Matter under Extreme Transient conditions investigated with Free Electron Laser radiation at FERMI

2013/2014Subject to high-intensity ultra-short light pulses, solid materials are driven into short-living states of extremely high temperatures and pressures. Creating such unique states under controlled laboratory conditions and addressing their physical properties with an adequate time resolution to follow their fast-evolving dynamics has been a primary objective of pump-and-probe studies employing the high-peak-power femtosecond optical lasers that have become of increasingly widespread availability during the last three decades. Operative since few years, free electron lasers (FELs) add - to the desired properties of high brilliance, short time duration and coherence proper of those lasers - the tunable energy up to the extreme ultraviolet (EUV) and X-ray spectral ranges which is typical of synchrotron light sources, thus offering a number of advantages on both the fronts of pumping and probing. This thesis describes the efforts that have been undertaken towards the establishment of a new experimental ground for the study of matter under extreme transient conditions at the TIMEX beamline of the free electron laser FERMI in Trieste and discusses my understanding of a number of seminal investigations. Characterization of the exotic states swiftly reached by FEL-excited metallic samples is addressed through single-shot measurements of their EUV reflectance and absorbance. In particular, we provide the first demonstration of EUV absorption near edge spectroscopy at high energy densities and with sub- 100 fs time resolution. The combination of these features is only possible at FEL sources, but makes FERMI truly unique when further considering that here it comes without a need for monochromatization thanks to its seeded nature which makes the delivered pulses almost Fourier-transform limited in bandwidth. The measured dependence on irradiation fluence of the transient absorbance and reflectance of Ti for selected wavelengths across the M2,3 absorption edge and near the plasma frequency respectively is reported, revealing an ultrafast modification of the electronic structure compatible with a single FEL pulse having turned within about 100 fs room-temperature solid-density Ti into a hot free electron gas coexisting in highly non-equilibrium conditions with a cold ion lattice. An interpretive perspective is proposed that - based on simplified, yet reasonable, Drude-like models - allows to gain information on the average free electron temperature and density and the average ionization state of the generated dense plasma. Ultra-fast high-energy-density self-transmission measurements on Al at a few photon energies between the plasma frequency and the L2,3 absorption edge are presented, evidencing for the first time a non-monotonic trend of the EUV transmission as a function of fluence, that are seen to be interpretable - within a three-channel model - as resulting from an interplay between saturation effects and absorption cross-section variations due to electron heating. Higher fluence conditions are shown to induce almost full transparency in an Al-Mg-Al stack over an entire energy region across the Mg L2,3 absorption edge. An analytical model is formulated, that tentatively permits to deduce from the data an estimate for the first ionization energy of warm dense Mg. The responses of insulator and semiconductor samples to FEL and laser excitations - from the initial non-equilibrium stage, through the subsequent relaxation dynamics, to possible phase transformations - are explored through pump-and-probe experiments. Here, FEL pulses are exploited in combination with pulses from the same infrared laser that serves to initiate the FEL emission process, which ensures a natural synchronization between the two radiation sources, - again - a unique feature of seeded FELs. Measurements of transient optical reflectivity on FEL-pumped Si3N4 samples are offered as a demonstration of the nearly jitter-free pump-probe capabilities of FERMI, thus far unbeaten among FEL facilities. The results of a systematic study of the optical response of this material to FEL irradiation for different pump wavelengths and intensities, probe polarizations, sample thicknesses and types of substrate in both reflection and transmission simultaneously are discussed, hinting at the role of interference effects at play due to the different penetration depths of the EUV FEL and infrared laser pulses. Transient EUV transmissions for Ge at its M4,5 edge within a laser pump / FEL probe configuration and for Si at its L2,3 edge within a FEL pump / FEL probe configuration yield a time and energy -resolved view on femtosecond pulse -induced melting of semiconductors. We show that, while able to follow the excitation and de-excitation dynamics of the electronic population in the valence band through below- absorption edge data, we can infer dynamical structural information through on- absorption edge data, in which we recognize the fingerprint of a partial band gap closure associated with the transition to a liquid state of increased metallic character. ------ Sottoposti ad impulsi luminosi ultra-intensi ed ultra-brevi, i materiali solidi sono temporaneamente portati ad altissime temperature e pressioni. Generare tali stati in controllate condizioni di laboratorio ed investigarne le proprietà fisiche con una risoluzione temporale adeguata a seguirne la rapida evoluzione è stato un obiettivo primario degli esperimenti di tipo ``pump-and-probe'' facenti uso dei laser ottici al femtosecondo ed elevate potenze di picco che sono divenuti di crescentemente diffusa disponibilità nel corso degli scorsi tre decenni. Operativi soltanto da pochi anni, i laser ad elettroni liberi (FEL) uniscono - alle desiderate proprietà di alta brillanza, breve durata temporale e coerenza propri di questi laser - l'energia variabile fin negli intervalli spettrali dell'estremo ultravioletto (EUV) e dei raggi X che è tipica delle sorgenti di sincrotrone, offrendo in tal modo una serie di vantaggi sia per quanto riguarda la generazione che la caratterizzazione dei suddetti stati. Questa tesi descrive gli sforzi che sono stati intrapresi verso il consolidamento di una nuova piattaforma sperimentale per lo studio della materia in condizioni estreme e transienti presso la linea di luce TIMEX del laser ad elettroni liberi FERMI in funzione a Trieste e discute la raggiunta comprensione di un certo numero di indagini pionieristiche. Stati esotici istantaneamente raggiunti da campioni metallici in seguito ad eccitazione FEL sono caratterizzati attraverso misure a singolo impulso delle loro riflettanza ed assorbanza EUV. In particolare, diamo la prima dimostrazione di spettroscopia di assorbimento EUV vicino soglia ad alte densità energetiche e con risoluzione temporale inferiore ai 100 fs. La combinazione di queste caratteristiche è disponibile solamente presso sorgenti FEL, ma rende FERMI unico qualora, in aggiunta, si consideri che qui si presenta senza bisogno di monocromatizzazione grazie alla natura ``seeded'' che rende gli impulsi rilasciati quasi Fourier-transform limited in larghezza di banda. Viene riportata la dipendenza dalla fluenza di irraggiamento dell' assorbanza e della riflettanza transienti del Ti per selezionate lunghezze d'onda sulla soglia M2,3 di assorbimento e vicino alla frequenza di plasma, rispettivamente. Essa rivela una modificazione ultraveloce della struttura elettronica compatibile con uno scenario in cui un singolo impulso FEL, entro la sua durata di circa 100 fs, trasforma il titanio a temperatura ambiente in un gas caldo di elettroni liberi coesistente in condizioni altamente fuori dall'equilibrio con un reticolo freddo di ioni. Si propone una prospettiva interpretativa che, basata su semplificati ma ragionevoli modelli simil-Drude, permette di ricavare informazioni sulla temperatura e sulla densità elettronica medie e sullo stato di carica medio del plasma denso generato. Sono presentate misure di trasmissione di impulsi ultra-veloci e ad alta densità di energia su un campione di Al ad alcune energie fotoniche tra la frequenza di plasma e la soglia di assorbimento L2,3. Queste evidenziano per la prima volta un andamento non-monotonico della trasmissione EUV in funzione della fluenza, che giudichiamo interpretabile – nell'ambito di un modello a tre canali – come risultante da un effetto di saturazione congiunto ad una variazione della sezione d'urto d'assorbimento dovuta al riscaldamento elettronico. Si mostra come condizioni di ancora più elevata fluenza inducono una trasparenza quasi completa in un campione Al-Mg-Al sull'intero intervallo di energie a cavallo della soglia di assorbimento L2,3 dell'Mg. Viene formulato un modello analitico che, tentativamente, permette di dedurre dai dati una stima per l'energia di prima ionizzazione dell'Mg in condizioni di plasma denso. Le risposte di campioni isolanti e semiconduttori ad eccitazioni FEL e laser - dall'iniziale fase di non-equilibrio, attraverso le successive dinamiche di rilassamento, alle possibili trasformazioni di fase - sono esplorate attraverso misure pump-and-probe. In questo caso, gli impulsi FEL sono sfruttati in combinazione con impulsi dallo stesso laser infrarosso che funge da iniziatore del processo di emissione FEL, il che assicura una naturale sincronizzazione tra le due sorgenti di radiazione, una caratteristica - di nuovo - esclusiva dei FEL seeded. Misure di riflettività ottica transiente su campioni di Si3N4 pompati dal FEL sono offerte come dimostrazione della capacità ad oggi imbattuta di FERMI di offrire una coppia di impulsi laser / FEL praticamente liberi da jitter. Sono discussi i risultati di uno studio sistematico della risposta ottica di questo materiale all'irraggiamento FEL a diverse lunghezze d'onda ed intensità di pump, diverse polarizzazioni di probe, diversi spessori del campione e diversi tipi di substrato, sia in riflessione che in trasmissione simultaneamente. Essi conducono a riconoscere il ruolo giocato da effetti di interferenza a causa delle differenti lunghezze di penetrazione tra impulsi EUV FEL ed infrarossi. Le trasmissioni EUV transienti collezionate per il Ge alla sua soglia M4,5 in una configurazione laser pump / FEL probe e per il Si alla sua soglia L2,3 in una configurazione FEL pump / FEL probe restituiscono una visione risolta sia in tempo che in energia del fenomeno di fusione dei semiconduttori indotto da impulsi al femtosecondo. Mostriamo come sia possibile seguire le dinamiche di eccitazione e diseccitazione della popolazione elettronica nella banda di valenza attraverso dati sotto soglia di assorbimento e come possiamo invece inferire informazioni di dinamica strutturale da dati sulla soglia di assorbimento, negli quali riteniamo di riconoscere l'impronta della chiusura parziale della “band gap” associata alla transizione verso uno stato liquido di aumentato carattere metallico.XXVI Ciclo198

LowDosePES: the low-dose photoelectron spectroscopy end-station at the PM4 beamline at BESSY II

A brief description of the main equipment at the Low Dose PhotoElectron Spectroscopy end-station is given, and a few possible applications highlighted

Long-lived nonthermal electron distribution in aluminum excited by femtosecond extreme ultraviolet radiation

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev et al., Phys. Rev. Lett. 107, 165003 (2011)], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior

Long-Term Degradation Mechanisms in Application-Implemented Radical Thin Films

Blatter radical derivatives are very attractive due to their potential applications, ranging from batteries to quantum technologies. In this work, we focus on the latest insights regarding the fundamental mechanisms of radical thin film (long-term) degradation, by comparing two Blatter radical derivatives. We find that the interaction with different contaminants (such as atomic H, Ar, N, and O and molecular H2, N2, O2, H2O, and NH2) affects the chemical and magnetic properties of the thin films upon air exposure. Also, the radical-specific site, where the contaminant interaction takes place, plays a role. Atomic H and NH2 are detrimental to the magnetic properties of Blatter radicals, while the presence of molecular water influences more specifically the magnetic properties of the diradical thin films, and it is believed to be the major cause of the shorter diradical thin film lifetime in air

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical, using two methods: by chemisorption from the radical solution and by on surface chemical derivatization from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. The monolayers are characterized by air and beam stability unprecedented for films of organic radicals. Over very long beam exposure we observed a dynamic nature of the radical-Au complex. The results clearly indicate that (mono)layers of PTM; radical derivatives have the necessary stability to stand device applications

Towards jitter-free pump-probe measurements at seeded free electron laser facilities

X-ray free electron lasers (FEL) coupled with optical lasers have opened unprecedented opportunities for studying ultrafast dynamics in matter. The major challenge in pump-probe experiments using FEL and optical lasers is synchronizing the arrival time of the two pulses. Here we report a technique that benefits from the seeded-FEL scheme and uses the optical seed laser for nearly jitter-free pump-probe experiments. Timing jitter as small as 6 fs has been achieved and confirmed by measurements of FEL-induced transient reflectivity changes of Si3N4 using both collinear and non-collinear geometries. Planned improvements of the experimental set-up are expected to further reduce the timing jitter between the two pulses down to fs level

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical using two methods: by chemisorption from the radical solution and by on-surface chemical derivation from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. We have used a robust method that can be applied to a variety of materials to assess the stability of the functionalized interface. The monolayers are characterized by air and X-ray beam stability unprecedented for films of organic radicals. Over very long X-ray beam exposure we observed a dynamic nature of the radical–Au complex. The results clearly indicate that (mono)layers of PTM radical derivatives have the necessary stability to withstand device applications.The authors would like to thank Helmholtz-Zentrum Berlin (HZB) for providing beamtime at BESSY II (Berlin, Germany), and Hilmar Adler, Elke Nadler, and Sergio Naselli for technical support. J. A. de S. is enrolled in the Materials Science PhD program of UAB. J. A. de S. thanks the Spanish Ministry for an FPI fellowship. This work was funded by the Spanish Ministry project FANCYCTQ2016-80030-R and GENESIS PID2019-111682RB-100, the Generalitat de Catalunya (2017SGR918) and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (SEV-2015-0496), the CSIC with the i-Link+ 2018 (Ref. LINKA20128) and CIBERBBN. Financial support from HZB and German Research Foundation (DFG) under the contract CA852/11-1 is gratefully acknowledged.Peer reviewe