307 research outputs found
Dust as interstellar catalyst I. Quantifying the chemical desorption process
Context. The presence of dust in the interstellar medium has profound
consequences on the chemical composition of regions where stars are forming.
Recent observations show that many species formed onto dust are populating the
gas phase, especially in cold environments where UV and CR induced photons do
not account for such processes. Aims. The aim of this paper is to understand
and quantify the process that releases solid species into the gas phase, the
so-called chemical desorption process, so that an explicit formula can be
derived that can be included into astrochemical models. Methods. We present a
collection of experimental results of more than 10 reactive systems. For each
reaction, different substrates such as oxidized graphite and compact amorphous
water ice are used. We derive a formula to reproduce the efficiencies of the
chemical desorption process, which considers the equipartition of the energy of
newly formed products, followed by classical bounce on the surface. In part II
we extend these results to astrophysical conditions. Results. The equipartition
of energy describes correctly the chemical desorption process on bare surfaces.
On icy surfaces, the chemical desorption process is much less efficient and a
better description of the interaction with the surface is still needed.
Conclusions. We show that the mechanism that directly transforms solid species
to gas phase species is efficient for many reactions.Comment: Accepted for publication in A&
A review of residual stress analysis using thermoelastic techniques
Thermoelastic Stress Analysis (TSA) is a full-field technique for experimental stress analysis
that is based on infra-red thermography. The technique has proved to be extremely effective for
studying elastic stress fields and is now well established. It is based on the measurement of the
temperature change that occurs as a result of a stress change. As residual stress is essentially a
mean stress it is accepted that the linear form of the TSA relationship cannot be used to
evaluate residual stresses. However, there are situations where this linear relationship is not
valid or departures in material properties due to manufacturing procedures have enabled
evaluations of residual stresses. The purpose of this paper is to review the current status of
using a TSA based approach for the evaluation of residual stresses and to provide some
examples of where promising results have been obtained
Progress in strain monitoring of tapestries
This paper reports interdisciplinary
research between conservators and
engineers designed to enhance the
long-term conservation of tapestries
(tapestry-weave hangings) on longterm
display. The aim is to monitor,
measure and document the strain
experienced by different areas of a
tapestry while it is hanging on display.
Initial research has established that
damage can be identified in the early
stages of its inception, i.e., before it is
visible to the naked eye. The paper also
reports initial results of strain data
visualisation that allows curators and
conservators to examine how strain
develops, thereby facilitating
predictions about the changes in the
form or condition of the tapestry.
Strain data visualisation also allows the
strain process to be recorded, thereby
facilitating the effective documentation
of display methods and conservation
interventions. The paper reports the
use of point measurements (using
silica optical fibre sensors) and full-field
monitoring (using 3-D
photogrammetry with digital image
correlation (DIC))
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
High-resolution spectroscopy of triplet states of Rb2 by femtosecond pump-probe photoionization of doped helium nanodroplets
The dynamics of vibrational wave packets in triplet states of rubidium dimers
(Rb2) formed on helium nanodroplets are studied using femtosecond pump-probe
photoionization spectroscopy. Due to fast desorption of the excited Rb2
molecules off the droplets and due to their low internal temperature, wave
packet oscillations can be followed up to very long pump-probe delay times
>1.5ns. In the first excited triplet state (1)^3\Sigma_g^+, full and fractional
revivals are observed with high contrast. Fourier analysis provides
high-resolution vibrational spectra which are in excellent agreement with ab
initio calculations
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
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
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