316 research outputs found
Time-resolved velocity map imaging of methyl elimination from photoexcited anisole
To date, H-atom elimination from heteroaromatic molecules following UV excitation has been extensively studied, with the focus on key biological molecules such as chromophores of DNA bases and amino acids. Extending these studies to look at elimination of other non-hydride photoproducts is essential in creating a more complete picture of the photochemistry of these biomolecules in the gas-phase. To this effect, CH3 elimination in anisole has been studied using time resolved velocity map imaging (TR-VMI) for the first time, providing both time and energy information on the dynamics following photoexcitation at 200 nm. The extra dimension of energy afforded by these measurements has enabled us to address the role of πσ* states in the excited state dynamics of anisole as compared to the hydride counterpart (phenol), providing strong evidence to suggest that only CH3 fragments eliminated with high kinetic energy are due to direct dissociation involving a 1πσ* state. These measurements also suggest that indirect mechanisms such as statistical unimolecular decay could be contributing to the dynamics at much longer times
Ultrasound as a technology of reassurance? How pregnant women and health care professionals articulate ultrasound reassurance and its limitations
The premise that ultrasound technologies provide reassurance for pregnant women is well‐rehearsed. However, there has been little research about how this reassurance is articulated and understood by both expectant mothers and health care professionals. In this article, we draw on two qualitative UK studies to explore the salience of ultrasound reassurance to women's pregnancy experiences whilst highlighting issues around articulation and silence. Specifically, we capture how expectant parents express a general need for reassurance and how visualisation and the conduct of professionals have a crucial role to play in accomplishing a sense of reassurance. We also explore how professionals have ambiguities about the relationship between ultrasound and reassurance, and how they subsequently articulate reassurance to expectant mothers. By bringing two studies together, we take a broad perspectival view of how gaps and silences within the discourse of ultrasound reassurance leave the claims made for ultrasound as a technology of reassurance unchallenged. Finally, we explore the implications this can have for women's experiences of pregnancy and health care professionals’ practices
Classifying Four-Body Convex Central Configurations
We classify the full set of convex central configurations in the Newtonian
four-body problem. Particular attention is given to configurations possessing
some type of symmetry or defining geometric property. Special cases considered
include kite, trapezoidal, co-circular, equidiagonal, orthodiagonal, and
bisecting-diagonal configurations. Good coordinates for describing the set are
established. We use them to prove that the set of four-body convex central
configurations with positive masses is three-dimensional, a graph over a domain
that is the union of elementary regions in .Comment: 28 pages, 14 figure
Divide-and-Conquer Monte Carlo Fusion
Combining several (sample approximations of) distributions, which we term
sub-posteriors, into a single distribution proportional to their product, is a
common challenge. For instance, in distributed `big data' problems, or when
working under multi-party privacy constraints. Many existing approaches resort
to approximating the individual sub-posteriors for practical necessity, then
representing the resulting approximate posterior. The quality of the posterior
approximation for these approaches is poor when the sub-posteriors fall
out-with a narrow range of distributional form. Recently, a Fusion approach has
been proposed which finds a direct and exact Monte Carlo approximation of the
posterior (as opposed to the sub-posteriors), circumventing the drawbacks of
approximate approaches. Unfortunately, existing Fusion approaches have a number
of computational limitations, particularly when unifying a large number of
sub-posteriors. In this paper, we generalise the theory underpinning existing
Fusion approaches, and embed the resulting methodology within a recursive
divide-and-conquer sequential Monte Carlo paradigm. This ultimately leads to a
competitive Fusion approach, which is robust to increasing numbers of
sub-posteriors.Comment: 24 pages, 8 figure
Towards Understanding Photodegradation Pathways in Lignins:The Role of Intramolecular Hydrogen Bonding in Excited States
The photoinduced dynamics of the lignin building blocks syringol, guaiacol, and phenol were studied using time-resolved ion yield spectroscopy and velocity map ion imaging. Following irradiation of syringol and guaiacol with a broad-band femtosecond ultraviolet laser pulse, a coherent superposition of out-of-plane OH torsion and/or OMe torsion/flapping motions is created in the first excited 1ππ* (S1) state, resulting in a vibrational wavepacket, which is probed by virtue of a dramatic nonplanar → planar geometry change upon photoionization from S1 to the ground state of the cation (D0). Any similar quantum beat pattern is absent in phenol. In syringol, the nonplanar geometry in S1 is pronounced enough to reduce the degree of intramolecular H bonding (between OH and OMe groups), enabling H atom elimination from the OH group. For guaiacol, H bonding is preserved after excitation, despite the nonplanar geometry in S1, and prevents O–H bond fission. This behavior affects the propensities for forming undesired phenoxyl radical sites in these three lignin chromophores and provides important insight into their relative “photostabilities” within the larger biopolymer
Bayesian inference for high-dimensional discrete-time epidemic models: spatial dynamics of the UK COVID-19 outbreak
In the event of a disease outbreak emergency, such as COVID-19, the ability
to construct detailed stochastic models of infection spread is key to
determining crucial policy-relevant metrics such as the reproduction number,
true prevalence of infection, and the contribution of population
characteristics to transmission. In particular, the interaction between space
and human mobility is key to prioritising outbreak control resources to
appropriate areas of the country. Model-based epidemiological intelligence must
therefore be provided in a timely fashion so that resources can be adapted to a
changing disease landscape quickly. The utility of these models is reliant on
fast and accurate parameter inference, with the ability to account for large
amount of censored data to ensure estimation is unbiased. Yet methods to fit
detailed spatial epidemic models to national-level population sizes currently
do not exist due to the difficulty of marginalising over the censored data. In
this paper we develop a Bayesian data-augmentation method which operates on a
stochastic spatial metapopulation SEIR state-transition model, using
model-constrained Metropolis-Hastings samplers to improve the efficiency of an
MCMC algorithm. Coupling this method with state-of-the-art GPU acceleration
enabled us to provide nightly analyses of the UK COVID-19 outbreak, with timely
information made available for disease nowcasting and forecasting purposes
Quasi-stationary Monte Carlo and the ScaLE Algorithm
This paper introduces a class of Monte Carlo algorithms which are based upon simulating a Markov process whose quasi-stationary distribution coincides with a distribution of interest. This differs fundamentally from, say, current Markov chain Monte Carlo methods which simulate a Markov chain whose stationary distribution is the target. We show how to approximate distributions of interest by carefully combining sequential Monte Carlo methods with methodology for the exact simulation of diffusions. The methodology introduced here is particularly promising in that it is applicable to the same class of problems as gradient based Markov chain Monte Carlo algorithms but entirely circumvents the need to conduct Metropolis-Hastings type accept/reject steps whilst retaining exactness: the paper gives theoretical guarantees ensuring the algorithm has the correct limiting target distribution. Furthermore, this methodology is highly amenable to big data problems. By employing a modification to existing naıve sub-sampling and control variate techniques it is possible to obtain an algorithm which is still exact but has sub-linear iterative cost as a function of data size
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