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Time-resolved and Imaging Techniques for Photoionization Studies of Atoms, Molecules and Clusters
Full text link2013/2014Photoionization studies of atoms, molecules and clusters by means of VUV and soft X-ray radiation have greatly benefited from the advance in new experimental methods. In particular new generation high-brightness light sources such as Synchrotrons and Free Electron Lasers (FEL) have increased the variety of experiments and also the range of accessible phenomena. Nevertheless a parallel development in experimental techniques and readout systems is needed as well.
My doctoral project was carried out at Elettra, Trieste, the Italian national synchrotron radiation (SR) laboratory. It is integrated into the activities of the local Atomic and Molecular Physics group, which aims at studying the electronic structure of isolated systems of increasing complexity such as atoms, molecules and clusters in the gas phase. At Elettra, in fact, the opportunity of combined access to the GasPhase beamline of the Elettra storage ring and to the Low Density Matter beamline of the FERMI FEL source allows covering a wide spectrum of single and multiphoton processes by means of advanced photoionization methods, for a thorough description of both energetics and dynamics of isolated systems.
The collaboration between the Atomic and Molecular Physics group, and the Elettra Instrumentation and Detector Laboratory, has resulted in a prototype set-up, built around a Velocity Map Imaging (VMI) spectrometer, with the flexibility to perform synchrotron radiation (SR) as well as FEL experiments, just by changing the last stage of detection:
-In SR experiments, at the GasPhase beamline, a crossed-delay-line detector is used, coupled to a 4-channel time-to-digital converter that reconstructs the arrival positions of charged particles. The VMI electron spectrometer can operate in tandem with an ion Time-of-Flight (TOF) mass spectrometer. Such a system allows PhotoElectron-PhotoIon-Coincidence (PEPICO) spectroscopy of atoms, molecules and clusters, by correlating the kinetic energy and emission angle of photoelectrons with ions of a specific mass;
- FEL experiments notably differ from SR experiments in the much higher rate of events produced and detected, which forces one to forfeit coincidence detection. At the Low Density Matter (LDM) beamline of the free electron laser FERMI, a Micro Channel Plate (MCP), a phosphor screen and a CCD camera are used; the system is capable of shot-by-shot collection of practically all events, albeit without time resolution. Femtosecond pump-and-probe experiments can be performed to access the electron dynamics.
Within this framework, the core of my experimental thesis is based on imaging and electron-ion coincidence techniques to investigate the energetics, the angular distribution and also dynamics of electrons and ions emitted upon VUV irradiation of atomic, molecular, and cluster targets, in parallel to a characterization of the light source used.
A thorough characterization of the two experimental set-ups has been performed and presented in this thesis work, as well as some recent results carried out with both set-ups with the purpose of exploiting the capabilities of the detection systems. In particular, helium clusters mass-correlated photoelectron images obtained with the SR, by means of PEPICO spectroscopy, have been reported. Atomic clusters are aggregates containing from few to a few thousand atoms. By studying the properties of clusters, one obtains information on the early stage of the growth of matter, and on the evolution of the properties towards the bulk. In this work detailed insight into the dynamics of photoexcitation and ionization of pure He clusters has also been gained; in particular we have observed inelastic collisions of outgoing photoelectrons with the surrounding He atoms in the clusters.
With the same SR setup, but by means of the triple coincidence PhotoElectron-PhotoIon-PhotoIon technique (PEPIPICO), the Interatomic Coulombic Decay (ICD) in rare gas clusters has been studied. ICD is a decay channel occurring in weakly bound systems, where electronic energy
is transferred (via virtual photon exchange) from an inner valence excited atom to a neighboring atom, which releases it by emitting a low energy electron. Predicted theoretically at the end of last century by Cederbaum, ICD has been more recently confirmed experimentally and, since then, it has become the object of numerous studies. ICD in fact appears to play a major role in a
large numbers of systems; e.g. low energetic ICD electrons produced in an aqueous environment have been demonstrated to be responsible for much of biological radiation damage. In this thesis experimental results on the energy distribution of the electrons emitted in the ICD process in rare gas dimers are reported.
In addition I have described pump and probe experiments carried out with the FEL setup on helium atomic targets, where the set-up has been exploited for the temporal characterization of the FEL light of FERMI. In fact, photoionization of simple atomic targets is usually one of the most used tools to characterize not only the detector response, but also the properties of the incident light.
Moreover at the LDM beamline novel multiphoton processes occurring in helium clusters have been investigated and discussed in this thesis work. The results obtained have revealed a novel collective ionization mechanism based on resonant excitation of clusters.
Because new generation SR and FEL sources require novel concepts of photon beam diagnostics, arising from the need for an accurate monitoring of position, intensity, and temporal structure, recent research has focused on novel high radiation hardness devices. Thanks to its extreme physical and electronic properties, diamond is the most promising material for the production of semitransparent in-situ photon beam monitors. A small section of this thesis work has been dedicated to illustrate preliminary results on diamond photon beam monitors.Studi di fotoionizzazione di atomi, molecole e cluster mediante radiazione VUV e raggi X molli hanno notevolmente beneficiato degli sviluppi fatti in nuove metodologie sperimentali. In particolare, l’utilizzo di nuove generazioni di sorgenti di luce ad alta luminosità, come sincrotroni e Free Electron Laser (FEL) ha aumentato la varietà di esperimenti e anche la gamma di fenomeni accessibili. Tuttavia a fronte di tali sviluppi è necessario anche un parallelo sviluppo delle tecniche sperimentali e dei sistemi di lettura.
Il mio progetto di dottorato è stato effettuato presso Elettra, il laboratorio italiano di radiazione di sincrotrone (SR) a Trieste. Tale progetto è stato integrato nelle attività del gruppo locale di Fisica Atomica e Molecolare. L’obiettivo di questo gruppo è quello di studiare la struttura elettronica di sistemi isolati a complessità crescente, come atomi, molecole e cluster in fase gassosa.
I clusters atomici sono aggregati contenenti da pochi a qualche migliaio di atomi. Studiando le proprietà dei clusters si ottengono informazioni importanti sulla fase iniziale di aggregazione e crescita della materia, nonchè sull'evoluzione delle proprietà verso lo stato solido.
Ad Elettra la possibilità di accedere sia alla beamline GasPhase della sorgente di radiazione Elettra sia alla beamline Low Density Matter (LDM) della sorgente Free Electron Laser (FEL) FERMI permette una analisi ad ampio spettro dei processi mediante l’utilizzo di metodi avanzati di fotoionizzazione sia in singolo e che in multi fotone. Questo consente una descrizione complementare sia dell’energetica che della dinamica dei sistemi isolati sotto studio.
La collaborazione tra il gruppo di Fisica Atomica e Molecolare ed il laboratorio di Instrumentation and Detection di Elettra, ha portato allo sviluppo di un prototipo di setup, costruito intorno ad uno spettrometro Velocity Map Imaging (VMI), con la flessibilità per effettuare sia esperimenti con radiazione di sincrotrone sia esperimenti FEL, semplicemente cambiando la flangia dello stadio di rivelazione:
- Per gli esperimenti SR, alla linea di luce GasPhase, viene utilizzato come rivelatore una crossed delay line accoppiato ad un time-to-digital converter a 4 canali che ricostruisce le posizioni di arrivo delle particelle cariche. Lo spettrometro di fotoelettroni VMI può operare in tandem con uno spettrometro a tempo di volo (TOF) per gli ioni. Tale sistema permette la correlazione della energia cinetica e dell’angolo di emissione dei fotoelettroni con specie ioniche di massa specifica. Con questo sistema è quindi possibile studiare la struttura elettronica di atomi, molecole e cluster mediante la tecnica di spettroscopia in coincidenza PEPICO (PhotoElectron-PhotoIon-Coincidence);
- Gli esperimenti FEL differiscono, in particolare, dagli esperimenti SR per il tasso molto più elevato di eventi realizzati e rilevati, costringendo però a rinunciare all’analisi degli eventi in coincidenza. Alla linea di luce LDM del FEL FERMI, viene quindi utilizzando un rivelatore diverso da quello sopra descritto, basato su un Micro Channel Plate (MCP), uno schermo al fosforo ed una telecamera CCD; il sistema è in grado di rivelare shot-by-shot praticamente tutti gli eventi, tuttavia senza risoluzione temporale. Tale sistema permette la inoltre la realizzazione di esperimenti in configurazione pump-and-probe, con risoluzione del femtosecondo, consentendo così di accedere alla dinamica degli elettroni.
In questo contesto, il nucleo della mia tesi sperimentale è basato su tecniche di imaging e coincidenza elettrone-ione dedicate allo studio dell'energetica, della distribuzione angolare e anche delle dinamiche di elettroni e ioni emessi in seguito ad irraggiamento VUV di bersagli atomici, molecolari e clusters, in parallelo alla caratterizzazione della sorgente luminosa utilizzata.
In particolare è stata eseguita una caratterizzazione completa ed approfondita dei due apparati sperimentali. Caratterizzazione presentata in questo lavoro di tesi insieme ad alcuni risultati recenti ottenuti con entrambi i setups con l'obiettivo di studiare inoltre anche le capacità dei sistemi di rilevazione. In particolare, sono stati riportati risultati sullo studio di cluster di Elio con radiazione di sincotrone, mediante spettroscopia PEPICO, analizzando le immagini ottenute dei fotoelettroni correlati con specifiche masse ioniche. In questo lavoro viene riportato uno studio dettagliato delle dinamiche di fotoeccitazione e ionizzazione di cluster di Elio puri; in particolare sono stati osservati effetti di collisione anelastica dei fotoelettroni uscenti con gli atomi circostanti di Elio all’interno del cluster.
Con la stesso apparato sperimentale per applicazioni SR, ma stavolta con l’utilizzo di tecniche in coincidenza tripla PEPIPICO (PhotoElectron-PhotoIon-PhotoIon), è stato inoltre studiato il processo di decadimento ICD (Interatomic Coulombic Decay) che si verifica in cluster di gas rari.
Il processo ICD è un canale di decadimento che avviene in sistemi debolmente legati, in cui l'energia dell’elettrone viene trasferita (tramite scambio di fotoni virtuali) da una shell di valenza interna di un atomo eccitato ad un atomo vicino, il quale a sua volta rilassa emettendo un elettrone di bassa energia. Questo meccanismo di decadimento, previsto teoricamente alla fine del secolo scorso da Cederbaum, è stato più di recente confermato sperimentalmente e, da allora, è diventato oggetto di numerosi studi. Il processo ICD sembra infatti giocare un ruolo importante in un un gran numero di sistemi; ad esempio, è stato dimostrato che elettroni ICD di bassa energia prodotti in un ambiente acquoso giocano un ruolo molto importante nel danno biologico provocato da radiazioni. In questa tesi sono stati riportati quindi risultati sperimentali sulla distribuzione di energia degli elettroni emessi nel processo ICD in dimeri di gas rari. Inoltre in questo lavoro di tesi sono riportati esperimenti in Pump-and-Probe, effettuati stavolta con il setup per applicazioni FEL, condotti su bersagli atomici di Elio; in particolare, tale sistema è stato utilizzato per la caratterizzazione temporale della luce FEL di FERMI. Infatti, la fotoionizzazione di semplici bersagli atomici è solitamente uno degli strumenti più utilizzati per caratterizzare non solo la risposta del rivelatore, ma anche le proprietà della luce incidente.
In aggiunta, sempre sulla linea LDM, sono stati studiati e discussi nuovi processi di ionizazzione a più fotoni osservati in cluster di Elio. I risultati ottenuti, e riportati in questa tesi, hanno rivelato un nuovo meccanismo di ionizzazione collettiva basato sull’eccitazione risonante del cluster.
Infine, poichè le nuove generazioni di sincrotroni e le nuove sorgenti FEL richiedono nuove diagnostiche per caratterizzare la luce incidente, derivanti dalla necessità di un accurato monitoraggio della posizione, dell’intensità e della struttura temporale, recentemente la ricerca si è concentrata su nuovi dispositivi in gradi di resistere ad elevate radiazioni. Grazie all’unicità delle sue proprietà fisiche e chimiche, il diamante si è rivelato come il materiale più promettente per la produzione di monitor semitrasparenti in-situ di fotoni. Una piccola sezione di questo lavoro di tesi è stata quindi dedicata ai risultati preliminari ottenuti sul diamante come monitor di fascio di fotoni.XXVII Ciclo198
Experimental investigation of the interatomic Coulombic decay in NeAr dimers
No full textThe kinetic-energy distribution of interatomic and -molecular Coulombic decay (ICD) electrons emitted following photoionization of 2s electrons from Ne atoms in NeAr dimers has been measured in a synchrotron radiation experiment with a velocity-map-imaging photoelectron-photoion-coincidence spectrometer. The position of the peak of the experimental distribution agrees well with calculations. The broadening of the distribution to high energies with respect to calculations of the ICD spectrum of the NeAr v=0 ground state is explained by the contribution to the spectrum due to the population in the excited vibrational states v=1 and v=2 of the electron ground state. On the other hand, the broadening on the low-energy side of the measured distribution may be explained by the dimer contracting in the intermediate state before ICD occurs, electrons coming from the ICD to spin-orbit split excited ion states, or a combination of these effects
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
Fast Synchrotron and FEL Beam Monitors Based on Single Crystal Diamond Detectors and InGaAs/InAlAs Quantum-Well Devices
No full textSimultaneous photon-beam position and intensity monitoring is becoming of increasing importance for new-generation synchrotron-radiation (SR) sources and free-electron lasers (FEL). Thus, novel concepts of beam diagnostics are required in order to keep such beams under control.
From this perspective diamond is a promising material for the production of semitransparent in situ photon-beam monitors which can withstand the high dose rates occurring in such radiation facilities. Here, we report on the development of freestanding, single-crystal chemical-vapor-deposited diamond detectors with segmented electrodes.
Due to their direct, low-energy band gap, InGaAs quantum-well (QW) devices operated at room temperature may be used as fast detectors for photons ranging from visible to X-ray. These features are valuable in low-energy and time-resolved FEL applications. In particular, a novel segmented InGaAs/InAlAs device has been developed and will be discussed.
Dedicated measurements carried out on both these devices at the Elettra Synchrotron show their capability to monitor the position and the intensity of the photon beam with bunch-by-bunch temporal performances. Furthermore, preliminary tests have been performed on diamond detectors at the Fermi FEL, extracting quantitative intensity and position information for 100-fs-wide FEL pulses with a photon energy of 28.8 eV
The Role of the Partner Atom and Resonant Excitation Energy in Interatomic Coulombic Decay in Rare Gas Dimers
No full textWe provide experimental evidence for interatomic Coulombic decay (ICD) in mixed rare gas dimers following resonant Auger decay. A velocity map imaging apparatus together with a cooled supersonic beam containing Ar2 and NeAr dimers was used to record the energy and angular distributions of electrons in coincidence with two mass selected ions following the excitation of a number of resonances converging to the Ar 2p3/2–1 threshold using synchrotron radiation. It is shown that the ICD process can be controlled by the choice of the partner atom in the dimer or of the resonance that triggers the resonant Auger deca
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
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