Context. 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 of the hydrogenation of CO ices provided controversial results about the formation rate of methanol. Aims. 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. To fully understand the laboratory data, the results are interpreted theoretically by means of continuous-time, random-walk Monte Carlo simulations. Methods. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with H atoms at room temperature. The reaction of CO + H into H 2 CO and subsequently CH 3 OH 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 according to their mass spectra and to determine their abundance. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Results. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (T = 12−20 K). We confirm that the discrepancy between the two Japanese studies is caused mainly by a difference in the applied hydrogen atom flux, as proposed by Hidaka and coworkers. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. Temperature-dependent reaction barriers and diffusion rates are inferred using a Monte Carlo physical chemical model. The model is extended to interstellar conditions to compare with observational H 2 CO/CH 3 OH data.

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, O 2 , or O 3 on icy grains, 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 O 2 ice is bombarded by H or D atoms under ultrahigh 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 H 2 O (D 2 O) with a rate that is surprisingly independent of temperature. This formation route converts O 2 into H 2 O via H 2 O 2 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.

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Lyman α), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (A 1 Π at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10 −5 molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower (< 10 −6 molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH 3 OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH 3 , OH, H 2 CO and CH 3 O/CH 2 OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH 3 O, therefore opening new gas phase chemical routes for the formation of complex molecules.

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, O 2 , and O 3 . In a previous paper [Ioppolo et al., Astrophys. J, 2008, 686, 1474 we discussed an unexpected nonstandard zeroth order H 2 O 2 production behaviour in O 2 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 O 2 surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O 2 deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O 2 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+O 2 ) or less efficient (H+OH and H 2 +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. Laboratory experiments show that the formation of water in space is much more complex (green + red arrows) than previously thought (just green).Due to a copyright agreement we are not allowed to publish the full paper on arXiv.org. Please look here for the paper. We apologise for any inconvenience.

Context. Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims. N 2 and O 2 are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure N 2 and O 2 thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic N 2 and O 2 isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results. N 2 photodesorption mainly occurs through excitation of the b 1 Π u state and subsequent desorption of surface molecules. The observed vibronic structure in the N 2 photodesorption spectrum, together with the absence of N 3 formation, supports that the photodesorption mechanism of N 2 is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, O 2 photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions. Photodesorption rates of N 2 and O 2 integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between 10 −3 and 10 −2 photodesorbed molecules per incoming photon.

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, O 2 , O 3 channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H 2 O production behavior in O 2 ice hydrogenation experiments compared to the first-order H 2 CO and CH 3 OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O 2 ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O 2 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 O 2 ice. We conclude that the competition between reaction and diffusion of the H atoms into the O 2 ice explains the unexpected H 2 O and H 2 O 2 formation behavior. In addition, we show that the proposed O 2 hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O 3 in the ice upon H-atom exposure proves that the O 2 channel is not an isolated route. Furthermore, the addition of H 2 molecules is found not to have a measurable effect on the O 2 reaction channel.The penetration depth of cold H atoms into solid oxygen is affected by the competition between reaction and diffusion and increases with the temperature.Due to a copyright agreement we are not allowed to publish the full paper on arXiv.org. Please look here for the paper. We apologise for any inconvenience.

Carbon monoxide is after H(2) the most abundant molecule identified in the interstellar medium (ISM), and is used as a major tracer for the gas phase physical conditions. Accreted at the surface of water-rich icy grains, CO is considered to be the starting point of a complex organic--presumably prebiotic--chemistry. Non-thermal desorption processes, and especially photodesorption by UV photons, are seen as the main cause that drives the gas-to-ice CO balance in the colder parts of the ISM. The process is known to be efficient and wavelength-dependent, but, the underlying mechanism and the physical-chemical parameters governing the photodesorption are still largely unknown. Using monochromatized photons from a synchrotron beamline, we reveal that the molecular mechanism responsible for CO photoejection is an indirect, (sub)surface-located process. The local environment of the molecules plays a key role in the photodesorption efficiency, and is quenched by at least an order of magnitude for CO interacting with a water ice surface.