Resonant Lifetime of Core-Excited Organic Adsorbates from First Principles

We investigate by first-principles simulations the resonant electron-transfer lifetime from the excited state of an organic adsorbate to a semiconductor surface, namely isonicotinic acid on rutile TiO$_2$(110). The molecule-substrate interaction is described using density functional theory, while the effect of a truly semi-infinite substrate is taken into account by Green's function techniques. Excitonic effects due to the presence of core-excited atoms in the molecule are shown to be instrumental to understand the electron-transfer times measured using the so-called core-hole-clock technique. In particular, for the isonicotinic acid on TiO$_2$(110), we find that the charge injection from the LUMO is quenched since this state lies within the substrate band gap. We compute the resonant charge-transfer times from LUMO+1 and LUMO+2, and systematically investigate the dependence of the elastic lifetimes of these states on the alignment among adsorbate and substrate states.Comment: 24 pages, 6 figures, to appear in Journal of Physical Chemistry

Ancient evolutionary origin of vertebrate enteric neurons from trunk-derived neural crest

The enteric nervous system of jawed vertebrates arises primarily from vagal neural crest cells that migrate to the foregut and subsequently colonize and innervate the entire gastrointestinal tract. Here we examine development of the enteric nervous system in the basal jawless vertebrate the sea lamprey (Petromyzon marinus) to gain insight into its evolutionary origin. Surprisingly, we find no evidence for the existence of a vagally derived enteric neural crest population in the lamprey. Rather, labelling with the lipophilic dye DiI shows that late-migrating cells, originating from the trunk neural tube and associated with nerve fibres, differentiate into neurons within the gut wall and typhlosole. We propose that these trunk-derived neural crest cells may be homologous to Schwann cell precursors, recently shown in mammalian embryos to populate post-embryonic parasympathetic ganglia, including enteric ganglia. Our results suggest that neural-crest-derived Schwann cell precursors made an important contribution to the ancient enteric nervous system of early jawless vertebrates, a role that was largely subsumed by vagal neural crest cells in early gnathostomes

High-Resolution Near-Edge X-ray Absorption Fine Structure Study of Condensed Polyacenes

International audienceWe present a systematic study of high-resolution NEXAFS spectra of well-ordered films of condensed benzene and polyacenes, namely naphthalene, anthracene, tetracene, and pentacene. An increased spectral complexity with increasing molecular size is observed: NEXAFS features decrease in intensity and moves to lower photon energy (redshift), as the size of the aromatic system grows. Moreover, a second group of transitions arises. The dichroism in the C K-edge spectra increases with the molecular size. While benzene molecules are randomly oriented, the polyacenes are preferentially lying flat with increasing molecular size. Vibrational fine structures coupled to the C 1s π* transitions are apparent for all investigated molecules. The energy position of the onset of the first resonance decreases from 284.86 eV for benzene to 283.26 eV for pentacene. Calculations of absolute band envelopes with TDDFT followed by the analysis of the transition densities were performed for the whole series of molecules, revealing the nature of the spectroscopic features