12,705 research outputs found
Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH_3-Si(111) and CD_3-Si(111) surfaces
The surface structure and vibrational dynamics of CH_3–Si(111) and CD_3–Si(111) surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye–Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular (θ_i=θ_f) decay revealed
perpendicular mean-square displacements of 1.0 x 10^(−5) Å^2 K^(−1) for the CH_3–Si(111) surface and 1.2 x 10^(−5) Å^2 K^(−1) for the CD_3–Si(111) surface, and a He-surface attractive well depth of ~7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH_3–Si(111) surface and 824 K for the CD_3–Si(111) surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through
the Si–C local molecular modes. The parallel mean-square displacements were 7.1 x 10^(−4) and 7.2 x 10^(−4) Å^2 K^(−1) for the CH_3–Si(111) and CD_3–Si(111) surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si–CH_3 bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces
Physics of Solid and Liquid Alkali Halide Surfaces Near the Melting Point
This paper presents a broad theoretical and simulation study of the high
temperature behavior of crystalline alkali halide surfaces typified by
NaCl(100), of the liquid NaCl surface near freezing, and of the very unusual
partial wetting of the solid surface by the melt. Simulations are conducted
using two-body rigid ion BMHFT potentials, with full treatment of long-range
Coulomb forces. After a preliminary check of the description of bulk NaCl
provided by these potentials, which seems generally good even at the melting
point, we carry out a new investigation of solid and liquid surfaces. Solid
NaCl(100) is found in this model to be very anharmonic and yet exceptionally
stable when hot. It is predicted by a thermodynamic integration calculation of
the surface free energy that NaCl(100) should be a well ordered, non-melting
surface, metastable even well above the melting point. By contrast, the
simulated liquid NaCl surface is found to exhibit large thermal fluctuations
and no layering order. In spite of that, it is shown to possess a relatively
large surface free energy. The latter is traced to a surface entropy deficit,
reflecting some kind of surface short range order. Finally, the solid-liquid
interface free energy is derived through Young's equation from direct
simulation of partial wetting of NaCl(100) by a liquid droplet. It is concluded
that three elements, namely the exceptional anharmonic stability of the solid
(100) surface, the molecular short range order at the liquid surface, and the
costly solid liquid interface, all conspire to cause the anomalously poor
wetting of the (100) surface by its own melt in the BMHFT model of NaCl -- and
most likely also in real alkali halide surfaces.Comment: modified version of JCP 123, 164701 15 pages, 25 figure
Positron-molecule interactions: resonant attachment, annihilation, and bound states
This article presents an overview of current understanding of the interaction
of low-energy positrons with molecules with emphasis on resonances, positron
attachment and annihilation. Annihilation rates measured as a function of
positron energy reveal the presence of vibrational Feshbach resonances (VFR)
for many polyatomic molecules. These resonances lead to strong enhancement of
the annihilation rates. They also provide evidence that positrons bind to many
molecular species. A quantitative theory of VFR-mediated attachment to small
molecules is presented. It is tested successfully for selected molecules (e.g.,
methyl halides and methanol) where all modes couple to the positron continuum.
Combination and overtone resonances are observed and their role is elucidated.
In larger molecules, annihilation rates from VFR far exceed those explicable on
the basis of single-mode resonances. These enhancements increase rapidly with
the number of vibrational degrees of freedom. While the details are as yet
unclear, intramolecular vibrational energy redistribution to states that do not
couple directly to the positron continuum appears to be responsible for these
enhanced annihilation rates. Downshifts of the VFR from the vibrational mode
energies have provided binding energies for thirty species. Their dependence
upon molecular parameters and their relationship to positron-atom and
positron-molecule binding energy calculations are discussed. Feshbach
resonances and positron binding to molecules are compared with the analogous
electron-molecule (negative ion) cases. The relationship of VFR-mediated
annihilation to other phenomena such as Doppler-broadening of the gamma-ray
annihilation spectra, annihilation of thermalized positrons in gases, and
annihilation-induced fragmentation of molecules is discussed.Comment: 50 pages, 40 figure
Detection of a branched alkyl molecule in the interstellar medium: iso-propyl cyanide
The largest non-cyclic molecules detected in the interstellar medium (ISM)
are organic with a straight-chain carbon backbone. We report an interstellar
detection of a branched alkyl molecule, iso-propyl cyanide (i-C3H7CN), with an
abundance 0.4 times that of its straight-chain structural isomer. This
detection suggests that branched carbon-chain molecules may be generally
abundant in the ISM. Our astrochemical model indicates that both isomers are
produced within or upon dust grain ice mantles through the addition of
molecular radicals, albeit via differing reaction pathways. The production of
iso-propyl cyanide appears to require the addition of a functional group to a
non-terminal carbon in the chain. Its detection therefore bodes well for the
presence in the ISM of amino acids, for which such side-chain structure is a
key characteristic.Comment: This is the author's version of the work. It is posted here by
permission of the AAAS for non-commercial use. The definitive version was
published in Science 345, 1584 (2014), doi:10.1126/science.125667
Femtosecond wave packet spectroscopy: Coherences, the potential, and structural determination
Recently, we presented a formalism for extracting highly resolved spectral information and the potential of bound isolated systems from coherent ultrafast laser experiments, using I2 as a model system [Gruebele et al., Chem. Phys. Lett. 166, 459 (1990)]. The key to this approach is the formation of coherent wave packets on the potential energy curve (or surface) of interest, and the measurement of their scalar and vector properties. Here we give a full account of the method by analyzing the coherences of the wave packet in the temporal transients of molecules excited by ultrashort laser pulses, either at room temperature, or in a molecular beam. From this, some general considerations for properly treating temporal data can be derived. We also present a direct inversion to the potential and quantum and classical calculations for comparison with the experiments
The Development of a Fiber Optic Raman Temperature Measurement System for Rocket Flows
A fiberoptic Raman diagnostic system for H2/O2 rocket flows is currently under development. This system is designed for measurement of temperature and major species concentration in the combustion chamber and part of the nozzle of a 100 Newton thrust rocket currently undergoing testing. This paper describes a measurement system based on the spontaneous Raman scattering phenomenon. An analysis of the principles behind the technique is given. Software is developed to measure temperature and major species concentrations by comparing theoretical Raman scattering spectra with experimentally obtained spectra. Equipment selection and experimental approach are summarized. This experimental program is part of a program, which is in progress, to evaluate Navier-Stokes based analyses for this class of rocket
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