19,726 research outputs found
Ten-dimensional wave packet simulations of methane scattering
We present results of wavepacket simulations of scattering of an oriented
methane molecule from a flat surface including all nine internal vibrations. At
a translational energy up to 96 kJ/mol we find that the scattering is almost
completely elastic. Vibrational excitations when the molecule hits the surface
and the corresponding deformation depend on generic features of the potential
energy surface. In particular, our simulation indicate that for methane to
dissociate the interaction of the molecule with the surface should lead to an
elongated equilibrium C--H bond length close to the surface.Comment: RevTeX 15 pages, 3 eps figures: This article may be found at
http://link.aip.org/link/?jcp/109/1966
Energy dissipation and scattering angle distribution analysis of the classical trajectory calculations of methane scattering from a Ni(111) surface
We present classical trajectory calculations of the rotational vibrational
scattering of a non-rigid methane molecule from a Ni(111) surface. Energy
dissipation and scattering angles have been studied as a function of the
translational kinetic energy, the incidence angle, the (rotational) nozzle
temperature, and the surface temperature. Scattering angles are somewhat
towards the surface for the incidence angles of 30, 45, and 60 degree at a
translational energy of 96 kJ/mol. Energy loss is primarily from the normal
component of the translational energy. It is transfered for somewhat more than
half to the surface and the rest is transfered mostly to rotational motion. The
spread in the change of translational energy has a basis in the spread of the
transfer to rotational energy, and can be enhanced by raising of the surface
temperature through the transfer process to the surface motion.Comment: 8 pages REVTeX, 5 figures (eps
Ab-initio coupled-cluster effective interactions for the shell model: Application to neutron-rich oxygen and carbon isotopes
We derive and compute effective valence-space shell-model interactions from
ab-initio coupled-cluster theory and apply them to open-shell and neutron-rich
oxygen and carbon isotopes. Our shell-model interactions are based on
nucleon-nucleon and three-nucleon forces from chiral effective-field theory. We
compute the energies of ground and low-lying states, and find good agreement
with experiment. In particular our calculations are consistent with the N=14,
16 shell closures in oxygen-22 and oxygen-24, while for carbon-20 the
corresponding N=14 closure is weaker. We find good agreement between our
coupled-cluster effective-interaction results with those obtained from standard
single-reference coupled-cluster calculations for up to eight valence neutrons
Molecular Lines as Diagnostics of High Redshift Objects
Models are presented for CO rotational line emission by high redshift
starburst galaxies. The influence of the cosmic microwave background on the
thermal balance and the level populations of atomic and molecular species is
explicitly included. Predictions are made for the observability of starburst
galaxies through line and continuum emission between z=5 and z=30. It is found
that the Millimeter Array could detect a starburst galaxy with ~10^5 Orion
regions, corresponding to a star formation rate of about 30 Mo yr^{-1}, equally
well at z=5 or z=30 due to the increasing cosmic microwave background
temperature with redshift. Line emission is a potentially more powerful probe
than dust continuum emission of very high redshift objects.Comment: 15 pages LaTex, uses aasms4.sty, Accepted by ApJ
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
Non-perturbative renormalization of the axial current with improved Wilson quarks
We present a new normalization condition for the axial current, which is
derived from the PCAC relation with non-vanishing mass.
Using this condition reduces the O(r_0 m) corrections to the axial current
normalization constant Z_A for an easier chiral extrapolation in the cases,
where simulations at zero quark-mass are not possible. The method described
here also serves as a preparation for a determination of Z_A in the full
two-flavor theory.Comment: 3 pages, 3 figures, Lattice2003(improve
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
- …