provides the most precise test of the theory of relativistic M1 decay and is sensitive for the first time to corrections of O(Zα${)}^2$ to the leading term.

We have studied the temporal profile of the picosecond laser pulse scattered from random media in the backward direction in two angular regions: Coherent (-1≤θ≤1 mrad) and diffuse (7.5≤θ≤9.5 mrad), using a streak camera with overall 15-ps time resolution. To give a good description of the temporal profile of the scattered pulse, in addition to a diffusion term, terms from the maximally crossed diagrams are required to account for the coherent enhancement of scattered intensity in the backward direction due to time-reversal symmetry.

We estimate the number of degrees of freedom in intermittent fully developed turbulence using the multifractal formalism. We find that for any reasonable distribution f(α), the number of degrees of freedom is decreased by intermittency for space dimensionality D<4, and increased for D>4. This is in agreement with a result of Kraichnan [Phys. Fluids 28, 10 (1985)] obtained for the special case of fractally homogeneous turbulence. We calculate the reduction in attractor size in three dimensions and find it to be very small. The possible dynamical significance of our result is briefly discussed.

A nonlinear stability analysis by Demay and Iooss of flow between concentric rotating cylinders (the Couette-Taylor system) predicted a transition from the basic flow to a state with ribbons, which are traveling waves in the azimuthal direction but standing waves in the axial direction. We have observed the transition to ribbons in laboratory experiments and numerical simulations, and the measured wave speeds are in accord with those obtained numerically.

The collapse transition of a polymer chain on a two-dimensional lattice with directed bonds, the Manhattan lattice, is shown to occur at temperature $T_{\theta }$=2ɛ/ln2, where ɛ is the attractive energy between nearest-neighbor pairs of monomers. The exact tricritical exponents are ${\nu }_t$=(4/7 and ${\gamma }_t$=(6/7. The latter result differs from two recently proposed values of ${\gamma }_t$.

The phase diagram of the liquid crystal n-propyl-4’-n-decyloxy-biphenyl-4-carboxylate [3(10)OBC] and 4-propionyl-4’-n-heptanoyloxy-azobenzene (PHOAB) binary mixture has been investigated by heat-capacity studies and optical observations. The heat-capacity data reveal a continuous change in the character of the smectic-A–hexatic-B phase transition which is continuous in 3(10)OBC and becomes first order in PHOAB. It is proposed that this crossover effect is due to the coupling of the spatial fluctuations of the smectic layers to the hexatic parameter.

Using optical second-harmonic generation, we have measured the surface coverage, binding energy, adsorption and desorption, and orientation of a liquid-crystal monolayer on clean glass and silane-treated glass. The results show that the short-range surface interaction does not necessarily dictate the bulk alignment of a liquid crystal.

It is shown that, in the long-time limit, d-dimensional diffusion in a Gaussian random potential has the logarithm of the average population ln 〈P〉 growing as $t^{(2\mathrm{}\mathrm{-}d/2)}$. The dimension d=4 is critical. For d≥4, 〈P〉 only grows as a power of t. Numerical simulations have confirmed this result.