Dust as interstellar catalyst I. Quantifying the chemical desorption process

Context. The presence of dust in the interstellar medium has profound consequences on the chemical composition of regions where stars are forming. Recent observations show that many species formed onto dust are populating the gas phase, especially in cold environments where UV and CR induced photons do not account for such processes. Aims. The aim of this paper is to understand and quantify the process that releases solid species into the gas phase, the so-called chemical desorption process, so that an explicit formula can be derived that can be included into astrochemical models. Methods. We present a collection of experimental results of more than 10 reactive systems. For each reaction, different substrates such as oxidized graphite and compact amorphous water ice are used. We derive a formula to reproduce the efficiencies of the chemical desorption process, which considers the equipartition of the energy of newly formed products, followed by classical bounce on the surface. In part II we extend these results to astrophysical conditions. Results. The equipartition of energy describes correctly the chemical desorption process on bare surfaces. On icy surfaces, the chemical desorption process is much less efficient and a better description of the interaction with the surface is still needed. Conclusions. We show that the mechanism that directly transforms solid species to gas phase species is efficient for many reactions.Comment: Accepted for publication in A&

Composite pulses in Hyper-Ramsey spectroscopy for the next generation of atomic clocks

The next generation of atomic frequency standards based on an ensemble of neutral atoms or a single-ion will provide very stringent tests in metrology, applied and fundamental physics requiring a new step in very precise control of external systematic corrections. In the proceedings of the 8th Symposium on Frequency Standards and Metrology, we present a generalization of the recent Hyper-Ramsey spectroscopy with separated oscillating fields using composites pulses in order to suppress field frequency shifts induced by the interrogation laser itself. Sequences of laser pulses including specific selection of phases, frequency detunings and durations are elaborated to generate spectroscopic signals with a strong reduction of the light-shift perturbation by off resonant states. New optical clocks based on weakly allowed or completely forbidden transitions in atoms, ions, molecules and nuclei will benefit from these generalized Ramsey schemes to reach relative accuracies well below the 10$^{-18}$ level.Comment: accepted as proceedings of the 8th Symposium on Frequency Standards and Metrology (Potsdam Germany, 12-16 october 2015

Oxygen diffusion and reactivity at low temperature on bare amorphous olivine-type silicate

The mobility of O atoms at very low temperatures is not generally taken into account, despite O diffusion would add to a series of processes leading to the observed rich molecular diversity in space. We present a study of the mobility and reactivity of O atoms on an amorphous silicate surface. Our results are in the form of RAIRS and temperature-programmed desorption spectra of O2 and O3 produced via two pathways: O + O and O2 + O, investigated in a submonolayer regime and in the range of temperature between 6.5 and 30 K. All the experiments show that ozone is formed efficiently on silicate at any surface temperature between 6.5 and 30 K. The derived upper limit for the activation barriers of O + O and O2 + O reactions is 150 K/kb. Ozone formation at low temperatures indicates that fast diffusion of O atoms is at play even at 6.5 K. Through a series of rate equations included in our model, we also address the reaction mechanisms and show that neither the Eley Rideal nor the Hot atom mechanisms alone can explain the experimental values. The rate of diffusion of O atoms, based on modeling results, is much higher than the one generally expected, and the diffusive process proceeds via the Langmuir-Hinshelwood mechanism enhanced by tunnelling. In fact, quantum effects turn out to be a key factor that cannot be neglected in our simulations. Astrophysically, efficient O3 formation on interstellar dust grains would imply the presence of huge reservoirs of oxygen atoms. Since O3 is a reservoir of elementary oxygen, and also of OH via its hydrogenation, it could explain the observed concomitance of CO2 and H2O in the ices.Comment: 28 pages, 14 figure

Propagation of heavy baryons in heavy-ion collisions

The drag and diffusion coefficients of heavy baryons ($\Lambda_c$ and $\Lambda_b$) in the hadronic phase created in the latter stage of the heavy-ion collisions at RHIC and LHC energies have been evaluated recently. In this work we compute some experimental observables, such as the nuclear suppression factor $R_{AA}$ and the elliptic flow $v_2$ of heavy baryons at RHIC and LHC energies, highlighting the role of the hadronic phase contribution to these observables, which are going to be measured at Run 3 of LHC. For the time evolution of the heavy quarks in the QGP and heavy baryons in the hadronic phase we use the Langevin dynamics. For the hadronization of the heavy quarks to heavy baryons we employ Peterson fragmentation functions. We observe a strong suppression of both the $\Lambda_c$ and $\Lambda_b$. We find that the hadronic medium has a sizable impact on the heavy-baryon elliptic flow whereas the impact of hadronic medium rescattering is almost unnoticeable on the nuclear suppression factor. We evaluate the $\Lambda_c/D$ ratio at RHIC and LHC. We find that $\Lambda_c/D$ ratio remain unaffected due to the hadronic phase rescattering which enable it as a nobel probe of QGP phase dynamics along with its hadronization.Comment: 22 pages, 15 figure

First record of Sphaeralcea bonariensis (Cav.) Griseb. (Malvaceae) as a casual alien species in the Mediterranean area

Sphaeralcea bonariensis (Cav.) Griseb. is here reported for the first time as alien species in Italy and in the Mediterranean area. It is a perennial shrub native to Southern America, where it is often considered a ruderal species frequent in cultivated areas. It was found in a population of a few individuals within Opuntia ficus-indica groves of Rocca Palumba territory (Western Sicily). Actually, due to short-term observational period and the low number of plants, Sphaeralcea bonariensis should be considered as a casual species in Italy. The ecological characteristics of the growing stand and the consistency of the population, as well as the possible causes of its introduction, are briefly discussed

On optical activity of Er⁺³ ions in Si-rich SiO₂ waveguides

Photoluminescence spectroscopy was used to explore the optical activity of Er³⁺ ions in Si-rich SiO₂waveguides prepared by ion implantation. Measurements were performed for a series of materials characterized by different Si excess levels, Er concentrations, and annealing temperatures. The highest fraction of optically active Er³⁺ ions which can be efficiently activated by nonresonant pumping was found to be 2.6%. This was realized in a waveguide with an Er concentration of [Er]=10¹⁸cm⁻³ and Si excess of 20%, annealed at 900°C. This optical activity level is insufficient to realize optical gain. It is therefore clear that further material improvement is needed before optical amplification in SiO₂:Er matrices sensitized by Si nanocrystals/nanoclusters can be achieved

A phenobarbital overdose: a case report

Background: Phenobarbital is a long-acting barbiturate, responsible for many cases of poisoning, from unintentional overdose or attempted suicide. We report a case of phenobarbital overdose in a patient with history of depression. Patients and Methods: A 60 year old woman was admitted to our Internal Medicine Unit for drowsiness, irritability, difficulties in the maintenance of an upright position, dysphasia and weakness. She was suffering from depression and epilepsy and treated with phenobarbital 150 mg/die. Results: At the admittance, she had high fever and neck stiffness; phenobarbital serum levels were 71.2 mcg/ml (3 times u.n.l.); aminotransferases were 12-17u.n.l. Arterial blood pressure was 80/50 mmHg. An inflammatory meningeal process was excluded by lumbar puncture; a brain and spinal cord CT scan excluded spine bone lesions and ischemic stroke. In the suspect of an overdose, a protocol of urine alkalinization was applied resulting in a reduction of phenobarbital levels below the therapeutic range in about 6 days, with state of consciousness, cognitive and behavioral functions improvement. A rapid normalization in aminotransferases levels was noted and serology for hepatitis viruses (HAV, HBV, CMV, EBV, HSV) resulted negative. Conclusions: In our patient phenobarbital was responsible for stupor, hypotension, hypertonicity and aminotransferases elevation, whereas fever was due to a concomitant pulmonary inflammatory process resolved after antibiotic therapy. Despite the use of these drugs has been progressively reduced, the number overdose reports remains still hig

Acetaldehyde binding energies: a coupled experimental and theoretical study

Acetaldehyde is one of the most common and abundant gaseous interstellar complex organic molecules, found in cold and hot regions of the molecular interstellar medium. Its presence in the gas-phase depends on the chemical formation and destruction routes, and its binding energy (BE) governs whether acetaldehyde remains frozen onto the interstellar dust grains or not. In this work, we report a combined study of the acetaldehyde BE obtained via laboratory TPD (Temperature Programmed Desorption) experiments and theoretical quantum chemical computations. BEs have been measured and computed as a pure acetaldehyde ice and as mixed with both polycrystalline and amorphous water ice. Both calculations and experiments found a BE distribution on amorphous solid water that covers the 4000--6000 K range, when a pre-exponential factor of $1.1\times 10^{18}s^{-1}$ is used for the interpretation of the experiments. We discuss in detail the importance of using a consistent couple of BE and pre-exponential factor values when comparing experiments and computations, as well as when introducing them in astrochemical models. Based on the comparison of the acetaldehyde BEs measured and computed in the present work with those of other species, we predict that acetaldehyde is less volatile than formaldehyde, but much more than water, methanol, ethanol, and formamide. We discuss the astrochemical implications of our findings and how recent astronomical high spatial resolution observations show a chemical differentiation involving acetaldehyde, which can easily explained as due to the different BEs of the observed molecules.Comment: 12 pages, 6 figure