516 research outputs found
Quantum Zeno-based control mechanism for molecular fragmentation
A quantum control mechanism is proposed for molecular fragmentation processes
within a scenario grounded on the quantum Zeno effect. In particular, we focus
on the van der Waals Ne-Br complex, which displays two competing
dissociation channels via vibrational and electronic predissociation.
Accordingly, realistic three dimensional wave packet simulations are carried
out by using ab initio interaction potentials recently obtained to reproduce
available experimental data. Two numerical models to simulate the repeated
measurements are reported and analyzed. It is found that the otherwise fast
vibrational predissociation is slowed down in favor of the slow electronic
(double fragmentation) predissociation, which is enhanced by several orders of
magnitude. Based on these theoretical predictions, some hints to
experimentalists to confirm their validity are also proposed.Comment: 4 pages, 3 figure
The electron-phonon coupling strength at metal surfaces directly determined from the Helium atom scattering Debye-Waller factor
A new quantum-theoretical derivation of the elastic and inelastic scattering
probability of He atoms from a metal surface, where the energy and momentum
exchange with the phonon gas can only occur through the mediation of the
surface free-electron density, shows that the Debye-Waller exponent is directly
proportional to the electron-phonon mass coupling constant . The
comparison between the values of extracted from existing data on the
Debye-Waller factor for various metal surfaces and the values known
from literature indicates a substantial agreement, which opens the possibility
of directly extracting the electron-phonon coupling strength in quasi-2D
conducting systems from the temperature or incident energy dependence of the
elastic Helium atom scattering intensities.Comment: 14 pages, 2 figures, 1 tabl
Quantum Zeno and anti-Zeno effects in surface diffusion of interacting adsorbates
Surface diffusion of interacting adsorbates is here analyzed within the
context of two fundamental phenomena of quantum dynamics, namely the quantum
Zeno effect and the anti-Zeno effect. The physical implications of these
effects are introduced here in a rather simple and general manner within the
framework of non-selective measurements and for two (surface) temperature
regimes: high and very low (including zero temperature). The quantum
intermediate scattering function describing the adsorbate diffusion process is
then evaluated for flat surfaces, since it is fully analytical in this case.
Finally, a generalization to corrugated surfaces is also discussed. In this
regard, it is found that, considering a Markovian framework and high surface
temperatures, the anti-Zeno effect has already been observed, though not
recognized as such.Comment: 17 pages, 1 figur
Quasi-elastic peak lineshapes in adsorbate diffusion on nearly flat surfaces at low coverages: the motional narrowing effect in Xe on Pt(111)
Quasi-elastic helium atom scattering measurements have provided clear
evidence for a two-dimensional free gas of Xe atoms on Pt(111) at low
coverages. Increasing the friction due to the surface, a gradual change of the
shape of the quasi-elastic peak is predicted and analyzed for this system in
terms of the so-called motional narrowing effect. The type of analysis
presented here for the quasi-elastic peak should be prior to any deconvolution
procedure carried out in order to better extract information from the process,
e.g. diffusion coefficients and jump distributions. Moreover, this analysis
also provides conditions for the free gas regime different than those reported
earlier.Comment: 12 pages, 4 figures (revised version
Sucrose homeostasis: Mechanisms and opportunity in crop yield improvement
Sugar homeostasis is a critical feature of biological systems. In humans, raised and dysregulated blood sugar
is a serious health issue. In plants, directed changes in sucrose homeostasis and allocation represent opportunities in crop improvement. Plant tissue sucrose varies more than blood glucose and is found at higher concentrations (cytosol and phloem ca. 100 mM v 3.9–6.9 mM for blood glucose). Tissue sucrose varies with developmental stage and environment, but cytosol and phloem exhibit tight sucrose control. Sucrose homeostasis is a consequence of the integration of photosynthesis, synthesis of storage end products such as starch, transport of sucrose to sinks and sink metabolism. Trehalose 6-phosphate (T6P)-SnRK1 and TOR play central, still emerging roles in regulating and coordinating these processes. Overall, tissue sucrose levels are more strongly related to growth than to photosynthesis. As a key sucrose signal,T6P regulates sucrose levels, transport and metabolic pathways to coordinate source and sink at a whole plant level. Emerging evidence shows that T6P interacts with meristems. With careful targeting, T6P manipulation through exploiting natural variation, chemical intervention and genetic modification is delivering benefits for crop yields. Regulation of cereal grain set, filling and retention may be the most strategically important aspect of sucrose allocation and homeostasis for food security
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