1,265 research outputs found
Coupled Electronic and Nuclear Motions during Azobenzene Photoisomerization Monitored by Ultrafast Electron Diffraction
Ultrafast electron diffraction is a powerful technique that can resolve molecular structures with femtosecond and angstrom resolutions. We demonstrate theoretically how it can be used to monitor conical intersection dynamics in molecules. Specific contributions to the signal are identified which vanish in the absence of vibronic coherence and offer a direct window into conical intersection paths. A special focus is on hybrid scattering from nuclei and electrons, a process that is unique to electron (rather than X-ray) diffraction and monitors the strongly coupled nuclear and electronic motions in the vicinity of conical intersections. An application is made to the cis to trans isomerization of azobenzene, computed with exact quantum dynamics wavepacket propagation in a reactive two-dimensional nuclear space
Resonant electron transmission through a finite quantum spin chain
Electron transport in a finite one dimensional quantum spin chain (with
ferromagnetic exchange) is studied within an exchange Hamiltonian. Spin
transfer coefficients strongly depend on the sign of the exchange
constant. For a ferromagnetic coupling, they exhibit a novel resonant pattern,
reflecting the salient features of the combined electron-spin system. Spin-flip
processes are inelastic and feasible at finite voltage or at finite
temperature.Comment: 4 pages including 4 .eps figure
Helping education undergraduates to use appropriate criteria for evaluating accounts of motivation
The aim of the study was to compare students in a control group with those in a treatment group with respect to evaluative comments on psychological accounts of motivation. The treatment group systematically scrutinized the nature and interpretation of evidence that supported different accounts, and the assumptions, logic, coherence and clarity of accounts. Content analysis of 74 scripts (using three categories) showed that the control group students made more assertions than either evidential or evaluative points, whereas the treatment group used evaluative statements as often as they used assertion. The findings provide support for privileging activities that develop understanding of how knowledge might be contested, and suggest a need for further research on pedagogies to serve this end. The idea is considered that such understanding has a pivotal role in the development of critical thinking
Experienced tutors' deployment of thinking skills and what might be entailed in enhancing such skills
In the context of research that reports weaknesses in adults' critical thinking skills, the primary aim was to examine adults' use of critical thinking skills that are described in taxonomies and to identify areas for development. Position papers written by an opportunity sample of 32 experienced adult educators formed the data for a descriptive sample survey design intended to reveal participants' use of critical thinking skills. Each 6000-word paper was written during a development programme that supported such skills. A content analysis of the papers revealed that when participants drew on personal and published ideas about learning to derive their proposals for change, they accepted the ideas uncritically, thereby implying that they might find it difficult to help learners to examine ideas critically. The evidence supports research that implies that critical thinking skills are unlikely to develop unless overall course design privileges the development of epistemological understanding (King and Kitchener 1994, Kuhn 1999). A fundamental assumption underlying the study is that this understanding influences effective citizenship and personal development, as well as employability. A proposition that merits attention in future research is that the development of epistemological understanding is largely neglected in current curricula in formal post-16 education
BMQ
BMQ: Boston Medical Quarterly was published from 1950-1966 by the Boston University School of Medicine and the Massachusetts Memorial Hospitals
A New Approach to Measuring Estrogen Exposure and Metabolism in Epidemiologic Studies
Endogenous estrogen plays an integral role in the etiology of breast and endometrial cancer, and conceivably ovarian cancer. However, the underlying mechanisms and the importance of patterns of estrogen metabolism and specific estrogen metabolites have not been adequately explored. Long-standing hypotheses, derived from laboratory experiments, have not been tested in epidemiologic research because of the lack of robust, rapid, accurate measurement techniques appropriate for large-scale studies. We have developed a stable isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS(2)) method that can measure concurrently all 15 estrogens and estrogen metabolites (EM) in urine and serum with high sensitivity (level of detection=2.5-3.0fmol EM/mL serum), specificity, accuracy, and precision [laboratory coefficients of variation (CV\u27s) \u3c or =5% for nearly all EM]. The assay requires only extraction, a single chemical derivatization, and less than 0.5mL of serum or urine. By incorporating enzymatic hydrolysis, the assay measures total (glucuronidated+sulfated+unconjugated) EM. If the hydrolysis step is omitted, the assay measures unconjugated EM. Interindividual differences in urinary EM concentrations (pg/mL creatinine), which reflect total EM production, were consistently large, with a range of 10-100-fold for nearly all EM in premenopausal and postmenopausal women and men. Correlational analyses indicated that urinary estrone and estradiol, the most commonly measured EM, do not accurately represent levels of total urinary EM or of the other EM. In serum, all 15 EM were detected as conjugates, but only 5 were detected in unconjugated form. When we compared our assay methods with indirect radioimmunoassays for estrone, estradiol, and estriol and enzyme-linked immunosorbent assays for 2-hydroxyestrone and 16alpha-hydroxyestrone, ranking of individuals agreed well for premenopausal women [Spearman r (r(s))=0.8-0.9], but only moderately for postmenopausal women (r(s)=0.4-0.8). Our absolute readings were consistently lower, especially at the low concentrations characteristic of postmenopausal women, possibly because of improved specificity. We are currently applying our EM measurement techniques in several epidemiologic studies of premenopausal and postmenopausal breast cancer
Centerscope
Centerscope, formerly Scope, was published by the Boston University Medical Center "to communicate the concern of the Medical Center for the development and maintenance of improved health care in contemporary society.
Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of FineâParticle GrainâSize Variations
Fineâgrained, Tiâpoor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grainâsize variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellierâtype paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequencyâdependent susceptibility, and thermomagnetic data show that sample grainâsize distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 ÎŒT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional qualityâcontrol tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermalâchemical remanent magnetization if microcrystal growth continued at T \u3c Tc and/or the section is affected by postâemplacement vaporâphase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction
Current collection to long conductors with wide geometries for bare electrodynamic tether applications - A laboratory update
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77101/1/AIAA-2002-1123-228.pd
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