Time-resolved and Imaging Techniques for Photoionization Studies of Atoms, Molecules and Clusters

2013/2014Photoionization 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

Photophysics of indole upon x-ray absorption

A photofragmentation study of gas-phase indole (C$_8$H$_7$N) upon single-photon ionization at a photon energy of 420 eV is presented. Indole was primarily inner-shell ionized at its nitrogen and carbon $1s$ orbitals. Electrons and ions were measured in coincidence by means of velocity map imaging. The angular relationship between ionic fragments is discussed along with the possibility to use the angle-resolved coincidence detection to perform experiments on molecules that are strongly oriented in their recoil-frame. The coincident measurement of electrons and ions revealed fragmentation-pathway-dependent electron spectra, linking the structural fragmentation dynamics to different electronic excitations. Evidence for photoelectron-impact self-ionization was observed.Comment: 11 pages, 6 figure

589 External validation of the increased wall thickness score for the diagnosis of cardiac amyloidosis

Abstract Aims This study aimed to validate the increased wall thickness (IWT) score, a multiparametric echocardiographic score to facilitate diagnosis of cardiac amyloidosis (CA), in an independent population of patients with increased LV wall thickness suspicious for CA. Methods and results Between January 2019 and December 2020, 152 consecutive patients with increased LV wall thickness suspicious for CA were included. For all patient, the multiparametric echocardiographic score (IWT score) was calculated. To validate the diagnostic accuracy of an IWT score ≥8 to predict the diagnosis of CA, sensibility (Se), specificity (Sp), positive predictive value (PPV), negative predictive value (NPV), and predictive accuracy (PA) were calculated. Among the 152 patients included in the study, 50 (33%) were diagnosed as CA, 25 (16%) had severe aortic stenosis, 25 (16%) had hypertensive remodelling, and 52 (34%) had hypertrophic cardiomyopathy. Among the 50 and 102 patients with and without CA, 19 (38%) and 1 (1%) showed an IWT score ≥8, respectively. Overall, the diagnostic accuracy of an IWT score ≥8 for the diagnosis of CA in our population was the following: Se 38% (95% CI: 25–53%); Sp 99% (95% CI: 95–100%); PPV 95% (95% CI: 72–99%); NPV 77% (95% CI: 73–80%); PA 79% (95% CI: 72–85%). Conclusions This study reports the first external validation of the IWT score for the diagnosis of CA in patients with increased LV wall thickness. A score ≥8 showed a high Sp, PPV and PA, suggesting that the IWT score can be used to identify CA patients in those with increased LV wall thickness

Reduced proteasome activity in the aging brain results in ribosome stoichiometry loss and aggregation.

A progressive loss of protein homeostasis is characteristic of aging and a driver of neurodegeneration. To investigate this process quantitatively, we characterized proteome dynamics during brain aging in the short-lived vertebrate Nothobranchius furzeri combining transcriptomics and proteomics. We detected a progressive reduction in the correlation between protein and mRNA, mainly due to post-transcriptional mechanisms that account for over 40% of the age-regulated proteins. These changes cause a progressive loss of stoichiometry in several protein complexes, including ribosomes, which show impaired assembly/disassembly and are enriched in protein aggregates in old brains. Mechanistically, we show that reduction of proteasome activity is an early event during brain aging and is sufficient to induce proteomic signatures of aging and loss of stoichiometry in vivo. Using longitudinal transcriptomic data, we show that the magnitude of early life decline in proteasome levels is a major risk factor for mortality. Our work defines causative events in the aging process that can be targeted to prevent loss of protein homeostasis and delay the onset of age-related neurodegeneration

Carbon and Nitrogen K-Edge NEXAFS Spectra of Indole, 2,3-Dihydro-7-azaindole, and 3-Formylindole

The near-edge X-ray absorption fine structure (NEXAFS) spectra of indole, 2,3-dihydro-7-azaindole, and 3-formylindole in the gas phase have been measured at the carbon and nitrogen K-edges. The spectral features have been interpreted based on density functional theory (DFT) calculations within the transition potential (TP) scheme, which is accurate enough for a general description of the measured C 1s NEXAFS spectra as well as for the assignment of the most relevant features. For the nitrogen K-edge, the agreement between experimental data and theoretical spectra calculated with TP-DFT was not quite satisfactory. This discrepancy was mainly attributed to the many-body effects associated with the excitation of the core electron, which are better described using the time-dependent density functional theory (TDDFT) with the range-separated hybrid functional CAM-B3LYP. An assignment of the measured N 1s NEXAFS spectral features has been proposed together with a complete description of the observed resonances. Intense transitions from core levels to unoccupied antibonding π* states as well as several transitions with mixed-valence/Rydberg or pure Rydberg character have been observed in the C and N K-edge spectra of all investigated indoles

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10$^{11}$ photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH$_3$I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses

Three-Dimensional Shapes of Spinning Helium Nanodroplets

A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion have been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large dataset allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale

Molecular Signature of Biological Aggressiveness in Clear Cell Sarcoma of the Kidney (CCSK)

: Clear cell sarcoma of the kidney (CCSK) is a rare pediatric renal tumor with a worse prognosis than Wilms' tumor. Although recently, BCOR internal tandem duplication (ITD) has been found as a driver mutation in more than 80% of cases, a deep molecular characterization of this tumor is still lacking, as well as its correlation with the clinical course. The aim of this study was to investigate the differential molecular signature between metastatic and localized BCOR-ITD-positive CCSK at diagnosis. Whole-exome sequencing (WES) and whole-transcriptome sequencing (WTS) were performed on six localized and three metastatic BCOR-ITD-positive CCSKs, confirming that this tumor carries a low mutational burden. No significant recurrences of somatic or germline mutations other than BCOR-ITD were identified among the evaluated samples. Supervised analysis of gene expression data showed enrichment of hundreds of genes, with a significant overrepresentation of the MAPK signaling pathway in metastatic cases (p < 0.0001). Within the molecular signature of metastatic CCSK, five genes were highly and significantly over-expressed: FGF3, VEGFA, SPP1, ADM, and JUND. The role of FGF3 in the acquisition of a more aggressive phenotype was investigated in a cell model system obtained by introducing the ITD into the last exon of BCOR by Crispr/Cas9 gene editing of the HEK-293 cell line. Treatment with FGF3 of BCOR-ITD HEK-293 cell line induced a significant increase in cell migration versus both untreated and scramble cell clone. The identification of over-expressed genes in metastatic CCSKs, with a particular focus on FGF3, could offer new prognostic and therapeutic targets in more aggressive cases

Deep neural networks for classifying complex features in diffraction images

Intense short-wavelength pulses from free-electron lasers and high-harmonic-generation sources enable diffractive imaging of individual nano-sized objects with a single x-ray laser shot. The enormous data sets with up to several million diffraction patterns represent a severe problem for data analysis, due to the high dimensionality of imaging data. Feature recognition and selection is a crucial step to reduce the dimensionality. Usually, custom-made algorithms are developed at a considerable effort to approximate the particular features connected to an individual specimen, but facing different experimental conditions, these approaches do not generalize well. On the other hand, deep neural networks are the principal instrument for today's revolution in automated image recognition, a development that has not been adapted to its full potential for data analysis in science. We recently published in Langbehn et al. (Phys. Rev. Lett. 121, 255301 (2018)) the first application of a deep neural network as a feature extractor for wide-angle diffraction images of helium nanodroplets. Here we present the setup, our modifications and the training process of the deep neural network for diffraction image classification and its systematic benchmarking. We find that deep neural networks significantly outperform previous attempts for sorting and classifying complex diffraction patterns and are a significant improvement for the much-needed assistance during post-processing of large amounts of experimental coherent diffraction imaging data.Comment: Published Version. Github code available at: https://github.com/julian-carpenter/airyne