208 research outputs found
R-matrix calculation of electron collisions with electronically excited O2 molecules
Low-energy electron collisions with O molecules are studied using the
fixed-bond R-matrix method. In addition to the O ground
state, integrated cross sections are calculated for elecron collisions with the
and excited states of O molecules. 13
target electronic states of O are included in the model within a valence
configuration interaction representations of the target states. Elastic cross
sections for the and excited states are
similar to the cross sections for the ground state. As in
case of excitation from the state, the O
resonance makes the dominant contribution to excitation cross sections from the
and states. The magnitude of excitation
cross sections from the state to the
state is about 10 time larger than the corresponding cross sections from the
to the state. For this
transition, our cross section at
4.5 eV agrees well with the available experimental value. These results should
be important for models of plasma discharge chemistry which often requires
cross sections between the excited electronic states of O.Comment: 26 pages, 10 figure
QED correction for H
A quantum electrodynamics (QED) correction surface for the simplest
polyatomic and polyelectronic system H is computed using an approximate
procedure. This surface is used to calculate the shifts to vibration-rotation
energy levels due to QED; such shifts have a magnitude of up to 0.25 cm
for vibrational levels up to 15~000 cm and are expected to have an
accuracy of about 0.02 cm. Combining the new H QED correction
surface with existing highly accurate Born-Oppenheimer (BO), relativistic and
adiabatic components suggests that deviations of the resulting {\it ab initio}
energy levels from observed ones are largely due to non-adiabatic effects
High accuracy calculations of the rotation-vibration spectrum of H
Calculation of the rotation-vibration spectrum of H3+, as well as of its
deuterated isotopologues, with near-spectroscopic accuracy requires the
development of sophisticated theoretical models, methods, and codes. The
present paper reviews the state-of-the-art in these fields. Computation of
rovibrational states on a given potential energy surface (PES) has now become
standard for triatomic molecules, at least up to intermediate energies, due to
developments achieved by the present authors and others. However, highly
accurate Born--Oppenheimer energies leading to highly accurate PESs are not
accessible even for this two-electron system using conventional electronic
structure procedures e.g., configuration-interaction or coupled-cluster
techniques with extrapolation to the complete basis set limit). For this
purpose highly specialized techniques must be used, e.g., those employing
explicitly correlated Gaussians and nonlinear parameter optimizations. It has
also become evident that a very dense grid of \ai\ points is required to obtain
reliable representations of the computed points extending from the minimum to
the asymptotic limits. Furthermore, adiabatic, relativistic, and QED correction
terms need to be considered to achieve near-spectroscopic accuracy during
calculation of the rotation-vibration spectrum of H3+. The remaining and most
intractable problem is then the treatment of the effects of non-adiabatic
coupling on the rovibrational energies, which, in the worst cases, may lead to
corrections on the order of several \cm. A promising way of handling this
difficulty is the further development of effective, motion- or even
coordinate-dependent, masses and mass surfaces. Finally, the unresolved
challenge of how to describe and elucidate the experimental pre-dissociation
spectra of H and its isotopologues is discussed.Comment: Topical review to be published in J Phys B: At Mol Opt Phy
Low and intermediate energy electron collisions with the C molecular anion
Calculations are presented which use the molecular R-matrix with
pseudo-states (MRMPS) method to treat electron impact electron detachment and
electronic excitation of the carbon dimer anion. Resonances are found above the
ionisation threshold of C with , and
symmetry. These are shape resonances trapped by the effect of an attractive
polarisation potential competing with a repulsive Coulomb interaction. The
resonances are found to give structure in the detachment cross section
similar to that observed experimentally. Both excitation and detachment cross
sections are found to be dominated by large impact parameter collisions whose
contribution is modelled using the Born approximation.Comment: 18 pages, 5 figures constructed from 8 file
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
High accuracy CO line intensities determined from theory and experiment
Atmospheric CO concentrations are being closely monitored by remote
sensing experiments which rely on knowing line intensities with an uncertainty
of 0.5\%\ or better. Most available laboratory measurements have uncertainties
much larger than this. We report a joint experimental and theoretical study
providing rotation-vibration line intensities with the required accuracy. The
{\it ab initio} calculations are extendible to all atmospherically important
bands of CO and to its isotologues. As such they will form the basis for
detailed CO spectroscopic line lists for future studies.Comment: 5 pages, 2 figures, 1 tabl
A room temperature CO line list with ab initio computed intensities
Atmospheric carbon dioxide concentrations are being closely monitored by
remote sensing experiments which rely on knowing line intensities with an
uncertainty of 0.5% or better. We report a theoretical study providing
rotation-vibration line intensities substantially within the required accuracy
based on the use of a highly accurate {\it ab initio} dipole moment surface
(DMS). The theoretical model developed is used to compute CO intensities
with uncertainty estimates informed by cross comparing line lists calculated
using pairs of potential energy surfaces (PES) and DMS's of similar high
quality. This yields lines sensitivities which are utilized in reliability
analysis of our results. The final outcome is compared to recent accurate
measurements as well as the HITRAN2012 database. Transition frequencies are
obtained from effective Hamiltonian calculations to produce a comprehensive
line list covering all CO transitions below 8000 cm
and stronger than 10 cm / molecule at ~
- …