We report a combined experimental and computational study of the near-edge X-ray absorption fine structure (NEXAFS) spectrum of the propargyl radical, C3H3●. As a central intermediate in the formation of polycyclic aromatic hydrocarbons, the propargyl radical is a species of considerable relevance in combustion and astrochemistry and was here generated by pyrolysis from propargyl bromide. The NEXAFS spectrum shows a pronounced band at 282.2 eV corresponding to transitions from carbon 1s orbitals to singly occupied molecular orbitals. Ab initio calculations reveal that two transitions to the lowest-lying states 1 A12 and 2 A12, which take place from the C1s orbital of the two terminal carbon atoms, contribute to this band. In addition, a 420 meV spacing of the first band is visible and is assigned to a vibrational progression in the symmetric CH2 stretch. Transitions at higher energies are also described reasonably well by theory. The fragmentation pattern was investigated at the different resonant transitions and shows the cleavage of one as well as both C-C bonds.

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 vibrational circular dichroism (VCD) spectra of (R)-2-chloromethyl-oxirane and (R)-2-chloromethyl-thiirane have been recorded in a vast spectral region including mid-IR, CH-stretching fundamentals and bending-CC/CO(CS) overtone/combination regions in the mid-IR, and CH-stretching overtone/combination regions in the near infrared (NIR). The presence of sulfur is associated with intensification of the NIR-VCD spectra, similarly to what is monitored with electronic circular dichroism (ECD) and optical rotatory dispersion (ORD). DFT calculations, dealing with anharmonicity at the GVPT2 level or based on the local mode approximation, permit to correctly predict the large majority of observed VCD and IR/NIR bands and to explain the role of various molecular moieties. Also, ORD data are better interpreted by including anharmonic vibrational contributions.

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.

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.

Double ionization spectra of isothiocyanic acid (HNCS) have been measured using multi-electron and multi-ion coincidence techniques combined with high-level theoretical calculations. The adiabatic double ionization energy of HNCS is found at 27.1 +/- 0.1 eV and is associated with the formation of the X 3A '' ground state of HNCS2+. The characteristics of different dissociation channels are examined and compared to the results of electronic structure calculations obtained by systematically elongating the three bonds H-NCS, HN-CS, and HNC-S. For instance, the adiabatic double ionization energy of the NCS fragment is deduced to be 30.95 +/- 0.5 eV. In addition, the C+ and NS+ dissociation channels are of particular interest, possibly indicating the involvement of a structural rearrangement process upon doubly ionizing HNCS.

We report the x-ray absorption spectrum (XAS) of the tert-butyl radical, C4H9. The radical was generated pyrolytically from azo-tert-butane, and the XAS of the pure radical was obtained by subtraction of spectra recorded at different temperatures. The bands in the XAS were assigned by ab initio calculations that are in very good agreement with the experimental data. The lowest energy signal in the XAS is assigned to the C1s electron transition from the central carbon atom to the singly occupied molecular orbital (SOMO), while higher transitions correspond to C1s excitations from terminal carbon atoms. Furthermore, we investigated the fragmentation of the radical following resonant C1s excitation by electron-ion-coincidence spectroscopy. Several fragmentation channels were identified. The C1s excitation of the terminal carbons is associated with a stronger fragmentation tendency compared to the lowest C1s excitation of the central carbon into the SOMO. For this core excited state, we still observe an intact parent ion, C4H9+, and a comparatively higher tendency to dissociate into CH3+ + C3H6(+).

The behavior of nitrosyl chloride (ClNO) exposed to ionizing radiation was studied by direct probing valence-shell electrons in temporal coincidence with ions originating from the fragmentation process of the transient ClNO2+. Such a molecular dication was produced by double photoionization with synchrotron radiation in the 24–70 eV photon energy range. The experiment has been conducted at the Elettra Synchrotron Facility of Basovizza (Trieste, Italy) using a light beam linearly polarized with the direction of the polarization vector parallel to the ClNO molecular beam axis. ClNO molecules crossing the photon beam at right angles in the scattering region are generated by effusive expansion and randomly oriented. The threshold energy for the double ionization of ClNO (30.1 ± 0.1 eV) and six dissociation channels producing NO+/Cl+, N+/Cl+, N+/O+, O+/Cl+, ClN+/O+, NO+/Cl2+ ion pairs, with their relative abundance and threshold energies, have been measured.

Gas phase C$_{70}$ molecules have been ionized with single photons of energies between 16 eV and 70 eV and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions from 55 eV. The low kinetic energy parts of the spectra are explained with thermal emission of transient hot electrons. Deviations at high photon energies are used to determine a value for the initial electron equilibration time. We propose a generally applicable mechanism, named Resonance Ionization Shadowing, for the creation of hot electrons by absorption of above-threshold energy photons.

The present paper reports on an experimental and computational study aimed at characterizing the microscopic fragmentation dynamics of molecules in space that are subject to double photoionization phenomena by UV and EUV (Extreme Ultraviolet) photons. This kind of processes happening in planetary ionospheres and interstellar medium are simulated at the GASPHASE and CIRCULAR POLARIZATION beamlines of the ELETTRA Synchrotron Facility of Basovizza, Trieste (Italy) using the ARPES (Angle Resolved PhotoEmission Spectroscopy) end station: a molecular beam apparatus coupled with the PEPIPICO (Photoelectron-Photoion-Photoion Coincidence) technique and the TOF (Time-of-Flight) mass spectrometry. The used synchrotron radiation is in the 25.0–45.0 eV photon energy range. Preliminary data are presented for the case of the double photoionization of allene molecules whose photoinduced decomposition highlights 8 different open two-body fragmentation channels and 3 three-body dissociations. For each investigated dissociation channels threshold energies, relative cross sections and the appearing energy for the allene (C3H4)2+ dication as a function of the photon energy have been measured. Based on a Monte Carlo trajectory simulation, the computational analysis of the PEPIPICO spectra experimentally collected which is still in progress allows the possible extraction of the kinetic energy released (KER) and the angular distributions for the final fragment ions. These observables are very important to rationalize the physical chemistry of the elementary processes induced by the interaction between molecules and ionizing radiations in space.

The potential for selective bond breaking of a small molecule was investigated with electron spectroscopy and electron-ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photoionization and resonant core excitation at the N 1s- and O 1s-edges, followed by the emission of resonant-Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states. The states are of special relevance for investigating how distinct electronic configurations influence selective bond breaking. The electron-ion coincidence measurements provided branching fractions of the produced ion fragments as a function of electron binding energy. They explicitly demonstrate how the final electronic states created after photoionization and RA decay influence fragmentation. In particular, we observed a significantly different branching fraction for spectator states compared with participator states. In addition, it was also observed that the bonds broken for the spectator states correlate with the antibonding nature of the spectator-electron orbital.

Ionization cross sections for Ne*( 3 P 2,0 )–HX (X = Cl, Br) chemi-ionizations are presented in the 0.02–0.5 eV collision energy range.

Vibrational circular dichroism (VCD) spectra and the corresponding IR spectra of the chiral isomers of methyloxirane and of methylthiirane have been reinvestigated, both experimentally and theoretically, with particular attention to accounting for anharmonic corrections, as calculated by the GVPT2 approach. De novo recorded VCD spectra in the near IR (NIR) range regarding CH-stretching overtone transitions, together with the corresponding NIR absorption spectra, were also considered and accounted for, both with the GVPT2 and with the local mode approaches. Comparison of the two methods has permitted us to better describe the nature of active "anharmonic" modes in the two molecules and the role of mechanical and electrical anharmonicity in determining the intensities of VCD and IR/NIR data. Finally, two nonstandard IR/NIR regions have been investigated: the first one about ≈2000 cm-1, involving mostly two-quanta bending mode transitions, the second one between 7000 and 7500 cm-1 involving three-quanta transitions containing CH-stretching overtones and HCC/HCH bending modes.

Interferometric pump-probe experiments in the extreme ultraviolet (XUV) domain are experimentally very challenging due to the high phase stability required between the XUV pulses. Recently, an efficient phase stabilization scheme was introduced for seeded XUV free electron lasers (FELs) combining shot-to-shot phase modulation with lock-in detection. This method stabilized the seed laser beampath on the fundamental ultraviolet wavelength to a high degree. Here, we extend this scheme including the stabilization of the XUV beampath, incorporating phase fluctuations from the FEL high gain harmonic generation process. Our analysis reveals a clear signal improvement with the new method compared to the previous stabilization scheme.

Using time-of-flight multiple electron and ion coincidence techniques in combination with a helium gas discharge lamp and synchrotron radiation, the double ionisation spectrum of disulfur (S $$_2$$ ) and the subsequent fragmentation dynamics of its dication are investigated. The S $$_2$$ sample was produced by heating mercury sulfide (HgS), whose vapour at a suitably chosen temperature consists primarily of two constituents: S $$_2$$ and atomic Hg. A multi-particle-coincidence technique is thus particularly useful for retrieving spectra of S $$_2$$ from ionisation of the mixed vapour. The results obtained are compared with detailed calculations of the electronic structure and potential energy curves of S $$_2^{2+}$$ which are also presented. These computations are carried out using configuration interaction methodology. The experimental results are interpreted with and strongly supported by the computational results.

Abstract Using time-of-flight multiple electron and ion coincidence techniques in combination with a helium gas discharge lamp and synchrotron radiation, the double ionisation spectrum of disulfur (S2) and the subsequent fragmentation dynamics of its dication are investigated. The S2 sample was produced by heating mercury sulfide (HgS), whose vapour at a suitably chosen temperature consists primarily of two constituents: S2 and atomic Hg. A multi-particle-coincidence technique is thus particularly useful for retrieving spectra of S2 from ionisation of the mixed vapour. The results obtained are compared with detailed calculations of the electronic structure and potential energy curves of S2+2 which are also presented. These computations are carried out using configuration interaction methodology. The experimental results are interpreted with and strongly supported by the computational results.

Double and triple ionization of allene are investigated using electron-electron, ion-ion, electron-electron-ion and electron-electron-ion-ion (ee, ii, eei, eeii) coincidence spectroscopies at selected photon energies. The results provide supporting evidence for a previously proposed roaming mechanism in H$_3^+$ formation by double ionisation. The lowest vertical double ionization energy is found to be 28.5 eV, while adiabatic double ionisation is not accessed by vertical ionisation at the neutral geometry. The triple ionization energy is found to be close to 50 eV in agreement with theoretical predictions. The doubly charged parent ion is stable up to about 2 eV above threshold, after which dissociations by charge separation and by double charge retention occur with comparable intensities. Fragmentation to H$^+$ + C$_3$H$_3^+$ starts immediately above threshold as a slow (metastable) decay with 130+/-10 ns mean lifetime.

Interatomic processes play a crucial role in weakly bound complexes exposed to ionizing radiation; therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high-resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 fs, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process, where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles, which drastically accelerates ICD.

High-intensity extreme ultraviolet (XUV) pulses from a free-electron laser can be used to create a nanoplasma in clusters. In reference Michiels et al (2020 Phys. Chem. Chem. Phys. 22 7828–34) we investigated the formation of excited states in an XUV-induced nanoplasma in ammonia clusters. In the present article we expand our previous study with a detailed analysis of the nanoplasma evolution and ion kinetics. We use a time-delayed UV laser as probe to ionize excited states of H and H 2 + in the XUV-induced plasma. Employing covariance mapping techniques, we show that the correlated emission of protons plays an important role in the plasma dynamics. The time-dependent kinetic energy of the ions created by the probe laser is measured, revealing the charge neutralization of the cluster happens on a sub-picosecond timescale. Furthermore, we observe ro-vibrationally excited molecular hydrogen ions H 2 + * being ejected from the clusters. We rationalize our data through a qualitative model of a finite-size non-thermal plasma.

When weakly-bound complexes are multiply excited by intense electromagnetic radiation, energy can be exchanged between neighboring atoms through a type of resonant interatomic Coulombic decay (ICD). This decay mechanism due to multiple excitations has been predicted to be relatively slow, typically lasting tens to hundreds of picoseconds. Here, we directly measure the ICD timescale in resonantly excited helium droplets using a high resolution, tunable, extreme ultraviolet free electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as fast as 400 femtoseconds, and to only present a weak dependence on the density of the excited states. Using a combination of time dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles which drastically accelerates ICD.