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Infrared waveform synthesis for applications in attosecond science

Infrared waveform synthesis for applications in attosecond science

Soon after it was realized that other branches of the attosecond physics, namely time- resolved attosecond polarization spectroscopy [12], may largely benefit from the availability of the sources with lower (in comparison to ∼ 1.6 eV (780 nm)) photon energies. The tech- nique analyze the imprint that the carrier dynamics in scope leaves on the transmitted electric field. Several experiments have concentrated on scrutinizing the band gap dynam- ics of dielectric materials, including silicon oxide, that are of interest and importance for application to light–wave–governed signal processing. Performing similar investigations with the conventional semiconductor materials that have a much lower band gap, how- ever, is less sensible due to the high photon energy of the excitation pulses that bleaches the material and hinders the unambiguous interpretation of the measurements along with damaging the sample. Few–cycle sources with the lower excitation energies are more ben- eficial in this respect. Apart from the central wavelength, another crucial requirement is, however, a phase–stability of the driver that is needed for the field–sensitive measurements. In the lack of broadband infrared gain media, generation of few–cycle phase-stable pulses in the infrared is substantially more challenging than in case of Ti:Sapphire–based systems. The access to the wavelengths in the infrared is possible through difference frequency generation. This path in some cases also enables passive CEP–stabilization of the generated spectrum via phase cancellation in the DFG process – pivotal requirement for field–resolved measurements – and due to that this approach quickly became a method of choice [84, 85]. Subsequent amplification in one or more OPA stages [46, 86, 87, 60, 61] boosts the output energy by several orders of magnitude.
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Tunneling Time in Attosecond Experiments and Time Operator in Quantum Mechanics

Tunneling Time in Attosecond Experiments and Time Operator in Quantum Mechanics

Abstract: Attosecond science is of a fundamental interest in physics. The measurement of the tunneling time in attosecond experiments, offers a fruitful opportunity to understand the role of time in quantum mechanics (QM). We discuss in this paper our tunneling time model in relation to two time operator definitions introduced by Bauer and Aharonov–Bohm. We found that both definitions can be generalized to the same type of time operator. Moreover, we found that the introduction of a phenomenological parameter by Bauer to fit the experimental data is unnecessary. The issue is resolved with our tunneling model by considering the correct barrier width, which avoids a misleading interpretation of the experimental data. Our analysis shows that the use of the so-called classical barrier width, to be precise, is incorrect.
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Synthesis and control of attosecond light transients

Synthesis and control of attosecond light transients

The accurate characterization of ultrafast laser pulses is, nowadays, an important asset in ultrafast science. Several techniques have been developed to this end. The autocorrelation-based techniques, for example, are one of the most commonly used. Moreover, more sophisticated techniques, such as frequency-resolved opti- cal gating (FROG) [60] or spectral phase interferometry for direct electric field reconstruction (SPIDER) [61], have advanced the field of ultrafast metrology even further, since they have allowed access into the spectral phase of the pulse. In the last decade, attosecond science has made possible yet another essential technique, namely “attosecond streaking”. This technique utilizes attosecond extreme ultra- violet pulses as samplers of the instantaneous field waveform of a pulse and is virtually insensitive to the pulse bandwidths. In this chapter, will be given a con- cise description of these techniques, especially due to the fact that they are critical for the work presented in this thesis
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Attosecond dynamics of collective electron effects in nanostructures and molecules

Attosecond dynamics of collective electron effects in nanostructures and molecules

After Einstein’s explanation of the photoelectric effect by the quantum nature of light [71], the photoionization (PI) process became one of the most-studied phenomena in physics. Based on Hertz’s experimental findings [64], for a long time, the electron-detachment from bound states in atomic or solid state targets was considered to happen instantaneous. However, pioneering theoretical works by Eisenbud [132], Wigner [130] and Smith [133] posed first questions on this simple picture of photoionization. The experimental verification became possible only re- cently with the advent of attosecond science [3], where attosecond streaking spectroscopy was applied to investigate the detachment of electrons from different initial states of atomic neon. In their experimental study, Schultze et al. obtained a time delay of around 20 as between the emission of photoelectrons from neon 2s and 2p orbitals [14]. Based on numerous theoretical calculations (see e.g. Ref. [134] for an overview), and under consideration of influences from the experimental method, the time delay is interpreted as Eisenbud-Wigner-Smith (EWS) delay, which originates from phase shifts accumulated by the photoelectron wave packet while propagating through the binding potential of the atomic nucleus during the ionization pro- cess. In reverse, the interpretation implies, that time delays measured in attosecond streaking experiments hold information about the orbital potential landscape encountered by the pho- toelectrons during the photoionization process. Given the high temporal resolution, not only static orbital potentials can be detected but even dynamic changes upon NIR interaction or collective electron interactions are acquired directly in the time domain.
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Attosecond Electron Spectroscopy

Attosecond Electron Spectroscopy

Attosecond science comprises two frontiers: (i) the generation and characterization of increasingly intense, energetic, short and isolated attosecond pulses; and (ii) the design of experiments to probe physical systems on the attosecond time scale, the holy grail being the attosecond pump-attosecond probe time-resolved spectroscopic measurement. The second frontier offers a deeper understanding of the temporal behavior of the microcosm, but relies on advancements made in the first one. At present, both of these frontiers heavily rely on the attosecond streaking technique, which consists in energy-resolving photoelectrons ejected by an attosecond extreme ultraviolet pulse, in the presence of a phase-stabilized and temporally synchronized near-infrared field. Although it was originally devised as a means to characterize attosecond pulses, this measurement technique has even produced new discoveries in atomic and solid-state physics, due to pioneering experiments by M. Drescher, A. Cavalieri, G. Sansone, M. Schultze, and others, and has inspired novel theories of laser-dressed photoionization by V. S. Yakovlev, A. Scrinzi, O. Smirnova, M. Y. Ivanov and others.
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Attosecond dynamics of nano-localized fields probed by photoelectron spectroscopy

Attosecond dynamics of nano-localized fields probed by photoelectron spectroscopy

The goal of this work is to investigate the combination of these two major developments, namely the application of femto- and attosecond laser science in the nanoworld. One key subject is the interaction of intense few-cycle laser fields with (dielectric) nanosystems. For single atoms in intense light fields, there is today a basic understanding of the ionization and subsequent processes [15] which serves as the backbone of modern attosecond science. A number of studies extend these concepts to many body systems. It was found that the ionization dynamics of molecules can also be controlled by the laser waveform [16]. The in- teraction of atomic clusters with intense laser fields creates essentially a nanoplasma, which challenges the theoretical models [17]. A first question that will be addressed in this work is to what extent the ionization dynamics will be modified by the near-field localization of light at the nanoparticle surface and the collective response of ionized electrons. Compared to atoms or molecules the potential landscape differs strongly, being highly asymmetric at the surface and likely to be further influenced by free charges. These issues are addressed in this work by studying the photoemission from dielectric silicon dioxide nanoparticles. Special emphasis is placed on the carrier-envelope-phase of the driving laser field. Knowl- edge of the waveform defined by the CEP greatly enhances the information obtained from the experiment as one can directly identify the field driven processes.
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Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

the few-femtosecond or even attosecond domain. Hence, avoiding space charge, i.e. using single-electron pulses, is a crucial requirement for achieving ultimate temporal resolutions with UED [ ]. With absent broadening due to space charge, only the dispersion of vacuum for electrons and their energy spread have to be taken into account. Reducing the energy spread of the electrons by minimizing their excess kinetic energy after photoemission [ – ] not only increases the coherence but also reduces the temporal broadening of single-electron pulses due to dispersion [ ]. In the absence of space charge, dispersion compensation with time-varying longitudi- nal electric fields transforms the phase space of the electrons almost linearly (cf. section ), allowing for temporal compression to nearly arbitrarily short pulse du- rations at the expense of an accordingly increased energy spread [ , – ]. This approach is demonstrated in this work. However, the limitation to one electron per pulse constitutes a considerable constraint for practical UED. For an adequate signal-to-noise ratio of a diffraction image, 10 5 –10 6 electrons (or more, depending on the studied system and desired resolution) need to be collected by the detector [ ]. This necessitates very long acquisition times or very high pulse repetition rates in the 100 kHz to MHz range, respectively, when using single-electron pulses. The studied dynamics are therefore limited to reversible processes and highly stable specimens capable of withstanding millions of consecutive pump-probe cycles [ , ]. When using optical pump pulses for excitation at high repetition rates and nanometer thin samples for UED in transmission, thermal damage can be a substantial issue [ ].
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Photoemission electron microscopy for nanoscale imaging and attosecond control of light-matter interaction at metal surfaces

Photoemission electron microscopy for nanoscale imaging and attosecond control of light-matter interaction at metal surfaces

reduced from some tens of hours down to several minutes, facilitating reasonably fast and reliable measurements. For instance, it has been shown that the image acquisi- tion time was reduced by a factor of ∼10 when the repetition rate was increased by a factor of ∼200 while the spatial resolution was improved about 3-fold [ , ]. Nevertheless, we foresee possible new challenges, which are not present in the case of PEEM experiments using femtosecond laser pulses. The first and foremost issue is chromatic aberrations caused by the large energy bandwidth of XUV-generated photoelectrons. Fortunately, this can be resolved by applying energy-filtering in our ToF-PEEM to particularly select the primary valence band electrons from the sec- ondary electron background, which we have demonstrated in this work. The next steps towards the realization of atto-PEEM will encompass energy-filtered imag- ing of fast valence band electrons and a spatial resolution down to 100 nm or even lower when low-megahertz attosecond XUV sources are available. Alternatively, an aberration-corrected PEEM equipped with an energy analyzer [ ] can be employed specifically for high-megahertz XUV sources since it does not depend on repetition rate. This type of PEEM has been proven to achieve a theoretical spatial resolution of ∼4 nm [ ], which will possibly enable energy-resolved imaging of the instanta- neous electron acceleration in a highly localized nanoplasmonic field within a ∼1 nm region, as originally proposed by M. I. Stockman et al. [ ]. Recent theoretical [ , ] and experimental studies [ ] of nanoplasmonic streaking without a microscope suggest lowering the stringent requirement for probing plasmonic fields in the in- stantaneous regime as it was demonstrated that a reconstruction of the fields is still attainable in the classical oscillatory regime.
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Carrier-envelope phase control for the advancement of attosecond pulse generation

Carrier-envelope phase control for the advancement of attosecond pulse generation

An array of concepts exist that aim to generate attosecond pulses at megahertz repetition rate, all of which are based on diode-pumped laser systems in combination with a scalable thermal management. This chapter gave an overview of the concepts and introduced the various points at which CEP stabilisation is a crucial factor for further progress. Exemplary experiments on CEO frequency measurement and stabilisation were conducted. The systems in question cover two laser architectures that can be expected to be at the core of a future source of megahertz XUV radiation. First, the output of a thin-disk laser was shown to be compressible to few-cycle duration, with CEP stabilisation being the logical next step. Although the cavity-external technique was found inapplicable in this case, the experiment later led to the first demonstration of a CEP-stable thin-disk laser. In conjunction with external compression, the laser provided waveform-controlled few-cycle pulses comparable to that of state-of-the-art Ti:sa oscillators, but at two orders of magnitude higher energy and average power. Another attractive future application of such an oscillator is intracavity high-harmonic generation. With moderate power scaling, the presented laser system would provide the necessary intensity level, leading to an XUV source of considerably reduced complexity as opposed to amplifiers or enhancement cavities. In combination with novel approaches for CEP stabilisation [Bal14], it might even provide phase-stable XUV pulses. The second system presented in this chapter was based on a master-oscillator power-amplifier architecture that combined a conventional bulk oscillator with two stages of fibre CPA. It was intended for the use of EC seeding, an approach that perfectly complements the low efficiency of HHG. However, this scenario calls for simultaneous stabilisation of both frequency comb parameters, and thus places numerous constraints on the employed seed laser. Detection of the CEO frequency was demonstrated by use of a nonlinear fibre amplifier, but only radian-level phase stability of the CEO could be achieved in a feed-back configuration. However, even this coarse level of stabilisation was sufficient for a visible improvement of the EC lock parameters. The findings highlight the necessity for full comb stabilisation in order to reach cavity enhancement of pulses lasting as short as a few tens of femtoseconds. The obstacles encountered in CEP-stabilisation of the system could be traced back to the fibre amplifier stages, and clues for further investigation were given. Mitigation of these obstacles can be expected to pave the way towards the EC locking of pulses approaching the 10-20 fs range. In combination with gating techniques and novel output coupling methods, cavity enhancement could even make isolated attosecond pulses available at megahertz repetition rate.
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Generation, characterization and sub-cycle shaping of intense, few-cycle light waveforms for attosecond spectroscopy

Generation, characterization and sub-cycle shaping of intense, few-cycle light waveforms for attosecond spectroscopy

On even smaller dimensions, on the level of atoms, for our studies attosecond res- olution becomes necessary. For example, the quantum-mechanical "breathing" of the superposition state of the 1S-2S levels of Hydrogen has an associated period of 400 as. Going more and more deeper inside atoms, where the energy spacing becomes larger and larger we even start to approach the atomic unit of time, which is 24 as. Attosecond physics in its current implementation which relies on attosecond streaking uses both a laser pulse (probe) that has a strong, high gradient electrical eld, with a rise-time al- ready in the attosecond range, and an attosecond pulse (pump) that can liberate bound electrons. In fact, such a scheme is used not only for tracking the evolution of funda- mental processes but also for characterizing both of these pulses [ 7 , 8 ]. It is basically a variation of the streak camera [ 9 ], where the time-evolution of a light pulse is mapped into a distribution of electrons in space by a fast-varying electrical eld. In the version of its attosecond implementation, it is the high gradient laser electric eld that acts as the streaking eld to create in energy a distribution of the photoelectrons. Attosecond streaking has allowed us so far to access a plethora of electron dynamics, the rst one being that of Auger-decay [ 10 ], while the latest ones being shake-up [ 11 ] (both connected to the rearrangement of the electrons after photoionization), and energy-band dependant photoemission in solids [ 12 ], to name a few. More experiments are expected to follow as collective motion of electrons (plasmons) on surfaces [ 13 ], charge-transfer in molecular structures, electron-electron interactions all happen on an attosecond time-scale.
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A laser source for the generation of intense attosecond pulses and its first applications

A laser source for the generation of intense attosecond pulses and its first applications

In order to increase the output power of the Ti:Sa laser system a second cryogenic cooled amplifier stage was added. Two different dispersion management setups were tested. The first used a grism stretcher and glass block compressor. Although the efficiency of the compressor was, with more than 85 %, very high the beam was distorted by the transmission through the glass blocks and good pulse compression was limited to half the available power. The second setup used a glass stretcher and transmission gratings for compression. The efficiency is comparable and the beam quality after compression was improved by a lot. By fine-tuning the Dazzler, a 3 mJ 22 fs 4 kHz system was set up. The pulses are CE-phase stabilized, broadened and compressed to sub-2-cycle pulse duration with more than 1 mJ pulse energy pro- viding a powerful laser source for nonlinear applications especially HHG and thus attosecond spectroscopy. Quite a few laser systems with more than 1 mJ and a pulse duration with less than 6 fs have been published. Mashiko et al. [ 72 ] show a phase stable 1 kHz laser system with 1.2 mJ and 5.6 fs pulses from a CPA stage cooled with liquid nitrogen. A very similar system was presented by Sung et al. [ 73 ]. Very recently Jullien et al. [ 74 ] presented a contrast cleaned 22 fs 8 mJ Ti:Sa.-laser system at 1 kHz with phase stabilization. If they find an efficient way to broaden this pulse they might be able to generate 4 mJ sub-5 fs pulses soon.
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Attosecond nonlinear spectroscopy of core-shell excitations in atomic and solid state systems

Attosecond nonlinear spectroscopy of core-shell excitations in atomic and solid state systems

Based on Equation (5.10), it is possible to derive an expression similar to the one used in Frequency Resolved Optical Gating for the Complete reconstruction of Attosecond Bursts (FROG-CRAB) [55], and in principle to apply a principal component generalized projections algorithm (PGCPA) [120] to extract both the phase and amplitude gate, and the gated dipole as a function of time. This derivation is developed in Appendix B. Our few attempts to use it have failed in the case of decoherence times longer than the optical pulse. This is due to the slight difference in mathematical formulation with a FROG expression. Moreover, tests on perfect numerical data have revealed that the method is not sensitive enough to allow the PGCPA algorithm to converge towards satisfying pump and gate fields. It is however a quite general concept which could be investigated further as an extension of the work presented here.
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Science NextGen Voices: Science-inspired sustainable behavior.

Science NextGen Voices: Science-inspired sustainable behavior.

Science diplomacy uses scientific partner- ships among nations to solve common problems. As a researcher in a war-torn country, I strive to translate this con- cept into reality by making changes at the national level through international scientific collaborations. My research on infectious diseases necessitates cross-boundary efforts. Through build- ing effective collaborations, I have implemented several projects funded by international organizations to ease the suffering of underserved communities trapped by conflict and war.

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Multilayer Mirrors for Attosecond Pulse Shaping between 30 and 200 eV

Multilayer Mirrors for Attosecond Pulse Shaping between 30 and 200 eV

The ultimate goal is the design of optics for the absolute control over all relevant attosecond pulse parameters. These are namely the central energy, the spectral shape, the spectral and temporal phase and the pulse duration. For the best compromise between these, detailed knowledge about how the spectral and temporal properties of a pulse are connected is required; most importantly how the spectral bandwidth and the spectral phase influence the pulse length (section 2.1). In section 2.2 properties of the XU V spectral range in which all presented attosecond experiments take place are summarized; different attosecond XU V optics are ana- lyzed according to their applicability in attosecond physics. The focus lies here on reflective multilayer coatings as introduced in sec- tion 2.2.4 as XU V optics with the highest tunability. Finally, in section 2.3 the principle of single attosecond pulse generation, their measurement via electron streaking and the experimental set-up, which is used for the following experiments is explained.
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Governing Science

Governing Science

Was the increase of low-impact publications in Australia caused by a funding formula that rewards number of publications [Butler 2003], by the preceding higher education reforms that confronted a large number of academics with increased expectations to conduct research and publish (a change described by Meek 1991), or by an increasing competition for project grants that emphasises track record, i.e. prior publication (a development described by Gl €aser and Laudel 2007; Gl€aser et al. 2010 )? Is it possible to ascribe the increase in “good” Danish publications to the performance-based funding scheme [Ingwersen and Larsen 2014]? The authors excluded changes in research funding, the increase in academic staff, and the internal dynamics of the Web of Science database as explanations because the growth in productivity and impact exceeds the other trends. They do not consider the combined effect of these trends or the increasing concentration of research funding, either as an effect of competitive grant funding or due to the new programme for the funding of “Centers of Excellence”, which was launched in the same year as the performance-based funding scheme [Langfeldt et al. 2015]. They describe but do not explain the observation that at least one university does not conform to the pattern. Aagaard and colleagues conducted the most complex study of this type to date. They analysed the introduction of the Norwegian publication indicator [Schneider et al. 2014; Aagaard 2015; Aagaard et al. 2015]. The authors combined bibliometric studies, surveys, and interviews in order to assess the impact of this indicator’s introduction on research. “Research” was operationalised as publication behaviour.
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How earth science has become a social science

How earth science has become a social science

The incorporation of social perspectives into the earth sciencea is thus al- most certainly a good idea, and how it is being done surely a topic for further examination. Yet at the same time, it would be difficult to describe these trends as “discipline-disregarding,” as most of the practitioners involved still consider what they do to be earth science. The IPCC, as I have stressed, maintains an organizational structure that presupposes a clear separation between the physi- cal and social sciences; seismologists still call themselves seismologists; hy- drologists are still hydrologists. One sees only modest attempts on the part of earth scientific journals or societies to change their names or compass to reflect a broader concern with topics that fall outside traditional definitions of earth science. 14 Rather, it seems more accurate to suggest that at least some earth scientists are doing social science without quite acknowledging that this is what they are doing, without adequate training and understanding of social phenom- ena, and, in the worst cases, without respect for colleagues who have greater experience and insights into the workings of social systems. In the process, they may do it poorly, to the detriment of the society that they believe them- selves to be serving.
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Open Science

Open Science

wir uns auf den Start des Datenrepositoriums KonDATA, der unmittelbar bevorsteht. Darüber hinaus soll die Universität eine Open-Science- Policy bekommen, wir entwickeln Konzepte zu Publikationsdienstleistungen und haben den Open-Access-Publikationsfonds auch auf Monografien ausgeweitet. Nicht zuletzt starten 2021 mehrere neu eingeworbene Projekte – ein spannendes Jahr ist garantiert.

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Open Science und Citizen Science als symbiotische Beziehung? Eine Gegenüberstellung von Konzepten

Open Science und Citizen Science als symbiotische Beziehung? Eine Gegenüberstellung von Konzepten

Werkzeugen. Für eine Umsetzung von Aktivitäten in diesem Schnittfeld sind erweiterte Anreizsysteme und die Bedarfe der unterschiedlich verfassten Communities einzubeziehen. Wesent­ licher für Citizen Science aber ist, dass die Möglichkeit der Be­ teiligung erweitert wird und sich Qualität nicht nur auf die Daten, sondern auch auf die Art der Zusammenarbeit bezieht. Ressour­ cen werden nicht nur benötigt, um die entsprechenden Infra­ strukturen auszubauen, sondern auch, um Kommunikation und Inklusion im Hinblick auf verschiedene soziale Gruppen und ihrer Interessen in der Gesellschaft zu fördern. Hier können die Open­Science­Strategen von den Citizen­Science­Communities lernen und Bürgerinnen und Bürger nicht nur als Zulieferer von Daten betrachten, sondern stärker als Treiber von Innovation im Hinblick auf einen Beitrag zum großen Ganzen: einer gesell­ schaftlichen Transformation hin zu mehr Nachhaltigkeit. In die­ sem Zusammenhang ist auch mehr Austausch zwischen akade­ mischer Wissenschafts­ und Technikforschung und den Praxis­ kontexten der Citizen Science wünschenswert (Mahr et al. im Druck), um kritische Reflexion stärker in den jeweiligen Diskur­ sen zu verankern, von Forschungsergebnissen aus anderen Pra­ xisfeldern zu profitieren und zukünftige Entwicklungen verant­ wortlich zu gestalten.
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Data Science

Data Science

the regular article acceptance rate was approx. 33 % while the extended abstract acceptance rate was approx. 63 %. Given the exceptionally high quality of submissions on already published works, the relatively high acceptance rate among extended abstracts was to be expected. The extended abstracts eventually included in the proceedings describe works that published in some of the worldŠs most prestigious data science outlets, including venues like NeurIPS, SIGKDD, IEEE BigData, WWW, IEEE Big Data, Hypertext, ICDM, ICLR, SIGSPATIAL, IEEE DSAA, Machine Learning, and ESWC. We see this as testimony to the exceptional quality of research done within the German, Swiss, and Austrian data science community and we thank all authors for their valuable contributions to this track.
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Different images of science at Nordic science centres

Different images of science at Nordic science centres

Lately, museums and science centres have been criticized and questioned when science has been presented in a too narrow-minded way (Pedretti, 2002; Menved and Oatley, 2000; Frø y land and Henriksen, 2003). Pedretti (2002) contends that many museums and science centres just show ”the wonders of science”, i.e. an unproblematic, product-focused way that shows the ”good things” we humans have accomplished through science. She argues that there is a need for change; a need for diverting attention away from the wonders of science to exhibitions related to contemporary and sometimes even controversial science. Such exhibitions enhance learning through an increased attention on context - not only the context in which science operates, but also the visitors’ contexts. By promoting a public debate about science, and not just presenting scientific facts, it entails understanding the nature, processes and achievements of science. It also entails critiquing the institution and practice of science (Pedretti 2002). Other scholars argue for integrating experiences from museums or science exhibitions into the visitors’ every-day life, linked to different social and cultural activities. This places scientific principles in more familiar contexts and could provide a starting point for reflecting on 7
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