arXiv subjects

H. Linnartz

Publications and source records attributed to H. Linnartz.

66 records · Page 4Linked to original sources

arXiv2010

Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration

Water is the main component of interstellar ice mantles, is abundant in the solar system and is a crucial ingredient for life. The formation of this molecule in the interstellar medium cannot be explained by gas-phase chemistry only and its surface hydrogenation formation routes at low temperatures (O, O2, O3 channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H2O production behavior in O2 ice hydrogenation experiments compared to the first-order H2CO and CH3OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O2 ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O2 ice under fully controlled conditions, while the changes are followed by means of reflection absorption infrared spectroscopy (RAIRS). The H-atom penetration mechanism is systematically studied by varying the temperature, thickness and structure of the O2 ice. We conclude that the competition between reaction and diffusion of the H atoms into the O2 ice explains the unexpected H2O and H2O2 formation behavior. In addition, we show that the proposed O2 hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O3 in the ice upon H-atom exposure proves that the O2 channel is not an isolated route. Furthermore, the addition of H2 molecules is found not to have a measurable effect on the O2 reaction channel.

astro-ph.SR↗

arXiv2010

Water formation at low temperatures by surface O2 hydrogenation II: the reaction network

Water is abundantly present in the Universe. It is the main component of interstellar ice mantles and a key ingredient for life. Water in space is mainly formed through surface reactions. Three formation routes have been proposed in the past: hydrogenation of surface O, O2, and O3. In a previous paper [Ioppolo et al., Astrophys. J., 2008, 686, 1474] we discussed an unexpected non-standard zeroth-order H2O2 production behaviour in O2 hydrogenation experiments, which suggests that the proposed reaction network is not complete, and that the reaction channels are probably more interconnected than previously thought. In this paper we aim to derive the full reaction scheme for O2 surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O2 deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O2 ratios are used to trace different stages in the hydrogenation network. The chemical changes in the forming ice are followed by means of reflection absorption infrared spectroscopy (RAIRS). New reaction paths are revealed as compared to previous experiments. Several reaction steps prove to be much more efficient (H + O2) or less efficient (H + OH and H2 + OH) than originally thought. These are the main conclusions of this work and the extended network concluded here will have profound implications for models that describe the formation of water in space.

astro-ph.SR↗

arXiv2010

A coincidence between a hydrocarbon plasma absorption spectrum and the lambda 5450 DIB

The aim of this work is to link the broad lambda 5450 diffuse interstellar band (DIB) to a laboratory spectrum recorded through an expanding acetylene plasma. Cavity ring-down direct absorption spectra and astronomical observations of HD 183143 with the HERMES spectrograph on the Mercator Telescope in La Palma and the McKellar spectrograph on the DAO 1.2 m Telescope are compared. In the 543-547 nm region a broad band is measured with a band maximum at 545 nm and FWHM of 1.03(0.1) nm coinciding with a well-known diffuse interstellar band at lambda 5450 with FWHM of 0.953 nm. A coincidence is found between the laboratory and the two independent observational studies obtained at higher spectral resolution. This result is important, as a match between a laboratory spectrum and a - potentially lifetime broadened - DIB is found. A series of additional experiments has been performed in order to unambiguously identify the laboratory carrier of this band. This has not been possible. The laboratory results, however, restrict the carrier to a molecular transient, consisting of carbon and hydrogen.

astro-ph.IM↗

arXiv2009

Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry

Context. Icy dust grains play an important role in the formation of complex inter- and circumstellar molecules. Observational studies show that polycyclic aromatic hydrocarbons (PAHs) are abundantly present in the ISM in the gas phase. It is likely that these non-volatile species freeze out onto dust grains as well and participate in the astrochemical solid-state network, but experimental PAH ice studies are largely lacking. Methods. Near UV/VIS spectroscopy is used to track the in situ VUV driven photochemistry of pyrene containing ices at temperatures ranging from 10 to 125 K. Results. The main photoproducts of VUV photolyzed pyrene ices are spectroscopically identified and their band positions are listed for two host ices, \water and CO. Pyrene ionisation is found to be most efficient in \water ices at low temperatures. The reaction products, triplet pyrene and the 1-hydro-1-pyrenyl radical are most efficiently formed in higher temperature water ices and in low temperature CO ice. Formation routes and band strength information of the identified species are discussed. Additionally, the oscillator strengths of Py, Py^+ and PyH are derived and a quantitative kinetic analysis is performed by fitting a chemical reaction network to the experimental data. Conclusions. Pyrene is efficiently ionised in water ice at temperatures below 50 K. Hydrogenation reactions dominate the chemistry in low temperature CO ice with trace amounts of water. The results are put in an astrophysical context by determining the importance of PAH ionisation in a molecular cloud. The photoprocessing of a sample PAH in ice described in this manuscript indicates that PAH photoprocessing in the solid state should also be taken into account in astrochemical models.

astro-ph.SR↗

arXiv2009

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.

astro-ph.GA↗

arXiv2009

Real-time optical spectroscopy of VUV irradiated pyrene:H_2O interstellar ice

This paper describes a near-UV/VIS study of a pyrene:H_2O interstellar ice analogue at 10 K using optical absorption spectroscopy. A new experimental approach makes it possible to irradiate the sample with vacuum ultraviolet (VUV) light (7-10.5 eV) while simultaneously recording spectra in the 240-1000 nm range with subsecond time resolution. Both spectroscopic and dynamic information on VUV processed ices are obtained in this way. This provides a powerful tool to follow, in-situ and in real time, the photophysical and photochemical processes induced by VUV irradiation of a polycyclic aromatic hydrocarbon containing inter- and circumstellar ice analogue. Results on the VUV photolysis of a prototype sample - strongly diluted pyrene in H_2O ice - are presented. In addition to the pyrene cation (Py+), other products - hydroxypyrene (PyOH), possibly hydroxypyrene cation (PyOH+), and pyrene/pyrenolate anion (Py-/PyO-) - are observed. It is found that the charge remains localized in the ice, also after the VUV irradiation is stopped. The astrochemical implications and observational constraints are discussed.

astro-ph.IM↗

arXiv2009

A Zero-Gravity Instrument to Study Low Velocity Collisions of Fragile Particles at Low Temperatures

We discuss the design, operation, and performance of a vacuum setup constructed for use in zero (or reduced) gravity conditions to initiate collisions of fragile millimeter-sized particles at low velocity and temperature. Such particles are typically found in many astronomical settings and in regions of planet formation. The instrument has participated in four parabolic flight campaigns to date, operating for a total of 2.4 hours in reduced gravity conditions and successfully recording over 300 separate collisions of loosely packed dust aggregates and ice samples. The imparted particle velocities achieved range from 0.03-0.28 m s^-1 and a high-speed, high-resolution camera captures the events at 107 frames per second from two viewing angles separated by either 48.8 or 60.0 degrees. The particles can be stored inside the experiment vacuum chamber at temperatures of 80-300 K for several uninterrupted hours using a built-in thermal accumulation system. The copper structure allows cooling down to cryogenic temperatures before commencement of the experiments. Throughout the parabolic flight campaigns, add-ons and modifications have been made, illustrating the instrument flexibility in the study of small particle collisions.

physics.space-ph↗

arXiv2009

Hydrogenation reactions in interstellar CO ice analogues

Hydrogenation reactions of CO in inter- and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups on the hydrogenation of CO ices resulted in controversial results on the formation rate of methanol. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with room temperature H atoms. The reaction of CO + H into H2CO and subsequently CH3OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement a temperature programmed desorption experiment is performed to identify the produced species. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (12-20 K). We confirm, as proposed by Hidaka et al., that the discrepancy between the two Japanese studies is mainly due to a difference in the applied hydrogen atom flux. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. In order to fully understand the laboratory data, the experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Temperature-dependent reaction barriers and diffusion rates are inferred using this model. The model is extended to interstellar conditions to compare with observational H2CO/CH3OH data.

astro-ph.SR↗

arXiv2008

Laboratory evidence for efficient water formation in interstellar ices

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O2, or O3, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O2 ice is bombarded by H- or D-atoms under ultra high vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H2O (D2O) with a rate that is surprisingly independent of temperature. This formation route converts O2 into H2O via H2O2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.

astro-ph↗

arXiv2007

H-atom bombardment of CO2, HCOOH and CH3CHO containing ices

Context: Hydrogenation reactions are expected to be among the most important surface reactions on interstellar ices. However, solid state astrochemical laboratory data on reactions of H-atoms with common interstellar ice constituents are largely lacking. Aims: The goal of our laboratory work is to determine whether and how carbon dioxide (CO2), formic acid (HCOOH) and acetaldehyde (CH3CHO) react with H-atoms in the solid state at low temperatures and to derive reaction rates and production yields. Methods: Pure CO2, HCOOH and CH3CHO interstellar ice analogues are bombarded by H-atoms in an ultra-high vacuum experiment. The ices are monitored by reflection absorption infrared spectroscopy and the reaction products are detected in the gas phase through temperature programmed desorption to determine the destruction and formation yields as well as the corresponding reaction rates. Results: Within the sensitivity of our set-up we conclude that H-atom bombardment of pure CO2 and HCOOH ice does not result in detectable reaction products. The upper limits on the reaction rates are less or equal to 7e(-17) cm^2/s which make it unlikely that these species play a major role in the formation of more complex organics in interstellar ices due to reactions with H-atoms. In contrast, CH3CHO does react with H-atoms. At most 20% is hydrogenated to ethanol (C2H5OH) and a second reaction route leads to the break-up of the C-C bond to form solid state CH4 (~20%) as well as H2CO and CH3OH (15-50%). The methane production yield is expected to be equal to the summed yield of H2CO and CH3OH and therefore CH4 most likely evaporates partly after formation due to the high exothermicity of the reaction. The reaction rates for CH3CHO destruction depend on ice temperature and not on ice thickness.

astro-ph↗

arXiv2007

Infrared spectroscopy of HCOOH in interstellar ice analogues

Context: HCOOH is one of the more common species in interstellar ices with abundances of 1-5% with respect to solid H2O. Aims: This study aims at characterizing the HCOOH spectral features in astrophysically relevant ice mixtures in order to interpret astronomical data. Methods: The ices are grown under high vacuum conditions and spectra are recorded in transmission using a Fourier transform infrared spectrometer. Pure HCOOH ices deposited at 15 K and 145 K are studied, as well as binary and tertiary mixtures containing H2O, CO, CO2 and CH3OH. The mixture concentrations are varied from 50:50% to ~10:90% for HCOOH:H2O. Binary mixtures of HCOOH:X and tertiary mixtures of HCOOH:H2O:X with X = CO, CO2, and CH3OH, are studied for concentrations of ~10:90% and ~7:67:26%, respectively. Results: Pure HCOOH ice spectra show broad bands which split around 120 K due to the conversion of a dimer to a chain-structure. Broad single component bands are found for mixtures with H2O. Additional spectral components are present in mixtures with CO, CO2 and CH3OH. The resulting peak position, full width at half maximum and band strength depend strongly on ice structure, temperature, matrix constituents and the HCOOH concentration. Comparison of the solid HCOOH 5.9, 7.2, and 8.1 micron features with astronomical data toward the low mass source HH 46 and high mass source W 33A shows that spectra of binary mixtures do not reproduce the observed ice features. However, our tertiary mixtures especially with CH3OH match the astronomical data very well. Thus interstellar HCOOH is most likely present in tertiary or more complex mixtures with H2O, CH3OH and potentially also CO or CO2, providing constraints on its formation.

astro-ph↗

arXiv2007

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.

astro-ph↗