553 research outputs found
Theory of collision-induced translation-rotation spectra: H2-He
This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.29.595.An adiabatic quantal theory of spectral line shapes in collision-induced absorption and emission is presented which incorporates the induced translation-rotation and translation-vibration spectra. The generalization to account for the anisotropy of the scattering potential is given. Calculations are carried out of the collision-induced absorption spectra of He in collisions with H2 with ab initio electric dipole functions and realistic potentials. The anisotropy of the interaction potential is small and is not included in the calculations. The predicted spectra are in satisfactory agreement with experimental data though some deviations occur which may be significant. The rotational line shapes have exponential wings and are not Lorentzian. The connection between the quantal and classical theories is written out explicitly for the isotropic overlap induction
Renormalization--Group Solutions for Yukawa Potential
The self--similar renormalization group is used to obtain expressions for the
spectrum of the Hamiltonian with the Yukawa potential. The critical screening
parameter above which there are no bound states is also obtained by this
method. The approach presented illustrates that one can achieve good accuracy
without involving extensive numerical calculations, but invoking instead the
renormalization--group techniques.Comment: 1 file, 12 pages, RevTe
Reconnection in the ISM
We discuss the role of ambipolar diffusion for simple reconnection in a
partially ionized gas, following the reconnection geometry of Parker and Sweet.
When the recombination time is short the mobility and reconnection of the
magnetic field is substantially enhanced as matter escapes from the
reconnection region via ambipolar diffusion. Our analysis shows that in the
interstellar medium it is the recombination rate that usually limits the rate
of reconnection. We show that heating effects can reduce this rate by
increasing the recombination time and raising the local ion pressure. In the
colder parts of the ISM, when temperatures are or the order of 100K or less, we
obtain a significant enhancement over the usual Sweet-Parker rate, but only in
dense molecular clouds will the reconnection velocity exceed 0.001 times the
Alfv\'en speed. The ratio of the ion orbital radius to the reconnection layer
thickness is typically a few percent, except in dense molecular clouds where it
can approach unity. We briefly discuss prospects for obtaining much faster
reconnection speeds in astrophysical plasmas.Comment: 25 pages, 1 figure, submitted to Ap
Ab initio determination of polarizabilities and van der Waals coefficients of Li atoms using the relativistic CCSD(T) method
We report a new technique to determine the van der Waals coeffcients of
lithium (Li) atoms based on the relativistic coupled-cluster theory. These
quantities are determined using the imaginary parts of the scalar dipole and
quadrupole polarizabilities, which are evaluated using the approach that we
have proposed in [1]. Our procedure is fully ab initio, and avoids the
sum-over-the-states method. We present the dipole and quadrupole
polarizabilities of many of the low-lying excited states of Li. Also, the
off-diagonal dipole and quadrupole polarizabilites between some of the
low-lying states of Li are calculated.Comment: 8 pages, 4 figure
Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the
physical properties of the molecular material in these reflection nebulae. Our
observations of NGC 2023 and NGC 7023 and the physical parameters we derive for
these nebulae are in good agreement with previous studies. Both NGC 1333 and
NGC 2068 have no previously-published analysis of near-infrared spectra. Our
study reveals that the rotational-vibrational states of molecular hydrogen in
NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We
determine that the relatively weak UV field illuminating NGC 1333 is the
primary cause of the difference. Further, we find that the density of the
emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4
cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC
7023 and NGC 2023. Our model fits to this nebula show that the bright,
H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to
what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023
show significant changes in both the near-infrared continuum and H_2 intensity
along the slit and offsets between the peaks of the H_2 and continuum emission.
We find that these brightness changes may correspond to real changes in the
density and temperatures of the emitting region, although uncertainties in the
total column of emitting material along a given line of sight complicates the
interpretation. The spatial difference in the peak of the H_2 and near-infrared
continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum
is due to a material which can survive closer to the star than H_2 can.Comment: Submitted for publication in ApJ. 34 pages including 12 embedded
postscript figures. Also available at
http://www.astronomy.ohio-state.edu/~martini/pub
Retarded long-range potentials for the alkali-metal atoms and a perfectly conducting wall
The retarded long-range potentials for hydrogen and alkali-metal atoms in
their ground states and a perfectly conducting wall are calculated. The
potentials are given over a wide range of atom-wall distances and the validity
of the approximations used is established.Comment: RevTeX, epsf, 11 pages, 2 fig
Diagonalization of multicomponent wave equations with a Born-Oppenheimer example
A general method to decouple multicomponent linear wave equations is presented. First, the Weyl calculus is used to transform operator relations into relations between c-number valued matrices. Then it is shown that the symbol representing the wave operator can be diagonalized systematically up to arbitrary order in an appropriate expansion parameter. After transforming the symbols back to operators, the original problem is reduced to solving a set of linear uncoupled scalar wave equations. The procedure is exemplified for a particle with a Born-Oppenheimer-type Hamiltonian valid through second order in h. The resulting effective scalar Hamiltonians are seen to contain an additional velocity-dependent potential. This contribution has not been reported in recent studies investigating the adiabatic motion of a neutral particle moving in an inhomogeneous magnetic field. Finally, the relation of the general method to standard quantum-mechanical perturbation theory is discussed
Site-Specific Metal Chelation Facilitates the Unveiling of Hidden Coordination Sites in an Fe II/Fe III -Seamed Pyrogallol[4]arene Nanocapsule
Under suitable conditions, C-alkylpyrogallol[4]arenes
(PgCs) arrange into spherical metal–organic nanocapsules (MONCs)
upon coordination to appropriate metal ions. Herein we present the
synthesis and structural characterization of a novel FeII/FeIII-seamed MONC, as well as studies related to its
electrochemical and magnetic behaviors. Unlike other MONCs that are
assembled through 24 metal ions, this nanocapsule comprises 32 Fe
ions, uncovering 8 additional coordination sites situated between
the constituent PgC subunits. The FeII ions are likely
formed by the reducing ability of DMF used in the synthesis, representing
a novel synthetic route toward polynuclear mixed-valence MONCs
Visible, EUV, and X-ray Spectroscopy at the NIST EBIT Facility
After a brief introduction to the NIST EBIT facility, we present the results
of three different types of experiments that have been carried out there
recently: EUV and visible spectroscopy in support of the microelectronics
industry, laboratory astrophysics using an x-ray microcalorimeter, and charge
exchange studies using extracted beams of highly charged ions.Comment: 10 page
Laboratory evidence for the non-detection of excited nascent H2 in dark clouds
There has always been a great deal of interest in the formation of H2 as well
as in the binding energy released upon its formation on the surface of dust
grains. The present work aims at collecting experimental evidence for how the
bond energy budget of H2 is distributed between the reaction site and the
internal energy of the molecule. So far, the non-detection of excited nascent
H2 in dense quiescent clouds could be a sign that either predictions of
emission line intensities are not correct or the de-excitation of the newly
formed molecules proceeds rapidly on the grain surface itself. In this letter
we present experimental evidence that interstellar molecular hydrogen is formed
and then rapidly de-excited on the surface of porous water ice mantles. In
addition, although we detect ro-vibrationally excited nascent molecules
desorbing from a bare non-porous (compact) water ice film, we demonstrate that
the amount of excited nascent hydrogen molecules is significantly reduced no
matter the morphology of the water ice substrate at 10 K (both on non-porous
and on porous water ice) in a regime of high molecular coverage as is the case
in dark molecular clouds.Comment: 15 pages, 3 figures, to be published in MNRA
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