Magnetic bottle electron spectroscopy of atomic reactions triggered by a pulsed electron beam

Barba, Žiga (author), Žitnik, Matjaž (mentor)

In the last 20–30 years the magnetic bottle electron spectrometer (MBES) has become a well established tool to research many-particle inner-shell vacancy decay by multi-electron coincidence methods in atoms and molecules. We report on a new version of such a spectrometer that we have designed at our laboratory where for the first time, instead of photons, electrons are used as the excitation source. This was achieved by positioning the electron source behind the permanent magnet setup aligning it parallel to the spectrometer axis which allowed 3–5 % of electrons to pass through a channel in the soft iron core that concentrates the magnetic field lines towards the target region achieving a magnetic field density of 600 mT. Short, nanosecond electron pulses, necessary for the operation of the spectrometer, were produced by sweeping the continuous beam across a narrow aperture at the electron source exit. Using numerical models we simulated how the different apparatus components would perform. These findings pointed out that careful alignment of all the components (the electron source, the permanent magnet set-up, the drift field and the electron detector) is crucial in optimizing the performance of the spectrometer. We report on the first experiment using the new spectrometer, where 800 eV electrons were scattered on argon. In the scattered, emitted and Auger electron kinetic spectra, which were calculated from electron time of flights, we can clearly distinguish several characteristic features: 3p and 2p ionization peaks and the L- MM Auger signal. From the results we estimate the energy resolution of 1.5 % after the electrons travel along the 2 m long drift tube. By analyzing two-, three- and four-electron coincidences we reduced the background further and resolved a few additional weaker spectral components that belong to more complex decay processes, such as the Coster-Kronig 2s vacancy decay, where the atom ejects four electrons with different energies in such a way, that the sum of their energies remains constant. Comparison to the theoretical BEB (Binary-encounter Bethe) scattering model showed agreement with our experimental data. The total electron detection efficiency $\eta \approx$ 0.23 was found to be constant in the investigated 0–0.8 keV energy range. This is somewhat lower than reported for other MBES set-ups where a practically 70 % efficiency is assumed. The discrepancy is most likely due to a relatively large target volume and the remaining spectrometer misalignment.

• Language: English
• Keywords: electron spectroscopy, TOF spectroscopy, electron-electron coincidences
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-121519
• COBISS.SI-ID: 24015363
• Publication date in RUL: 13.10.2020

Secondary language

• Language: Slovenian
• Title: Elektronska spektroskopija atomskih reakcij z magnetno steklenico in sunkovnim elektronskim vzbujanjem

Abstract

Elektronski spektrometri na magnetno steklenico s fotonskim vzbujanjem so se v zadnjih 40 letih uveljavili kot koristno orodje za raziskave večdelčnih razpadov notranjih vrzeli v atomih in molekulah. V disertaciji poročamo o različici spektrometra kjer smo za vzbujanje tarče prvič uporabili elektrone. Elektronski vir smo usmerili vzdolž glavne osi spektrometra, tako da 3–5 % delež elektronov preleti kanal v jedru iz mehkega železa, ki v tarčo usmerja magnetno polje z gostoto 600 mT. Kratke, nanosekundne sunke elektronov, ki so potrebni za delovanje spektrometra, smo pripravili s hitrim odklanjanjem stalnega žarka preko majhne odprtine na izhodu iz elektronskega vira. Z numeričnimi modeli smo simulirali delovanje posameznih komponent spektrometra in rezultate primerjali z meritvami. Pri tem se je pokazalo, da je za optimalno delovanje spektrometra ključna natančna poravnava vseh njegovih komponent: elektronskega vira, sistema permanentnih magnetov, usmerjevalnega šibkega magnetnega polja ter detektorja za elektrone. Poročamo o prvi študiji z novim spektrometrom, kjer smo opazovali sipanje elektronov s kinetično energijo 800 eV na argonu. V energijskih spektrih sipanih, izbitih in Augerjevih elektronov, ki smo jih preračunali iz njihovih časov preleta, se jasno vidi nekaj izrazitih struktur: ionizacijska vrhova 3p in 2p ter signal Augerjevih elektronov L-MM. Meritve pokažejo, da je energijska ločljivost okrog 1.5 %, pri čemer je pot, ki jo preletijo elektroni dolga približno 2 m. Z analizo dvo-, tro- in štirielektronskih koincidenc smo dodatno zmanjšali ozadje in iz meritev razločili še nekaj šibkejših spektralnih komponent, ki pripadajo kompleksnejšim razpadnim procesom, recimo razpadu vrzeli 2s s prehodom Coster-Kronig, kjer atom odda 4 elektrone z različnimi energijami, in sicer tako, da se ohranja vsota energij. Primerjava s teoretičnim modelom sipanja elektronov BEB kaže dobro ujemanje z našimi meritvami. Iz rezultatov analize sledi, da je povprečna verjetnost za detekcijo elektrona, ki se izseva v tarči, okrog $\eta \approx$ 0,23 in je dokaj neodvisna od energije na območju od 0 do 0,8 keV. Številka je nekoliko nižja od tistih v literaturi, kjer spektrometri na magnetno steklenico s svetlobnim vzbujanjem dosegajo izkoristek 70 %. Razlika je posledica uporabe dokaj razsežne plinske tarče pri vzbujanju z elektroni ter nepopolne poravnave posameznih komponent spektrometra.

• Keywords: elektronska spektroskopija, spektroskopija na čas preleta, večelektronske koincidence