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Aliphatics prevail in asteroids, comets, meteorites and other bodies in our solar system. They are also found in the interstellar and circumstellar media both in gas-phase and in dust grains. Among aliphatics, linear alkanes (n-CnH2n+2) are known to survive in carbonaceous chondrites in hundreds to thousands of parts per billion, encompassing sequences from CH4 to n-C31H64. Despite being systematically detected, the mechanism responsible for their formation in meteorites has yet to be identified. Based on advanced laboratory astrochemistry simulations, we propose a gas-phase synthesis mechanism for n-alkanes starting from carbon and hydrogen under conditions of temperature and pressure that mimic those found in carbon-rich circumstellar envelopes. We characterize the analogs generated in a customized sputter gas aggregation source using a combination of atomically precise scanning tunneling microscopy, non-contact atomic force microscopy and ex-situ gas chromatography-mass spectrometry. Within the formed carbon nanostructures, we identify the presence of n-alkanes with sizes ranging from n-C8H18 to n-C32H66. Ab-initio calculations of formation free energies, kinetic barriers, and kinetic chemical network modelling lead us to propose a gas-phase growth mechanism for the formation of large n-alkanes based on methyl-methylene addition (MMA). In this process, methylene serves as both a reagent and a catalyst for carbon chain growth. Our study provides evidence of an aliphatic gas-phase synthesis mechanism around evolved stars and provides a potential explanation for its presence in interstellar dust and meteorites. Extraterrestrial organic matter found in meteorites may hold a unique record of its synthesis, and chemical and thermal alterations in the parent body, however, the origin of such aliphatics remains enigmatic. Here, the authors propose sequential gas-phase methyl-methylene addition growth of n-C8H18 to n-C32H66 alkanes based on a series of sputter gas aggregation source experiments and DFT calculations. [ABSTRACT FROM AUTHOR]

The hydrogen (H) dimer structuresformed upon room-temperatureH adsorption on single layer graphene (SLG) grown on SiC(0001) areaddressed using a combined theoretical–experimental approach.Our study includes density functional theory (DFT) calculations forthe full (6√3 × 6√3)R30° unit cell of theSLG/SiC(0001) substrate and atomically resolved scanning tunnelingmicroscopy images determining simultaneously the graphene latticeand the internal structure of the H adsorbates. We show that H atomsnormally group in chemisorbed coupled structures of different sizesand orientations. We make an atomic scale determination of the moststable experimental geometries, the small dimers and ellipsoid-shapedfeatures, and we assign them to hydrogen adsorbed in para dimers andortho dimers configuration, respectively, through comparison withthe theory. [ABSTRACT FROM AUTHOR]

Sykes et al. [Proa Natl. Acad. Sei. U.S.A. 102, 17907 (2005)] have reported how electrons injected from a scanning tunneling microscope modify the diffusion rates of H buried beneath Pd(lll). A key point in that experiment is the symmetry between positive and negative voltages for H extraction, which is difficult to explain in view of the large asymmetry in Pd between the electron and hole densities of states. Combining concepts from the theory of ballistic electron microscopy and electron-phonon scattering we show that H diffusion is driven by the s-band electrons only, which explains the observed symmetry. [ABSTRACT FROM AUTHOR]