1,721,183 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Light-induced ultrafast tunneling dynamics of a many-electron system: from weak to strong fields
No full textPhotoionization is one of the most possible processes that can happen when light interacts with anelectronic system. This seemingly simple phenomenon has expanded into a rich playground thanksto the rapid development of ultraintense and ultrashort light-source technology.In this doctoral dissertation, we investigate photoionization dynamics that can be measured ortriggered by advanced light sources. We focus on the theoretical understanding of (1) photoionizationprocesses assisted by ultrafast electron tunneling, i.e., short-lived shape-type resonance processes, and(2) properties that emerge as the size of the electronic system increases, with the heavy atom xenonserving as a benchmark system. To this end, many-electron Schrödinger equation is solved fromfirst principles using a combination of wave-packet propagation and non-Hermitian resonance-statetechniques. Two applications are presented in two distinct regimes of light-matter coupling.In the first application, we study the resonance dynamics of xenon induced by extreme ultraviolet(XUV) light in the perturbative regime of light-matter coupling. Here, the linear response of xenonis known to exhibit a plasma-like, collective feature, the giant dipole resonance (GDR). We find thatthe GDR is composed of two short-lived resonance states. Albeit hidden in XUV linear spectroscopy,this resonance substructure has been resolved lately using XUV nonlinear spectroscopy. In addition,by analyzing all types of resonance states and then their spectroscopic fingerprints, we explain thelimitation of linear-response studies and reveal the existence of collective multipole resonances. Ourwork offers a new way to understand collective electronic behavior in and beyond the linear regime.In the second application, we study the resonance dynamics of xenon induced by near infrared(NIR) light in the nonperturbative regime of light-matter coupling. Here, strong-field ionization (SFI),a process of both fundamental and technological importance, is examined inside each laser subcycle.In our experiment-theory collaboration, the SFI dynamics is followed by attosecond transient absorptionspectroscopy and is found to show an unexpected oscillatory character. With the aid of theory,we identify polarization as the origin of this oscillation. While the role of polarization had so far beenunnoticed in the strong-field regime, it comes into light due to the high polarizability of xenon and isexpected to influence the strongly driven dynamics of extended electronic systems
Analytical and computational investigations in atomic attosecond physics
No full textUnderstanding the interaction of matter with ultrashort attosecond light pulse provides fundamental insights into the structure of matter. The electronic properties are reflected in wave packets dynamics. The ability to trace the evolution of electrons in wave packets is a big challenge. It requires ultrashort pulse duration and precise pulse characterization. In this thesis the time-delayed scanning of the atom is used to (1) characterize the inner atomic dynamics, via excitation of the atom by a time-delayed attosecond extreme ultraviolet (XUV) and femtosecond optical field, and to (2) characterize a laser field, via multiphoton ionization of the atom by the autocorrelation function of the field.
The first part of the thesis explores the capability of attosecond transient absorption spectroscopy to characterize the dynamics of inner-shell-excited systems. I discuss an unusual kind of pump-probe experiment, where information is gained from the absorption spectrum of an attosecond XUV pulse, which serves as a pump pulse at the same time. The optical pulse in this kind of experiment gives a reference time that provides a possibility to measure the time evolution of a system of interest. In the study, I use different theoretical approaches, treating one or both of the pulses as perturbative or non-perturbative. I present an analytical theory of attosecond transient absorption spectroscopy for perturbatively dressed systems and illustrate how the attosecond transient absorption signal reveals the real-time attosecond dynamics of the atom. In addition, I apply our study to atomic Xe and compare the theoretical predictions with experimental results.
In the second part of the thesis a new method for laser pulse characterization is presented. It is based on a machine learning algorithm and is used to study multiphoton autocorrelations in Ar. I analyze the time-delay dependence of the ionization probability for given laser pulse parameters, such as photon energy, pulse intensity and pulse duration. Taking into consideration the mapping between the ionization-probability time-delay pattern and pulse parameters I use a machine-learning algorithm to retrieve the best approximation function for the laser pulse from experimentally measured multiphoton autocorrelation in Ar
Analytical and computational investigations in atomic attosecond physics
No full textUnderstanding the interaction of matter with ultrashort attosecond light pulse provides fundamental insights into the structure of matter. The electronic properties are reflected in wave packets dynamics. The ability to trace the evolution of electrons in wave packets is a big challenge. It requires ultrashort pulse duration and precise pulse characterization. In this thesis the time-delayed scanning of the atom is used to (1) characterize the inner atomic dynamics, via excitation of the atom by a time-delayed attosecond extreme ultraviolet (XUV) and femtosecond optical field, and to (2) characterize a laser field, via multiphoton ionization of the atom by the autocorrelation function of the field.The first part of the thesis explores the capability of attosecond transient absorption spectroscopy to characterize the dynamics of inner-shell-excited systems. I discuss an unusual kind of pump-probe experiment, where information is gained from the absorption spectrum of an attosecond XUV pulse, which serves as a pump pulse at the same time. The optical pulse in this kind of experiment gives a reference time that provides a possibility to measure the time evolution of a system of interest. In the study, I use different theoretical approaches, treating one or both of the pulses as perturbative or non-perturbative. I present an analytical theory of attosecond transient absorption spectroscopy for perturbatively dressed systems and illustrate how the attosecond transient absorption signal reveals the real-time attosecond dynamics of the atom. In addition, I apply our study to atomic Xe and compare the theoretical predictions with experimental results.In the second part of the thesis a new method for laser pulse characterization is presented. It is based on a machine learning algorithm and is used to study multiphoton autocorrelations in Ar. I analyze the time-delay dependence of the ionization probability for given laser pulse parameters, such as photon energy, pulse intensity and pulse duration. Taking into consideration the mapping between the ionization-probability time-delay pattern and pulse parameters I use a machine-learning algorithm to retrieve the best approximation function for the laser pulse from experimentally measured multiphoton autocorrelation in Ar
Röntgeninduzierte Vielphotonenionisierungsdynamiken: Quantenzustandsaufgelöste Berechnungen und maschineller Lernansatz
No full textX-ray free-electron lasers (XFELs) offer unique opportunities for unraveling ultrafast dynamics in matter and for imaging biomolecules with almost atomic resolution. For these attractive applications of XFELs, deepening our understanding of the interaction of high-intensity X rays with individual atoms is essential. In a single intense X-ray pulse, atoms are highly ionized by multiple sequences of one-photon ionization events and accompanying decay processes. These X-ray multiphoton ionization dynamics are commonly simulated by a rate-equation approach for electronic configurations. However, the configuration-based rate-equation approach does not include individual quantum states and, thereby, is unsuitable for studying the electron-cloud alignment of the produced atomic ions. It has been well-known that atomic photoionization can align an electron cloud, initially being perfectly spherically symmetric. But it is not clear how the alignment of the electron cloud evolves during X-ray multiphoton ionization dynamics. However, such a study requires a computationally expensive description of individual quantum states and quantum-state-resolved atomic transitions and includes solving rate equations in a generally extremely large space of states. In this thesis, I present a comprehensive framework for quantum-state-resolved calculations of X-ray multiphoton ionization dynamics of atoms and apply machine learning for handling the high computational cost.
To this end, a quantum-state-resolved electronic-structure framework for isolated atoms and atomic ions is introduced in the first part of the thesis. This framework uses first-order many-body perturbation theory, which improves accuracy of transition energies. In addition, I employ quantum-state-resolved electronic-structure calculations to study how much the electron cloud of argon ions can be aligned through a single X-ray-induced atomic transition. A nonnegligible degree of alignment is observed.
Combining the quantum-state-resolved electronic-structure framework with a Monte Carlo rate-equation method in a follow-up study enables me to calculate quantum-state-resolved X-ray multiphoton ionization dynamics. Results for neon atoms demonstrate that state-resolved calculations provide similar charge-state distributions, but more precise information about resonant excitations and electron and photon spectra than the common configuration-based calculations. Moreover, calculated time-resolved spectra of electrons and photons present detailed insight into ultrafast dynamics of state-resolved X-ray multiphoton ionization. However, performing such state-resolved calculations for atoms much heavier than neon is very costly due to extremely time-consuming state-resolved calculations of a very large number of atomic transition parameters.
Because of this limitation, I next present a strategy that embeds machine-learning models for predicting atomic transition parameters into the state-resolved calculation of X-ray multiphoton ionization dynamics. As potential machine-learning models, I discuss feedforward neural networks and random forest regressors, which exhibit a similarly acceptable, but limited accuracy. In addition, fully calculated and machine-learning-based charge-state distributions and electron and photon spectra are compared for argon atoms. The comparison demonstrates that the machine-learning strategy works in principle and that the performance, in terms of charge-state distributions and electron and photon spectra, is good.
Lastly, I apply the state-resolved X-ray multiphoton ionization dynamics calculations to explore the possibility to align the electron cloud of argon ions through a linearly polarized XFEL pulse. The induced X-ray multiphoton ionization dynamics generate ions in a wide range of charge states with nonzero orbital- and spin-angular momentum. While the electron-cloud alignment is suppressed with progressing ionization dynamics when averaging over all individual quantum states, the simulations clearly demonstrate nonnegligible electron-cloud alignment for orbital-angular-momentum- and charge-resolved states.
Overall, this thesis contributes to a deeper understanding of the interaction of XFEL pulses with atoms by providing more accurate state-resolved information, complemented by insight into electron-cloud alignment dynamics. It also establishes a first step toward computationally efficient calculations of X-ray multiphoton ionization dynamics for easily examining a variety of atoms and XFEL beam parameters in the future
Extension of the ring polymer molecular dynamics approach to state-selective molecular reactive scattering simulations
No full textThe state-resolved knowledge of molecular scattering dynamics is paramount to the understanding and control of chemical reactions. The ideal approach to rigorously simulate such systems is a full-dimensional treatment with quantum mechanical
calculations. Current generation of computers allows such calculations to be carried out up to six atoms for zero total angular momentum. It is not likely that the computational power will allow significant progress in the near future, as the computational time grows exponentially with the number of degrees of freedom.
Thus, there is a demand for alternative methods that would allow the inclusion of quantum effects while remaining computationally efficient.
This thesis consists of the development of such an approach. Inspired by the success of the efficient ring polymer molecular dynamics (RPMD) approach applied to quantum systems at thermal equilibrium, I extend the scope of RPMD to stateselective chemistry.
I start by presenting the method development groundwork toward microcanonical simulations of triatomic reactions using RPMD. I expose the steps to prepare a specific initial vibrational state for a diatomic molecule. The assessment of the method revolves around the computation of integral cross sections (ICS) in the ring polymer phase space. I report an ansatz for the reciprocal temperature depending on the characteristics of the reactions. The benchmark reactions Mu/H/D +H2 with H2 either in its ground (v = 0) or first excited vibrational state (v = 1) are chosen to test the method. Good agreement for the Mu/H/D + H2(v = 0, 1) reactions with exact quantum scattering calculations is found. It is shown that RPMD can describe to a good approximation zero-point energy (ZPE) effects and tunneling effects for these reactions while remaining computationally efficient. Following these encouraging preliminary results, the robustness and applicability of the method is further tested. I present a detailed review of the approach. The reactions Mu/H/D/Cl/F+H2(v = 0, 1) are considered over a wide range of collision energies. The accuracy and stability of the vibrational excitation scheme are tested. It is found that the ICS results are strongly sensitive to the spring constant of the ring polymers. A refined ansatz for the reciprocal temperature is suggested. I observe a convergence of the ICS above a number of beads that depends on the characteristics of the diatomic reactant. I show that the tunneling contributions to the reactivity stem from the spatial extension of both ring polymer reactants. However, it is found that RPMD is unable to describe dynamical resonance effects. Shortcomings in the vibrational excitation scheme at low collision energies are reported.
Finally, I further test the approach with larger and more intricate systems. In particular, the reactions F,H+CH4 and its isotopic variants for the case of the ground state CH4 and CHD3 and, in the presence of the C-H excited stretch in CHD3. Accurate RPMD ICS results for most of the aforementioned reactions are reported.
It is found that the ZPE leakage problem usually present in classical dynamics is prevented. It is found that the vibrational excitation scheme is accurate for most purposes except for reactions involving a very low energy barrier
A simulation framework for studying high intensity x-ray induced dynamics and scattering patterns from nanocrystals
No full textA standard method of reconstructing the structure of a protein in its crystalline phase is by x-ray diffraction. New generation x-ray sources, the X-ray free-electron lasers (XFEL), provide novel opportunities for biomolecular structure determination. The extreme intensity and ultrashort pulse duration of an XFEL pulse make it feasible to extend the diffraction technique towards nano sized crystals. However, during a high-intensity measurement, significant atomic and electronic dynamics occur that affect the diffraction signal. Simulations of the ionization dynamics of an irradiated nanocrystal and the diffraction pattern formed are computationally expensive. To overcome this bottleneck, I have developed a methodology implemented as computer codes. I have applied the methodology for specific problems: for identifying the characteristic features of the spatial beam profile imprinted on the scattering pattern, analyzing effective form factors at high intensity and in studies of high energy density plasma formation
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