1,174 research outputs found
UV photolysis of 4-iodo-, 4-bromo-, and 4-chlorophenol::Competition between C-Y (Y = halogen) and O-H bond fission
Consumer Return Chronology Alters Recovery Trajectory of Stream Ecosystem Structure and Function Following Drought
Consumers are increasingly being recognized as important drivers of ecological succession, yet it is still hard to predict the nature and direction of consumer effects in nonequilibrium environments. We used stream consumer exclosures and large outdoor mesocosms to study the impact of macroconsumers (i.e., fish and crayfish) on recovery of intermittent prairie streams after drying. In the stream, macroconsumers altered system recovery trajectory by decreasing algal and macroinvertebrate biomass, primary productivity, and benthic nutrient uptake rates. However, macroconsumer influence was transient, and differences between exclosures and controls disappeared after 35 days. Introducing and removing macroconsumers after 28 days resulted mainly in changes to macroinvertebrates. In mesocosms, a dominant consumer (the grazing minnow Phoxinus erythrogaster) reduced macroinvertebrate biomass but had little effect on algal assemblage structure and ecosystem rates during recovery. The weak effect of P. erythrogaster in mesocosms, in contrast to the strong consumer effect in the natural stream, suggests that both timing and diversity of returning consumers are important to their overall influence on stream recovery patterns. Although we found that consumers significantly altered ecosystem structure and function in a system experiencing rapid changes in abiotic and biotic factors following disturbance, consumer effects diminished over time and trajectories converged to similar states with respect to primary producers, in spite of differences in consumer colonization history. Thus, consumer impacts can be substantial in recovering ecosystems and are likely to be dependent on the disturbance regime and diversity of the consumer community
Evolutionary Dynamics While Trapped in Resonance: A Keplerian Binary System Perturbed by Gravitational Radiation
The method of averaging is used to investigate the phenomenon of capture into
resonance for a model that describes a Keplerian binary system influenced by
radiation damping and external normally incident periodic gravitational
radiation. The dynamical evolution of the binary orbit while trapped in
resonance is elucidated using the second order partially averaged system. This
method provides a theoretical framework that can be used to explain the main
evolutionary dynamics of a physical system that has been trapped in resonance.Comment: REVTEX Style, Submitte
Probing Photochemically and Thermally Induced Isomerization Reactions in α-Pyrone
The
isomerization dynamics of α-pyrone dissolved in CH<sub>3</sub>CN have been probed by femtosecond 267 nm pump/broadband infrared
(IR) probe spectroscopy. A novel experimental setup allowed the populations
of the parent molecule and ring-opened photoproducts to be monitored
over pump/probe time delays ranging between 2 ps and 100 μs
within a single experiment, and at 5 different temperatures between
0 and 40 °C. The photochemically prepared α-pyroneÂ(S<sub>1</sub>) molecules decay rapidly (<10 ps) through internal conversion
to the S<sub>0</sub> potential energy surface, with an initial quantum
yield for parent molecule re-formation of ∼60%. Probing the
antisymmetric ketene stretch region (2100–2150 cm<sup>–1</sup>) confirms the presence of at least two ring-opened photoproducts,
which are assumed to have an <i>E</i>-configuration with
respect to the central Cî—»C double bond. These ketenes are observed
to undergo two distinct, thermally driven, isomerization processes
which occur on the nanosecond and microsecond time scales, respectively.
The former reaction is ascribed to thermalization of the initially
prepared <i>E-</i>isomer populations, while the slower (microsecond)
process involves rotation around the central Cî—»C double bond
leading to formation of <i>Z</i>-isomers. Subsequent rapid <i>Z</i> → <i>Z</i> isomerizations (occurring
on a nanosecond time scale) result in ring-closure and a second, longer
time recovery of parent molecule population. By determining rates
as a function of the sample temperature, barrier heights of 0.23(3)
eV and 0.43(2) eV are obtained for the <i>E</i> → <i>E</i> and <i>E</i> → <i>Z</i> transformations,
respectively
Tracking a Paterno-Buchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies
Breakdown of Conformal Invariance at Strongly Random Critical Points
We consider the breakdown of conformal and scale invariance in random systems
with strongly random critical points. Extending previous results on
one-dimensional systems, we provide an example of a three-dimensional system
which has a strongly random critical point. The average correlation functions
of this system demonstrate a breakdown of conformal invariance, while the
typical correlation functions demonstrate a breakdown of scale invariance. The
breakdown of conformal invariance is due to the vanishing of the correlation
functions at the infinite disorder fixed point, causing the critical
correlation functions to be controlled by a dangerously irrelevant operator
describing the approach to the fixed point. We relate the computation of
average correlation functions to a problem of persistence in the RG flow.Comment: 9 page
The Renormalization Group and Singular Perturbations: Multiple-Scales, Boundary Layers and Reductive Perturbation Theory
Perturbative renormalization group theory is developed as a unified tool for
global asymptotic analysis. With numerous examples, we illustrate its
application to ordinary differential equation problems involving multiple
scales, boundary layers with technically difficult asymptotic matching, and WKB
analysis. In contrast to conventional methods, the renormalization group
approach requires neither {\it ad hoc\/} assumptions about the structure of
perturbation series nor the use of asymptotic matching. Our renormalization
group approach provides approximate solutions which are practically superior to
those obtained conventionally, although the latter can be reproduced, if
desired, by appropriate expansion of the renormalization group approximant. We
show that the renormalization group equation may be interpreted as an amplitude
equation, and from this point of view develop reductive perturbation theory for
partial differential equations describing spatially-extended systems near
bifurcation points, deriving both amplitude equations and the center manifold.Comment: 44 pages, 2 Postscript figures, macro \uiucmac.tex available at macro
archives or at ftp://gijoe.mrl.uiuc.edu/pu
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