113 research outputs found
Use of the Frank sequence in pulsed EPR
The Frank polyphase sequence has been applied to pulsed EPR of triarylmethyl radicals at 256 MHz (9.1 mT magnetic field), using 256 phase pulses. In EPR, as in NMR, use of a Frank sequence of phase steps permits pulsed FID signal acquisition with very low power microwave/RF pulses (ca. 1.5 mW in the application reported here) relative to standard pulsed EPR. A 0.2 mM aqueous solution of a triarylmethyl radical was studied using a 16 mm diameter cross loop resonator to isolate the EPR signal detection system from the incident pulses
On the existence of a modal antinomy
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43821/1/11229_2004_Article_BF00636296.pd
Rapid-Scan EPR of Immobilized Nitroxides
X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293 K. Scan widths were 155 G with 13.4 kHz scan frequency for 14N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100 G with 20.9 kHz scan frequency for 15N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10 magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes
Rapid-Scan EPR of Immobilized Nitroxides
X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293 K. Scan widths were 155 G with 13.4 kHz scan frequency for 14N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100 G with 20.9 kHz scan frequency for 15N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10 magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes
Stochastic Dynamics of Lexicon Learning in an Uncertain and Nonuniform World
We study the time taken by a language learner to correctly identify the
meaning of all words in a lexicon under conditions where many plausible
meanings can be inferred whenever a word is uttered. We show that the most
basic form of cross-situational learning - whereby information from multiple
episodes is combined to eliminate incorrect meanings - can perform badly when
words are learned independently and meanings are drawn from a nonuniform
distribution. If learners further assume that no two words share a common
meaning, we find a phase transition between a maximally-efficient learning
regime, where the learning time is reduced to the shortest it can possibly be,
and a partially-efficient regime where incorrect candidate meanings for words
persist at late times. We obtain exact results for the word-learning process
through an equivalence to a statistical mechanical problem of enumerating loops
in the space of word-meaning mappings.Comment: 7 pages, 3 figures. Version 2 contains additional discussion and will
appear in Phys. Rev. Let
Linguistics and natural logic
Evidence is presented to show that the role of a generative grammar of a natural language is not merely to generate the grammatical sentences of that language, but also to relate them to their logical forms. The notion of logical form is to be made sense of in terms a ‘natural logic’, a logical for natural language, whose goals are to express all concepts capable of being expressed in natural language, to characterize all the valid inferences that can be made in natural language, and to mesh with adequate linguistic descriptions of all natural languages. The latter requirement imposes empirical linguistic constraints on natural logic. A number of examples are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43819/1/11229_2004_Article_BF00413602.pd
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