177 research outputs found
Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nano-droplets
The spectroscopy of free radicals and radical containing entrance-channel
complexes embedded in superfluid helium nano-droplets is reviewed. The
collection of dopants inside individual droplets in the beam represents a
micro-canonical ensemble, and as such each droplet may be considered an
isolated cryo-reactor. The unique properties of the droplets, namely their low
temperature (0.4 K) and fast cooling rates ( K s) provides
novel opportunities for the formation and high-resolution studies of molecular
complexes containing one or more free radicals. The production methods of
radicals are discussed in light of their applicability for embedding the
radicals in helium droplets. The spectroscopic studies performed to date on
molecular radicals and on entrance / exit-channel complexes of radicals with
stable molecules are detailed. The observed complexes provide new information
on the potential energy surfaces of several fundamental chemical reactions and
on the intermolecular interactions present in open-shell systems. Prospects of
further experiments of radicals embedded in helium droplets are discussed,
especially the possibilities to prepare and study high-energy structures and
their controlled manipulation, as well as the possibility of fundamental
physics experiments.Comment: 25 pages, 12 figures, 4 tables (RevTeX
Structures of the linear silicon carbides SiC 4 and SiC 6 : Isotopic substitution and Ab Initio theory
V. D. Gordon,a) E. S. Nathan, A. J. Apponi, M. C. McCarthy, and P. Thaddeus are with
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 and
Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge,
Massachusetts 02138
-- P. Botschwina is with the
Institut fu¨r Physikalische Chemie der Universita¨t Go¨ttingen, Tammannstr, 6. D-37077 Go¨ttingen, GermanyThe structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination
of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of
all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform
microwave spectrometer. Rotational constants obtained by least-squares fitting transition
frequencies were used to derive experimental structures; except for those nearest the center of mass,
individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium
structures were derived by converting the experimental rotational constants to equilibrium constants
using the vibration–rotation coupling constants from coupled-cluster calculations, including
connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies
were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type
doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were
assigned to v6 , a bending mode calculated to lie about 205 cm -1 above the ground state. A
recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American
Institute of Physics. @S0021-9606~00!01537-3#Chemistr
Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Mrk 273, and IRAS 17208-0014, based on HCN to HCO+ Ratios
We present interferometric observations of three ultraluminous infrared
galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm
wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and
HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was
clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission
was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN
to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs
were derived and compared with those of lower luminosity galaxies known to be
dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk
273, where there is evidence for obscured AGNs from previous observations at
other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range
found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a
powerful obscured AGN has been unclear, the ratio (1.7) is in between the
observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+
brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence
of an HCN abundance enhancement, which is expected from chemical effects of the
central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed
millimeter interferometric method based on HCN/HCO+ ratios may be an effective
tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially
because of the negligible dust extinction at these wavelengths.Comment: 15 pages (emulateapj.sty), 8 figures (figures 1-5 resolution
reduced), Accepted for publication in Astronomical Journal, A PDF file with
high resolution is availble at
http://optik2.mtk.nao.ac.jp/~imanishi/Paper/HCN/HCN.pd
Chemical differentiation in regions of high-mass star formation I. CS, dust and N2H^+ in southern sources
Aims. Our goals are to compare the CS, N2H+ and dust distributions in a
representative sample of high-mass star forming dense cores and to determine
the physical and chemical properties of these cores. Methods. We compare the
results of CS(5-4) and 1.2 mm continuum mapping of twelve dense cores from the
southern hemisphere presented in this work, in combination with our previous
N2H+(1-0) and CS(2-1) data. We use numerical modeling of molecular excitation
to estimate physical parameters of the cores. Results. Most of the maps have
several emission peaks (clumps). We derive basic physical parameters of the
clumps and estimate CS and N2H+ abundances. Masses calculated from LVG
densities are higher than CS virial masses and masses derived from continuum
data, implying small-scale clumpiness of the cores. For most of the objects,
the CS and continuum peaks are close to the IRAS point source positions. The
CS(5-4) intensities correlate with continuum fluxes per beam in all cases, but
only in five cases with the N2H+(1-0) intensities. The study of spatial
variations of molecular integrated intensity ratios to continuum fluxes reveals
that I(N2H+)/F{1.2} ratios drop towards the CS peaks for most of the sources,
which can be due to a N2H+ abundance decrease. For CS(5-4), the I(CS)/F{1.2}
ratios show no clear trends with distance from the CS peaks, while for CS(2-1)
such ratios drop towards these peaks. Possible explanations of these results
are considered. The analysis of normalized velocity differences between CS and
N2H+ lines has not revealed indications of systematic motions towards CS peaks.Comment: 13 pages, 5 figures, accepted by Astronomy and Astrophysic
Astronomical identification of CN-, the smallest observed molecular anion
We present the first astronomical detection of a diatomic negative ion, the
cyanide anion CN-, as well as quantum mechanical calculations of the excitation
of this anion through collisions with para-H2. CN- is identified through the
observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star
envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles
indicate that CN-, like the large anion C6H-, is formed in the outer regions of
the envelope. Chemical and excitation model calculations suggest that this
species forms from the reaction of large carbon anions with N atoms, rather
than from the radiative attachment of an electron to CN, as is the case for
large molecular anions. The unexpectedly large abundance derived for CN-, 0.25
% relative to CN, makes likely its detection in other astronomical sources. A
parallel search for the small anion C2H- remains so far unconclusive, despite
the previous tentative identification of the J = 1-0 rotational transition. The
abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 %
relative to C2H.Comment: 5 pages, 4 figures; accepted for publication in A&A Letter
Calculations of the Far-Wing Line Profiles of Sodium and Potassium in the Atmospheres of Substellar-Mass Objects
At the low temperatures achieved in cool brown dwarf and hot giant planet
atmospheres, the less refractory neutral alkali metals assume an
uncharacteristically prominent role in spectrum formation. In particular, the
wings of the Na-D (5890 \AA) and K I (7700 \AA) resonance lines come to define
the continuum and dominate the spectrum of T dwarfs from 0.4 to 1.0 \mic.
Whereas in standard stellar atmospheres the strengths and shapes of the wings
of atomic spectral lines are rarely needed beyond 25 \AA of a line center, in
brown dwarfs the far wings of the Na and K resonance lines out to 1000's of \AA
detunings are important. Using standard quantum chemical codes and the Unified
Franck-Condon model for line profiles in the quasi-static limit, we calculate
the interaction potentials and the wing line shapes for the dominant Na and K
resonance lines in H- and helium-rich atmospheres. Our theory has natural
absorption profile cutoffs, has no free parameters, and is readily adapted to
spectral synthesis calculations for stars, brown dwarfs, and planets with
effective temperatures below 2000 Kelvin.Comment: 14 pages, Latex, 7 figures in JPEG format, accepted for publication
in the Astrophysical Journa
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