85 research outputs found
CO ice photodesorption: A wavelength-dependent study
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 (Ly-alpha),
where CO barely absorbs, compared to the high yields found at wavelengths
coinciding with transitions into the first electronic state of CO (singulet Pi
at 150 nm); the CO photodesorption rates depend strongly on the UV profiles
encountered in different star formation environments.Comment: 5 pages, 2 figures, published in ApJ
A new equipment for continuous measurement of methane production in a batch in vitro rumen system
A new rumen batch fermentation system that allows continuous measures of total gas (GP) and methane production (MP) was tested. The fermentation system is composed of glass bottles connected to gas counters (Ritter Apparatebau GmbH & Co. KG) and an infrared gas analyser that measures the methane concentration. The system allows direct and continuous measurement of GP and MP for accurate kinetic studies. The aim of the work was to test the rumen fermentation system and compare the GP and MP kinetics obtained. Barley meal (BM), alfalfa hay (AH), corn silage (CS), and soya bean hulls (SH) were used as substrates in four consecutive fermentation runs. Cumulative volumes of GP and MP and the percentage of methane on total GP were recorded continuously until 48 h and average values at 1 h intervals were fitted with an exponential model with a lag phase reaching a good fit (R2 > 0.992). GP and MP reached the highest plateau levels for SH (1836 and 370 ml, respectively; p < 0.01) and the lowest for AH (1000 and 233 ml, respectively). The remaining substrates showed intermediate values. MP kinetics showed a discrete lag phase (from 0.09 to 1.12 h), whereas it was equal to zero for the total GP (except for SH). The methane concentration in gas flowing increased rapidly at the beginning of fermentation (from 0.35 to 0.95 h−1) and reached a plateau after approximately 8–12 h. In conclusion, the rumen fermentation system evaluated generates methane data comparable to those reported in the literature and allows simple continuous measurement of methane release throughout fermentation
SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest
Article / Letter to editorSterrewach
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.Comment: 1 page, 1 figur
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.Comment: 1 page, 1 figur
Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
UV ice photodesorption is an important non-thermal desorption pathway in many
interstellar environments that has been invoked to explain observations of cold
molecules in disks, clouds and cloud cores. Systematic laboratory studies of
the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have
been performed at the DESIRS vacuum UV beamline of the synchrotron facility
SOLEIL. The photodesorption spectral analysis demonstrates that the
photodesorption process is indirect, i.e. the desorption is induced by a photon
absorption in sub-surface molecular layers, while only surface molecules are
actually desorbing. The photodesorption spectra of CO and N2 in binary ices
therefore depend on the absorption spectra of the dominant species in the
subsurface ice layer, which implies that the photodesorption efficiency and
energy dependence are dramatically different for mixed and layered ices
compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO
will effectively quench CO photodesorption, while enhancing N2 photodesorption
by a factors of a few (compared to the pure ices) when the ice is exposed to a
typical dark cloud UV field, which may help to explain the different
distributions of CO and N2H+ in molecular cloud cores. This indirect
photodesorption mechanism may also explain observations of small amounts of
complex organics in cold interstellar environments.Comment: 21 pages 5 figure
Photoionization spectroscopy of CH3C3N in the vacuum-ultraviolet range
International audienceUsing vacuum-ultraviolet (VUV) synchrotron radiation, threshold and dissociative photoionization of cyanopropyne (CH3C3N) in the gas phase have been studied from 86 000 cm−1 up to 180 000 cm−1 by recording Threshold-PhotoElectron Spectrum (TPES) and PhotoIon Yield (PIY). Ionization energies of the four lowest electronic states X̃+2E,Ã+2A1,B̃+2E and C̃+ of CH3C3N+ are derived from the TPES with a better accuracy than previously reported. The adiabatic ionization potential of CH3C3N is measured as 86872±20 cm−1. A description of the vibrational structure of these states is proposed leading to the first determination of the vibrational frequencies for most modes. The vibrational assignments of the X̃+ state are supported by density functional theory calculations. In addition, dissociative photoionization spectra have been recorded for several cationic fragments in the range 12–15.5 eV (96 790–125 000 cm−1) and they bring new information on the photophysics of CH3C3N+. Threshold energies for the cationic dissociative channels leading to CH2C3N+, CHC3N+, HC3H+, HCNH+ and CH3+ have been measured for the first time and are compared with quantum chemical calculations
Heat stress and feeding behaviour of dairy cows in late lactation
Heat stress is one of the most important problems that dairy cows have to face and the use of cooling systems is becoming more and more important. The first reaction that has the animal to cope with the environmental variations is to modify its behaviour. This study was aimed to investigate the effect of heat stress and a cooling system on the feeding behaviour of Italian Holstein Friesian dairy cows in late lactation. Two experiments were performed. In the first experiment, eight dairy cows were firstly kept 7 d under thermoneutral condition, and then under mild heat stress (temperature humidity index, THI, ranging between 72 and 78) for others 7 d. The second experiment consisted of 8 dairy cows used in a two-period cross-over design where the treatment was the use or not of a sprinkler system for cooling cows under mild heat stress. Cows were equipped with a noseband pressure sensor able to detect rumination and eating time, number of rumination and eating chews, number of rumination boluses and rumination intensity. Heat stress reduced rumination time, number of rumination chews and boluses (p <.05), and tended to reduce the number of eating chews (p <.10). Cooled cows increased rumination and eating time (p <.05), rumination intensity (p <.01), and the number of rumination and eating chews (p <.05). In conclusion, feeding behaviour was deeply influenced even by mild heat stress, which was effectively improved by the use of a sprinkler system.HIGHLIGHTS Mild heat stress reduced rumination time, number of rumination chews and boluses of dairy cows in late lactation Cooling cows with sprinklers was effective in alleviating heat stress in terms of feeding behaviour
Wavelength-Dependent UV Photodesorption of Pure and Ices
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: and 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 and 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 and isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: photodesorption mainly occurs through excitation of the state and subsequent desorption of surface molecules. The observed vibronic structure in the photodesorption spectrum, together with the absence of formation, supports that the photodesorption mechanism of is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of and integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between and photodesorbed molecules per incoming photon.Astronom
SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest
A new ultrahigh vacuum experiment is described to study atom and radical addition reactions in interstellar ice analogues for astronomically relevant temperatures. The new setup – SURFace REaction SImulation DEvice (SURFRESIDE2) – allows a systematic investigation of solid state pathways resulting in the formation of molecules of astrophysical interest. The implementation of a double beam line makes it possible to expose deposited ice molecules to different atoms and/or radicals sequentially or at the same time. Special efforts are made to perform experiments under fully controlled laboratory conditions, including precise atom flux determinations, in order to characterize reaction channels quantitatively. In this way, we can compare and combine different surface reaction channels with the aim to unravel the solid state processes at play in space. Results are constrained in situ by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer using reflection absorption infrared spectroscopy and temperature programmed desorption, respectively. The performance of the new setup is demonstrated on the example of carbon dioxide formation by comparing the efficiency through two different solid state channels (CO + OH → CO_2 + H and CO + O → CO_2) for which different addition products are needed. The potential of SURFRESIDE2 to study complex molecule formation, including nitrogen containing (prebiotic) compounds, is discussed
- …