2,116 research outputs found
Ab initio Molecular Dynamics in Adaptive Coordinates
We present a new formulation of ab initio molecular dynamics which exploits
the efficiency of plane waves in adaptive curvilinear coordinates, and thus
provides an accurate treatment of first-row elements. The method is used to
perform a molecular dynamics simulation of the CO_2 molecule, and allows to
reproduce detailed features of its vibrational spectrum such as the splitting
of the Raman sigma+_g mode caused by Fermi resonance. This new approach opens
the way to highly accurate ab initio simulations of organic compounds.Comment: 11 pages, 3 PostScript figure
On-chip spectroscopy with thermally-tuned high-Q photonic crystal cavities
Spectroscopic methods are a sensitive way to determine the chemical
composition of potentially hazardous materials. Here, we demonstrate that
thermally-tuned high-Q photonic crystal cavities can be used as a compact
high-resolution on-chip spectrometer. We have used such a chip-scale
spectrometer to measure the absorption spectra of both acetylene and hydrogen
cyanide in the 1550 nm spectral band, and show that we can discriminate between
the two chemical species even though the two materials have spectral features
in the same spectral region. Our results pave the way for the development of
chip-size chemical sensors that can detect toxic substances
Dynamics of Bulk vs. Nanoscale WS_2: Local Strain and Charging Effects
We measured the infrared vibrational properties of bulk and nanoparticle
WS in order to investigate the structure-property relations in these novel
materials. In addition to the symmetry-breaking effects of local strain,
nanoparticle curvature modifies the local charging environment of the bulk
material. Performing a charge analysis on the \emph{xy}-polarized E
vibrational mode, we find an approximate 1.5:1 intralayer charge difference
between the layered 2H material and inorganic fullerene-like (IF)
nanoparticles. This effective charge difference may impact the solid-state
lubrication properties of nanoscale metal dichalcogenides.Comment: 6 pages, 5 figure
High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N
The first comprehensive high-resolution photoabsorption spectrum of 14N15N
has been recorded using the Fourier-transform spectrometer attached to the
Desirs beamline at the Soleil synchrotron. Observations are made in the extreme
ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed
absorption lines have been assigned to 25 bands and reduced to a set of
transition energies, f values, and linewidths. This analysis has verified the
predictions of a theoretical model of N2 that simulates its photoabsorption and
photodissociation cross section by solution of an isotopomer independent
formulation of the coupled-channel Schroedinger equation. The mass dependence
of predissociation linewidths and oscillator strengths is clearly evident and
many local perturbations of transition energies, strengths, and widths within
individual rotational series have been observed.Comment: 14 pages, 8 figures, one data archiv
Infrared spectroscopy of diatomic molecules - a fractional calculus approach
The eigenvalue spectrum of the fractional quantum harmonic oscillator is
calculated numerically solving the fractional Schr\"odinger equation based on
the Riemann and Caputo definition of a fractional derivative. The fractional
approach allows a smooth transition between vibrational and rotational type
spectra, which is shown to be an appropriate tool to analyze IR spectra of
diatomic molecules.Comment: revised + extended version, 9 pages, 6 figure
Characterization of Central Iowa Forests with Permanent Plots
We describe a standard methodology for establishing and inventorying the woody vegetation in 0.1-ha permanent forest plots and we utilize the data from 86 plots to characterize the mature forest vegetation of central Iowa. Obvious differences existed between bottomland hardwoods and upland forest stands, but disturbed uplands contained species typical of bottomlands. Little-disturbed upland forest plots included a variety of species associations, but variation among plots was continuous. The trees dominating the mature upland forests of central Iowa grow in a wide variety of habitats, with the result that predictability of stand structure at any location is limited. Stands on northerly and southerly aspects did differ consistently from one another, but other aspects were not intermediate in character. The overall structure of the forest stands we inventoried was typical of old secondary forests. Even the oldest forests of central Iowa are still relatively young, and their structure and composition is likely to change in the coming decades. The permanent plots we established will allow for documentation and analysis of this change
Infrared emission spectrum and potentials of and states of Xe excimers produced by electron impact
We present an investigation of the Xe excimer emission spectrum
observed in the near infrared range about 7800 cm in pure Xe gas and in
an Ar (90%) --Xe (10%) mixture and obtained by exciting the gas with energetic
electrons. The Franck--Condon simulation of the spectrum shape suggests that
emission stems from a bound--free molecular transition never studied before.
The states involved are assigned as the bound state with atomic limit and the dissociative state with limit. Comparison with the spectrum simulated by using theoretical
potentials shows that the dissociative one does not reproduce correctly the
spectrum features.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Strong fragmentation of low-energy electromagnetic excitation strength in Sn
Results of nuclear resonance fluorescence experiments on Sn are
reported. More than 50 transitions with MeV were
detected indicating a strong fragmentation of the electromagnetic excitation
strength. For the first time microscopic calculations making use of a complete
configuration space for low-lying states are performed in heavy odd-mass
spherical nuclei. The theoretical predictions are in good agreement with the
data. It is concluded that although the E1 transitions are the strongest ones
also M1 and E2 decays contribute substantially to the observed spectra. In
contrast to the neighboring even Sn, in Sn the
component of the two-phonon quintuplet built on top of
the 1/2 ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure
A theory of intense-field dynamic alignment and high harmonic generation from coherently rotating molecules and interpretation of intense-field ultrafast pump-probe experiments
A theory of ultra-fast pump-probe experiments proposed by us earlier [F.H.M.
Faisal et al., Phys. Rev. Lett. 98, 143001 (2007) and F.H.M. Faisal and A.
Abdurrouf, Phys. Rev. Lett. 100, 123005 (2008)] is developed here fully and
applied to investigate the phenomena of dynamic alignment and high harmonic
generation (HHG) from coherently rotating linear molecules. The theory provides
essentially analytical results for the signals that allow us to investigate the
simultaneous dependence of the HHG signals on the two externally available
control parameters, namely, the relative angle between the polarizations, and
the delay-time between the two pulses. It is applied to investigate the
characteristics of high harmonic emission from nitrogen and oxygen molecules
that have been observed experimentally in a number of laboratories. The results
obtained both in the time-domain and in the frequency-domain are compared with
the observed characteristics as well as directly with the data and are found to
agree remarkably well. In addition we have predicted the existence of a "magic"
polarization angle at which all modulations of the harmonic emission from
nitrogen molecule changes to a steady emission at the harmonic frequency. Among
other things we have also shown a correlation between the existence of the
"magic" or critical polarization angles and the symmetry of the active
molecular orbitals, that is deemed to be useful in connection with the "inverse
problem" of molecular imaging from the HHG data.Comment: 31 pages, 22 figures, and 140 equation
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