UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments

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.Comment: 13 pages, 2 figures, 1 tabl

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

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

Indirect Ultraviolet Photodesorption from CO:N2 Binary Ices — An Efficient Grain-Gas Process

Ultraviolet (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 sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with 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 factor of a few (compared with 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.Astronom

Prothèses totales de genou, faut-il utiliser les clichés en stress ? (à propos d'une série de 58 prothèses totales de genou sur grande déformation en genu valgum)

CAEN-BU Médecine pharmacie (141182102) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Réactions de métathèse catalysée par le ruthénium, application à la synthèse de divinylcétones et de spirolactones (Mathieu Michaut)

Au cours de ce travail, nous nous sommes intéressés à la réaction de métathèse qui a connu un essor considérable durant ces dix dernières années. Dans une première partie, nous avons appliqué la réaction d'ouverture de cycle par métathèse croisée à plusieurs dérivés cétaliques de la cyclopropénone. Cette réaction d'ouverture est très sélective et permet d'accéder aux 1,4-divinylcétones protégées, avec une configuration majoritairement E au niveau de la double liaison disubstituée nouvellement formée. L'utilisation du citronellène ou d'énynes a, en outre, permis de mettre au point des réactions tandems d'ouverture / fermeture de cycle ou de double ouverture de cycle. Une étude complémentaire a permis d'appréhender les différents aspects de cette réaction complexe. Da ns un second temps, nous avons mis au point des systèmes rapides et efficaces de diverses spirolactones. Cette approche fait appel à une double condensation d'organosilanes allyliques ou d'organomagnésiens sur des anhydrides cycliques, suivie d'une métathèse cyclisante. D'autres spirolactones tricycliques ont pu être obtenues par condensation du 1,8-triméthylsilylocta-2,6-diène sur l'anhydride succinique, suivie d'une alkylation diastérosélective et d'une réaction de métathèse cyclisante. Enfin, l'action du chlorure de cyclooctadiényle ruthénium sur les lactones diallyliques entraîne une réaction de cycloisomérisation diastérosélective conduisant à des spirolactones comportant un motif exométhylène.AIX-MARSEILLE3-BU Sc.St Jérô (130552102) / SudocSudocFranceF

1,2,3-Triazole‑gold(I)-triethylphosphine derivatives of nutrients as new antimicrobials against antibiotic resistant Gram-positive pathogens

International audienceThe ongoing rise of bacterial resistance against antibiotics is an incentive to develop radically new antibacterial molecules. Innovative organo-gold(I) antibacterial compounds were synthesized by click chemistry with triethylphosphine‑gold(I) azides and alkyne-functionalized metabolites like amino acids, nicotinic acid and biotin. The resulting organo‑gold(I) compounds exhibited remarkable activities against a panel of Gram-positive pathogens (Staphyloccocus sp., Enterococcus sp. and Clostridium difficile) but failed to inhibit the growth of Gram-negative bacilli (e.g., Escherichia coli) under the conditions used (8 μg. mL−1 maximum)

Comparison of medetomidine-morphine and medetomidine-methadone for sedation, isoflurane requirement and postoperative analgesia in dogs undergoing laparoscopy.

OBJECTIVE To compare the effects of intravenous (IV) medetomidine-morphine and medetomidine-methadone on preoperative sedation, isoflurane requirements and postoperative analgesia in dogs undergoing laparoscopic surgery. STUDY DESIGN Randomized, crossover trial. ANIMALS Twelve adult Beagle dogs weighing 15.1 ± 4.1 kg. METHODS Dogs were administered medetomidine (2.5 μg kg(-1) ) IV 5 minutes before either methadone (MET) or morphine (MOR) (0.3 mg kg(-1) ) IV. Anaesthesia was induced with propofol, maintained with isoflurane in oxygen, and depth was clinically assessed and adjusted by an anaesthetist blinded to the treatment. Animals underwent laparoscopic abdominal biopsies. Sedation and nausea scores, pulse rate (PR), respiratory rate (fR ), noninvasive systolic arterial blood pressure (SAP), rectal temperature (RT) and pain scores were recorded before drug administration, 5 minutes after medetomidine injection and 10 minutes after opioid administration. Propofol dose, PR, fR , SAP, oesophageal temperature (TOES ), end-tidal carbon dioxide and end-tidal isoflurane concentration (Fe'Iso) were recorded intraoperatively. Pain scores, PR, fR , SAP and RT were recorded 10 minutes after extubation, every hour for 6 hours, then at 8, 18 and 24 hours. The experiment was repeated with the other drug 1 month later. RESULTS Nine dogs completed the study. After opioid administration and intraoperatively, PR, but not SAP, was significantly lower in MET. Fe'Iso was significantly lower in MET. Temperature decreased in both treatments. Pain scores were significantly higher in MOR at 3 hours after extubation, but not at other time points. Two dogs required rescue analgesia; one with both treatments and one in MOR. CONCLUSION AND CLINICAL RELEVANCE At the dose used, sedation produced by both drugs when combined with medetomidine was equivalent, while volatile anaesthetic requirements and PR perioperatively were lower with methadone. Postoperative analgesia was deemed to be adequate for laparoscopy with either protocol, although methadone provided better analgesia 3 hours after surgery