In this work, the production of the cyanoacetylene dication and the characterization of its fragmentation dynamics followed by Coulomb explosion are presented, as well as its possible role in astrochemistry. The molecular dication (HC3N)2+ was formed by double photoionization of the neutral precursor molecule HC3N after its in-situ synthesis. The dissociation dynamics has been studied using the photoelectron-photoion-photoion coincidence (PEPIPICO) technique coupled with time-of-flight (TOF) mass spectrometry and synchrotron radiation in the photon energy range of 30.0–50.0 eV. Preliminary results, analyzed by a Monte Carlo computational approach, are presented regarding the threshold energy for the formation of the molecular dication (HC3N)2+ and of all the open fragmentation channels resulting from its Coulomb explosion. Furthermore, a more in-depth analysis of the experimental data is outlined that can determine: i) the relative cross sections of the observed fragmentation channels as a function of photon energy; ii) the kinetic energy released for the formation of each fragment ion. This work is in progress in our laboratory and highlights its relevance for fully characterizing the energetics and microscopic dynamics of a process important for both environmental chemistry and astrochemistry.

The ionization mechanisms of small H2O/D2O clusters embedded in helium nanodroplets (HNDs) irradiated with extreme ultraviolet photons of energy hν = 21.6 eV are investigated using Penning ionization electron-ion coincidence spectroscopy. Both protonated (H2O)n-1H+/(D2O)n-1D+ (n = 3-6) and unprotonated (H2O)n+/(D2O)n+ (n = 2-5) cluster ions were observed. Penning ionization electron spectra (PIES) measured in coincidence with water cluster ions being emitted from H2O/D2O clusters doped in both large and small HNDs are analyzed, and compared with photoelectron-photoion coincidence spectra measured for free H2O/D2O clusters at hν = 20.6 eV. While the 1b1 outer-valence electron signal associated with direct photoionization of free water clusters suggests that the most abundant cluster ions result from cluster ion fragmentation, the droplet-correlated 1b1 outer-valence electron signal indicates that water cluster ions of various sizes remain largely intact, rather than fragmenting, following their creation by Penning ionization. Quantum chemical calculations on unprotonated clusters confirm the likely coexistence of proton-transferred and hemibonded isomers under the cryogenic environment of HNDs, at least for the trimer and the tetramer.

The microhydration of rock salt (NaCl) molecules was investigated theoretically by density-functional theory and force field calculations and experimentally by high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. The calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at -15. However, it takes water molecules to form a complete solvation shell around the Cl- anion and as many as to fully hydrate the Na+ cation, thus the entire NaCl molecule. Although NaCl molecules are predicted to be fully submerged inside the droplets, the PIES of NaCl are highly resolved, in stark contrast to other molecular species. Codoping the droplets with a controlled number of -10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for n greater than or similar to 30, in line with the calculations.

Pentafluorophenyl triflate has been explored as a highly absorbing neutral photoacid generator (PAG) candidate for next generation chemically amplified resists used in extreme ultraviolet (EUV) lithography. Although increased fluorination enhances EUV absorption, this study demonstrates that such an approach does not necessarily improve photoacid generation efficiency. Using photoelectron-photoion coincidence (PEPICO) spectroscopy at the 92 eV photon energy of the EUV scanners in combination with quantum chemical calculations, the dissociative photoionization of pentafluorophenyl triflate was systematically investigated. The photoionization mass spectrum reveals extensive fragmentation, with the parent ion contributing only 3.1

The microhydration of rock salt (NaCl) molecules was investigated using high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. Although model calculations predict that NaCl molecules are fully submerged inside the droplets, PIES of NaCl are highly resolved, in stark contrast to other molecular species. Co-doping the droplets with a controlled number of n=5–10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for n≳ 30. Accompanying density-functional theory (DFT) and force field calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at n=12–15. However, it takes n≈ 17 water molecules to form a complete solvation shell around the Cl^- anion and as many as n≈ 34 to fully hydrate the Na^+ cation, thus the entire NaCl molecule, which rationalizes the experimental findings.

In this work, the synthesis performed for neutral precursors (isothiocyanic acid, carbon suboxide and cyanoacetylene) used in studies of double photoionization spectroscopy with synchrotron radiation are presented. They were performed starting from few data present in the literature by modifying the procedure in order to obtain a better yield of the desired product with an optimized timing. All the synthetic procedures are related to chemical species of great relevance from an environmental and astrochemical point of view and not commercially available. They can be easily reproduced by any researcher interested in studying the chemical species that are the subject of this work.

Due to the smaller size of nickel compared to palladium, the C–S cross-coupling of sterically challenging aryl electrophiles with alkyl thiols under nickel catalysis remained elusive. Herein, we report the nickel-catalyzed cross-coupling of alkyl thiols with aryl triflates bearing functional groups in ortho-position relative to the leaving group using Ni(cod)2/DPEphos (L1) or dppbz (L2) as the catalytic system. For substrates featuring non-coordinating ortho-substituents, the reaction operates under mild conditions using L1, while for electrophiles bearing coordinating groups, the ligand L2 and elevated temperatures are required. The synthetic utility could be demonstrated on numerous examples, including biologically relevant compounds, and on larger scale. Instead of Ni(cod)2, more cost-efficient Ni(OAc)2 can also be employed in the presence of zinc as reductant. Furthermore, insights into the reaction mechanism were obtained by competition experiments, isolation of organometallic intermediates and computations.

A combined experimental and computational approach aimed at systematically exploring unimolecular reaction channels of ionized gas-phase systems—including rearrangements, intramolecular bonds formation, and fragmentation—has been applied to 5- and 6-benzyluracil (5BU and 6BU) molecules, which are used as molecular models of photo-induced interactions between nucleobases and aromatic aminoacids. Photoelectron–photoion coincidence experiments offer a unique view of energy-selected fragmentation channels, while ab initio molecular dynamics of the ionic system, performed with semi-empirical potentials and for long simulation times, can extensively explore possible unimolecular reactive pathways, revealing the detailed molecular identity of the products. Results show that, at lower binding energies, ionized 5- and 6-benzyluracil molecules do not fragment but may nevertheless rearrange to give, mainly in 6BU, cross-linked products. This suggests that ionization can efficiently promote covalent cross-links between interacting proteins and nucleic acids, which can be exploited to isolate and characterize such transient complexes occurring in cellular environments. At higher binding energies, however, fragmentation channels are predominant, mainly involving the decomposition and photo-damage of the uracil moiety. Several fragmentation products have been characterized, and the differences in fragment abundances and fragmentation mechanisms between the two benzyluracil isomers have been outlined.

We report a nickel-catalyzed cross-coupling between aryl (pseudo)halides and thiols under mechanochemical conditions. The reaction tolerated both alkyl and aryl thiols and required a lower catalyst loading than the same catalyst under homogeneous conditions (2.5 vs 5 mol %). Moreover, the reaction can be set up under air, greatly simplifying and shortening the reaction setup. Notably, trialkyl amines proved most efficient as a base under mechanochemical conditions. This class of bases has been rarely used in C-S cross-couplings before. The applicability of this system was demonstrated on numerous examples, including compounds relevant to materials science and pharmaceuticals. While mechanistic studies revealed that the reaction is mainly driven by thermal energy produced by the mixer mill, mechanical force appeared to accelerate the reaction, enabling otherwise unfeasible substrates. Using neat NMR and DSC techniques, it could be shown that the catalyst is initially deactivated by excess thiol, forming a mixture of nickel(hydrido)(thiolate) species. Upon reduction by zinc, the active catalyst is formed, able to engage in a Ni(0)/Ni(II) catalytic cycle.

The development of extreme ultraviolet sources for nanolithography is enabling the production of integrated circuits with feature sizes of less than 10 nm. This necessitates simultaneously the optimization of photoresist materials adapted to the high photon energy and low flux of these sources to ensure a precise pattern transfer and guarantee a high throughput. A common type of EUV resists are chemically amplified resists (CAR), containing a (co)polymer, which plays the main role in the pattern transfer through a deprotection reaction leading to a solubility switch. The composition of CARs and the ionizing nature of EUV light lead to a complex chemistry induced by photons and electrons in the resist film. This makes it difficult to grasp the full reaction mechanism. Therefore, the isolated role of the photoionization process on the copolymer of typical CARs containing poly(hydroxy styrene)/poly(tert-butyl methacrylate) (PHS/PBMA) is studied. Gas phase photoelectron photoion coincidence (PEPICO) experiments on the proxy molecules 4-isopropyl phenol (IPP) and tert-butyl methacrylate (tBMA) are conducted employing synchrotron radiation, which yield deep insights into dissociative photoionization. While the phenolic moiety in IPP undergoes fragmentation only to a small degree, the dissociative photoionization of the ester group in tBMA leads to several important fragments that are mostly unfavorable for an efficient solubility switch. By comparing these insights to EUV photoemission and desorption experiments on PHS and PBMA thin films in solid phase, a better understanding of the initial photoionization reaction in the exposure mechanism of CARs is obtained. Combining this knowledge with further complementary experiments is crucial to develop higher performing EUV photoresists for future technology nodes.

The photofragmentation of halothane (CF3CHBrCl) was studied with synchrotron radiation by photoionization efficiency (PIE) measurements and photoelectron-photoion coincidence (PEPICO) experiments, as well as by a theoretical exploration of potential energy surfaces. Among the other fragments, the formation of the CHClF+ and CHBrF+ ions, which involves the transfer of a F atom between the two moieties of the parent molecule, was observed. To understand the mechanisms leading to the halogen migration, a detailed theoretical study of the production of CHClF+, m/z 67+, based on DFT calculations and natural bond orbital (NBO) analysis was conducted. The results contribute to the understanding of the photochemistry of halothane, its polluting behavior in the high atmosphere, and the formation of highly reactive species.

We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase using time-resolved core-level photoelectron spectroscopy together with high-level calculations. The dynamics was investigated by trajectory surface hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics probed on the oxygen, nitrogen, and carbon sites by core electron spectroscopy. We find that the main de-excitation channel of the initially excited S2(ππ*) state involves internal conversion to the S1(nπ*) state with a time constant of 17 ± 4 fs, while a portion of S2(ππ*) population returns directly to the ground state by internal conversion. We find no evidence that the S1(nπ*) state decays to the ground state; instead, it decays to triplet states with a time constant of 1.6 ± 0.4 ps. Oscillations of the S1(nπ*) state O 1s intensity as a function of time correlate with those of calculated C4═O8 and C5═C6 bond lengths, which undergo a sudden expansion following the initial π → π* excitation. Our calculations support our interpretation of the data and provide detailed insight into the relaxation processes of uracil.

In this paper is presented a summarizing description of the collection and computational analysis of the relevant data recorded following a double photoionization experiment on molecular species of interest in astrochemistry. In particular, the computational procedure used to analyze experimental data collected in the double photoionization of allene molecules by tunable radiation in the 25.0–45.0 photon energy range of the GASPHASE and CiPo beamlines of the ELETTRA synchrotron facility of Basovizza (Trieste, Italy) is outlined and discussed. Data presented are recorded using the ARPES (Angle Resolved PhotoEmission Spectroscopy) end station, which is a molecular beam apparatus coupled with a PEPIPICO (Photoelectron-Photoion-Photoion Coincidence) technique and TOF (Time-of-Flight) mass spectrometry, in three different beamtimes.

Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound complexes exposed to intense or high-energy radiation. So far, neutral or ionic atoms or molecules have been prepared in singly excited electron or hole states that can transfer energy to neighboring centers and cause ionization and radiation damage. Here we demonstrate that a doubly excited atom, despite its extremely short lifetime, can decay by ICD; evidenced by high-resolution photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find that ICD proceeds by relaxation into excited He^{*}He^{+} atom-pair states, in agreement with calculations. The ability of inducing ICD by resonant excitation far above the single-ionization threshold opens opportunities for controlling radiation damage to a high degree of element specificity and spectral selectivity.

The goals of this work are to attempt to decipher if an aniline dication can isomerize to a picoline dication in a given astrochemical environment and if the dissociation of such dications could be a source of kinetically hot fragment ions, some of which could be of significance in the interstellar medium. Toward this purpose, the VUV-induced dication dissociation was investigated experimentally using ion-ion coincidence and computationally by optimizing various pathways. Contrary to previous reports, we show here that the dication of aniline is structurally too weak to retain its ring structure while following the dissociation pathways. A fragile open ring structure could lead to all the experimentally observed pathways of noticeable intensity. The significance of this, especially in terms of molecular dynamics, can be assessed by the fact that all the transformations were facilitated by specific hydrogen migration. A clear selectivity is seen where the dication of aniline was found to prefer a rearrangement of hydrogen within the ring rather than transferring from nitrogen to the ring, which is conventionally expected and has to do with the charge state and charge localization.

The exposure of molecules to attosecond extreme-ultraviolet (XUV) pulses offers a unique opportunity to study the early stages of coupled electron-nuclear dynamics in which the role played by the different degrees of freedom is beyond standard chemical intuition. We investigate, both experimentally and theoretically, the first steps of charge-transfer processes initiated by prompt ionization in prototype donor-pi-acceptor molecules, namely nitroanilines. Time-resolved measurement of this process is performed by combining attosecond XUV-pump/few-femtosecond infrared-probe spectroscopy with advanced many-body quantum chemistry calculations. We show that a concerted nuclear and electronic motion drives electron transfer from the donor group on a sub-10-fs timescale. This is followed by a sub-30-fs relaxation process due to the probing of the continuously spreading nuclear wave packet in the excited electronic states of the molecular cation. These findings shed light on the role played by electron-nuclear coupling in donor-pi-acceptor systems in response to photoionization.

The fragmentation of three cyclic dipeptides (c-Glycil-Phenylalanine, c-Tryptophan-Tyrosine and c-Tryptophan-Tryptophan), characterized by an aromatic side chain, has been investigated by synchrotron radiation and photoelectron-photoion coincidence (PEPICO) experiments, assisted by atomistic simulations. The PEPICO experiments show that the charged moiety containing the aromatic side chain is the main fragment in the three samples. The theoretical exploration of the potential energy surfaces has allowed to identify the possible fragmentation paths leading to the formation of these fragments. Then, the analysis of the differences in the electronic density distributions of the neutral molecule and the cation and a molecular dynamics simulation provided an understanding of the preferred localization of the positive charge on the aromatic side chain of the cyclic dipeptide.

The three-body dissociation dynamics of the dicationic camphor molecule (C10H16O2+) resulting from Auger decay are investigated using soft x-ray synchrotron radiation. A photoelectron-photoion-photoion coincidence method, a combination of a velocity map imaging spectrometer and a time-of-flight spectrometer is employed to measure the 3D momenta of ions detected in coincidence. The ion mass spectra and the ion-ion coincidence map at photon energies of 287.9 eV (below the C 1s ionization potential) and 292.4 eV (above the C 1s ionization potential for skeletal carbon) reveal that fragmentation depends on the final dicationic state rather than the initial excitation. Using the native frame method, three new fragmentation channels are discussed; (1) CH2CO+ + C7H11+ + CH3, (2) CH3+ + C7H11+ + CH2CO, and (3) C2H5+ + C6H9+ + CH2CO. The dominating nature of sequential decay with deferred charge separation is clearly evidenced in all three channels. The results are discussed based on the experimental angular distributions and momenta distributions, corroborated by geometry optimization of the ground, monocationic, and dicationic camphor molecule.

We investigate the ionisation and fragmentation dynamics of free camphor molecules and camphor-doped helium nanodroplets by extreme ultraviolet (EUV) and soft x-ray photons using velocity map imaging combined with photoelectron-photoion coincidence (VMI-PEPICO) spectroscopy. We notably find that the Penning ionisation of camphor in He nanodroplets at h nu=21.43 eV is soft with nearly identical Penning ionised electron spectra correlated to different molecular fragments. Further, fragmentation following ionisation at all photon energies exhibit a droplet-specific mass peak equivalent to the missing mass of CO, suggesting suppression of further fragmentation inside the droplet. This work reveals unusual features of photoionisation of organic molecules doped in He nanodroplet motivating further experimental and theoretical explorations.