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A MAJOR EVENT IN THE MORE THAN 6-year journey of the Gelileo spececraft to Jupiter was its encounter with Venus on 10 February 1990. The encounter provided an opportunity to [...]

COLLISIONS BETWEEN GAS-PHASE and liquid-phase molecules are the first step in establishing mechanical, thermal, and chemical equilibrium between gases and liquids. Gas-phase molecules striking a liquid can scatter immediately away [...]

Near ambient pressures, molecular diffusion dominates protonic diffusion in ice. Theoretical studies have predicted that protonic diffusion will dominate at high pressures in ice. We measured the protonic diffusion coefficient for the highest temperature molecular phase of ice VII at 400 kelvin over its entire stable pressure region. The values ranged from [10.sup.-17] to [10.sup.-15] square meters per second at pressures of 10 to 63 gigapascals. The diffusion coefficients extrapolated to high temperatures dose to the ice VII melting curve were less by a factor of [10.sup.2] to [10.sup.3] than a superionic criterion of ~[10.sup.-8] square meters per second, at which protons would diffuse freely., Protons can move in the hydrogen-bonded network of water molecules by transferring within a hydrogen bond and then jumping into another hydrogen bond by molecular rotation. Although this diffusion model [...]

Pulses of x-rays 300 femtoseconds in duration at a wavelength of 0.4 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly [...]

We observed coherent proton tunneling in the cyclic network of four hydrogen bonds in calix[4]arene. The tunneling frequency of 35 megahertz was revealed by a peak in the magnetic field dependence of the proton spin-lattice relaxation rate measured with field-cycling nuclear magnetic resonance in the solid state at temperatures below 80 kelvin. The amplitude of the coherent tunneling peak grows with temperature according to a Boltzmann law with energy D/[k.sub.B] = (125 ± 10) kelvin (where [k.sub.B] is Boltzmann's constant). The tunneling peak can be interpreted in the context of level crossings in the region where the tunneling frequency matches the proton Larmor frequency. The tunneling spectrum reveals fine structure that we attribute to coupling between the hydrogen bonds in the network. The characteristics of the tunneling peak are interpreted in the context of the potential energy surface experienced by the hydrogen atoms in the network., Quantum tunneling can occur when a particle with energy E encounters a potential barrier of magnitude [V.sub.0] greater than E. The tunneling probability that defines the tunneling frequency is an [...]

A full quantum dynamical study of the reactions of a hydrogen atom with water, on an accurate ab initio potential energy surface, is reported. The theoretical results are compared with [...]

Experiments, employing crossed molecular beams, with vibrational state resolution have been performed on the simplest four-atom reaction, OH + [D.sub.2] → HOD + D. In good agreement with the most [...]

The O(¹D) + [H.sub.2] → OH + H reaction, which proceeds mainly as an insertion reaction at a collisional energy of 1.3 kilocalories per mole, has been investigated with the high-resolution H atom Rydberg 'tagging' time-of-flight technique and the quasiclassical trajectory (QCT) method. Quantum state-resolved differential cross sections were measured for this prototype reaction. Different rotationally-vibrationally excited OH products have markedly different angular distributions, whereas the total reaction products are roughly forward and backward symmetric. Theoretical results obtained from QCT calculations indicate that this reaction is dominated by the insertion mechanism, with a small contribution from the collinear abstraction mechanism through quantum tunneling., The quantum-state distribution of a nascent chemical reaction product carries important information in a reactive scattering experiment. Information on the angular distribution of the reaction product is also crucial in [...]

An effective environmental force constant is introduced to quantify the molecular resilience (or its opposite, 'softness') of a protein structure and relate it to biological function and activity. Specific resilience-function relations were found in neutron-scattering experiments on purple membranes containing bacteriorhodopsin, the light-activated proton pump of halobacteria; the connection between resilience and stability is illustrated by a study of myoglobin in different environments. Important advantages of the neutron method are that it can characterize the dynamics of any type of biological sample--which need not be crystalline or monodisperse--and that it enables researchers to focus on the dynamics of specific parts of a complex structure with deuterium labeling., It is now well accepted that conformational flexibility is essential for enzyme catalysis and for biological molecular activity in general. But the concept of flexibility refers to molecular motions on [...]

Low-Energy Atom Scattering from Surfaces STRUCTURAL STUDIES OF SURFACES HAVE BEEN ONE OF THEmain avenues of research in surface science since the 1960's. From chemical studies of gas and liquid [...]

A new approach to radiation theory was published recently by Lieu and Axford in the Astrophysical Journal[1] The method is elegant in that it uses the concept of inverse Compton [...]

Playing Three-Dimensional Pool THIS GAME HAS NO EIGHT BALL and no corner pockets, but anyone who has knocked a few balls around a billiards table will recognize it on sight. [...]

Phonon Density of States and Specific Heat of Forsterite, Mg2SiO4 FORSTERITE IS THE MAGNESIUM END-memberof the olivine (Mg, Fe)2SiO4 series. Magnesium-rich olivine, (Mg0.9Fe0.1)2SiO4, is considered to be the major component [...]