33 research outputs found
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
Enhanced mesoscopic fluctuations in the crossover between random matrix ensembles
In random-matrix ensembles that interpolate between the three basic ensembles
(orthogonal, unitary, and symplectic), there exist correlations between
elements of the same eigenvector and between different eigenvectors. We study
such correlations, using a remarkable correspondence between the interpolating
ensembles late in the crossover and a basic ensemble of finite size. In small
metal grains or semiconductor quantum dots, the correlations between different
eigenvectors lead to enhanced fluctuations of the electron-electron interaction
matrix elements which become parametrically larger than the non-universal
fluctuations.Comment: 4 pages, RevTeX; 3 figure
Surface formation of hcooh at low temperature
Contains fulltext :
92389.pdf (publisher's version ) (Open Access)7 p
Selective excitation of parabolic Stark states in ion-atom collisions via the Paul-trap mechanism
Surface formation of co(2) ice at low temperatures
Contains fulltext :
92388.pdf (publisher's version ) (Open Access)8 p
Double K-shell excitation of Li by 10.6-MeV/nucleon N7+ projectiles
International audienceSingle and double K-shell excitation of Li by 10.6-MeV/nucleon N7+ projectiles is investigated by measuring Li autoionization spectra in the energy range from 50 to 90 eV. The ratio for double K-shell excitation to single K-shell excitation is determined to be about 0.36%. The measured ratio for double-to-single K-shell excitation of Li is about a factor of 2 larger than calculations based on independent interactions between the projectile nucleus and each of the Li target electrons, indicating that the electron-electron interaction plays a dominant role in the double K-shell excitation of Li in these collisions
Total Synthesis of Solandelactones A and B
The total synthesis of solandelactones A and B is presented. The eastern cyclopropyl moiety was prepared following an exceptionally short chemoenzymatic approach whereas enantioselective synthesis of the western side-chain relied on the application of diastereomerically pure allyl boronates. The natural products solandelactones A and B were isolated in good overall yields following convergence of each eastern and western element by application of the Nozaki–Hiyama–Kishi reaction