1,721,226 research outputs found
Velocity mapping of elementary bimolecular reactions
Full text linkA new and flexible velocity-map ion imaging apparatus, designed for the study of photodissociation processes and photon-initiated bimolecular reactions in a single molecular beam, has been constructed, developed and characterised. An image Legendre moment fitting analysis was developed to allow recovery of centre-of-mass (CM) angular scattering and kinetic energy release distributions from velocity-map ion images of the products of photon-initiated bimolecular reactions.The Legendre moment analysis methodology has been applied to images of the HCl(v' = 0,j' = 0-6) products of the reactions of Cl(²P3/2) atoms with ethane and n-butane at collision energies of 0.24 eV and 0.32 eV respectively. The Cl(²P3/2) reactants were generated by polarised laser photodissociation of Cl₂ at 355 nm. For reaction with ethane, the CM angular scattering distributions show a steady trend from forward scattering at low j' to more isotropic, but backward peaking, scattering at high j'. An impact parameter-based mechanism is proposed to account for the observed dynamics. Abstraction of a hydrogen atom from a primary carbon site in n-butane is seen to produce rotationally very cold HCl products that are forward scattered, whereas H atom abstraction from a secondary carbon site in n-butane yields more isotropically scattered HCl products formed with higher rotational excitation. A peripheral mechanism is proposed to operate for the primary abstraction channel, whilst a more rebound type mechanism is seen to account for the dynamics of the secondary abstraction channel. Around 22% and 30% of the available energy is found in internal modes of the alkyl radical co-products of the Cl + C₂H₆ and Cl + n-C₄H₁₀ reactions respectively. Possible sources of alkyl co-product excitation are discussed in each case.The hydrogen or deuterium atom abstraction reactions of Cl(²P3/2) with CH₄, CD₄ and CH₃D, have been studied at mean collision energies around 0.3 eV. Chlorine atom reactants were generated by polarised laser photodissociation of Cl₂ at 308 nm. The methyl radical products were detected using (2+1) resonance-enhanced multi-photon ionisation, coupled with velocity-map ion imaging. The laboratory frame speed distributions obtained from the images are in excellent agreement with previous work. The interpretation of the experiments is shown to be very sensitive to assumptions made about the reactant velocity distributions. If these are assumed to be narrow, the data are seen to suggest that a significant fraction of the product signal must arise from the reaction of Cl with vibrationally excited methane reactants. This conclusion is in agreement with previous photon-initiated reaction studies. However, by allowing for the spread in collision energies in the molecular beam, it is shown that it is possible to fit the data sensibly assuming reaction with vibrational ground state methane alone. CM angular scattering distributions thereby derived are presented for all three reactions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Novel probes of angular momentum polarization
No full textNew dynamical applications of quantum beat spectroscopy (QBS) to molecular dynamics are employed to probe the angular momentum polarization effects in photodissociation and molecular collisions. The magnitude and the dynamical behaviour of angular momentum alignment and orientation, two types of polarization, can be measured via QBS technique on a shot-by-shot basis. The first part of this thesis describes the experimental studies of collisional angular momentum depolarization for the electronically excited state radicals in the presence of the collider partners. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer (RET), and elastic collisions. Experimental results also have a fairly good agreement with the results of quasi-classical trajectory scattering calculations. Chapter 1 provides the brief theories about the application of the QBS technique and collisional depolarization. Chapter 2 describes the method and instrumentation employed in the experiments of this work. In Chapter 3, the QBS technique is used to measure the total elastic plus elastic depolarization rate constants under thermal conditions for NO(A,v=0) in the presence of He, Ar, N2, and O2. In the case of NO(A) with Ar, and particularly with He, collisional depolarization is significantly smaller than RET, reflecting the weak long-range forces in these systems. In the case of NO(A)+N2/O2, collisional depolarization and RET are comparable, reflecting the relatively strong long-range forces in these systems. In Chapter 4, the QBS technique is used to measure the elastic and inelastic depolarization and total RET rate constants for OH(A,v=0) under thermal conditions in the presence of He and Ar, as well as the total depolarization rate constants under superthermal conditions. In the case of OH(A)+He, elastic depolarization is sensitive to the N rotational state, and inelastic depolarization is strongly dependent on the collision energy. In the case of OH(A)+Ar, elastic depolarization is insensitive to N, and inelastic depolarization is less sensitive to the collision energy, reflecting that the relatively strong long-range force in OH(A)+Ar system. The second part of this thesis describes the experimental studies of photodissociation under thermal conditions. Chapter 5 provides a brief introduction about several polarization parameter formalisms used for photodissociation, and the incorporation of the QBS technique to measure these polarization parameters. In this thesis, most polarization parameters of the molecular photofragments are measured using the LIF method, and the QBS technique is used as a complementary tool to probe these polarization parameters. In Chapter 6, rotational orientation in the OH(X,v=0) photofragments from H2O2 photodissociation using circularly polarized light at 193 nm is observed. Although H2O2 can be excited to both the A and B electronic states by 193 nm, the observed orientation is only related to the A state dynamics. A proposed mechanism about the coupling between a polarized photon and the H2O2 parent rotation is simulated, and the good agreement between the experimental and simulation results further confirms the validity of this mechanism. In Chapter 7, rotational orientation in the NO(X,v) photofragments from NO2 photodissociation using circularly polarized light at 306 nm (v=0,1,2) and at 355 nm (v=0,1) is observed. Two possible mechanisms, the parent molecular rotation and the coherent effect between multiple electronic states, are discussed. NOCl is photodissociated using circularly polarized light at 306 nm, and NO(X,v) rotational distributions (v=0,1) and rotational orientation (v=0) are measured. For the case of NOCl, the generation of orientation is attributed to the coherent effect
The collision dynamics of OH(A)+H2
Full text linkThis thesis presents a joint experimental and theoretical study of a bimolecular collision between OH(A) and H2 diatoms. The study focuses on the relationship between the initial, j, and final rotational angular momentum, j'. This relationship is explored from both a scalar point of view by measuring rotational energy transfer (RET), and a vectorial viewpoint by considering the collisional depolarisation. The experimental technique used in this investigation, Zeeman quantum beat spectroscopy, is first demonstrated by applying it to the determination of the lab-frame orientation of OH(X) photofragments following the photolysis of H2O2. The H2O2 is photolysed by circularly-polarised light at 248 nm, and Zeeman quantum beat spectroscopy probes the angular momentum orientation as a function of the photofragment spin-rotation level. The results of this experiment are compared with orientation parameters predicted by a simulation that couples the rotation of the parent molecule to the torsional motion during bond cleavage. The calculations from the model agree qualitatively with those from the experiment. The Zeeman quantum beat spectroscopy technique is then used to monitor the evolution of angular momentum polarisation of OH(A) radicals during collisions with H2. The technique allows for the determination of depolarisation cross sections for oriented and aligned distributions, as a result of collisions with H2. Alongside this, cross sections for collisional quenching to non-reactive OH(X)+H2 and reactive H2O+H products are determined. By resolving the fuorescence with a monochromator the contributions to depolarisation from elastic collisions (the elastic depolarisation cross sections) are measured alongside cross sections for RET. Cross sections for total depolarisation and rotational energy transfer demonstrate only weak dependence on the rotational quantum number of the OH(A) radical, NOH. Competing quenching processes that fall with NOH are likely a considerable cause of this weak dependence. Furthermore, the polarisation of the angular momentum of OH(A) is randomised following RET. The elastic depolarisation cross sections make only a small contribution to the depolarisation and fall with increasing NOH. Collectively these trends have not been seen previously in similar studies on OH(A) collisions with atomic colliders. For the theoretical calculations, a four-atom quasi-classical trajectory (QCT) method has been developed, utilising Lagrangian multipliers to fix the OH(A) and H2 bonds. The calculations demonstrate that collisions involving the formation of complexes that survive for several rotational periods are prevalent in this collision system, and that these lead to large amounts of depolarisation. The calculations also demonstrate that RET in the H2 diatom supports higher levels of RET in OH(A) than seen in previous triatomic systems. Additionally, when one diatom is depolarised the accompanying diatom is typically also depolarised. These trends, at least in part, are owed to the highly attractive and anisotropic potential energy surface (PES) describing the interaction. The QCT calculations overestimate the experimentally-measured cross sections by more than a factor of 2. The calculations are adiabatic and do not account for the non-adiabatic activity associated with this collision system, and this is likely one cause of the discrepancies. In an attempt to further account for this overestimation, alternative angular momentum binning approaches for the QCT calculations are developed, but with limited success. Further exploration of the topology of the PES used in the calculations suggests that inadequacies in this surface are a major contributor to the discrepancies
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