4,017 research outputs found
Semiclassical and quantum polarons in crystaline acetanilide
Crystalline acetanilide is a an organic solid with peptide bond structure
similar to that of proteins. Two states appear in the amide I spectral region
having drastically different properties: one is strongly temperature dependent
and disappears at high temperatures while the other is stable at all
temperatures. Experimental and theoretical work over the past twenty five years
has assigned the former to a selftrapped state while the latter to an extended
free exciton state. In this article we review the experimental and theoretical
developments on acetanilide paying particular attention to issues that are
still pending. Although the interpretation of the states is experimentally
sound, we find that specific theoretical comprehension is still lacking. Among
the issues that that appear not well understood is the effective dimensionality
of the selftrapped polaron and free exciton states.Comment: 28 pages 13 figure
Generation of tunable subpicosecond light pulses in the midinfrared between 4.5 and 11.5 Mum
Stable subpicosecond infrared pulses in the spectral region of 4.5-11.5 ,um are generated by difference-frequency
mixing in AgGaS2. The system uses femtosecond pulses from a Ti:sapphire regenerative amplifier and from a
tunable traveling-wave dye laser. The infrared pulses have a duration of 400 fs, an energy of more than 10 nJ,
and a repetition rate of 1 kHz
Highly sensitive multichannel spectrometer for subpicosecond spectroscopy in the midinfrared
A spectrometer system is presented for time-resolved probing in the midinfrared between 5 and 11 /tLmw ith a
temporal resolution of better than 400 fs. Multichannel detection with HgCdTe detector arrays consisting of
ten elements in combination with a high repetition rate permits one to record weak absorbance changes with an
accuracy of 0.1 mOD
On the trophic fate of Phaeocystis pouchetii: VII. Sterols and fatty acids reveal sedimentation of Phaocystis-derived organic matter via krill fecal strings
As part of a joint project on the fate of phytoplankton in Balsfjorden in Northern Norway, we investigated the trophic fate and sedimentation potential of Phaeocystis pouchetii by tracing the transition of biomarker patterns from a phytoplankton bloom to sediment traps and during a gut passage experiment. The phytoplankton biomass during the spring bloom 1996 was dominated by colonial P. pouchetii (ca. 85 %) and four members of the diatom family Thalassiosiraceae (ca. 10%). Particulate organic carbon in sediment traps largely consisted of fecal material from the Arctic krill Thysanoessa sp.. Sterol and fatty acid biomarker patterns in the phytoplankton bloom could be reproduced by combining the individual biomarker patterns of the isolated phytoplankters P. pouchetii and Thalassiosira decipiens in a ratio of ca. 75:25. In a laboratory experiment, Arctic krill (Thysanoessa raschii) fed with similar efficiency on Phaeocystis colonies and the Thalassiosiraceae. During gut passage, the abundance of Thalassiosiraceae biomarkers in fecal strings increased relative to Phaeocystis biomarkers, while biomarkers from krill became dominant. This transition of biomarker patterns due to gut passage in T. raschii closely resembled the biomarker transition from the surface bloom to material in sediment traps at 40-170 m depth, which was mainly composed of krill fecal strings. We conclude that krill grazed efficiently on Phaeocystis colonies in Balsfjorden, and caused sedimentation of Phaeocystis-derived organic matter below the euphotic zone via fecal strings. Hence, both transfer to higher trophic levels and sedimentation of Phaeocystis-derived organic matter can be more effective than commonly believed
Perception of Bone Mineral Loss on Cone-Beam Computed Tomography (CBCT) & Digital Periapical (PA) Film: An Ex-Vivo Comparison
Using micro computed tomography (micro-CT) as a reference standard, the purpose of this investigation was to evaluate the influences of bone mineral loss and radiographic contrast in digital periapical (PA) film and cone-beam computed tomography (CBCT). Seven dog and human mandibles were sectioned, followed by modification via cortical bone drilling to create 248 lesions and/or trabecular bone acid demineralization to create 96 coronal slice pairs. Pretreatment and post-treatment radiographs (PA, CBCT, and/or micro-CT) were taken for lesion rating/perception in addition to two-dimensional (area) and three-dimensional (volumetric) measurements of 50 drilling lesions and 20 coronal slice pairs. 4-7% bone mineral loss is needed for radiographic perception of cortical bone drilling lesions (rating ≥3), and at least 17.34% bone mineral loss is needed for trabecular bone acid demineralization perception. Radiographic contrast indices were difficult to ascertain and may be related to regional differences and lesion anatomical location. This study suggests that even though %BML is a major factor in radiographic lesion perception, additional factors of regional and anatomical contrast influence lesion rating. With a limited sample size, further testing is needed to investigate cortical bone lesion perception differences between dog and human mandibles, to quantify radiographic contrast, and to identify more precise trabecular bone demineralization thresholds
Barrier crossing to the small Holstein polaron regime
We investigate the dimensionality effects of the Holstein polaron from the
fully quantum regime, where the crossover between large and small polaron
solutions is known to be continuous in all dimensions, into the limit described
by the semiclassical Discrete Nonlinear Schr\"odinger (DNLS) Equation, where
the crossover is continuous in 1D but discontinuous in higher dimensions. We
use exact numerics on one hand and a two variable parametrization of the
Toyozawa ansatz on the other in order to probe the crossover region in all
parameter regimes. We find that a barrier appears also in 1D separating the two
types of solutions, seemingly in contradiction to the common paradigm for the
DNLS according to which the crossover is barrier-free. We quantify the polaron
behavior in the crossover region as a function of the exciton overlap and find
that the barrier remains small in 1D and tunnelling through it is not
rate-limiting.Comment: 4 pages with 4 figure
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