1,715 research outputs found
A simplified picture for Pi electrons in conjugated polymers : from PPP Hamiltonian to an effective molecular crystal approach
An excitonic method proper to study conjugated oligomers and polymers is
described and its applicability tested on the ground state and first excited
states of trans-polyacetylene, taken as a model. From the Pariser-Parr-Pople
Hamiltonian, we derive an effective Hamiltonian based on a local description of
the polymer in term of monomers; the relevant electronic configurations are
build on a small number of pertinent local excitations. The intuitive and
simple microscopic physical picture given by our model supplement recent
results, such as the Rice and Garstein ones. Depending of the parameters, the
linear absorption appears dominated by an intense excitonic peak.Comment: 41 Pages, 6 postscript figure
Magnetic molecules created by hydrogenation of Polycyclic Aromatic Hydrocarbons
Present routes to produce magnetic organic-based materials adopt a common
strategy: the use of magnetic species (atoms, polyradicals, etc.) as building
blocks. We explore an alternative approach which consists of selective
hydrogenation of Polycyclic Aromatic Hydrocarbons. Self-Consistent-Field (SCF)
(Hartree-Fock and DFT) and multi-configurational (CISD and MCSCF) calculations
on coronene and corannulene, both hexa-hydrogenated, show that the formation of
stable high spin species is possible. The spin of the ground states is
discussed in terms of the Hund rule and Lieb's theorem for bipartite lattices
(alternant hydrocarbons in this case). This proposal opens a new door to
magnetism in the organic world.Comment: 6 pages, 4 figures and 2 table
Optical Absorption Study by Ab initio Downfolding Approach: Application to GaAs
We examine whether essence and quantitative aspects of electronic excitation
spectra are correctly captured by an effective low-energy model constructed
from an {\em ab initio} downfolding scheme. A global electronic structure is
first calculated by {\em ab initio} density-functional calculations with the
generalized gradient approximation. With the help of constrained density
functional theory, the low-energy effective Hamiltonian for bands near the
Fermi level is constructed by the downfolding procedure in the basis of
maximally localized Wannier functions. The excited states of this low-energy
effective Hamiltonian ascribed to an extended Hubbard model are calculated by
using a low-energy solver. As the solver, we employ the Hartree-Fock
approximation supplemented by the single-excitation configuration-interaction
method considering electron-hole interactions. The present three-stage method
is applied to GaAs, where eight bands are retained in the effective model after
the downfolding. The resulting spectra well reproduce the experimental results,
indicating that our downfolding scheme offers a satisfactory framework of the
electronic structure calculation, particularly for the excitations and dynamics
as well as for the ground state.Comment: 14 pages, 6 figures, and 1 tabl
Application of an Ultralight Aircraft to Aerial Surveys of Kangaroos on Grazing Properties
A Drifter ultralight aircraft was used as a platform for line-transect aerial surveys of three species of kangaroo in the sheep rangelands south-east of Blackall and north of Longreach in central-western Queensland in winter 1993 and 1994. Favourable comparisons between the results of ultralight surveys and those made from a helicopter flying the same transects and foot surveys along another set of transects, all within a few days of the ultralight survey, confirmed the expectation we had that an ultralight would be a satisfactory and much cheaper vehicle for conducting aerial surveys of kangaroos. The comparisons are even more favourable when data for the three species surveyed are combined, pointing to a problem in species identification and underlining the importance of using only experienced observers for aerial survey of kangaroos, whatever the platform. The use of an ultralight aircraft could have particular value where a comparatively small area, such as an individual sheep or cattle property, is under consideration. In this paper, we present the numerical comparisons, along with an evaluation of the practicability of using this type of aircraft. We also describe a possible future scenario in which an accreditation process could see approved kangaroo surveyors undertaking property assessments by ultralight, under contract to graziers or other interested parties
Ab initio equilibrium constants for H2O–H2O and H2O–CO2
Ab initio 6‐31G** electronic structure calculations have been used to determine the minimum energy geometries and vibrational frequencies of molecular clusters of water and carbon dioxide. Application of statistical thermodynamics leads to theoretical equilibrium constants for gas phase dimerization of water and the formation of an adduct of carbon dioxide with water.The low energy vibrations of the clusters lead to much larger contributions to the vibrational partitioning of the energy than do the fundamental vibrations of the monomeric species. A new ‘‘Harmonic‐Morse’’ formula is derived to estimate anharmonicity from optimized harmonic frequencies and two additional values on the potential surface for each vibration. These ab initiocalculations of equilibrium constants are very close to recent measurements and fall within the range of values obtained by other methods. This no‐parameter treatment gives excellent agreement for the equilibrium of H2O–CO2 near the supercritical fluid range of CO2 and suggests that a ‘‘Theory of Significant Clusters’’ may be extended to a model of supercritical fluids which includes the effects of anharmonicity
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
Dancing to a different tune: changing reproductive seasonality in an introduced chital deer population
Male and female reproductive behaviour is typically synchronised. In species such as those in the family Cervidae, reproductive timing is often cued by photoperiod, although in females, it can be dependent on body condition. When a species is introduced to a novel environment, the environment changes, or responses of the sexes to such cues differ, asynchronous reproductive behaviour between males and females may occur. We investigated the seasonality of reproductive behaviour in introduced chital deer in northern Queensland by examining male antler phase in relation to female conception rates. We then analysed the influence of different variables likely to affect the timing of male and female reproductive physiology. The lowest percentage of chital in hard antler in any 1 month in this study was 35% (Fig. 1), but the average value was closer to 50%, thus there was a seasonal peak in antler phase linked with photoperiod. Females conceived at any time of year, but were strongly influenced by the amount of rainfall 3 months prior to conception. This resulted in varying conception peaks year-to-year that often did not correspond to the male’s peak in hard antler. In this system, a proportion of males and females were physiologically and behaviourally ready to mate at any time of the year. We predict that differences in the timing of the peaks between the males and females will lead to increased reproductive skew (variation in reproductive success among individual males). This pattern may select for different mating strategies or physiological mechanisms to increase reproductive success
Dancing to a different tune: changing reproductive seasonality in an introduced chital deer population
Male and female reproductive behaviour is typically synchronised. In species such as those in the family Cervidae, reproductive timing is often cued by photoperiod, although in females, it can be dependent on body condition. When a species is introduced to a novel environment, the environment changes, or responses of the sexes to such cues differ, asynchronous reproductive behaviour between males and females may occur. We investigated the seasonality of reproductive behaviour in introduced chital deer in northern Queensland by examining male antler phase in relation to female conception rates. We then analysed the influence of different variables likely to affect the timing of male and female reproductive physiology. The lowest percentage of chital in hard antler in any 1 month in this study was 35% (Fig. 1), but the average value was closer to 50%, thus there was a seasonal peak in antler phase linked with photoperiod. Females conceived at any time of year, but were strongly influenced by the amount of rainfall 3 months prior to conception. This resulted in varying conception peaks year-to-year that often did not correspond to the male’s peak in hard antler. In this system, a proportion of males and females were physiologically and behaviourally ready to mate at any time of the year. We predict that differences in the timing of the peaks between the males and females will lead to increased reproductive skew (variation in reproductive success among individual males). This pattern may select for different mating strategies or physiological mechanisms to increase reproductive success.Fig. 1The average percentage of male chital deer in hard antler by month from 2014 to 2019 in north Queensland. Values above the bars indicate the total number of males that were sampled in each month and the error bars indicate the standard error. In the month with the lowest % males in hard antler in the entire study (November, 2017), 35% of males were in hard antle
Dancing to a different tune: changing reproductive seasonality in an introduced chital deer population
Male and female reproductive behaviour is typically synchronised. In species such as those in the family Cervidae, reproductive timing is often cued by photoperiod, although in females, it can be dependent on body condition. When a species is introduced to a novel environment, the environment changes, or responses of the sexes to such cues differ, asynchronous reproductive behaviour between males and females may occur. We investigated the seasonality of reproductive behaviour in introduced chital deer in northern Queensland by examining male antler phase in relation to female conception rates. We then analysed the influence of different variables likely to affect the timing of male and female reproductive physiology. The lowest percentage of chital in hard antler in any 1 month in this study was 35% (Fig. 1), but the average value was closer to 50%, thus there was a seasonal peak in antler phase linked with photoperiod. Females conceived at any time of year, but were strongly influenced by the amount of rainfall 3 months prior to conception. This resulted in varying conception peaks year-to-year that often did not correspond to the male’s peak in hard antler. In this system, a proportion of males and females were physiologically and behaviourally ready to mate at any time of the year. We predict that differences in the timing of the peaks between the males and females will lead to increased reproductive skew (variation in reproductive success among individual males). This pattern may select for different mating strategies or physiological mechanisms to increase reproductive success.Fig. 1The average percentage of male chital deer in hard antler by month from 2014 to 2019 in north Queensland. Values above the bars indicate the total number of males that were sampled in each month and the error bars indicate the standard error. In the month with the lowest % males in hard antler in the entire study (November, 2017), 35% of males were in hard antle
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