1,642 research outputs found
Transient chaos and resonant phase mixing in violent relaxation
This paper explores how orbits in a galactic potential can be impacted by
large amplitude time-dependences of the form that one might associate with
galaxy or halo formation or strong encounters between pairs of galaxies. A
period of time-dependence with a strong, possibly damped, oscillatory component
can give rise to large amounts of transient chaos, and it is argued that
chaotic phase mixing associated with this transient chaos could play a major
role in accounting for the speed and efficiency of violent relaxation. Analysis
of simple toy models involving time-dependent perturbations of an integrable
Plummer potential indicates that this chaos results from a broad, possibly
generic, resonance between the frequencies of the orbits and harmonics thereof
and the frequencies of the time-dependent perturbation. Numerical computations
of orbits in potentials exhibiting damped oscillations suggest that, within a
period of 10 dynamical times t_D or so, one could achieve simultaneously both
`near-complete' chaotic phase mixing and a nearly time-independent, integrable
end state.Comment: 11 pages and 12 figures: an extended version of the original
manuscript, containing a modified title, one new figure, and approximately
one page of additional text, to appear in Monthly Notices of the Royal
Astronomical Societ
Chirality and the origin of atmospheric humic-like substances
Aerosol water extracts and atmospheric humic-like substances (HULIS) obtained from PM2.5-fraction aerosol samples collected in a rural/continental background environment and in an urban environment in spring and summer, and at a tropical site that was heavily impacted by biomass burning were studied. HULIS was obtained as the water-soluble, methanol-elutable material isolated from a solid-phase extraction procedure. The mean organic matter-to-organic carbon mass conversion factor and the standard deviation of 2.04 +/- 0.06 were derived for HULIS from biomass burning. Mean atmospheric concentrations of HULIS for the rural and urban environments and for the biomass burning during daylight periods and nights, were 1.65, 2.2, 43, and 60 mu gm(-3), respectively. This and other abundances indicate that intense emission sources and/or formation mechanisms of HULIS operate in biomass burning. Mean contributions of C in HULIS (HULIS-C) to water-soluble organic carbon (WSOC) were 35, 48, 63, and 76%, respectively, for the sample set listed. HULIS-C is the major component of the WSOC in tropical biomass burning. The data also suggest that HULIS most likely do not share common origin in the three environments studied. Differentiation among the possible formation processes was attempted by investigating the optical activity of HULIS through their (electronic and vibrational) circular dichroism properties. The urban HULIS did not show optical activity, which is in line with the concept of their major airborne formation from anthropogenic aromatics. The rural HULIS revealed weak optical activity, which may be associated with one of their important formation pathways by photo-oxidation and oligomerisation, i.e., with the formation from chiral biogenic precursors with one of the enantiomers slightly enriched. The The biomass burning of HULIS exhibited a strong effect in the vibrational circular dichroism as a clear distinction from the other two types. This was related to the contribution of the thermal degradation products of lignins and cellulose. The biomass burning of HULIS resemble Suwannee River Fulvic Acid standard more closely in some aspects than the urban and rural types of HULIS, which may be related to their common origin from plant material
Generation of a stable low-frequency squeezed vacuum field with periodically-poled KTiOPO at 1064 nm
We report on the generation of a stable continuous-wave low-frequency
squeezed vacuum field with a squeezing level of dB at 1064 nm, the
wavelength at which laser interferometers for gravitational wave (GW) detection
operate, using periodically poled KTiOPO (PPKTP) in a sub-threshold optical
parametric oscillator. PPKTP has the advantages of higher nonlinearity, smaller
intra-crystal and pump-induced seed absorption, user-specified parametric
down-conversion temperature, wider temperature tuning range, and lower
susceptibility to thermal lensing over alternative nonlinear materials such as
MgO doped or periodically poled LiNbO, and is, therefore, an excellent
material for generation of squeezed vacuum fields for application to laser
interferometers for GW detection
Structure analysis of proteins, peptides and metal complexes by vibrational circular dichroism
There are two principal forms of vibrational optical activity (VOA), an IR form referred to as vibrational circular dichroism (VCD) and Raman form known as Raman optical activity (ROA). This paper reports examples of the application of VCD spectroscopy for the determination of the absolute configuration and conformation of chiral molecules, e. g. cyclic beta-lactams. VCD spectroscopy can be applied for the characterization of the conformation of proteins and peptides in solution. VCD based conformational analysis of cyclic peptides is discussed. Examples are the cyclic hexapeptide cyclo(Pro(2)-Gly-Pro(2)-Gly) and cyclic peptides comprising beta-homoamino acids (trans-2-aminocyclopentane or trans-2-aminocyclohexane carboxylic acid). Structure analysis by VCD of opiate peptides, glycopeptides, peptidomimetics and chiral transition metal complexes are also discussed
Does subsurface carbon influence the selectivity of Pd catalysts in the hydrogenation of pentyne?
The role of carbon species in heterogeneous catalytic processes: an in situ soft x-ray photoelectron spectroscopy study
High pressure X-ray photoelectron spectroscopy (XPS) is used to characterize heterogeneous catalytic processes. The success of the new technique based on the possibility to correlate the catalytic activity and the electronic structure of an active surface. The dynamic character of a catalyst surface can be demonstrated impressively by this technique. In this contribution the basics of high pressure XPS will be discussed. Three examples of heterogeneous catalytic reactions are presented in this contribution. The selective hydrogenation of 1-pentyne over Pd based catalysts and the dehydrogenation of n-butane and the oxidation of ethylene over V based catalysts. It is shown, that the formation of subsurface carbons plays an important role in all the examples. The incorporated carbon changes the electronic structure of the surface and so controls the selectivity of the reaction. A change of the educts in the reaction atmosphere induces modifications of the electronic surface structure of the operation catalysts
In situ XPS study on Mo-based oxides under conditions of selective oxidation of C3 hydrocarbons
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