165 research outputs found
Product-state distribution after isotopic substitution in ultracold atom-molecule collisions
We show that products of the isotopic substitution reactions in
experimentally accessible molecules such as NaK, RbCs, and SrF are cold
according to their translational energy below hundreds of mK. For these
chemical reactions, molecular products may occupy only the lowest rotational
states. We also discuss the possibility of controlling the chemical reactions
by the electric field in ultracold mixtures of molecules and atoms with low
kinetic energy release, where one of the constituent atoms of colliding
molecule is replaced by its isotope. This letter opens new avenues in
investigating the branching ratios of chemical reactions in ultracold
conditions
Light-assisted ion-neutral reactive processes in the cold regime: radiative molecule formation vs. charge exchange
We present a combined experimental and theoretical study of cold reactive
collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid
trap. We observe rich chemical dynamics which are interpreted in terms of
non-adiabatic and radiative charge exchange as well as radiative molecule
formation using high-level electronic structure calculations. We study the role
of light-assisted processes and show that the efficiency of the dominant
chemical pathways is considerably enhanced in excited reaction channels. Our
results illustrate the importance of radiative and non-radiative processes for
the cold chemistry occurring in ion-atom hybrid traps.Comment: 5 pages, 4 figure
Modeling interstellar amorphous solid water grains by tight-binding based methods: comparison between GFN-XTB and CCSD(T) results for water clusters
One believed path to Interstellar Complexes Organic Molecules (iCOMs)
formation inside the Interstellar Medium (ISM) is through chemical
recombination at the surface of amorphous solid water (ASW) mantle covering the
silicate-based core of the interstellar grains. The study of these iCOMs
formation and their binding energy to the ASW, using computational chemistry,
depends strongly on the ASW models used, as different models may exhibit sites
with different adsorbing features. ASW extended models are rare in the
literature because large sizes require very large computational resources when
quantum mechanical methods based on DFT are used. To circumvent this problem,
we propose to use the newly developed GFN-xTB Semi-empirical Quantum Mechanical
(SQM) methods from the Grimme's group. These methods are, at least, two orders
of magnitude faster than conventional DFT, only require modest central memory,
and in this paper we aim to benchmark their accuracy against rigorous and
resource hungry quantum mechanical methods. We focused on 38 water structures
studied by MP2 and CCSD(T) approaches comparing energetic and structures with
three levels of GFN-xTB parametrization (GFN0, GFN1, GFN2) methods. The
extremely good results obtained at the very cheap GFN-xTB level for both water
cluster structures and energetic paved the way towards the modeling of very
large AWS models of astrochemical interest.Comment: 9 pages, 4 figures, Submitted to LNCS (Springer) ICCSA202
Global analysis of data on the spin-orbit coupled and states of Cs2
We present experimentally derived potential curves and spin-orbit interaction
functions for the strongly perturbed and
states of the cesium dimer. The results are based on data from several sources.
Laser-induced fluorescence Fourier transform spectroscopy (LIF FTS) was used
some time ago in the Laboratoire Aim\'{e} Cotton primarily to study the state. More recent work at Tsinghua University provides
information from moderate resolution spectroscopy on the lowest levels of the
states as well as additional high resolution data. From
Innsbruck University, we have precision data obtained with cold Cs
molecules. Recent data from Temple University was obtained using the
optical-optical double resonance polarization spectroscopy technique, and
finally, a group at the University of Latvia has added additional LIF FTS data.
In the Hamiltonian matrix, we have used analytic potentials (the Expanded Morse
Oscillator form) with both finite-difference (FD) coupled-channels and discrete
variable representation (DVR) calculations of the term values. Fitted diagonal
and off-diagonal spin-orbit functions are obtained and compared with {\it ab
initio} results from Temple and Moscow State universities
Deceleration and trapping of heavy diatomic molecules using a ring-decelerator
We present an analysis of the deceleration and trapping of heavy diatomic
molecules in low-field seeking states by a moving electric potential. This
moving potential is created by a 'ring-decelerator', which consists of a series
of ring-shaped electrodes to which oscillating high voltages are applied.
Particle trajectory simulations have been used to analyze the deceleration and
trapping efficiency for a group of molecules that is of special interest for
precision measurements of fundamental discrete symmetries. For the typical case
of the SrF molecule in the (N,M) = (2, 0) state, the ring-decelerator is shown
to outperform traditional and alternate-gradient Stark decelerators by at least
an order of magnitude. If further cooled by a stage of laser cooling, the
decelerated molecules allow for a sensitivity gain in a parity violation
measurement, compared to a cryogenic molecular beam experiment, of almost two
orders of magnitude
Formation and interactions of cold and ultracold molecules: new challenges for interdisciplinary physics
Progress on researches in the field of molecules at cold and ultracold
temperatures is reported in this review. It covers extensively the experimental
methods to produce, detect and characterize cold and ultracold molecules
including association of ultracold atoms, deceleration by external fields and
kinematic cooling. Confinement of molecules in different kinds of traps is also
discussed. The basic theoretical issues related to the knowledge of the
molecular structure, the atom-molecule and molecule-molecule mutual
interactions, and to their possible manipulation and control with external
fields, are reviewed. A short discussion on the broad area of applications
completes the review.Comment: to appear in Reports on Progress in Physic
Nonexponetial relaxation of photoinduced conductance in organic field effect transistor
We report detailed studies of the slow relaxation of the photoinduced excess
charge carriers in organic metal-insulator-semiconductor field effect
transistors consisting of poly(3-hexylthiophene) as the active layer. The
relaxation process cannot be physically explained by processes, which lead to a
simple or a stretched-exponential decay behavior. Models based on serial
relaxation dynamics due to a hierarchy of systems with increasing spatial
separation of the photo-generated negative and positive charges are used to
explain the results. In order to explain the observed trend, the model is
further modified by introducing a gate voltage dependent coulombic distribution
manifested by the trapped negative charge carriers.Comment: 17 pages, 3 Figure
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ∼25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
Seeds of Life in Space (SOLIS). III. Zooming Into the Methanol Peak of the Prestellar Core L1544
Toward the prestellar core L1544, the methanol (CH3OH) emission forms an asymmetric ring around the core center, where CH3OH is mostly in solid form, with a clear peak at 4000 au to the northeast of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH3OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2 km s−1 and the velocity dispersion increases from subsonic to transonic toward the central zone of the core, where the velocity field also shows complex structure. This could be an indication of gentle accretion of material onto the core or the interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH3OH column density, N tot(CH3OH), profile has been derived with non-LTE radiative transfer modeling and compared with chemical models of a static core. The measured N tot(CH3OH) profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunneling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance
Seeds of Life in Space (SOLIS) VI. Chemical evolution of sulfuretted species along the outflows driven by the low-mass protostellar binary NGC1333-IRAS4A
Context: Low-mass protostars drive powerful molecular outflows that can be observed with millimetre and submillimetre telescopes.
Various sulfuretted species are known to be bright in shocks and could be used to infer the physical and chemical conditions throughout
the observed outflows.
Aims: The evolution of sulfur chemistry is studied along the outflows driven by the NGC 1333-IRAS4A protobinary system located
in the Perseus cloud to constrain the physical and chemical processes at work in shocks.
Methods: We observed various transitions from OCS, CS, SO, and SO2 towards NGC 1333-IRAS4A in the 1.3, 2, and 3 mm bands
using the IRAM NOrthern Extended Millimeter Array and we interpreted the observations through the use of the Paris-Durham shock
model.
Results: The targeted species clearly show different spatial emission along the two outflows driven by IRAS4A. OCS is brighter
on small and large scales along the south outflow driven by IRAS4A1, whereas SO2 is detected rather along the outflow driven by
IRAS4A2 that is extended along the north east–south west direction. SO is detected at extremely high radial velocity up to +25 km s−1
relative to the source velocity, clearly allowing us to distinguish the two outflows on small scales. Column density ratio maps estimated
from a rotational diagram analysis allowed us to confirm a clear gradient of the OCS/SO2 column density ratio between the IRAS4A1
and IRAS4A2 outflows. Analysis assuming non Local Thermodynamic Equilibrium of four SO2 transitions towards several SiO emission peaks suggests that the observed gas should be associated with densities higher than 105
cm−3
and relatively warm (T > 100 K)
temperatures in most cases.
Conclusions: The observed chemical differentiation between the two outflows of the IRAS4A system could be explained by a different chemical history. The outflow driven by IRAS4A1 is likely younger and more enriched in species initially formed in interstellar
ices, such as OCS, and recently sputtered into the shock gas. In contrast, the longer and likely older outflow triggered by IRAS4A2 is
more enriched in species that have a gas phase origin, such as SO2
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