381 research outputs found
Thermodynamics and dynamics of two-dimensional systems with dipole-like repulsive interactions
Thermodynamics and dynamics of a classical two-dimensional system with
dipole-like isotropic repulsive interactions are studied systematically using
extensive molecular dynamics (MD) simulations supplemented by appropriate
theoretical approximations. This interaction potential, which decays as an
inverse cube of the interparticle distance, belongs to the class of very soft
long-ranged interactions. As a result, the investigated system exhibits certain
universal properties that are also shared by other related soft-interacting
particle systems (like, for instance, the one-component plasma and weakly
screened Coulomb systems). These universalities are explored in this article to
construct a simple and reliable description of the system thermodynamics. In
particular, Helmholtz free energies of the fluid and solid phases are derived,
from which the location of the fluid-solid coexistence is determined. The
quasi-crystalline approximation is applied to the description of collective
modes in dipole fluids. Its simplification, previously validated on strongly
coupled plasma fluids, is used to derive explicit analytic dispersion relations
for the longitudinal and transverse wave modes, which compare satisfactory with
those obtained from direct MD simulations in the long-wavelength regime. Sound
velocities of the dipole fluids and solids are derived and analyzed.Comment: to be published in Phys. Rev.
Thermodynamics of two-dimensional Yukawa systems across coupling regimes
Thermodynamics of two-dimensional Yukawa (screened Coulomb or Debye-H\"uckel)
systems is studied systematically using molecular dynamics (MD) simulations.
Simulations cover very broad parameter range spanning from weakly coupled
gaseous states to strongly coupled fluid and crystalline states. Important
thermodynamic quantities such as internal energy and pressure are obtained and
accurate physically motivated fits are proposed. This allows us to put forward
simple practical expressions to describe thermodynamic properties of
two-dimensional Yukawa systems. For crystals, in addition to numerical
simulations, the recently developed shortest-graph interpolation method is
applied to describe pair correlations and hence thermodynamic properties. It is
shown that the finite-temperature effects can be accounted for by using simple
correction of peaks in the pair correlation function. The corresponding
correction coefficients are evaluated using MD simulation. The relevance of the
obtained results in the context of colloidal systems, complex (dusty) plasmas,
ions absorbed to interfaces in electrolytes is pointed out.Comment: 11 pages, 3 figures, 5 table
High Accuracy Molecular Line Lists for Studies of Exoplanets and Other Hot Atmospheres
The desire to characterize and model the atmospheres of the many extrasolar planets that have been discovered over the last three decades is a major driver of current astronomy. However, this goal is impacted by the lack of spectroscopic data on the molecules in question. As most atmospheres that can be studied are hot, some surprisingly so, this activity requires spectroscopic information not readily available from laboratory studies. This article will review the current status of available molecular spectroscopic data, usually presented as line lists, for studies of exoplanet atmospheres and, indeed, the atmospheres of other astronomical objects hotter than the Earth such as brown dwarfs, cool stars and even sunspots. Analysis of exoplanet transit spectra and the calculation of the relevant opacities often require huge datasets comprising billions of individual spectroscopic transitions. Conversely, the newly-developed high-resolution Doppler-shift spectroscopy technique has proved to be a powerful tool for detecting molecular species in exoplanet atmospheres, but relies on the use of smaller, highly accurate line lists. Methods of resolving issues arising from the competing demands of completeness versus accuracy for line lists are discussed
A variational model for the hyperfine resolved spectrum of VO in its ground electronic state
A variational model for the infra-red spectrum of VO is presented which aims to accurately predict the hyperfine structure within the VO X 4Σ− electronic ground state.
To give the correct electron spin splitting of the X 4Σ− state, electron spin dipolar
interaction within the ground state and the spin-orbit coupling between X 4Σ− and
two excited states, A 4Πand 1 2Σ+, are calculated ab initio alongside hyperfine interaction terms. Four hyperfine coupling terms are explicitly considered: Fermi-contact
interaction, electron spin-nuclear spin dipolar interaction, nuclear spin-rotation interaction and nuclear electric quadrupole interaction. These terms are included as part
of a full variational solution of the nuclear-motion Schrödinger equation performed
using program Duo, which is used to generate both hyperfine-resolved energy levels
and spectra. To improve the accuracy of the model, ab initio curves are subject to
small shifts. The energy levels generated by this model show good agreement with
the recently derived empirical term values. This and other comparisons validate both
our model and the recently developed hyperfine modules in Duo
ExoMol molecular line lists XXXVI: X 2Π– X 2Πand A 2Σ+ – X 2Πtransitions of SH
The GYT line list covering rotational, rovibrational and rovibronic
transitions of the mercapto radical SH is presented. This work extends and
replaces the SNaSH line list [Yurchenko et al., 2018, MNRAS, 478, 270] which
covers the ground (electronic) state only. This extension is
prompted by the tentative identification of the ultra-violet features of SH as
being of importance in the transmission spectrum of the ultra-hot Jupiter
exoplanet WASP-121b [Evans et al., 2018, AJ., 156, 283]. This GYT line list
model is generated by fitting empirical potential energy, spin-orbit and
electronic angular momenta functions to experimentally measured wavelengths
within the and states and to the
- band system using ab initio curves as a
starting reference point. The fits are compatible with the quoted uncertainty
of the experimental data used of 0.03 - 0.3 cm. The GYT line list
covers wavelengths longer than 0.256 m and includes 7686 rovibronic states
and 572 145 transitions for SH. Line lists for the SH, SH,
SH and SD isotopologues are generated including a consideration
of non-Born-Oppenheimer effects for SD. The line lists are available from the
CDS (http://cdsarc.u-strasbg.fr) and ExoMol (www.exomol.com) data bases
Blind extraction of an exoplanetary spectrum through Independent Component Analysis
Blind-source separation techniques are used to extract the transmission
spectrum of the hot-Jupiter HD189733b recorded by the Hubble/NICMOS instrument.
Such a 'blind' analysis of the data is based on the concept of independent
component analysis. The de-trending of Hubble/NICMOS data using the sole
assumption that nongaussian systematic noise is statistically independent from
the desired light-curve signals is presented. By not assuming any prior, nor
auxiliary information but the data themselves, it is shown that spectroscopic
errors only about 10 - 30% larger than parametric methods can be obtained for
11 spectral bins with bin sizes of ~0.09 microns. This represents a reasonable
trade-off between a higher degree of objectivity for the non-parametric methods
and smaller standard errors for the parametric de-trending. Results are
discussed in the light of previous analyses published in the literature. The
fact that three very different analysis techniques yield comparable spectra is
a strong indication of the stability of these results.Comment: ApJ accepte
Practical thermodynamics of Yukawa systems at strong coupling
Simple practical approach to estimate thermodynamic properties of strongly
coupled Yukawa systems, in both fluid and solid phases, is presented. The
accuracy of the approach is tested by extensive comparison with direct computer
simulation results (for fluids and solids) and the recently proposed
shortest-graph method (for solids). Possible applications to other systems of
softly repulsive particles are briefly discussed.Comment: Published in J. Chem. Phy
The nuclear-spin-forbidden rovibrational transitions of water from first principles
The water molecule occurs in two nuclear-spin isomers that differ by the value of the total nuclear spin of the hydrogen atoms, i.e., I = 0 for para-H2O and I = 1 for ortho-H2O. Spectroscopic transitions between rovibrational states of ortho and para water are extremely weak due to the tiny hyperfine nuclear-spin-rotation interaction of only ∼30 kHz and so far were not observed. We report the first comprehensive theoretical investigation of the hyperfine effects and ortho-para transitions in H216O due to nuclear-spin-rotation and spin-spin interactions. We also present the details of our newly developed general variational approach to the simulation of hyperfine effects in polyatomic molecules. Our results for water suggest that the strongest ortho-para transitions with room-temperature intensities on the order of 10−31 cm/molecule are about an order of magnitude larger than previously predicted values and should be detectable in the mid-infrared ν2 and near-infrared 2ν1 + ν2 and ν1 + ν2 + ν3 bands by current spectroscopy experiments
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