131 research outputs found
Wave Function Microscopy of Quasibound Atomic States
In the 1980s Demkov, Kondratovich, and Ostrovsky and Kondratovich and
Ostrovsky proposed an experiment based on the projection of slow electrons
emitted by a photoionized atom onto a position-sensitive detector. In the case
of resonant excitation, they predicted that the spatial electron distribution
on the detector should represent nothing else but a magnified image of the
projection of a quasibound electronic state. By exciting lithium atoms in the
presence of a static electric field, we present in this Letter the first
experimental photoionization wave function microscopy images where signatures
of quasibound states are evident. Characteristic resonant features, such as
(i) the abrupt change of the number of wave function nodes across a resonance
and (ii) the broadening of the outer ring of the image (associated with
tunneling ionization), are observed and interpreted via wave packet
propagation simulations and recently proposed resonance tunneling mechanisms.
The electron spatial distribution measured by our microscope is a direct
macroscopic image of the projection of the microscopic squared modulus of the
electron wave that is quasibound to the atom and constitutes the first
experimental realization of the experiment proposed 30 years ago
Wave-function imaging of quasibound and continuum Stark states
Photoionization of an atom in the presence of a uniform static electric field
provides the unique opportunity to expand and visualize the atomic wave
function at a macroscopic scale. In a number of seminal publications dating
back to the 1980s, Fabrikant, Demkov, Kondratovich, and Ostrovsky showed that
this goal could be achieved by projecting slow (meV) photoionized electrons
onto a position-sensitive detector and underlined the distinction between
continuum and resonant contributions. The uncovering of resonant signatures
was achieved fairly recently in experiments on the nonhydrogenic lithium atoms
[Cohen et al., Phys. Rev. Lett. 110, 183001 (2013)]. The purpose of the
present article is the general description of these findings, with emphasis on
the various manifestations of resonant character. From this point of view,
lithium has been chosen as an illustrative example between the two limiting
cases of hydrogen, where resonance effects are more easily identified, and
heavy atoms like xenon, where resonant effects were not observed
Darboux polynomials for Lotka-Volterra systems in three dimensions
We consider Lotka-Volterra systems in three dimensions depending on three
real parameters. By using elementary algebraic methods we classify the Darboux
polynomials (also known as second integrals) for such systems for various
values of the parameters, and give the explicit form of the corresponding
cofactors. More precisely, we show that a Darboux polynomial of degree greater
than one is reducible. In fact, it is a product of linear Darboux polynomials
and first integrals.Comment: 16 page
An integrating factor matrix method to find first integrals
In this paper we developed an integrating factor matrix method to derive
conditions for the existence of first integrals. We use this novel method to
obtain first integrals, along with the conditions for their existence, for two
and three dimensional Lotka-Volterra systems with constant terms. The results
are compared to previous results obtained by other methods
Regularity Properties and Pathologies of Position-Space Renormalization-Group Transformations
We reconsider the conceptual foundations of the renormalization-group (RG)
formalism, and prove some rigorous theorems on the regularity properties and
possible pathologies of the RG map. Regarding regularity, we show that the RG
map, defined on a suitable space of interactions (= formal Hamiltonians), is
always single-valued and Lipschitz continuous on its domain of definition. This
rules out a recently proposed scenario for the RG description of first-order
phase transitions. On the pathological side, we make rigorous some arguments of
Griffiths, Pearce and Israel, and prove in several cases that the renormalized
measure is not a Gibbs measure for any reasonable interaction. This means that
the RG map is ill-defined, and that the conventional RG description of
first-order phase transitions is not universally valid. For decimation or
Kadanoff transformations applied to the Ising model in dimension ,
these pathologies occur in a full neighborhood of the low-temperature part of the first-order
phase-transition surface. For block-averaging transformations applied to the
Ising model in dimension , the pathologies occur at low temperatures
for arbitrary magnetic-field strength. Pathologies may also occur in the
critical region for Ising models in dimension . We discuss in detail
the distinction between Gibbsian and non-Gibbsian measures, and give a rather
complete catalogue of the known examples. Finally, we discuss the heuristic and
numerical evidence on RG pathologies in the light of our rigorous theorems.Comment: 273 pages including 14 figures, Postscript, See also
ftp.scri.fsu.edu:hep-lat/papers/9210/9210032.ps.
Comparative Transcriptional Profiling of Bacillus cereus Sensu Lato Strains during Growth in CO2-Bicarbonate and Aerobic Atmospheres
Bacillus species are spore-forming bacteria that are ubiquitous in the environment and display a range of virulent and avirulent phenotypes. This range is particularly evident in the Bacillus cereus sensu lato group; where closely related strains cause anthrax, food-borne illnesses, and pneumonia, but can also be non-pathogenic. Although much of this phenotypic range can be attributed to the presence or absence of a few key virulence factors, there are other virulence-associated loci that are conserved throughout the B. cereus group, and we hypothesized that these genes may be regulated differently in pathogenic and non-pathogenic strains.Here we report transcriptional profiles of three closely related but phenotypically unique members of the Bacillus cereus group--a pneumonia-causing B. cereus strain (G9241), an attenuated strain of B. anthracis (Sterne 34F(2)), and an avirulent B. cereus strain (10987)--during exponential growth in two distinct atmospheric environments: 14% CO(2)/bicarbonate and ambient air. We show that the disease-causing Bacillus strains undergo more distinctive transcriptional changes between the two environments, and that the expression of plasmid-encoded virulence genes was increased exclusively in the CO(2) environment. We observed a core of conserved metabolic genes that were differentially expressed in all three strains in both conditions. Additionally, the expression profiles of putative virulence genes in G9241 suggest that this strain, unlike Bacillus anthracis, may regulate gene expression with both PlcR and AtxA transcriptional regulators, each acting in a different environment.We have shown that homologous and even identical genes within the genomes of three closely related members of the B. cereus sensu lato group are in some instances regulated very differently, and that these differences can have important implications for virulence. This study provides insights into the evolution of the B. cereus group, and highlights the importance of looking beyond differences in gene content in comparative genomics studies
The apicomplexan plastid and its evolution
Protistan species belonging to the phylum Apicomplexa have a non-photosynthetic secondary plastid—the apicoplast. Although its tiny genome and even the entire nuclear genome has been sequenced for several organisms bearing the organelle, the reason for its existence remains largely obscure. Some of the functions of the apicoplast, including housekeeping ones, are significantly different from those of other plastids, possibly due to the organelle’s unique symbiotic origin
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