Molecular Fragmentation of Acetylene by VUV Double Photoionization

Acetylene is a simple molecule of interest for interstellar medium (ISM) and planetary atmospheres. The presence of C2H2 was detected by IR spectroscopic measurements. Acetylene was also found as a minor component in the atmosphere of gas giants like the planet Jupiter, in the atmosphere of Saturn's satellite Titan, and in comets, where photochemical experiments have demonstrated that this simple hydrocarbon is a likely precursor of C2, a widely observed component in such environments. It has to be noted that the presence in planetary atmospheres and ISM of Vacuum Ultra Violet (VUV) light's photons as well as cosmic rays makes highly probable the double photoionization of molecular species with the production of molecular dications producing subsequent dissociation into ionic fragments having a high kinetic energy content of several eV. This translational energy is sufficient in some cases to allow ions escape from the upper atmosphere of some planet of the Solar System, as Venus, Mars and Titan, into space. In this contribution we present the experimental study of the microscopic dynamics of the two-body dissociation reactions of the C2H2+2 dication, induced by the double ionization of acetylene molecules by VUV photons in the energy range of 31.9–50.0 eV. The photoionizing agent was a tunable synchrotron radiation beam, while ion products are revealed by coupling photoelectron-photoion-photoion-coincidence and ion imaging techniques. The measured angular distributions and kinetic energy of product ions exhibit significant changes (as the photon energy increases) for the three leading dissociation reactions producing H++C2H+, C++CH2+, and CH++CH+, providing detailed information on the fragmentation dynamics of the C2H22+ dication

Double photoionization of propylene oxide: a coincidence study of the ejection of a pair of valence-shell electrons

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV energy range, permits us (i) to observe six double ionization fragmentation channels, their relative yields being accounted for about two-thirds by the couple (C2H4+, CH2O+) and one-fifth by (C2H3+, CH3O+); (ii) to measure thresholds for their openings as a function of photon energy; and (iii) to unravel a pronounced bimodality for a kinetic-energy-released distribution, fingerprint of competitive non-adiabatic mechanisms

Studies of molecular photoionization of simple systems by advanced photon sources

This doctorate thesis reports on a variety of experimental investigations aiming to advance the understanding of fundamental processes in molecules and clusters by exploiting the properties of Synchrotron and FEL radiation: photoionization dynamics, double ionization, dissociation and molecular recognition were subject of investigation. The emphasis of the thesis lies on the application of advanced light sources in the study of photoionization processes in simple gas-phase molecules, with particular attention on chiro-optical properties of chiral systems. The valence photoionization dynamics of a chiral molecule, namely the epichlorohydrin molecule, was studied for the first time and a peculiar electron correlation effect was observed. The experimental data were supported by state-of-the-art theoretical calculations. VUV direct double ionization was studied for the methyl oxirane chiral molecule by the use of Photoelectron-Photoion-Photoion Coincidence spectroscopy using synchrotron radiation. The chiral recognition mechanism of 1-methoxy-2-propanol oligomers was studied by FEL based IRMPD-VUV vibrational spectroscopy, a technique that exploits the nature of the photoionization process in order to apply the IRMPD spectroscopy to systems of arbitrary structure. The collaboration between the Sapienza University of Rome, the CNRIOM institute, and the Elettra Instrumentation and Detector Laboratory, has resulted in the development of a position sensitive cross delay line electron detector integrated in an experimental apparatus with the flexibility to perform synchrotron radiation (SR) photoemission experiments on gasphase systems. The improvement of the apparatus detection system has stimulated the collaboration with the Theoretical Chemistry group of the University of Trieste, in a joint experimental and theoretical long-term research activity, whose first part was the study of the photoionization dynamics of the Osmium tetroxide molecule, a highly reactive tetraoxo complex

The Reactions of Dications with Neutral Species: Understanding Planetary Ionospheres

Doubly charged cations (dications) of molecular and atomic species are predicted to be influential in high-energy environments such as the interstellar medium, the ionospheres of planets and satellites, and plasmas. However, definitive detection of dications in these environments are not yet available and the presence of these ions is often overlooked. Early investigations of dication-neutral collisions, often at high collision energies, only resulted in the observation of electron-transfer reactivity. Modern experiments, using lower collision energies, have revealed a range of exotic chemistry such as bond-formation with rare gas elements. This chemistry, coupled with the significant abundance of dications predicted in ionospheres, suggests that these ions could play important roles in atmospheric processes. For example, dications could be involved in the chemistry of complex molecule assembly. The study of dications and their reactions is clearly important to understanding ionospheric processes in planets and satellites including the prebiotic Earth. This thesis explores the bimolecular reactivity of various dications with neutral species in order to better understand the processes occurring in the ionospheres of planets and satellites. The position-sensitive coincidence mass spectrometry technique employed in this work utilises coincident, position-sensitive, detection of ions to reveal comprehensive information concerning the dynamics and energetics of the consequences of dication-neutral reactions. Specifically, the reactions following collisions of Ar2+, S2+ and CH2CN2+ with atoms and small molecules have been investigated. These dication-neutral collision systems exhibit intriguing reactivity clearly demonstrating the diversity of dication chemistry. For example, many of the electron-transfer reactions observed show evidence of proceeding via collision complexes, contrary to the orthodox (direct) mechanism. Of the bond-forming reactions detected, those generating molecular species containing a rare gas, such as ArO+ and ArN+, are the most notable. Despite the observation of the involvement of collision complexes in electron-transfer, many of the bond-forming reactions described in this thesis have been shown to occur via direct mechanisms. The observation of bond-forming reactions and the involvement of collision complexes clearly shows the facility of dications to form associations despite their often-high potential energies

The gas phase reactivity of doubly–charged ions with neutral species

Doubly charged atomic and molecular species (dications) are potentially influential reactants in many environments, including our Earth’s own ionosphere and ionospheres of planets and moons such as Mars and Titan. However, the dication chemistry of these energized environments is far from understood. This thesis attempts to experimentally and theoretically characterize some dicationic reactions that could occur in such environments. Moreover, astro–chemists have pondered for decades how the polycyclic aromatic hydrocarbons and long chain alkynes thought to exist in the interstellar medium are synthesized. Reactions of dications with neutral molecules have been proposed as one possible mechanism for the formation of these large molecules. In this context, it is important to undertake a systematic experimental study of the reactions of dications with neutrals to discover the products that might be formed. Dication– neutral reaction dynamics are also not fully understood and are often surprising, meriting further study. A position–sensitive coincidence time–of–flight mass spectrometer has been used to probe the reactions of dications, including Ar2+, N22+, C2H22+, SF42+ and O22+, with a variety of neutrals. The experiment exploits the fact that dication reactions often result in a pair of product monocations. These pairs of charged products are detected in coincidence, on an event–by–event basis, allowing the reaction channels to be characterised. The position–sensitive detection provides data from which the reaction dynamics and energetics can be determined. One might anticipate that dication–neutral interactions would predominantly result in electron–transfer at large interspecies separation. However, this thesis presents results proving that the chemistry can be far more diverse, with bond–formation between the reactants competing favourably with the more dominant electron–transfer process

Molecules in Superfluid Helium Nanodroplets

This open access book covers recent advances in experiments using the ultra-cold, very weakly perturbing superfluid environment provided by helium nanodroplets for high resolution spectroscopic, structural and dynamic studies of molecules and synthetic clusters. The recent infra-red, UV-Vis studies of radicals, molecules, clusters, ions and biomolecules, as well as laser dynamical and laser orientational studies, are reviewed. The Coulomb explosion studies of the uniquely quantum structures of small helium clusters, X-ray imaging of large droplets and electron diffraction of embedded molecules are also described. Particular emphasis is given to the synthesis and detection of new species by mass spectrometry and deposition electron microscopy