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

NEXAFS spectroscopy and site-specific fragmentation of N -methylformamide, N,N -dimethylformamide, and N,N -dimethylacetamide

Near-edge X-ray absorption fine-structure (NEXAFS) spectra measured at the C, N, and O K-edges for three molecules containing the amide moiety, N-methylformamide (HCONHCH3), N,Ndimethylformamide (HCON(CH3)2), and N,N-dimethylacetamide (CH3CON(CH3)2) are presented. These molecules have similar structures and differ by the number of methyl groups located at the molecular ends. The fragmentation of these molecules after resonant excitation at different K-edge resonances is also investigated, using a 3D-ion imaging time-of-flight spectrometer. A comparison between the molecules with respect to the relative contributions of the fragments created upon excitation at distinct resonances reveals site-specific fragmentation. Further information about the character of the core-excitation and dissociation process is obtained from the angular distributions of the ion fragments

Single photon thermal ionization of C60

We report on experiments which show that C60 can ionize in an indirect, quasi-thermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 10^4 K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed

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

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

This article is part of the themed collections: PCCP Reviews and 2022 PCCP HOT.In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO2(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding Obr rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Tiint) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti5f). This configuration is associated with the formation of a new coordination complex, Ti–O–Ti5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti–O–Ti5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.A. V. is grateful to Markus K. Kremer for computational support. C. R. acknowledges the financial support through program MCIN/AEI/10.13039/501100011033 of Spain under grants PID2019-107338RB-C63 and PID2020-114252GB-I00. We acknowledge the CINECA award under the ISCRA initiative (code: HP10B7T83G) and the C3P computer facility at the University of Padova for the availability of high performance computing resources and support.Peer reviewe

On-surface products from de-fluorination of C60F48 on Ag(111): C60, C60Fx and silver fluoride formation

By employing diverse surface sensitive synchrotron radiation spectroscopies we demonstrate that the fluorine content of initial C60F48 deposited at room temperature on Ag(111) varies with molecular coverage. At the very early stages of deposition, C60F48 fully de-fluorinates and transforms into C60. Strong indications of silver fluoride formation are provided. The chemical footprint of fluorinated fullerenes emerges at relatively low molecular coverage indicating that the degree of fullerene de-fluorination decreases (from total to partial de-fluorination) as molecules are deposited. De-fluorination stops well before the substrate surface is completely covered by fullerenes. At the molecular level, the fluorine loss observed by spectroscopic techniques are supported by scanning tunneling microscopy imaging. Both molecules and metal surface are importantly involved in the process.This work has been supported by the Spanish Government under the projects PID2019-110907GB-I00, MAT2017-85089- C2-1-R (AEI/FEDER, UE) and the ‘‘Severo Ochoa’’ Program for Centres of Excellence in R&D (CEX2019-000917-S). This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 having benefitted from the access provided by CNR-IOM in Trieste and Grenoble within the framework of the NFFA-Europe Transnational Access Activity (ID: 929).Peer reviewe

Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography

The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems

Ordered assembly of non-planar vanadyl-tetraphenylporphyrins on ultra-thin iron oxide

Stabilizing ordered assemblies of molecules represents the first step towards the construction of molecular devices featuring hybrid (organic-inorganic) interfaces where molecules can be easily functionalized in view of specific applications. Molecular layers of planar metal-tetraphenylporphyrins (MTPP) grown on an ultrathin iron oxide [namely Fe(001)-p(1 x 1)O] show indeed a high degree of structural order. The generality of such a picture is tested by exploiting non-planar porphyrins, such as vanadyl-TPP (VOTPP). These molecules feature a VO2+ ion in their center, with the O atom protruding out of the plane of the porphyrin ring. In this work, by employing diffraction, photoemission and X-ray absorption, we prove that non-planar VOTPP can nevertheless form a square and ordered superstructure, where porphyrin molecules lie flat with respect to the underlying substrate. Ab initio density functional theory simulations are used to elucidate the V = O bond orientation with respect to the iron substrate

The Sea Bathing Safeguard Plan in Rimini

In this paper, the Safe Bathing Master Plan deliberated by the Municipality of Rimini, in Italy, is presented. As is well known, the main goal of a sewerage master plan is to avoid sanitary problems, prevent floods, reduce combined sewer overflows to receiving waters in wet weather thus improving the environmental quality both for inhabitants and tourists. Of course, such items become more severe with references to cities, as Rimini, for instance, located close to the sea and where the number of inhabitants strongly varies during the year time due to seasons\u2019 tourist inflows that impact on the carrying capacity. In the experience in Rimini, the iter for a new sewerage plan started more than 10 years ago, and in 2011 a new perspective has been chosen, together with a large involvement of the citizens in the decision making. In the present paper, first, the genesis of the plan is presented. Then, in section 2, details about the plan are given. A special section is dedicated to the mitigation of the impact of the construction sites on the citizenship. Finally, a discussion on the benefit of the plan is reported. At present, more than 60% of the constructions foreseen in the Safe Bathing Maserplan has been realized