4,922 research outputs found
The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)
The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are
determined computationally using Density Functional Theory. Trends in the band
positions and intensities as a function of PAH size, charge and geometry are
discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions
hold as with small PAHs.
This does not hold for the other features. The larger PAH cations and anions
produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron
becomes prominent and shifts slightly to the red. In addition, the average
anion peak falls slightly to the red of the average cation peak. The similarity
in behavior of the 7.8 and 8.6 micron bands with the astronomical observations
suggests that they arise from large, cationic and anionic PAHs, with the
specific peak position and profile reflecting the PAH cation to anion
concentration ratio and relative intensities of PAH size. Hence, the broad
astronomical 7.7 micron band is produced by a mixture of small and large PAH
cations and anions, with small and large PAHs contributing more to the 7.6 and
7.8 micron component respectively.
For the CH out-of-plane vibrations, the duo hydrogens couple with the solo
vibrations and produce bands that fall at wavelengths slightly different than
their counterparts in smaller PAHs. As a consequence, previously deduced PAH
structures are altered in favor of more compact and symmetric forms. In
addition, the overlap between the duo and trio bands may reproduce the
blue-shaded 12.8 micron profile.Comment: ApJ, 36 pages, 9 fig
Magnetic particles confined in a modulated channel: structural transitions tunable by tilting a magnetic field
The ground state of colloidal magnetic particles in a modulated channel are
investigated as function of the tilt angle of an applied magnetic field. The
particles are confined by a parabolic potential in the transversal direction
while in the axial direction a periodic substrate potential is present. By
using Monte Carlo (MC) simulations, we construct a phase diagram for the
different crystal structures as a function of the magnetic field orientation,
strength of the modulated potential and the commensurability factor of the
system. Interestingly, we found first and second order phase transitions
between different crystal structures, which can be manipulated by the
orientation of the external magnetic field. A re-entrant behavior is found
between two- and four-chain configurations, with continuous second order
transitions. Novel configurations are found consisting of frozen in solitons.
By changing the orientation and/or strength of the magnetic field and/or the
strength and the spatial frequency of the periodic substrate potential, the
system transits through different phases.Comment: Submitted to Phys. Rev. E (10 pages, 12 figures
Dirac and Klein-Gordon particles in one-dimensional periodic potentials
We evaluate the dispersion relation for massless fermions, described by the
Dirac equation, and for zero-spin bosons, described by the Klein-Gordon
equation, moving in two dimensions and in the presence of a one-dimensional
periodic potential. For massless fermions the dispersion relation shows a zero
gap for carriers with zero momentum in the direction parallel to the barriers
in agreement with the well-known "Klein paradox". Numerical results for the
energy spectrum and the density of states are presented. Those for fermions are
appropriate to graphene in which carriers behave relativistically with the
"light speed" replaced by the Fermi velocity. In addition, we evaluate the
transmission through a finite number of barriers for fermions and zero-spin
bosons and relate it with that through a superlattice.Comment: 9 pages, 12 figure
A Dust Twin of Cas A: Cool Dust and 21-micron Silicate Dust Feature in the Supernova Remnant G54.1+0.3
We present infrared (IR) and submillimeter observations of the Crab-like
supernova remnant (SNR) G54.1+0.3 including 350 micron (SHARC-II), 870 micron
(LABOCA), 70, 100, 160, 250, 350, 500 micron (Herschel) and 3-40 micron
(Spitzer). We detect dust features at 9, 11 and 21 micron and a long wavelength
continuum dust component. The 21 micron dust coincides with [Ar II] ejecta
emission, and the feature is remarkably similar to that in Cas A. The IRAC 8
micron image including Ar ejecta is distributed in a shell-like morphology
which is coincident with dust features, suggesting that dust has formed in the
ejecta. We create a cold dust map that shows excess emission in the
northwestern shell. We fit the spectral energy distribution of the SNR using
the continuous distributions of ellipsoidal (CDE) grain model of pre-solar
grain SiO2 that reproduces the 21 and 9 micron dust features and discuss grains
of SiC and PAH that may be responsible for the 10-13 micron dust features. To
reproduce the long-wavelength continuum, we explore models consisting of
different grains including Mg2SiO4, MgSiO3, Al2O3, FeS, carbon, and Fe3O4. We
tested a model with a temperature-dependent silicate absorption coefficient. We
detect cold dust (27-44 K) in the remnant, making this the fourth such SNR with
freshly-formed dust. The total dust mass in the SNR ranges from 0.08-0.9 Msun
depending on the grain composition, which is comparable to predicted masses
from theoretical models. Our estimated dust masses are consistent with the idea
that SNe are a significant source of dust in the early Universe.Comment: MNRAS: accepted on June 28, 2018 and published on July 4, 201
Structural and dynamical properties of a quasi-one-dimensional classical binary system
The ground state configurations and the \lq{}\lq{}normal\rq{}\rq{} mode
spectra of a -one-dimensional (Q1D) binary system of charged particles
interacting through a screened Coulomb potential are presented. The minimum
energy configurations were obtained analytically and independently through
molecular dynamic simulations. A rich variety of ordered structures were found
as a function of the screening parameter, the particle density, and the ratio
between the charges of the distinct types of particles. Continuous and
discontinuous structural transitions, as well as an unexpected symmetry
breaking in the charge distribution are observed when the density of the system
is changed. For near equal charges we found a disordered phase where a mixing
of the two types of particles occurs. The phonon dispersion curves were
calculated within the harmonic approximation for the one- and two-chain
structures.Comment: 11 pages, 11 fig
Simulation and experimental verification of W-band finite frequency selective surfaces on infinite background with 3D full wave solver NSPWMLFMA
We present the design, processing and testing of a W-band finite by infinite and a finite by finite Grounded Frequency Selective Surfaces (FSSs) on infinite background. The 3D full wave solver Nondirective Stable Plane Wave Multilevel Fast Multipole Algorithm (NSPWMLFMA) is used to simulate the FSSs. As NSPWMLFMA solver improves the complexity matrix-vector product in an iterative solver from O(N(2)) to O(N log N) which enables the solver to simulate finite arrays with faster execution time and manageable memory requirements. The simulation results were verified by comparing them with the experimental results. The comparisons demonstrate the accuracy of the NSPWMLFMA solver. We fabricated the corresponding FSS arrays on quartz substrate with photolithographic etching techniques and characterized the vector S-parameters with a free space Millimeter Wave Vector Network Analyzer (MVNA)
Normal modes of a quasi-one-dimensional multi-chain complex plasma
We studied equally charged particles, suspended in a complex plasma, which
move in a plane and interact with a screened Coulomb potential (Yukawa type)
and with an additional external confining parabolic potential in one direction,
that makes the system quasi-one-dimensional (Q1D). The normal modes of the
system are studied in the presence of dissipation. We also investigated how a
perpendicular magnetic field couples the phonon modes with each other. Two
different ways of exciting the normal modes are discussed: 1) a uniform
excitation of the Q1D lattice, and 2) a local forced excitation of the system
in which one particle is driven by e.g. a laser. Our results are in very good
agreement with recent experimental findings on a finite single chain system
(Phys. Rev. Lett. {\bf 91}, 255003 (2003)). Predictions are made for the normal
modes of multi-chain structures in the presence of damping.Comment: 15 pages, 14 figures, accepted for publication on PR
- …