2,911 research outputs found
Influence of a tight isotropic harmonic trap on photoassociation in ultracold homonuclear alkali gases
The influence of a tight isotropic harmonic trap on photoassociation of two
ultracold alkali atoms forming a homonuclear diatomic is investigated using
realistic atomic interaction potentials. Confinement of the initial atom pair
due to the trap leads to a uniform strong enhancement of the photoassociation
rate to most, but also to a strongly suppressed rate for some final states.
Thus tighter traps do not necessarily enhance the photoassociation rate. A
further massive enhancement of the rate is found for strong interatomic
interaction potentials. The details of this interaction play a minor role,
except for large repulsive interactions for which a sharp window occurs in the
photoassociation spectrum as is known from the trap-free case. A comparison
with simplified models describing the atomic interaction like the
pseudopotential approximation shows that they often provide reasonable
estimates for the trap-induced enhancement of the photoassociation rate even if
the predicted rates can be completely erroneous.Comment: 19 pages, 17 figure
Exploring the distribution and physiological roles of bacterial membrane lipids in the marine environment
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010Lipids have a legacy in the geologic record extending back to the Archaean.
Since the phylogenetic diversity of life is reflected in the structural diversity
of biomolecules, lipid biomarkers that are shown to be diagnostic of certain
organisms that carry out specific biochemical processes or that are
demonstrated to have unique physiological roles can be used to trace the
biogeochemical influence of bacteria in modern and ancient environments. In
this thesis I explore the application of two classes of bacterial membrane
lipids as biomarkers for marine biogeochemical processes in marine
environments: ladderanes and hopanoids. Through the detection of ladderane
lipids – biomarkers for anaerobic ammonium oxidizing (anammox) bacteria –
I demonstrate the presence and distribution of anammox bacteria in a
subterranean estuary. Through a survey of hopanoids in marine
environments and cultured marine cyanobacteria I show that hopanoids are
ubiquitous in the oceans and that their presence in ancient marine sediments
could provide information about biogeochemical processes in past
environments. Based on novel results demonstrating that hopanoids are
resistant to extraction by non-ionic detergent, I propose that they may play a
role in lipid ordering and the formation of putative lipid rafts in hopanoid-producing
bacteria.This work was supported by National Science Foundation grants to
R.E. Summons and T.I. Eglinton (ETBC 084990) an American Chemical
Society Petroleum Research Fund to R.E. Summons (ACS PRF 46838-AC2),
and a WHOI OLI Tropical Initiatives grant to T.I. Eglinton and J.
Waterbury. I received support from an NSF Graduate Research Fellowship,
Dan David Doctoral Fellowship, WHOI COI Student Fellowship, and a WHOI
Ocean Ventures Fund
Break-down of the single-active-electron approximation for one-photon ionization of the B state of H exposed to intense laser fields
Ionization, excitation, and de-excitation to the ground state is studied
theoretically for the first excited singlet state B of H
exposed to intense laser fields with photon energies in between about 3 eV and
13 eV. A parallel orientation of a linear polarized laser and the molecular
axis is considered. Within the dipole and the fixed-nuclei approximations the
time-dependent Schr\"odinger equation describing the electronic motion is
solved in full dimensionality and compared to simpler models. A dramatic
break-down of the single-active-electron approximation is found and explained
to be due to the inadequate description of the final continuum states.Comment: 9 pages, 4 figure
Accurate photoionisation cross section for He at non-resonant photon energies
The total single-photon ionisation cross section was calculated for helium
atoms in their ground state. Using a full configuration-interaction approach
the photoionisation cross section was extracted from the complex-scaled
resolvent. In the energy range from ionisation threshold to 59\,eV our results
agree with an earlier -spline based calculation in which the continuum is
box discretised within a relative error of in the non-resonant part of
the spectrum. Above the \He^{++} threshold our results agree on the other
hand very well to a recent Floquet calculation. Thus our calculation confirms
the previously reported deviations from the experimental reference data outside
the claimed error estimate. In order to extend the calculated spectrum to very
high energies, an analytical hydrogenic-type model tail is introduced that
should become asymptotically exact for infinite photon energies. Its
universality is investigated considering also H, Li, and HeH. With
the aid of the tail corrections to the dipole approximation are estimated.Comment: 20 pages, 7 figures, 2 table
Stopping power of antiprotons in H, H2, and He targets
The stopping power of antiprotons in atomic and molecular hydrogen as well as
helium was calculated in an impact-energy range from 1 keV to 6.4 MeV. In the
case of H2 and He the targets were described with a single-active electron
model centered on the target. The collision process was treated with the
close-coupling formulation of the impact-parameter method. An extensive
comparison of the present results with theoretical and experimental literature
data was performed in order to evaluate which of the partly disagreeing
theoretical and experimental data are most reliable. Furthermore, the size of
the corrections to the first-order stopping number, the average energy
transferred to the target electrons, and the relative importance of the
excitation and the ionization process for the energy loss of the projectile was
determined. Finally, the stopping power of the H, H2, and He targets were
directly compared revealing specific similarities and differences of the three
targets.Comment: v1: 12 pages, 8 figures, and 1 table v2: 15 pages, 9 figures, and 2
tables; extended discussion on IPM in Method; influence of double ionization
on stopping power discussed in Result
High School Students Embedded in Adult Community College Classes
Early college high schools were established as an initiative of the Bill and Melinda Gates Foundation with the goal for students of earning college credit and an associate degree while in high school. Many of these high school students attend college classes with adults, ages 18 and older, in the same class. Instructors are challenged to address these students’ diverse needs and diverse ways of learning. Young teenagers typically are told exactly what to learn and how it is to be learned; the adult learner, however, is much more independent and he or she learns and thinks differently based on more varied experiences. In this paper, the authors provide some concrete examples of instructional practices based on Knowles’ Model of Adult Learning
Exploring relationships between playspaces, pedagogy, and preschoolers’ play-based science and engineering practices
This manuscript reports the results of a research study exploring the ways in which physical space and teacher pedagogy are related to preschoolers’ engagement with science and engineering practices while at play. Using the Science and Engineering Practices Observation Protocol (SciEPOP), researchers captured children’s engagement with the eight science and engineering practices identified in the Next Generation Science Standards (NGSS). This study explores relationships between specific playspaces, materials, and pedagogical strategies, and children’s patterns of engagement with particular science and engineering practices during free play. There are notable differences in the spaces, materials, and pedagogies children encounter across the four participating preschools, and these differences suggest significant gaps in children’s opportunities to engage in and deepen their enactment of science and engineering practices. The authors present evidence in support of adaptive, personalized strategies for deepening children’s engagement with science through play, and raise questions about equity in early science learning environments that have implications both nationally and internationally for science education research, practice, and policy
Photoassociative spectroscopy at long range in ultracold strontium
We report photoassociative spectroscopy of Sr in a magneto-optical
trap operating on the intercombination line at 689 nm.
Photoassociative transitions are driven with a laser red-detuned by 600-2400
MHz from the atomic resonance at 461 nm. Photoassociation
takes place at extremely large internuclear separation, and the
photoassociative spectrum is strongly affected by relativistic retardation. A
fit of the transition frequencies determines the atomic lifetime
( ns) and resolves a discrepancy between experiment and
recent theoretical calculations.Comment: 4 pages, 4 figures, submitte
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