202 research outputs found
Possible contribution of olefins and heteroatoms to the Unidentified Infrared Bands
The current assignments of the 11.3 feature are shown to be incompatible with
observations. An unbiased survey of correlation charts suggests that the
olefinic group R2=CH2 is a good alternative candidate. For the 12.7 feature,
the best fits are provided by nitrites, R-O-N=O, and amines, R-N=H2. Sulfones,
SO2, exhibit strong features near 7.7 and 8.6 microns, which may contribute to
the UIBs. These additional functional groups are likely to be attached to the
main hydrocarbon dust skeleton previously hypothesized in the coal/kerogen
model. They provide further leeway in modelling the large variety of relative
band intensities observed in the sky.Comment: 5 pages, 1 figure, accepted by A and A Letter
Relaxation paths for single modes of vibrations in isolated molecules
A numerical simulation of vibrational excitation of molecules was devised,
and used to excite computational models of common molecules into a prescribed,
pure, normal vibration mode in the ground electronic state, with varying,
controlable energy content. The redistribution of this energy (either
non-chaotic or irreversible IVR) within the isolated, free molecule is then
followed in time with a view to determining the coupling strength between
modes. This work was triggered by the need to predict the general characters of
the infrared spectra to be expected from molecules in interstellar space, after
being excited by photon absorption or reaction with a radical. It is found that
IVR from a pure normal mode is very "restricted" indeed at energy contents of
one mode quantum or so. However, as this is increased, or when the excitation
is localized, our approach allows us to isolate, describe and quantify a number
of interesting phenomena, known to chemists and in non-linear mechanics, but
difficult to demonstrate experimentally: frequency dragging, mode locking or
quenching or, still, instability near a potential surface crossing, the first
step to generalized chaos as the energy content per mode is increased.Comment: 25 pages, 15 figures; accepted by J. Atom. Phys.
On the band-to-continuum intensity ratio in the infrared spectra of interstellar carbonaceous dust
Published interpretations of the relative intensity variations of the
Unidentified Infrared Bands (UIBs) and their underlying continuum are
discussed. An alternative model is proposed, in which a single carrier for both
emits a) mostly a continuum when it is electronically excited by photons
(visible or UV), or b) exclusively the UIBs, when only chemical energy is
deposited by H capture on its surface, inducing only nuclear vibrations. The
bands will dominate in atomic H regions but will be overcome by thermal
continuum radiation when the ambient field is strong but lacks dissociating
photons (900-1100 Angstroms). The model applies to PDRs as well as to limbs of
molecular clouds in the ISM and agrees quantitatively with recent satellite
observations. It gives indications on atomic H density and UIB intensity
provided the ambient radiation field is known. It invokes no chemical,
electronic, structural or size change in order to interpret the observed
intensity variations.Comment: 18 pages, 2 figure
Candidate carriers and synthetic spectra of the 21- and 30-mu protoplanetary nebular bands
Computational chemistry is used here to determine the vibrational line
spectrum of several candidate molecules. It is shown that the thiourea
functional group, associated with various carbonaceous structures (mainly
compact and linear aromatic clusters), is able to mimic the 21-m band
emitted by a number of proto-planetary nebulae. The combination of nitrogen and
sulphur in thiourea is the essential source of emission in this model: the band
disappears if these species are replaced by carbon.
The astronomical 21-m feature extends redward to merge with another
prominent band peaking between 25 and 30 m, also known as the 30-m
band. It is found that the latter can be modelled by the combined spectra of
aliphatic chains, made of CH groups, oxygen bridges and OH groups, which
provide the 30-m emission. The absence of oxygen all but extinguishes the
30-m emission. The emission between the 21- and 30-m bands is
provided mainly by thiourea attached to linear aromatic clusters.
The chemical software reveals that the essential role of the heteroatoms N, S
and O stems from their large electronic charge. It also allows to determine the
type of atomic vibration responsible for the different lines of each structure,
which helps selecting the most relevant structures.
A total of 22 structures have been selected here, but their list is far from
being exhaustive; they are only intended as examples of 3 generic classes. When
background dust emission is added, model spectra are obtained, which are able
to satisfactorily reproduce recent observations of proto-planetary nebulae.
The relative numbers of atomic species used in this model are typically
H:C:O:N:S=53:36:8:2:1.Comment: 9 pages, 14 figure
Anomalous spin density distribution on oxygen and Ru in CaSrRuO: A polarised neutron diffraction study
By means of polarized neutron diffraction in a magnetic field of 7.0 T at 1.6
K an anomalously large magnetization density is observed on the in-plane oxygen
in CaSrRuO. Field-induced moments of different ions are
determined by refinement on the flipping ratios, yielding =
0.346(11) , = 0.076(6) and = 0.009(6)
. The moment on the oxygen arises from the strong hybridization
between the Ru-4d and O-2p orbitals. %The maximum entropy method is used for
the %reconstruction of the magnetization density and reveals a strongly
anisotropic The maximum entropy magnetization density reconstruction reveals a
strongly anisotropic density at the Ru site, consistent with the distribution
of the {\it xy} ( band) {\it d}-orbitals.Comment: 4 pages 3 figure
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
Spin density distribution in a partially magnetized organic quantum magnet
Polarized neutron diffraction experiments on an organic magnetic material
reveal a highly skewed distribution of spin density within the magnetic
molecular unit. The very large magnitude of the observed effect is due to
quantum spin fluctuations. The data are in quantitative agreement with direct
diagonalization results for a model spin Hamiltonian, and provide insight on
the actual microscopic origin of the relevant exchange interactions.Comment: 5 pages 4 figure
Classical, non-linear, internal dynamics of large, isolated, vibrationally excited molecules
This work reports numerical experiments intended to clarify the internal
equilibration process in large molecules, following vibrational excitation. A
model of amorphous and oxygenated hydrocarbon macromolecule (about 500
atoms)--simulating interstellar dust-- is built up by means of a chemical
simulation code. Its structure is optimized, and its normal modes determined.
About 4.5 eV of potential energy is then deposited locally by perturbing one of
the C-H peripheral bonds, thus simulating the capture of a free H atom by a
dangling C bond. The ensuing relaxation of the system is followed for up to 300
ps, using a molecular mechanics code. When steady state is reached, spectra and
time correlation functions of kinetic energy and bond length fluctuations
indicate that most normal modes have been activated, but the motion remains
quasi-periodic or regular. By contrast, when the molecule is violently excited
or embedded in a thermal bath (modelled by Langevin dynamics), the same markers
clearly depict chaotic motions. Thus it appears that even such a large system
of oscillators is unable to provide the equivalent of a thermal bath to any one
of these, unless there are strong resonances between some of them. In general,
therefore, an energy of a few eV's deposited in an isolated molecule will not
be immediately thermalized. This conclusion is of consequence for the
interpretation of astronomical UIB spectra.
Key Words:IS dust--UIBs--Excitation, relaxation processes.Comment: 19 pages, 9 figures, J. of Phys. B 2002, vol 35(17
Spatial Separation of the 3.29 micron Emission Feature and Associated 2 micron Continuum in NGC 7023
We present a new 0.9" resolution 3.29 micron narrowband image of the
reflection nebula NGC 7023. We find that the 3.29 micron IEF in NGC 7023 is
brightest in narrow filaments NW of the illuminating star. These filaments have
been seen in images of K', molecular hydrogen emission lines, the 6.2 and 11.3
micron IEFs, and HCO+. We also detect 3.29 micron emission faintly but
distinctly between the filaments and the star. The 3.29 micron image is in
contrast to narrowband images at 2.09, 2.14, and 2.18 micron, which show an
extended emission peak midway between the filaments and the star, and much
fainter emission near the filaments. The [2.18]-[3.29] color shows a wide
variation, ranging from 3.4-3.6 mag at the 2 micron continuum peak to 5.5 mag
in the filaments. We observe [2.18]-[3.29] to increase smoothly with increasing
distance from the star, up until the filament, suggesting that the main
difference between the spatial distributions of the 2 micron continuum and the
the 3.29 micron emission is related to the incident stellar flux. Our result
suggests that the 3.29 micron IEF carriers are likely to be distinct from, but
related to, the 2 micron continuum emitters. Our finding also imply that, in
NGC 7023, the 2 micron continuum emitters are mainly associated with HI, while
the 3.29 micron IEF carriers are primarily found in warm molecular hydrogen,
but that both can survive in HI or molecular hydrogen. (abridged)Comment: to appear in ApJ, including 1 table and 8 figures, high resolution
figures available at http://www.ast.cam.ac.uk/~jin/n7023
Aspects of Discrete Breathers and New Directions
We describe results concerning the existence proofs of Discrete Breathers
(DBs) in the two classes of dynamical systems with optical linear phonons and
with acoustic linear phonons. A standard approach is by continuation of DBs
from an anticontinuous limit. A new approach, which is purely variational, is
presented. We also review some numerical results on intraband DBs in random
nonlinear systems. Some non-conventional physical applications of DBs are
suggested. One of them is understanding slow relaxation properties of glassy
materials. Another one concerns energy focusing and transport in biomolecules
by targeted energy transfer of DBs. A similar theory could be used for
describing targeted charge transfer of nonlinear electrons (polarons) and, more
generally, for targeted transfer of several excitations (e.g. Davydov soliton).Comment: to appear in the Proceedings of NATO Advanced Research Workshop
"Nonlinearity and Disorder: Theory and Applications",
Tashkent,Uzbekistan,October 1-6, 200
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