280 research outputs found
Assessment of foam fracture in sandwich beams using thermoelastic stress analysis
Thermoelastic Stress Analysis (TSA) has been well established for determining crack-tip parameters in metallic materials. This paper examines its ability to determine accurately the crack-tip parameters for PVC foam used in sandwich structures
Structure of the Alkali-metal-atom-Strontium molecular ions: towards photoassociation and formation of cold molecular ions
The potential energy curves, permanent and transition dipole moments, and the
static dipolar polarizability, of molecular ions composed of one alkali-metal
atom and a Strontium ion are determined with a quantum chemistry approach. The
molecular ions are treated as effective two-electron systems and are treated
using effective core potentials including core polarization, large gaussian
basis sets, and full configuration interaction. In the perspective of upcoming
experiments aiming at merging cold atom and cold ion traps, possible paths for
radiative charge exchange, photoassociation of a cold Lithium or Rubidium atom
and a Strontium ion are discussed, as well as the formation of stable molecular
ions
Calculation of accurate permanent dipole moments of the lowest states of heteronuclear alkali dimers using extended basis sets
The obtention of ultracold samples of dipolar molecules is a current
challenge which requires an accurate knowledge of their electronic properties
to guide the ongoing experiments. In this paper, we systematically investigate
the ground state and the lowest triplet state of mixed alkali dimers (involving
Li, Na, K, Rb, Cs) using a standard quantum chemistry approach based on
pseudopotentials for atomic core representation, gaussian basis sets, and
effective terms for core polarization effects. We emphasize on the convergence
of the results for permanent dipole moments regarding the size of the gaussian
basis set, and we discuss their predicted accuracy by comparing to other
theoretical calculations or available experimental values. We also revisit the
difficulty to compare computed potential curves among published papers, due to
the differences in the modelization of core-core interaction.Comment: accepted to J. Chem. Phy
Strain monitoring of tapestries: results of a three-year research project
The outcomes of an interdisciplinary research project between conservators and engineers investigating the strain experienced by different areas of a tapestry are described. Two techniques were used: full-field monitoring using digital image correlation (DIC) and point measurements using optical fibre sensors. Results showed that it is possible to quantify the global strain across a discrete area of a tapestry using DIC; optical fibre and other sensors were used to validate the DIC. Strain maps created by the DIC depict areas of high and low strain and can be overlaid on images of the tapestry, creating a useful visual tool for conservators, custodians and the general public. DIC identifies areas of high strain not obvious to the naked eye. The equipment can be used in situ in a historic house. In addition the work demonstrated the close relationship between relative humidity and strain
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
Oxygen diffusion and reactivity at low temperature on bare amorphous olivine-type silicate
The mobility of O atoms at very low temperatures is not generally taken into
account, despite O diffusion would add to a series of processes leading to the
observed rich molecular diversity in space. We present a study of the mobility
and reactivity of O atoms on an amorphous silicate surface. Our results are in
the form of RAIRS and temperature-programmed desorption spectra of O2 and O3
produced via two pathways: O + O and O2 + O, investigated in a submonolayer
regime and in the range of temperature between 6.5 and 30 K. All the
experiments show that ozone is formed efficiently on silicate at any surface
temperature between 6.5 and 30 K. The derived upper limit for the activation
barriers of O + O and O2 + O reactions is 150 K/kb. Ozone formation at low
temperatures indicates that fast diffusion of O atoms is at play even at 6.5 K.
Through a series of rate equations included in our model, we also address the
reaction mechanisms and show that neither the Eley Rideal nor the Hot atom
mechanisms alone can explain the experimental values. The rate of diffusion of
O atoms, based on modeling results, is much higher than the one generally
expected, and the diffusive process proceeds via the Langmuir-Hinshelwood
mechanism enhanced by tunnelling. In fact, quantum effects turn out to be a key
factor that cannot be neglected in our simulations. Astrophysically, efficient
O3 formation on interstellar dust grains would imply the presence of huge
reservoirs of oxygen atoms. Since O3 is a reservoir of elementary oxygen, and
also of OH via its hydrogenation, it could explain the observed concomitance of
CO2 and H2O in the ices.Comment: 28 pages, 14 figure
KRb Feshbach Resonances: Modeling the interatomic potential
We have observed 28 heteronuclear Feshbach resonances in 10 spin combinations
of the hyperfine ground states of a 40K 87Rb mixture. The measurements were
performed by observing the loss rates from an atomic mixture at magnetic fields
between 0 and 700 G. This data was used to significantly refine an interatomic
potential derived from molecular spectroscopy, yielding a highly consistent
model of the KRb interaction. Thus, the measured resonances can be assigned to
the corresponding molecular states. In addition, this potential allows for an
accurate calculation of the energy differences between highly excited levels
and the rovibrational ground level. This information is of particular relevance
for the formation of deeply bound heteronuclear molecules. Finally, the model
is used to predict Feshbach resonances in mixtures of 87Rb combined with 39K or
41K.Comment: 4 pages, 3 figure
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
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