74 research outputs found
Photodissociation of water in crystalline ice: a molecular dynamics study
Ultraviolet irradiation of ice is of great interest for understanding the
chemistry in both atmospheric and astrophysical environments. In interstellar
space, photodissociation of H2O molecules can be a driving force behind the
chemistry on icy dust grains in dense, cold molecular clouds even though the
flux of UV photons is extremely low. The mechanisms of such photoinduced
processes are poorly understood, however. In this work the photodissociation
dynamics of a water molecule in crystalline ice at 10 K is studied
computationally using classical molecular dynamics. Photodissociation in the
first bilayer leads mainly to H atoms desorbing (65%), while in the third
bilayer trapping of H and OH dominates (51%). The kinetic energy distribution
of the desorbing H atoms is much broader than that for the corresponding
gas-phase photodissociation. The H atoms on average move 11 Angstroms before
becoming trapped, while OH radicals typically move 2 Angstroms. In accordance
with experiments a blueshift of the absorption spectrum is obtained relative to
gas-phase water.Comment: 23 pages, 5 figure
The effect of surface relaxation on the N-2 dissociation rate on stepped Ru: A Transition State Theory Study
van Harrevelt R, Honkala K, Norskov JK, Manthe U. The effect of surface relaxation on the N2 dissociation rate on stepped Ru: A Transition State Theory Study. Journal of Chemical Physics. 2006;124(2):026102: 026102
The reaction rate for dissociative adsorption of N-2 on stepped Ru(0001): Six-dimensional quantum calculations
van Harrevelt R, Honkala K, Norskov JK, Manthe U. The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): Six-dimensional quantum calculations. Journal of Chemical Physics. 2005;122(23): 234702.Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N-2 on stepped Ru(0001) are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small. (C) 2005 American Institute of Physics
Photodesorption of water ice: a molecular dynamics study
Absorption of ultraviolet radiation by water ice coating interstellar grains
can lead to dissociation and desorption of the ice molecules. These processes
are thought to be important in the gas-grain chemistry in molecular clouds and
protoplanetary disks, but very few quantitative studies exist. We compute the
photodesorption efficiencies of amorphous water ice and elucidate the
mechanisms by which desorption occurs. Classical molecular dynamics
calculations were performed for a compact amorphous ice surface at 10 K thought
to be representative of interstellar ice. Dissociation and desorption of H2O
molecules in the top six monolayers are considered following absorption into
the first excited electronic state with photons in the 1300-1500 Angstrom
range. The trajectories of the H and OH photofragments are followed until they
escape or become trapped in the ice. The probability for H2O desorption per
absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to
0.03% in the next two monolayers, and is negligible deeper into the ice. The
main H2O removal mechanism in the top two monolayers is through separate
desorption of H and OH fragments. Removal of H2O molecules from the ice, either
as H2O itself or its products, has a total probability of 2-3% per absorbed UV
photon in the top two monolayers. In the third monolayer the probability is
about 1% and deeper into the ice the probability of photodesorption falling to
insignificant numbers. The probability of any removal of H2O per incident
photon is estimated to be 3.7x10^-4, with the probability for photodesorption
of intact H2O molecules being 1.4x10^-4 per incident photon. When no desorption
occurs, the H and OH products can travel up to 70 and 60 Angstroms inside or on
top of the surface during which they can react with other species.Comment: 12 pages, 10 figures, A&A, in pres
OH emission from warm and dense gas in the Orion Bar PDR
As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present
the first detection of the hydroxyl radical (OH) towards the Orion Bar
photodissociation region (PDR). Five OH rotational Lambda-doublets involving
energy levels out to E_u/k~511 K have been detected (at ~65, ~79, ~84, ~119 and
~163um). The total intensity of the OH lines is I(OH)~5x10^-4 erg s^-1 cm^-2
sr^-1. The observed emission of rotationally excited OH lines is extended and
correlates well with the high-J CO and CH^+ J=3-2 line emission (but apparently
not with water vapour), pointing towards a common origin. Nonlocal, non-LTE
radiative transfer models including excitation by the ambient FIR radiation
field suggest that OH arises in a small filling factor component of warm
(Tk~160-220 K) and dense (n_H~10^{6-7} cm^-3) gas with source-averaged OH
column densities of ~10^15 cm^-2. High density and temperature photochemical
models predict such enhanced OH columns at low depths (A_V<1) and small spatial
scales (~10^15 cm), where OH formation is driven by gas-phase endothermic
reactions of atomic oxygen with molecular hydrogen. We interpret the extended
OH emission as coming from unresolved structures exposed to far-ultraviolet
(FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and
not from the lower density "interclump" medium. Photodissociation leads to
OH/H2O abundance ratios (>1) much higher than those expected in equally warm
regions without enhanced FUV radiation fields.Comment: Accepted for publication in A&A Letters. Figure B.2. is bitmapped to
lower resolutio
Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation
The HDO/H2O ratio measured in interstellar gas is often used to draw conclusions on the formation and evolution of water in star-forming regions and, by comparison with cometary data, on the origin of water on Earth. In cold cores and in the outer regions of protoplanetary disks, an important source of gas-phase water comes from photodesorption of water ice. This research note presents fitting formulae for implementation in astrochemical models using previously computed photodesorption efficiencies for all water ice isotopologues obtained with classical molecular dynamics simulations. The results are used to investigate to what extent the gas-phase HDO/H2O ratio reflects that present in the ice or whether fractionation can occur during the photodesorption process. Probabilities for the top four monolayers are presented for photodesorption of X (X = H, D) atoms, OX radicals, and X2O and HDO molecules following photodissociation of H2O, D2O, and HDO in H2O amorphous ice at ice temperatures from 10-100 K. Significant isotope effects are found for all possible products: (1) H atom photodesorption probabilities from H2O ice are larger than those for D atom photodesorption from D2O ice by a factor of 1.1; the ratio of H and D photodesorbed upon HDO photodissociation is a factor of 2. This process will enrich the ice in deuterium atoms over time; (2) the OD/OH photodesorption ratio upon D2O and H2O photodissociation is on average a factor of 2, but the OD/OH photodesorption ratio upon HDO photodissociation is almost constant at unity for all ice temperatures; (3) D atoms are more effective in kicking out neighbouring water molecules than H atoms. However, the ratio of the photodesorbed HDO and H2O molecules is equal to the HDO/H2O ratio in the ice, therefore, there is no isotope fractionation when HDO and H2O photodesorb from the ice. Nevertheless, the enrichment of the ice in D atoms due to photodesorption can over time lead to an enhanced HDO/H2O ratio in the ice, and, when photodesorbed, also in the gas. The extent to which the ortho/para ratio of H2O can be modified by the photodesorption process is discussed briefly as well
Photodissociation of small carbonaceous molecules of astrophysical interest
Astronomical observations have shown that small carbonaceous molecules can
persist in interstellar clouds exposed to intense ultraviolet radiation.
Current astrochemical models lack quantitative information on photodissociation
rates in order to interpret these data. We here present ab initio
multi-reference configuration-interaction calculations of the vertical
excitation energies, transition dipole moments and oscillator strengths for a
number of astrophysically relevant molecules: C3, C4, C2H, l- and c-C3H, l- and
c-C3H2, HC3H, l-C4H and l-C5H. Highly excited states up to the 9'th root of
each symmetry are computed, and several new states with large oscillator
strengths are found below the ionization potentials. These data are used to
calculate upper limits on photodissociation rates in the unattenuated
interstellar radiation field by assuming that all absorptions above the
dissociation limit lead to dissociation.Comment: Full tables, rates and cross sections are posted at
http://www.strw.leidenuniv.nl/~ewine/phot
Recursive formulation of the multiconfigurational time-dependent Hartree method for fermions, bosons and mixtures thereof in terms of one-body density operators
The multiconfigurational time-dependent Hartree method (MCTDH) [Chem. Phys.
Lett. {\bf 165}, 73 (1990); J. Chem. Phys. {\bf 97}, 3199 (1992)] is
celebrating nowadays entering its third decade of tackling numerically-exactly
a broad range of correlated multi-dimensional non-equilibrium quantum dynamical
systems. Taking in recent years particles' statistics explicitly into account,
within the MCTDH for fermions (MCTDHF) and for bosons (MCTDHB), has opened up
further opportunities to treat larger systems of interacting identical
particles, primarily in laser-atom and cold-atom physics. With the increase of
experimental capabilities to simultaneously trap mixtures of two, three, and
possibly even multiple kinds of interacting composite identical particles
together, we set up the stage in the present work and specify the MCTDH method
for such cases. Explicitly, the MCTDH method for systems with three kinds of
identical particles interacting via all combinations of two- and three-body
forces is presented, and the resulting equations-of-motion are briefly
discussed. All four possible mixtures of fermions and bosons are presented in a
unified manner. Particular attention is paid to represent the coefficients'
part of the equations-of-motion in a compact recursive form in terms of
one-body density operators only. The recursion utilizes the recently proposed
Combinadic-based mapping for fermionic and bosonic operators in Fock space
[Phys. Rev. A {\bf 81}, 022124 (2010)] and successfully applied and implemented
within MCTDHB. Our work sheds new light on the representation of the
coefficients' part in MCTDHF and MCTDHB without resorting to the matrix
elements of the many-body Hamiltonian with respect to the time-dependent
configurations. It suggests a recipe for efficient implementation of the
schemes derived here for mixtures which is suitable for parallelization.Comment: 43 page
Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes
We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re
- …