Ultrafast dynamics of small quantum systems studied using electron-ion coincidence spectroscopy

Studying how small quantum systems, like molecules and clusters, interact with X-rays is crucial to understanding the ultrafast processes that occur in nature on incredibly short timescales, ranging from femtoseconds to picoseconds. X-rays excite small quantum systems to unstable core hole states, leading to a cascade of phenomena, including Auger decay, nuclear rearrangement, and dissociation. The dissociation of molecules is influenced by the initial site of X-ray excitation, as well as the properties of the Auger populated states, such as charge localization and internal energy. In clusters, the dissociation process depends on intermolecular interactions, cluster size, and geometry. The interplay between electronic and nuclear dynamics in core-excited/ionized molecules and clusters is a critical factor that needs to be assessed. This thesis investigates X-ray-induced fragmentation of molecular adamantane and CO2 clusters using synchrotron radiation. The kinematics of molecular and cluster fragmentation is measured using advanced techniques, such as 3D momentum imaging of the ion fragments and multiparticle coincidence spectroscopy. Site-selective fragmentation of the carbon cage of the adamantane molecule is studied using Auger-electron Photoion coincidence spectroscopy, revealing the influence of the core-hole site on the Auger decay and dissociation process. Statistical data analysis treatment is developed and implemented to remove background contamination in the coincidence data using experimental random coincidences. The results highlight that the fragmentation of adamantane cation and dication is a complex dynamical process with competing relaxation pathways involving cage opening, hydrogen migration, and carbon-carbon bond breaking. Additionally, the thesis investigates the photoreactions of core-ionized CO2 clusters, reporting a significantly increased production of O2+ compared to isolated CO2 molecules. Through quantum chemistry calculations and multi-coincidence 3D momentum imaging, the study determined that the enhanced production of O2+ is due to a size-dependent structural transition of the clusters. The study also proposes two relevant photoreactions involving intermolecular interactions. This thesis highlights the complexity of core-hole dynamics in molecular and cluster chemistry and emphasizes the need for meticulous experimental and theoretical investigations of the underlying mechanisms. It also discusses the relevance of the results in the context of X-ray-induced astrochemistry. Indeed, the experiments presented are conducted in vacuum chambers in a controlled environment and can crudely replicate the conditions found in astrophysical environments. From the adamantane study, we conclude that X-ray absorption emphatically results in dissociation into smaller hydrocarbons and low photostability can play a part in the absence of diamondoids in the interstellar medium. From the CO2 clusters study, we found an enhancement in the O2+ yield, which can significantly influence the ion balance in CO2-rich atmospheres like Mars

Hubble Space Telescope Ultraviolet Spectroscopy of Fourteen Low-Redshift Quasars

We present low-resolution ultraviolet spectra of 14 low redshift (z<0.8) quasars observed with HST/STIS as part of a Snap project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the CIV emission line. Nine of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars including two broad absorption line (BAL) quasars (HE0143-3535, HE0436-2614), one quasar with a mini-BAL (HE1105-0746), and one quasar with associated narrow absorption (HE0409-5004). These BAL quasars are among the brightest known (though not the most luminous) since they lie at z<0.8. We compare the properties of these BAL quasars to the z1.4 Large Bright Quasar samples. By design, our objects sample luminosities in between these two surveys, and our four absorbed objects are consistent with the v ~ L^0.62 relation derived by Laor & Brandt (2002). Another quasar, HE0441-2826, contains extremely weak emission lines and our spectrum is consistent with a simple power-law continuum. The quasar is radio-loud, but has a steep spectral index and a lobe-dominated morphology, which argues against it being a blazar. The unusual spectrum of this quasar resembles the spectra of the quasars PG1407+265, SDSSJ1136+0242, and PKS1004+13 for which several possible explanations have been entertained.Comment: Uses aastex.cls, 21 pages in preprint mode, including 6 figures and 2 tables; accepted for publication in The Astronomical Journal (projected vol 133

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

The origin of enhanced O2+ production from photoionized CO2 clusters

CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes. In the Martian atmosphere, CO2 clusters are predicted to exist at high altitudes motivating a deeper understanding of their photochemistry. In this joint experimental-theoretical study, we investigate the photoreactions of CO2 clusters (≤2 nm) induced by soft X-ray ionisation. We observe dramatically enhanced production of O2- from photoionized CO2 clusters compared to the case of the isolated molecule and identify two relevant reactions. Using quantum chemistry calculations and multi-coincidence mass spectrometry, we pinpoint the origin of this enhancement: A size-dependent structural transition of the clusters from a covalently bonded arrangement to a weakly bonded polyhedral geometry that activates an exothermic reaction producing O+2. Our results unambiguously demonstrate that the photochemistry of small clusters/particles will likely have a strong influence on the ion balance in atmospheres

Resonant inner-shell photofragmentation of adamantane (C10H16)

International audienceAdamantane, the smallest diamondoid molecule with a symmetrical cage, contains two distinct carbon sites, CH and CH$_2$. The ionization/excitation of the molecule leads to the cage opening and strong structural reorganization. While theoretical predictions suggest that the carbon site CH primarily causes the cage opening, the role of the other CH$_2$ site remains unclear. In this study, we used advanced experimental Auger electron-ion coincidence techniques and theoretical calculations to investigate the fragmentation dynamics of adamantane after resonant inner-shell photoexcitation. Our results demonstrate that some fragmentation channels exhibit site-sensitivity of the initial core-hole location, indicating that different carbon site excitations could lead to unique cage opening mechanisms

Coincidence study of core-ionized adamantane: site-sensitivity within a carbon cage?

International audienceWe investigate the fragmentation dynamics of adamantane dications produced after core-ionization at the carbon edge followed by Auger decay. The combination of high-resolution electron spectroscopy, energy-resolved electron-ion multi-coincidence spectroscopy and different theoretical models allows us to give a complete characterization of the processes involved after ionization. We show that energy-and site-sensitivity is observed even for a highly-symmetric molecule that lacks any unique atomic site