36,098 research outputs found
Isotopic and spin selectivity of H_2 adsorbed in bundles of carbon nanotubes
Due to its large surface area and strongly attractive potential, a bundle of
carbon nanotubes is an ideal substrate material for gas storage. In addition,
adsorption in nanotubes can be exploited in order to separate the components of
a mixture. In this paper, we investigate the preferential adsorption of D_2
versus H_2(isotope selectivity) and of ortho versus para(spin selectivity)
molecules confined in the one-dimensional grooves and interstitial channels of
carbon nanotube bundles. We perform selectivity calculations in the low
coverage regime, neglecting interactions between adsorbate molecules. We find
substantial spin selectivity for a range of temperatures up to 100 K, and even
greater isotope selectivity for an extended range of temperatures,up to 300 K.
This isotope selectivity is consistent with recent experimental data, which
exhibit a large difference between the isosteric heats of D_2 and H_2 adsorbed
in these bundles.Comment: Paper submitted to Phys.Rev. B; 17 pages, 2 tables, 6 figure
Ion-Molecule Reactions in Unsaturated Hydrocarbons: Allene, Propyne, Diacetylene, and Vinylacetylene
Ion-molecule reactions in allene, propyne, diacetylene, and vinylacetylene (1-buten-3-yne) have been studied at near-thermal energies by the technique of ion cyclotron resonance mass spectrometry. Rate coefficients and branching ratios are reported for the reactions of C_3H^+_n (n = 1-4) with allene and propyne and for the reactions of C_4H^+_n (n = 0-5) with diacetylene and vinylacetylene. Branching ratios are also given for the reactions of C_4H^+_n, C_5H_n, and C_6H^+_n with propyne and for reactions of C_6H^+_n with diacetylene and vinylacetylene. More than 90% of the reactive channels lead to product ions having a larger carbon skeleton than the reactant ion. Evidence for ions with the same m/e ratio having differing reactivities was obtained for C_3H^+_3, C_6H^+_7, and C_7H^+_7. Ion reaction sequences in allene and propyne were followed at higher pressures (l0^(-4) torr) to investigate secondary, tertiary, and higher order processes
Glass-like Thermal-Transport in Symmetry-Broken Clathrates
We present the quantitative interpretation for the glass-like behavior of
thermal conductivities for type-I clathrate compounds involving
off-centered guest ions. It is shown that the dipole-dipole interaction
generated in cage/guest-ion systems is crucial to reproduce the characteristics
of thermal conductivities for these symmetry-broken clathrates. The above
scenario also explains well the difference of between the -type
and the -type -BGS found recently by K. Suekuni et al. [Phys. Rev. B,
77 (2008) 235119.
Laser-heater assisted plasma channel formation in capillary discharge waveguides
A method of creating plasma channels with controllable depth and transverse
profile for the guiding of short, high power laser pulses for efficient
electron acceleration is proposed. The plasma channel produced by the
hydrogen-filled capillary discharge waveguide is modified by a ns-scale laser
pulse, which heats the electrons near the capillary axis. This interaction
creates a deeper plasma channel within the capillary discharge that evolves on
a ns-time scale, allowing laser beams with smaller spot sizes than would
otherwise be possible in the unmodified capillary discharge.Comment: 5 pages, 3 figure
Negative specific heat in a thermodynamic model of multifragmentation
We consider a soluble model of multifragmentation which is similar in spirit
to many models which have been used to fit intermediate energy heavy ion
collision data. In this model is always positive but for finite nuclei
can be negative for some temperatures and pressures. Furthermore,
negative values of can be obtained in canonical treatment. One does not
need to use the microcanonical ensemble. Negative values for can persist
for systems as large as 200 paticles but this depends upon parameters used in
the model calculation. As expected, negative specific heats are absent in the
thermodynamic limit.Comment: Revtex, 13 pages including 6 figure
Additives That Prevent Or Reverse Cathode Aging In Drift Chambers With Helium-Isobutane Gas
Noise and Malter breakdown have been studied at high rates in a test chamber
having the same cell structure and gas as in the BaBar drift chamber. The
chamber was first damaged by exposing it to a high source level at an elevated
high voltage, until its operating current at normal voltages was below
0.5nA/cm. Additives such as water or alcohol allowed the damaged chamber to
operate at 25 nA/cm, but when the additive was removed the operating point
reverted to the original low value. However with 0.02% to 0.05% oxygen or 5%
carbon dioxide the chamber could operate at more than 25 nA/cm, and continued
to operate at this level even after the additive was removed. This shows for
the first time that running with an O2 or CO2 additive at high ionisation
levels can cure a damaged chamber from breakdown problems.Comment: There were typos: 0.2%-0.5% oxygen should be 0.02%-0.05% oxygen.
Values in the Table were O
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