1,723,567 research outputs found
Optimal-continuum and multicentered Gaussian basis sets for high-harmonic generation spectroscopy
No full text.High-harmonic generation (HHG) is used to produce coherent XUV and soft X-ray radiation with atto-second resolution and is a sensitive tool for probing atomic and molecular structures. In this work, we have used time-dependent configuration interaction with a Gaussian basis set to compute the HHG spectrum of the hydrogen atom. To get a correct description of the HHG optical spectrum, the Gaussian basis set has to provide an accurate representation of the bound and the continuum states. Two strategies have been proposed: (1) multicentered (defining ghost atoms around the hydrogen) and (2) optimal-continuum Gaussian basis sets. We have systematically investigated these two approaches for the hydrogen atom, which permits a non-biased analysis of the basis set. Several basis sets have been constructed and tested by combining multicentered and optimal-continuum functions together in order to obtain a reliable and accurate Gaussian basis set to be used for HHG. We have studied the effect of changing the number of ghost atoms and the distance between the ghosts and the hydrogen atom, with and without optimal-continuum Gaussian functions. We conclude that multicentered basis sets are less efficient than basis sets using only optimal-continuum Gaussian functions for a proper description of HHG
Time-dependent ab initio approaches for high-harmonic generation spectroscopy
No full textHigh-harmonic generation (HHG) is a nonlinear physical process used for the production of ultrashort pulses in XUV region, which are then used for investigating ultrafast phenomena in time-resolved spectroscopies. Moreover, HHG signal itself encodes information on electronic structure and dynamics of the target, possibly coupled to the nuclear degrees of freedom. Investigating HHG signal leads to HHG spectroscopy, which is applied to atoms, molecules, solids and recently also to liquids. Analysing the number of generated harmonics, their intensity and shape gives a detailed insight of, e.g., ionisation and recombination channels occurring in the strong-field dynamics. A number of valuable theoretical models has been developed over the years to explain and interpret HHG features, with the three-step model being the most known one. Originally, these models neglect the complexity of the propagating electronic wavefunction, by only using an approximated formulation of ground and continuum states. Many effects unravelled by HHG spectroscopy are instead due to electron correlation effects, quantum interference, and Rydberg-state contributions, which are all properly captured by an ab initio electronic-structure approach. In this review we have collected recent advances in modelling HHG by means of ab initio time-dependent approaches relying on the propagation of the time-dependent Schrödinger equation (or derived equations) in presence of a very intense electromagnetic field. We limit ourselves to gas-phase atomic and molecular targets, and to solids. We focus on the various levels of theory employed for describing the electronic structure of the target, coupled with strong-field dynamics and ionisation approaches, and on the basis used to represent electronic states. Selected applications and perspectives for future developments are also given
Role of Inner Molecular Orbitals in High-Harmonic Generation Spectra of Aligned Uracil
No full textIn this work, we decompose the high-harmonic generation (HHG) signal of aligned gas-phase uracil into single molecular-orbital (MO) contributions. We compute HHG spectra for a pulse linearly polarized perpendicular to the molecular plane, with an intensity of 0.6 and 0.85 × 1014 W/cm2 and a wavelength of 800 nm. We use the real-time time-dependent Configuration Interaction with singles method, coupled to a Gaussian-based representation of the time-dependent wavefunction. The strong-field dynamics is affected by the energy of the ionization/recombination channels and by the coupling between the orbital symmetry and laser polarization. In the configuration studied here, we expect that π-type MOs favorably couple with the incoming pulse and play a substantial role in generating the HHG spectrum. Indeed, we show that HOMO, HOMO - 1, and HOMO - 4, which all are π-like, determine the intensity of harmonic peaks at different energies, while HOMO - 2 and HOMO - 3 provide a smaller contribution. It is worth mentioning that HOMO - 4 produces a stronger signal than that from HOMO - 1, even though the corresponding ionization energy, in an one-electron picture, is around 2.5 eV larger and more than 4 eV larger than the HOMO one
Detecting the minimum in argon high-harmonic generation spectrum using Gaussian basis sets
No full textCoupling the real-time description of the ultrafast electron dynamics in strong laser fields with quantum chemistry techniques still represents an open challenge for theoreticians. In this work, high-harmonic generation (HHG) spectrum of the argon atom has been computed by means of the time-dependent configuration with singly excited configurations approach (TDCIS), using Gaussian basis sets. We show that adding a number of continuum-optimal Gaussian functions to basis sets routinely used in quantum chemistry calculations provides the expected position of the intensity minimum in the HHG spectrum, for a given selection of laser intensities and ionisation parameters. Advantages and weaknesses of the proposed computational strategy are discussed. We give evidence here that Gaussian-based TDCIS simulations are accurate enough to correctly reproduce the features of ultrafast and highly nonlinear optical processes, as HHG
Facing Adversity at the University. A Case Study to Reflect on Pedagogical Challenges in Times of Pandemic Risk
No full textIn a few months, the coronavirus (COVID-19) affected a large part of the world, forcing a huge number of extremely different countries to face common challenges. One of the most interesting and crucial challenges is being played out in the educational field. This chapter focuses on describing how the University of Bologna (UNIBO) is facing the pandemic emergency by presenting, in particular, empirical research on a study program. By exploring the first results of a survey (pre- and post-emergency distance learning) addressed to a target group of Veterinary Medicine students, we are going to highlight some emerging trends and to reflect on the impact of this emergency on teaching and learning practices
Impact of assembly algorithms on end-to-end performance in Optical Burst Switched networks with different QoS classes
No full textThis paper investigates the performance of a complete OBS (Optical Burst Switched) network that implements the JET (Just Enough Time) reservation mechanism. The network under examination adopts a core node architecture with no fiber delay lines and a limited set of wavelength converters, while featuring an edge node architecture with a mix of input, output and shared buffers. We investigate the overall network performance and design, specifically focusing on burst delay and end-to-end TCP throughput. In order to study the OBS network behavior, we develop a modular and flexible simulation tool, that we call MOBSSIM (Modular OBS Simulator). MOBSSIM is the means to accurately build an arbitrary topology OBS network via its main functional blocks, edge and core routers: its sound degree of development enables us to accurately evaluate several figures of merit, e.g., burst blocking probability and delay, and also allows for a critical comparison of alternative design solutions in terms of assembly algorithms, reservation strategies and QoS oriented routing
L'istruzione degli alunni con disabilità, DSA e BES
No full textsi affronta, con attenzione all'evoluzione del dato normativo di riferimento, ma altresì alla tutela resa dalla qui abbondante giurisprudenza sia amministrativa che ordinaria, il tema dell'integrazione/inclusione scolastica di ragazzi/alunni portatori di handicap e disturbi specifici di apprendimento (quali la dislessia) e altri bisogni educativi special
TCP Performance for Vehicular Users Through Satellite Links
No full textIn this paper the reference system is given by wireless usersaboard of fast moving vehicles, such as trains, connected to theInternet by means of satellite links. Performance of several TCPflavors are investigated in this scenario using a PerformanceEnhancing Proxy architecture. TCP performance are reported also in presence of UDP flows and some enhancements, such as large initial window and window scale, and are evaluated in terms of throughput and fairness. Results show that TCP Vegas is the best performing when large initial window and window scale options areimplemented
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
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