557 research outputs found
Debunking \u3cem\u3eInstant Messenger\u3c/em\u3e Myths: Meeting Student Needs in a Digital Age
Experimental evidence for radiative attachment in astrochemistry from electron attachment to NCCCCN
Electron attachment to NCCCCN, dicyanoacetylene (2-butynedinitrile), has been observed. Metastable parent anions, NCCCCN_∗, with microsecond or longer lifetimes are formed close to 0 eV electron energy with a cross section of ≥0.25 2. The stability of NCCCCN suggests that radiative attachment to NCCCCN and similar _∗ °A linear carbon chain molecules may be an important mechanism for the formation of negatively charged molecular ions in astrophysical environments. CCCN_ and CN_ fragment anions are formed at ∼3 and ∼6 eV
Metastable anions of dinitrobenzene: resonances for electron attachment and kinetic energy release
Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB as well as several fragment anions. DNB, (DNB-H), (DNB-NO), (DNB-2NO), and (DNB-NO(2)) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels. (C) 2010 American Institute of Physics. [doi:10.1063/1.3514931
Electron-attachment rates for carbon-rich molecules in protoplanetary atmospheres: the role of chemical differences
The formation of anionic species in the interstellar medium from interaction
of linear molecules containing carbon, nitrogen and hydrogen as atomic
components (polyynes) with free electrons in the environment is modelled via a
quantum treatment of the collision dynamics. The ensuing integral cross
sections are employed to obtain the corresponding attachment rates over a broad
range of temperatures for the electrons. The calculations unequivocally show
that a parametrization form often employed for such rates yields a broad range
of values that turn out to be specific for each molecular species considered,
thus excluding using a unique set for the whole class of polyynes.Comment: accepted to be published on MNRA
(Meta-)stable reconstructions of the diamond(111) surface: interplay between diamond- and graphite-like bonding
Off-lattice Grand Canonical Monte Carlo simulations of the clean diamond
(111) surface, based on the effective many-body Brenner potential, yield the
Pandey reconstruction in agreement with \emph{ab-initio}
calculations and predict the existence of new meta-stable states, very near in
energy, with all surface atoms in three-fold graphite-like bonding. We believe
that the long-standing debate on the structural and electronic properties of
this surface could be solved by considering this type of carbon-specific
configurations.Comment: 4 pages + 4 figures, Phys. Rev. B Rapid Comm., in press (15Apr00).
For many additional details (animations, xyz files) see electronic supplement
to this paper at http://www.sci.kun.nl/tvs/carbon/meta.htm
High pressure effects on the photoluminescence intensity of sexithiophene single crystals
We report on the influence of interchain interactions on the light emission properties of a sexithiophene single crystal. The strength of the intermolecular interactions is controlled by applying hydrostatic pressure. The combined use of both steady-state and time-resolved photoluminescence techniques permits to show that the pressure-induced quenching of the photoluminescence is caused by a reduction of the radiative recombination rate
Distinct and Conserved Prominin-1/CD133–Positive Retinal Cell Populations Identified across Species
Besides being a marker of various somatic stem cells in mammals, prominin-1 (CD133) plays a role in maintaining the photoreceptor integrity since mutations in the PROM1 gene are linked with retinal degeneration. In spite of that, little information is available regarding its distribution in eyes of non-mammalian vertebrates endowed with high regenerative abilities. To address this subject, prominin-1 cognates were isolated from axolotl, zebrafish and chicken, and their retinal compartmentalization was investigated and compared to that of their mammalian orthologue. Interestingly, prominin-1 transcripts—except for the axolotl—were not strictly restricted to the outer nuclear layer (i.e., photoreceptor cells), but they also marked distinct subdivisions of the inner nuclear layer (INL). In zebrafish, where the prominin-1 gene is duplicated (i.e., prominin-1a and prominin-1b), a differential expression was noted for both paralogues within the INL being localized either to its vitreal or scleral subdivision, respectively. Interestingly, expression of prominin-1a within the former domain coincided with Pax-6–positive cells that are known to act as progenitors upon injury-induced retino-neurogenesis. A similar, but minute population of prominin-1–positive cells located at the vitreal side of the INL was also detected in developing and adult mice. In chicken, however, prominin-1–positive cells appeared to be aligned along the scleral side of the INL reminiscent of zebrafish prominin-1b. Taken together our data indicate that in addition to conserved expression of prominin-1 in photoreceptors, significant prominin-1–expressing non-photoreceptor retinal cell populations are present in the vertebrate eye that might represent potential sources of stem/progenitor cells for regenerative therapies
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers
We have measured the ratio, r = of the formation cross
section, of singlet () and triplet () excitons
from oppositely charged polarons in a large variety of -conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which depends linearly on ,
irrespective of the chain backbone structure. These results indicate that
-conjugated polymers have a clear advantage over small molecules in OLED
applications.Comment: 5 pages, 4 figure
Endogenous cholinergic inputs and local circuit mechanisms govern the phasic mesolimbic dopamine response to nicotine
Nicotine exerts its reinforcing action by stimulating nicotinic acetylcholine receptors (nAChRs) and boosting dopamine (DA) output from the ventral tegmental area (VTA). Recent data have led to a debate about the principal pathway of nicotine action: direct stimulation of the DAergic cells through nAChR activation, or disinhibition mediated through desensitization of nAChRs on GABAergic interneurons. We use a computational model of the VTA circuitry and nAChR function to shed light on this issue. Our model illustrates that the α4β2-containing nAChRs either on DA or GABA cells can mediate the acute effects of nicotine. We account for in vitro as well as in vivo data, and predict the conditions necessary for either direct stimulation or disinhibition to be at the origin of DA activity increases. We propose key experiments to disentangle the contribution of both mechanisms. We show that the rate of endogenous acetylcholine input crucially determines the evoked DA response for both mechanisms. Together our results delineate the mechanisms by which the VTA mediates the acute rewarding properties of nicotine and suggest an acetylcholine dependence hypothesis for nicotine reinforcement.Peer reviewe
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