1,573 research outputs found
Hybrid variation-perturbation method for calculating rovibrational energy levels of polyatomic molecules
A procedure for calculation of rotation-vibration states of medium sized
molecules is presented. It combines the advantages of variational calculations
and perturbation theory. The vibrational problem is solved by diagonalizing a
Hamiltonian matrix, which is partitioned into two sub-blocks. The first,
smaller sub-block includes matrix elements with the largest contribution to the
energy levels targeted in the calculations. The second, larger sub-block
comprises those basis states which have little effect on these energy levels.
Numerical perturbation theory, implemented as a Jacobi rotation, is used to
compute the contributions from the matrix elements of the second sub-block.
Only the first sub-block needs to be stored in memory and diagonalized.
Calculations of the vibrational-rotational energy levels also employ a
partitioning of the Hamiltonian matrix into sub-blocks, each of which
corresponds either to a single vibrational state or a set of resonating
vibrational states, with all associated rotational levels. Physically, this
partitioning is efficient when the Coriolis coupling between different
vibrational states is small. Numerical perturbation theory is used to include
the cross-contributions from different vibrational states. Separate individual
sub-blocks are then diagonalized, replacing the diagonalization of a large
Hamiltonian matrix with a number of small matrix diagonalizations. Numerical
examples show that the proposed hybrid variational-perturbation method greatly
speeds up the variational procedure without significant loss of precision for
both vibrational-rotational energy levels and transition intensities. The
hybrid scheme can be used for accurate nuclear motion calculations on molecules
with up to 15 atoms on currently available computers.Comment: Molecular Physics (Handy Special Issue), in pres
MARVEL analysis of the measured high-resolution rovibronic spectra of the calcium monohydroxide radical (CaOH)
The calcium monohydroxide radical (CaOH) is an important astrophysical
molecule relevant to cool stars and rocky exoplanets, amongst other
astronomical environments. Here, we present a consistent set of highly accurate
rovibronic (rotation-vibration-electronic) energy levels for the five lowest
electronic states (\tilde{X}\,^2\Sigma^+, \tilde{A}\,^2\Pi,
\tilde{B}\,^2\Sigma^+, \tilde{C}\,^2\Delta, \tilde{D}\,^2\Sigma^+) of
CaOH. A comprehensive analysis of the published spectroscopic literature on
this system has allowed 1955 energy levels to be determined from 3204
rovibronic experimental transitions, all with unique quantum number labelling
and measurement uncertainties. The dataset covers rotational excitation up to
for molecular states below 29\,000~cm. The analysis was
performed using the MARVEL algorithm, which is a robust procedure based on the
theory of spectroscopic networks. The dataset provided will significantly aid
future interstellar, circumstellar and atmospheric detections of CaOH, as well
as assisting in the design of efficient laser cooling schemes in ultracold
molecule research and precision tests of fundamental physics
ExoMol molecular line lists - XVI: The rotation-vibration spectrum of hot HS
This work presents the AYT2 line list: a comprehensive list of 114 million
HS vibration-rotation transitions computed using an
empirically-adjusted potential energy surface and an {\it ab initio} dipole
moment surface. The line list gives complete coverage up to 11000 \cm\
(wavelengths longer than 0.91 m) for temperatures up to 2000 K. Room
temperature spectra can be simulated up to 20000 \cm\ (0.5 m) but the
predictions at visible wavelengths are less reliable. AYT2 is made available in
electronic form as supplementary data to this article and at
\url{www.exomol.com}.Comment: 12 pages, 10 figures, 10 table
Collective modes of two-dimensional classical Coulomb fluids
Molecular dynamics simulations have been performed to investigate in detail
collective modes spectra of two-dimensional Coulomb fluids in a wide range of
coupling. The obtained dispersion relations are compared with theoretical
approaches based on quasi-crystalline approximation (QCA), also known as the
quasi-localized charge approximation (QLCA) in the plasma-related context. An
overall satisfactory agreement between theory and simulations is documented for
the longitudinal mode at moderate coupling and in the long-wavelength domain at
strong coupling. For the transverse mode, satisfactory agreement in the
long-wavelength domain is only reached at very strong coupling, when the cutoff
wave-number below which shear waves cannot propagate becomes small. The
dependence of the cutoff wave-number for shear waves on the coupling parameter
is obtained.Comment: 10 pages, 6 figure
A near infrared line list for \NH: Analysis of a Kitt Peak spectrum after 35 years
A Fourier Transform (FT) absorption spectrum of room temperature NH3 in the
region 7400 - 8600 cm-1 is analysed using a variational line list and ground
state energies determined using the MARVEL procedure. The spectrum was measured
by Dr Catherine de Bergh in 1980 and is available from the Kitt Peak data
center. The centers and intensities of 8468 ammonia lines were retrieved using
a multiline fitting procedure. 2474 lines are assigned to 21 bands providing
1692 experimental energies in the range 7000 - 9000 cm-1. The spectrum was
assigned by the joint use of the BYTe variational line list and combination
differences. The assignments and experimental energies presented in this work
are the first for ammonia in the region 7400 - 8600 cm-1, considerably
extending the range of known vibrational-excited statesComment: 27 pages, 6 table, 5 figures. Accepted for publication in Journal of
Molecular Spectroscop
First observation of electric-quadrupole infrared transitions in water vapour
Molecular absorption of infrared radiation is generally due to ro-vibrational
electric-dipole transitions. Electric-quadrupole transitions may still occur,
but they are typically a million times weaker than electric-dipole transitions,
rendering their observation extremely challenging. In polyatomic or polar
diatomic molecules, ro-vibrational quadrupole transitions have never been
observed. Here, we report the first direct detection of quadrupole transitions
in water vapor. The detected quadrupole lines have intensity largely above the
standard dipole intensity cut-off of spectroscopic databases and thus are
important for accurate atmospheric and astronomical remote sensing
ExoMol line lists -- XLI. High-temperature molecular line lists for the alkali metal hydroxides KOH and NaOH
Potassium hydroxide (KOH) and sodium hydroxide (NaOH) are expected to occur
in the atmospheres of hot rocky super-Earth exoplanets but a lack of
spectroscopic data is hampering their potential detection. Using robust
first-principles methodologies, comprehensive molecular line lists for KOH and
NaOH that are applicable for temperatures up to K are presented. The
KOH OYT4 line list covers the 0-6000 cm (wavelengths
m) range and comprises 38 billion transitions between 7.3 million energy
levels with rotational excitation up to . The NaOH OYT5 line list covers
the 0 - 9000 cm (wavelengths m) range and contains
almost 50 billion lines involving 7.9 million molecular states with rotational
excitation up to . The OYT4 and OYT5 line lists are available from the
ExoMol database at www.exomol.com and should greatly aid the study of hot rocky
exoplanets
ExoMol molecular line lists XXX: a complete high-accuracy line list for water
A new line list for HO is presented. This line list, which is
called POKAZATEL, includes transitions between rotation-vibrational energy
levels up to 41000 cm in energy and is the most complete to date. The
potential energy surface (PES) used for producing the line list was obtained by
fitting a high-quality ab initio PES to experimental energy levels with
energies of 41000 cm and for rotational excitations up to . The
final line list comprises all energy levels up to 41000 cm and
rotational angular momentum up to 72. An accurate ab initio dipole moment
surface (DMS) was used for the calculation of line intensities and reproduces
high-precision experimental intensity data with an accuracy close to 1 %. The
final line list uses empirical energy levels whenever they are available, to
ensure that line positions are reproduced as accurately as possible. The
POKAZATEL line list contains over 5 billion transitions and is available from
the ExoMol website (www.exomol.com) and the CDS database
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