Optimizing a microwave gas ion source for continuous-flow accelerator mass spectrometry

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of American Institute of Physics for personal use, not for redistribution. The definitive version was published in Review of Scientific Instruments 83 (2012): 02B304, doi:10.1063/1.3656408.A 2.45 GHz microwave ion source coupled with a magnesium charge exchange canal (CxC) has been successfully adapted to a large acceptance radiocarbon accelerator mass spectrometry system at the National Ocean Sciences AMS Facility (NOSAMS), Woods Hole Oceanographic Institution. CO2 samples from various preparation sources are injected into the source through a glass capillary at 370 µl/min. Routine system parameters are about 120 - 140 µA of negative 12C current after the CxC, leading to about 400 14C counts per second for a modern sample and implying a system efficiency of 0.2%. While these parameters already allow us to perform high quality AMS analyses on large samples, we are working on ways to improve the output of the ion source regarding emittance and efficiency. Modeling calculations suggest modifications in the extraction triode geometry, shape and size of the plasma chamber could improve emittance and hence ion transport efficiency. Results of experimental tests of these modifications are presented.This work has been supported by the US National Science Foundation through Cooperative Agreement OCE-0753487

Le FORUM, Vol. 39 No. 4

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Recognizing Emotions in a Foreign Language

Expressions of basic emotions (joy, sadness, anger, fear, disgust) can be recognized pan-culturally from the face and it is assumed that these emotions can be recognized from a speaker's voice, regardless of an individual's culture or linguistic ability. Here, we compared how monolingual speakers of Argentine Spanish recognize basic emotions from pseudo-utterances ("nonsense speech") produced in their native language and in three foreign languages (English, German, Arabic). Results indicated that vocal expressions of basic emotions could be decoded in each language condition at accuracy levels exceeding chance, although Spanish listeners performed significantly better overall in their native language ("in-group advantage"). Our findings argue that the ability to understand vocally-expressed emotions in speech is partly independent of linguistic ability and involves universal principles, although this ability is also shaped by linguistic and cultural variables

Le FORUM, Vol. 34 No. 2

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Le FORUM, Vol. 38 No. 4

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Le Forum, Vol. 41 No. 1

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Le FORUM, Vol. 37 No. 3

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The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site. The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4,000–6,000 years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular data set of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50° N to 70° S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising Δ14C values from -229 to -495‰) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24 ka for the most persistent DOM fraction. First order kinetic degradation rates for 1,557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of >100 ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment (“island of stability”). These most persistent compounds encompass only a narrow range of the elemental ratios H/C (average of 1.17 ± 0.13), and O/C (average of 0.52 ± 0.10) and molecular masses (360 ± 28 and 497 ± 51 Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3 ± 3.3 μM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, indicating average surface water DOM ages of 4,920 ± 180 a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index IDEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate

Stimulus set size modulates the sex–emotion interaction in face categorization

Previous research has shown that invariant facial features—for example, sex—and variant facial features—for example, emotional expressions—interact during face categorization. The nature of this interaction is a matter of dispute, however, and has been reported as either asymmetrical, such that sex cues influence emotion perception but emotional expressions do not affect the perception of sex, or symmetrical, such that sex and emotion cues each reciprocally influence the categorization of the other. In the present research, we identified stimulus set size as the critical factor leading to this disparity. Using faces drawn from different databases, in two separate experiments we replicated the finding of a symmetrical interaction between face sex and emotional expression when larger sets of posers were used. Using a subset of four posers, in the same setups, however, did not provide evidence for a symmetrical interaction, which is also consistent with prior research. This pattern of results suggests that different strategies may be used to categorize aspects of faces that are encountered repeatedly