1,926 research outputs found
Diffusion-desorption ratio of adsorbed CO and CO on water ice
Diffusion of atoms and molecules is a key process for the chemical evolution
in the star forming regions of the interstellar medium. Accurate data on the
mobility of many important interstellar species is however often not available
and this provides a serious limitation for the reliability of models describing
the physical and chemical processes in molecular clouds. Here we aim to provide
the astrochemical modeling community with reliable data on the ratio between
the energy barriers for diffusion and desorption for adsorbed CO and CO on
water ices. To this end, we use a fully atomistic, off-lattice kinetic Monte
Carlo technique to generate dynamical trajectories of CO and CO molecules
on the surface of crystalline ice at temperatures relevant for the interstellar
medium. The diffusion to desorption barrier ratios are determined to be 0.31
for CO and 0.39 for CO . These ratios can be directly used to improve the
accuracy of current gas-grain chemical models.Comment: 4 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
AIM: We have recently developed a microscopic Monte Carlo approach to study
surface chemistry on interstellar grains and the morphology of ice mantles. The
method is designed to eliminate the problems inherent in the rate-equation
formalism to surface chemistry. Here we report the first use of this method in
a chemical model of cold interstellar cloud cores that includes both gas-phase
and surface chemistry. The surface chemical network consists of a small number
of diffusive reactions that can produce molecular oxygen, water, carbon
dioxide, formaldehyde, methanol and assorted radicals. METHOD: The simulation
is started by running a gas-phase model including accretion onto grains but no
surface chemistry or evaporation. The starting surface consists of either flat
or rough olivine. We introduce the surface chemistry of the three species H, O
and CO in an iterative manner using our stochastic technique. Under the
conditions of the simulation, only atomic hydrogen can evaporate to a
significant extent. Although it has little effect on other gas-phase species,
the evaporation of atomic hydrogen changes its gas-phase abundance, which in
turn changes the flux of atomic hydrogen onto grains. The effect on the surface
chemistry is treated until convergence occurs. We neglect all non-thermal
desorptive processes. RESULTS: We determine the mantle abundances of assorted
molecules as a function of time through 2x10^5 yr. Our method also allows
determination of the abundance of each molecule in specific monolayers. The
mantle results can be compared with observations of water, carbon dioxide,
carbon monoxide, and methanol ices in the sources W33A and Elias 16. Other than
a slight underproduction of mantle CO, our results are in very good agreement
with observations.Comment: 13 pages, 7 figures, to be published in A. &
Long-timescale simulations of HO admolecule diffusion on Ice Ih(0001) surfaces
Long-timescale simulations of the diffusion of a HO admolecule on the
(0001) basal plane of ice Ih were carried out over a temperature range of 100
to 200 K using the adaptive kinetic Monte Carlo method and TIP4P/2005f
interaction potential function. The arrangement of dangling H atoms was varied
from the proton-disordered surface to the perfectly ordered Fletcher surface. A
large variety of sites was found leading to a broad distribution in adsorption
energy at both types of surfaces. Up to 4 % of the sites on the
proton-disordered surface have an adsorption energy exceeding the cohesive
energy of ice Ih. The mean squared displacement of a simulated trajectory at
175 K for the proton-disordered surface gave a diffusion constant of
610 cm/s, consistent with an upper bound previously reported
from experimental measurements. During the simulation, dangling H atoms were
found to rearrange so as to reduce clustering, thereby approaching a linear
Fletcher type arrangement. Diffusion on the perfectly ordered Fletcher surface
was estimated to be significantly faster, especially in the direction along the
rows of dangling hydrogen atoms. From simulations over the range in
temperature, an effective activation energy of diffusion was estimated to be
0.16 eV and 0.22 eV for diffusion parallel and perpendicular to the rows,
respectively. Even a slight disruption of the rows of the Fletcher surface made
the diffusion isotropic.Comment: 24 pages, 8 figures, 1 tabl
Interactions of adsorbed CO on water ice at low temperatures
We present a computational study into the adsorption properties of CO on
amorphous and crystalline water surfaces under astrophysically relevant
conditions. Water and carbon dioxide are two of the most dominant species in
the icy mantles of interstellar dust grains and a thorough understanding of
their solid phase interactions at low temperatures is crucial for understanding
the structural evolution of the ices due to thermal segregation. In this paper,
a new HO-CO interaction potential is proposed and used to model the
ballistic deposition of CO layers on water ice surfaces, and to study the
individual binding sites at low coverages. Contrary to recent experimental
results, we do not observe CO island formation on any type of water
substrate. Additionally, density functional theory calculations are performed
to assess the importance of induced electrostatic interactions.Comment: Accepted for publication in Physical Chemistry Chemical Physic
Putting Perspectives into Participation:Constructive Conflict Methodology for Problem Structuring in Stakeholder Dialogues
Berkhout, F.G.H. [Promotor]Midden, C.J.H. [Promotor]Leemans, R. [Promotor]Hirsschemoller, M. [Copromotor
A kinetic Monte Carlo study of desorption of H2 from graphite (0001)
The formation of H2 in the interstellar medium proceeds on the surfaces of
silicate or carbonaceous particles. To get a deeper insight of its formation on
the latter substrate, this letter focuses on H2 desorption from graphite (0001)
in Temperature-Programmed-Desorption Monte-Carlo simulations. The results are
compared to experimental results which show two main peaks and an intermediate
shoulder for high initial coverage. The simulation program includes barriers
obtained by ab-initio methods and is further optimised to match two independent
experimental observations. The simulations reproduce the two experimental
observed desorption peaks. Additionally, a possible origin of the intermediate
peak is given.Comment: 9 pages, 5 figures, Chem. Phys. Lett. in pres
Origins of Chevron Rollovers in Non-Two-State Protein Folding Kinetics
Chevron rollovers of some proteins imply that their logarithmic folding rates
are nonlinear in native stability. This is predicted by lattice and continuum
G\=o models to arise from diminished accessibilities of the ground state from
transiently populated compact conformations under strongly native conditions.
Despite these models' native-centric interactions, the slowdown is due partly
to kinetic trapping caused by some of the folding intermediates' nonnative
topologies. Notably, simple two-state folding kinetics of small single-domain
proteins are not reproduced by common G\=o-like schemes.Comment: 10 pages, 4 Postscript figures (will appear on PRL
Thermal H/D exchange in polar ice - deuteron scrambling in space
We have investigated the thermally induced proton/deuteron exchange in mixed
amorphous HO:DO ices by monitoring the change in intensity of
characteristic vibrational bending modes of HO, HDO, and DO with time
and as function of temperature. The experiments have been performed using an
ultra-high vacuum setup equipped with an infrared spectrometer that is used to
investigate the spectral evolution of homogeneously mixed ice upon
co-deposition in thin films, for temperatures in the 90 to 140 K domain. With
this non-energetic detection method we find a significantly lower activation
energy for H/D exchange -- K -- than previously reported. Very
likely this is due to the amorphous nature of the interstellar ice analogues
involved. This provides reactive timescales ( K)
fast enough for the process to be important in interstellar environments.
Consequently, an astronomical detection of DO will be even more challenging
because of its potential to react with HO to form HDO. Furthermore,
additional experiments, along with previous studies, show that proton/deuteron
swapping also occurs in ice mixtures of water with other hydrogen bonded
molecules, in particular on the OH and NH moieties. We conclude that H/D
exchange in ices is a more general process that should be incorporated into ice
models that are applied to protoplanetary disks or to simulate the warming up
of cometary ices in their passage of the perihelion, to examine the extent of
its influence on the final deuteron over hydrogen ratio.Comment: 10 pages, 8 figures, accepted for publication in MNRA
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