281 research outputs found
Creating Ground State Molecules with Optical Feshbach Resonances in Tight Traps
We propose to create ultracold ground state molecules in an atomic
Bose-Einstein condensate by adiabatic crossing of an optical Feshbach
resonance. We envision a scheme where the laser intensity and possibly also
frequency are linearly ramped over the resonance. Our calculations for
Rb show that for sufficiently tight traps it is possible to avoid
spontaneous emission while retaining adiabaticity, and conversion efficiencies
of up to 50% can be expected
Photoassociative creation of ultracold heteronuclear 6Li40K* molecules
We investigate the formation of weakly bound, electronically excited,
heteronuclear 6Li40K* molecules by single-photon photoassociation in a
magneto-optical trap. We performed trap loss spectroscopy within a range of 325
GHz below the Li(2S_(1/2))+K(4P_(3/2)) and Li(2S_(1/2))+K(4P_(1/2)) asymptotic
states and observed more than 60 resonances, which we identify as rovibrational
levels of 7 of 8 attractive long-range molecular potentials. The long-range
dispersion coefficients and rotational constants are derived. We find large
molecule formation rates of up to ~3.5x10^7s^(-1), which are shown to be
comparable to those for homonuclear 40K_2*. Using a theoretical model we infer
decay rates to the deeply bound electronic ground-state vibrational level
X^1\Sigma^+(v'=3) of ~5x10^4s^(-1). Our results pave the way for the production
of ultracold bosonic ground-state 6Li40K molecules which exhibit a large
intrinsic permanent electric dipole moment.Comment: 6 pages, 4 figures, submitted to EP
Potential energy and dipole moment surfaces of H3- molecule
A new potential energy surface for the electronic ground state of the
simplest triatomic anion H3- is determined for a large number of geometries.
Its accuracy is improved at short and large distances compared to previous
studies. The permanent dipole moment surface of the state is also computed for
the first time. Nine vibrational levels of H3- and fourteen levels of D3- are
obtained, bound by at most ~70 cm^{-1} and ~ 126 cm^{-1} respectively. These
results should guide the spectroscopic search of the H3- ion in cold gases
(below 100K) of molecular hydrogen in the presence of H3- ions
Experimental study of the binding energy of NH3 on different types of ice and its impact on the snow line of NH3 and H2O
N-bearing molecules (like N2H+ or NH3) are excellent tracers of high-density,
low-temperature regions like dense cloud cores and could shed light into
snowlines in protoplanetary disks and the chemical evolution of comets.
However, uncertainties exist about the grain surface chemistry of these
molecules -- which could play an important role in their formation and
evolution. This study explores experimentally the behaviour of NH on
surfaces mimicking grains under interstellar conditions alongside other major
interstellar ice components (ie. HO, CO, CO). We performed
co-deposition experiments using the Ultra High Vacuum (UHV) setup VENUS (VErs
des NoUvelles Syntheses) of NH along with other adsorbates (here, HO,
CO and CO) and performed Temperature Programmed Desorption (TPD) and
Temperature Programmed-During Exposure Desorption (TP-DED) experiments. We
obtained binding Energy (BE) distribution of NH on Crystalline Ice(CI) and
compact-Amorphous Solid Water (c-ASW) by analyses of the TPD profiles of NH3 on
the substrates. We observe a significant delay in the desorption and a decrease
in the desorption rate of NH when HO is introduced into the
co-deposited mixture of NH-Co or NH-CO, absent without
HO. Secondly, HO traps nearly 5-9 per cent of the co-deposited NH3,
released during water's amorphous-to-crystalline phase change. Thirdly, for CI,
we obtained a BE distribution between 3780K-4080K, and c-ASW between
3780K-5280K -- using a pre-exponential factor A = 1.94/s. We
conclude that NH behaviour is significantly influenced by the presence of
HO due to the formation of hydrogen bonds, in line with quantum
calculations. This interaction preserves NH on grain surfaces to higher
temperatures making it available to the central protostar in protoplanetary
disks. It also explains why NH freeze out in pre-stellar cores is
efficient
Microstructural Assessment of 316L Stainless Steel Using Infrared Thermography Based Measurement of Energy Dissipation Arising from Cyclic Loading
A procedure is developed that evaluates the energy dissipated from a material subject to cyclic loading and enables identification of the difference in material microstructure. It is demonstrated that the dissipated energy can be derived from specimens loaded in the elastic region using temperature measurements obtained by infrared thermography. To obtain accurate values of the small temperature changes resulting from the intrinsic dissipation below the yield point, a key part of the procedure is to eliminate the effect of external heat sources and sinks from the vicinity of the test specimen under investigation. To this end, a chamber was designed to minimise the external radiation whilst allowing the specimens to be cyclically loaded; the configuration of the chamber is described, alongside its integration into the procedure. A reference specimen was specifically introduced in the chamber to take into account the thermal exchanges between the specimen and the chamber environment. A data processing procedure, based on the thermomechanical heat diffusion equation, is applied to enable the dissipated energy to be derived from the temperature measurements. It is established that quantifying the amount of energy dissipation provides an opportunity to identify the material condition. The procedure is demonstrated on specimens made from 316L stainless steel containing a range of microstructures produced by different heat treatments. It is shown that the dissipative energy is dependent on the microstructure and that the dissipative source can be identified using the experimental procedure
Influence of a Feshbach resonance on the photoassociation of LiCs
We analyse the formation of ultracold 7Li133Cs molecules in the rovibrational
ground state through photoassociation into the B1Pi state, which has recently
been reported [J. Deiglmayr et al., Phys. Rev. Lett. 101, 133004 (2008)].
Absolute rate constants for photoassociation at large detunings from the atomic
asymptote are determined and are found to be surprisingly large. The
photoassociation process is modeled using a full coupled-channel calculation
for the continuum state, taking all relevant hyperfine states into account. The
enhancement of the photoassociation rate is found to be caused by an `echo' of
the triplet component in the singlet component of the scattering wave function
at the inner turning point of the lowest triplet a3Sigma+ potential. This
perturbation can be ascribed to the existence of a broad Feshbach resonance at
low scattering energies. Our results elucidate the important role of couplings
in the scattering wave function for the formation of deeply bound ground state
molecules via photoassociation.Comment: Added Erratum, 20 pages, 9 figure
Enhancement of the formation of ultracold Rb molecules due to resonant coupling
We have studied the effect of resonant electronic state coupling on the
formation of ultracold ground-state Rb. Ultracold Rb molecules
are formed by photoassociation (PA) to a coupled pair of states,
and , in the region below the
limit. Subsequent radiative decay produces high vibrational levels of the
ground state, . The population distribution of these state
vibrational levels is monitored by resonance-enhanced two-photon ionization
through the state. We find that the populations of vibrational
levels =112116 are far larger than can be accounted for by the
Franck-Condon factors for transitions with
the state treated as a single channel. Further, the
ground-state molecule population exhibits oscillatory behavior as the PA laser
is tuned through a succession of state vibrational levels. Both of
these effects are explained by a new calculation of transition amplitudes that
includes the resonant character of the spin-orbit coupling of the two
states. The resulting enhancement of more deeply bound ground-state molecule
formation will be useful for future experiments on ultracold molecules.Comment: 6 pages, 5 figures; corrected author lis
Electrostatic extraction of cold molecules from a cryogenic reservoir
We present a method which delivers a continuous, high-density beam of slow
and internally cold polar molecules. In our source, warm molecules are first
cooled by collisions with a cryogenic helium buffer gas. Cold molecules are
then extracted by means of an electrostatic quadrupole guide. For ND the
source produces fluxes up to molecules/s with
peak densities up to molecules/cm. For
HCO the population of rovibrational states is monitored by depletion
spectroscopy, resulting in single-state populations up to .Comment: 4 pages, 4 figures, changes to the text, updated figures and
reference
Thermoelastic stress and damage analysis using transient loading
Thermoelastic stress analysis (TSA) is often regarded as a laboratory based technique due to its requirement for a cyclic load. A modified methodology is proposed in which only a single transient load is used for the TSA measurement. Two methods of imparting the transient load are validated against calculations and the conventional TSA approach. Specimens with different damage severities are tested and it is shown that the modified TSA method has the potential to be applied in the field as a non-destructive evaluation too
Acetaldehyde binding energies: a coupled experimental and theoretical study
Acetaldehyde is one of the most common and abundant gaseous interstellar
complex organic molecules, found in cold and hot regions of the molecular
interstellar medium. Its presence in the gas-phase depends on the chemical
formation and destruction routes, and its binding energy (BE) governs whether
acetaldehyde remains frozen onto the interstellar dust grains or not. In this
work, we report a combined study of the acetaldehyde BE obtained via laboratory
TPD (Temperature Programmed Desorption) experiments and theoretical quantum
chemical computations. BEs have been measured and computed as a pure
acetaldehyde ice and as mixed with both polycrystalline and amorphous water
ice. Both calculations and experiments found a BE distribution on amorphous
solid water that covers the 4000--6000 K range, when a pre-exponential factor
of is used for the interpretation of the experiments.
We discuss in detail the importance of using a consistent couple of BE and
pre-exponential factor values when comparing experiments and computations, as
well as when introducing them in astrochemical models. Based on the comparison
of the acetaldehyde BEs measured and computed in the present work with those of
other species, we predict that acetaldehyde is less volatile than formaldehyde,
but much more than water, methanol, ethanol, and formamide. We discuss the
astrochemical implications of our findings and how recent astronomical high
spatial resolution observations show a chemical differentiation involving
acetaldehyde, which can easily explained as due to the different BEs of the
observed molecules.Comment: 12 pages, 6 figure
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