200 research outputs found
Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
(Abridged) Gas-phase complex organic molecules are commonly detected in the
warm inner regions of protostellar envelopes. Recent models show that
photochemistry in ices followed by desorption may explain the observed
abundances. This study aims to experimentally quantify the broad-band
UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K
under ultra-high vacuum conditions. The reaction products are mainly identified
by RAIRS and TPD experiments. Complex organics are readily formed in all
experiments, both during irradiation and during a slow warm-up of the ices to
200 K after the UV lamp is turned off. The relative abundances of photoproducts
depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or
CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3,
HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived
product-formation yields and their dependences on different experimental
parameters, such as the initial ice composition, are used to estimate the CH3OH
photodissociation branching ratios in ice and the relative diffusion barriers
of the formed radicals. The experiments show that ice photochemistry in CH3OH
ices is efficient enough to explain the observed abundances of complex organics
around protostars and that ratios of complex molecules can be used to constrain
their formation pathway.Comment: Accepted for publication in A&A. 65 pages including appendice
A Spectroscopic Survey of Electronic Transitions of CH, CH, and CD
Electronic spectra of CH are measured in the cm
domain using cavity ring-down spectroscopy of a supersonically expanding
hydrocarbon plasma. In total, 19 (sub)bands of CH are presented, all
probing the vibrational manifold of the B electronically excited state.
The assignments are guided by electronic spectra available from matrix
isolation work, isotopic substitution experiments (yielding also spectra for
CH and CD), predictions from ab initio calculations as well as
rotational fitting and vibrational contour simulations using the available
ground state parameters as obtained from microwave experiments. Besides the
origin band, three non-degenerate stretching vibrations along the
linear backbone of the CH molecule are assigned: the mode
associated with the C-C bond vibration and the and modes
associated with CC triple bonds. For the two lowest and
bending modes, a Renner-Teller analysis is performed identifying the
() and both () and
() components. In addition, two higher lying bending
modes are observed, which are tentatively assigned as ()
and () levels. In the excitation region below the first
non-degenerate vibration (), some transitions are
observed that are assigned as even combination modes of low-lying bending
vibrations. The same holds for a transition found above the
level. From these spectroscopic data and the vibronic analysis a
comprehensive energy level diagram for the B state of CH is derived
and presented.Comment: Accepted for publication in The Journal of Physical Chemistry A (26
July 2016
Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations
Aims. This article aims to provide an alternative method of measuring the
porosity of multi-phase composite ices from their refractive indices and of
characterising how the abundance of a premixed contaminant (e.g., CO2) affects
the porosity of water-rich ice mixtures during omni-directional deposition.
Methods. We combine optical laser interference and extended effective medium
approximations (EMAs) to measure the porosity of three astrophysically relevant
ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a
benchmarking test of this new laboratory-based method. Results. By
independently monitoring the O-H dangling modes of the different water-rich ice
mixtures, we confirm the porosities predicted by the extended EMAs. We also
demonstrate that CO2 premixed with water in the gas phase does not
significantly affect the ice morphology during omni-directional deposition, as
long as the physical conditions favourable to segregation are not reached. We
propose a mechanism in which CO2 molecules diffuse on the surface of the
growing ice sample prior to being incorporated into the bulk and then fill the
pores partly or completely, depending on the relative abundance and the growth
temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&
H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices: an extended view on complex organic molecule formation
Complex organic molecules (COMs) have been observed not only in the hot cores
surrounding low- and high- mass protostars, but also in cold dark clouds.
Therefore, it is interesting to understand how such species can be formed
without the presence of embedded energy sources. We present new laboratory
experiments on the low-temperature solid state formation of three complex
molecules: methyl formate (HC(O)OCH3), glycolaldehyde (HC(O)CH2OH) and ethylene
glycol (H2C(OH)CH2OH), through recombination of free radicals formed via H-atom
addition and abstraction reactions at different stages in the CO-H2CO-CH3OH
hydrogenation network at 15 K. The experiments extend previous CO hydrogenation
studies and aim at resembling the physical&chemical conditions typical of the
CO freeze-out stage in dark molecular clouds, when H2CO and CH3OH form by
recombination of accreting CO molecules and H-atoms on ice grains. We confirm
that H2CO, once formed through CO hydrogenation, not only yields CH3OH through
ongoing H-atom addition reactions, but is also subject to H-atom-induced
abstraction reactions, yielding CO again. In a similar way, H2CO is also formed
in abstraction reactions involving CH3OH. The dominant methanol H-atom
abstraction product is expected to be CH2OH, while H-atom additions to H2CO
should at least partially proceed through CH3O intermediate radicals. The
occurrence of H-atom abstraction reactions in ice mantles leads to more
reactive intermediates (HCO, CH3O and CH2OH) than previously thought, when
assuming sequential H-atom addition reactions only. This enhances the
probability to form COMs through radical-radical recombination without the need
of UV photolysis or cosmic rays as external triggers.Comment: 20 pages, 8 figure
Relevance of the H_2 + O reaction pathway for the surface formation of interstellar water. Combined experimental and modeling study
The formation of interstellar water is commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10–20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H_2 + O has been proposed to contribute significantly to the formation of water as well. However, gas-phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature-programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H_2 and O. These reactants were brought together in a matrix of CO_2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The water detected with the quadrupole mass spectrometer upon TPD was found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H_2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate was derived with a microscopic kinetic Monte Carlo model that converts the upper limit into the highest possible reaction rate. Incorporating this rate into simulation runs for astrochemically relevant parameters shows that the upper limit to the contribution of the reaction H_2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate, however, that this contribution is most likely much lower
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
Laser desorption time-of-flight mass spectrometry of ultraviolet photo-processed ices
A new ultra-high vacuum experiment is described that allows studying photo-induced chemical processes in interstellar ice analogues. MATRI2CES - a Mass Analytical Tool to study Reactions in Interstellar ICES applies a new concept by combining laser desorption and time-of-flight mass spectrometry with the ultimate goal to characterize in situ and in real time the solid state evolution of organic compounds upon UV photolysis for astronomically relevant ice mixtures and temperatures. The performance of the experimental setup is demonstrated by the kinetic analysis of the different photoproducts of pure methane (CH4) ice at 20 K. A quantitative approach provides formation yields of several new species with up to four carbon atoms. Convincing evidence is found for the formation of even larger species. Typical mass resolutions obtained range from M/M ∼320 to ∼400 for CH4 and argon, respectively. Additional tests show that the typical detection limit (in monolayers) is ≤0.02 ML, substantially more sensitive than the regular techniques used to investigate chemical processes in interstellar ices.Seventh Framework Programme (FP7)Laboratory astrophysics and astrochemistr
Desorption of CO and O2 interstellar ice analogs
Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds
through freeze-out processes. The aim of this work is to characterize
quantitatively the physical processes that are involved in the desorption
kinetics of CO-O2 ices by interpreting laboratory temperature programmed
desorption (TPD) data. This information is used to simulate the behavior of
CO-O2 ices under astrophysical conditions. The TPD spectra have been recorded
under ultra high vacuum conditions for pure, layered and mixed morphologies for
different thicknesses, temperatures and mixing ratios. An empirical kinetic
model is used to interpret the results and to provide input parameters for
astrophysical models. Binding energies are determined for different ice
morphologies. Independent of the ice morphology, the desorption of O2 is found
to follow 0th-order kinetics. Binding energies and temperature-dependent
sticking probabilities for CO-CO, O2-O2 and CO-O2 are determined. O2 is
slightly less volatile than CO, with binding energies of 912+-15 versus 858+-15
K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but
its binding energies are slightly increased compared with pure ice whereas
those for O2 are slightly decreased. Lower limits to the sticking probabilities
of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20K. The
balance between accretion and desorption is studied for O2 and CO in
astrophysically relevant scenarios. Only minor differences are found between
the two species, i.e., both desorb between 16 and 18K in typical environments
around young stars. Thus, clouds with significant abundances of gaseous CO are
unlikely to have large amounts of solid O2.Comment: 8 pages + 2 pages online material, 8 figures (1 online), accepted by
A&
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Laboratory experiments on interstellar ice analogs: The sticking and desorption of small physisorbed molecules
Molecular oxygen and nitrogen are difficult to observe since they are infrared inactive and radio quiet. The low O2 abundances found so far combined with general considerations of dense cloud conditions suggest molecular oxygen is frozen out at low temperatures (< 20 K) in the shielded inner regions of cloud cores. In solid form O2 and N2 can only be observed as adjuncts within other ice constituents, like CO. In this work we focus on fundamental properties of N2 and O2 in CO ice-gas systems, e.g. desorption characteristics and sticking probabilities at low temperatures for different ice morphologies
The formation of peptide-like molecules on interstellar dust grains
Molecules with an amide functional group resemble peptide bonds, the
molecular bridges that connect amino acids, and may thus be relevant in
processes that lead to the formation of life. In this study, the solid state
formation of some of the smallest amides is investigated in the laboratory. To
this end, CH:HNCO ice mixtures at 20 K are irradiated with far-UV
photons, where the radiation is used as a tool to produce the radicals required
for the formation of the amides. Products are identified and investigated with
infrared spectroscopy and temperature programmed desorption mass spectrometry.
The laboratory data show that NHCHO, CHNCO, NHC(O)NH,
CHC(O)NH and CHNH can simultaneously be formed. The
NHCO radical is found to be key in the formation of larger amides. In
parallel, ALMA observations towards the low-mass protostar IRAS 16293-2422B are
analysed in search of CHNHCHO (N-methylformamide) and
CHC(O)NH (acetamide). CHC(O)NH is tentatively detected
towards IRAS 16293-2422B at an abundance comparable with those found towards
high-mass sources. The combined laboratory and observational data indicates
that NHCHO and CHC(O)NH are chemically linked and form in the
ice mantles of interstellar dust grains. A solid-state reaction network for the
formation of these amides is proposed.Comment: Accepted for publication in MNRA
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