Electronic state spectroscopy of C₂Cl₄

The VUV spectrum of C2Cl4 is reported in the energy range 3.8-10.8 eV (325-115 nm). Several photoabsorption features are observed for the first time, including a very weak low-lying band which is provisionally attributed to a π → π* triplet transition. Recent ab initio calculations of the molecule’s electronic transitions [Arulmozhiraja et al. J. Chem. Phys. 129 (2008) 174506] provide the basis for the present assignments below 8.5 eV. An extended ndπ series is proposed to account for several higher-energy Rydberg bands. The identification of vibrational structure, dominated by symmetric C=C and CCl2 stretching in excitations from the HOMO, largely agrees with previous spectroscopic studies. The present absolute photoabsorption cross sections cover a wider energy range than the previous measurements and are used to calculate UV photolysis lifetimes of this aeronomic molecule at altitudes between 20 and 50 km

CNO- formation through selective bond cleavage

H- and CNO- site and bond selectivity formation is shown in the context of atom-molecule collisions.publishersversionpublishe

Electron transfer driven decomposition of adenine and selected analogs as probed by experimental and theoretical methods

We report on a combined experimental and theoretical study of electron transfer induced decomposition of adenine and a selection of analogue molecules in collisions with potassium atoms (K). Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6–68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine, adenine, 9-methyl adenine, 6-dimethyl adenine and 2-D adenine. Following our recent communication about selective hydrogen loss from the transient negative ions (TNI) produced in these collisions [T. Dunha et al. J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH2 loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K+ cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment the lowest unoccupied molecular orbitals accessed in the collision process

Novel experimental setup for time-of-flight mass spectrometry ion detection in collisions of anionic species with neutral gas-phase molecular targets

8 págs.; 4 figs.; Open Access funded by Creative Commons Atribution Licence 4.0We report a novel experimental setup for studying collision induced products resulting from the interaction of anionic beams with a neutral gas-phase molecular target. The precursor projectile was admitted into vacuum through a commercial pulsed valve, with the anionic beam produced in a hollow cathode discharge-induced plasma, and guided to the interaction region by a set of deflecting plates where it was made to interact with the target beam. Depending on the collision energy regime, negative and positive species can be formed in the collision region and ions were time-of-flight (TOF) mass-analysed. Here, we present data on O2 precursor projectile, where we show clear evidence of O– and O2 – formation from the hollow cathode source as well as preliminary results on the interaction of these anions with nitromethane, CH3NO2. The negative ions formed in such collisions were analysed using time-of-flight mass spectrometry. The five most dominant product anions were assigned to H–, O–, NO–, CNO– and CH3NO2 –.PLV acknowledges the Portuguese Foundation for Science and Technology (FCT-MEC) through SFRH/BSAB/105792/ 2014 during his sabbatical stay at CSIC, Madrid and the research grants PTDC/FIS-ATO/1832/2012 and UID/FIS/00068/ 2013. FFS acknowledges FCT-MEC through researcher grant IF-FCT IF/00380/2014. We also acknowledge the Spanish Ministerio de Economía y Competitividad (Project No. FIS 2012-31230). Some of this work forms part of the EU/ESF COST Actions CM1401 and CM1301, Our Astro-Chemical History and Chemistry for Electron-Induced Nanofabrication, respectively. LE-G and GG acknowledge the FP7-PEOPLE-2013-ITN research grant “Advanced Radiotherapy, Generated by Exploiting Nanoprocesses and Technologies (ARGENT)”.Peer Reviewe

Exploring the low-lying electronic states of C4H6OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

PLV also acknowledges his visiting professor position at Federal University of Paraná, Curitiba, Brazil. The authors wish to acknowledge the beam time at the ISA synchrotron, Aarhus University, Denmark. This contribution is also based upon work from the COST Action CA18212-Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). DD acknowledges support from the CaPPA project (Chemical and Physical Properties of the Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract no. ANR-10-LABX-005 ; the Région Hauts de France and the Ministère de l'Enseignement Supérieur et de la Recherche (CPER ECRIN) and the European Fund for Regional Economic Development for their financial support. This work was performed using HPC resources from GENCI-TGCC (Grant No. 2023–A0130801859 ) and the Centre de Ressources Informatiques (CRI) of the Université de Lille. Publisher Copyright: © 2024 The Author(s)Results of a detailed study on the electronic state spectroscopy of C4H6OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, have been obtained from high-resolution vacuum ultraviolet photoabsorption experiments together with quantum chemical calculations. The absolute photoabsorption cross-sections in the 3.7–10.7 eV energy range were obtained at the AU-UV beam line, ASTRID2 synchrotron radiation facility. The absorption spectra exhibit features due to transitions into valence and Rydberg states, superimposed on vibrational fine structure which appear much weaker in the photoabsorption spectrum of dihydro-3(2H)-thiophenone. Assignments have been proposed for some of the absorption bands with the aid of ab initio calculations at the equation-of-motion coupled-cluster singles and doubles level (EOM-CCSD) providing vertical excitation energies and oscillator strengths. The nature of the transitions was assessed by visual inspection of the natural orbitals for each transition and the average values from the electronic radial spatial extents of the electronic cloud. A comparison between the vibrational structure observed in the experimental spectra suggests relevant C[dbnd]O stretching excitations for both molecules, with important CH2 twisting and rocking modes for dihydro-2(3H)-thiophenone and ring stretching modes for dihydro-3(2H)-thiophenone. Photolysis lifetimes from 0 up to 50 km altitude in the Earth's atmosphere for both chemical compounds have been estimated from the absolute photoabsorption cross-sections.publishersversionpublishe

Threshold behavior in metastable dissociation of multi-photon ionized thymine and uracil

Microsecond-timescale HNCO loss has been observed from single-color multi-photon ionized pyrimidine nucleobases in the gas phase. Photon energy thresholds for the metastable channels have been measured at 5.55 ± 0.02 eV for thymine and 5.57 ± 0.02 eV for uracil. We argue that these results can be attributed to accessing the molecules’ S1 states with additional vibrational energy matching the threshold energy for HNCO loss from the radical cation. Combined with previous photoionization energies, this enables the S1 adiabatic energies to be deduced: 3.67 ± 0.07 eV for thymine and 3.77 ± 0.07 eV for uracil. These values are consistent with recent calculations

Multi-photon ionization and fragmentation of uracil: neutral excited-state ring opening and hydration effects

Multi-photon ionization (MPI) of the RNA base uracil has been studied in the wavelength range 220–270 nm, coinciding with excitation to the S2(ππ*) state. A fragment ion at m/z = 84 was produced by 2-photon absorption at wavelengths ≤232 nm and assigned to C3H4N2O+ following CO abstraction. This ion has not been observed in alternative dissociative ionization processes (notably electron impact) and its threshold is close to recent calculations of the minimum activation energy for a ring opening conical intersection to a σ(n-π)π* closed shell state. Moreover, the predicted ring opening transition leaves a CO group at one end of the isomer, apparently vulnerable to abstraction. An MPI mass spectrum of uracil-water clusters is presented for the first time and compared with an equivalent dry measurement. Hydration enhances certain fragment ion pathways (particularly C3H3NO+) but represses C3H4N2O+ production. This indicates that hydrogen bonding to water stabilizes uracil with respect to neutral excited-state ring opening

Communication: Site-selective bond excision of adenine upon electron transfer

This work demonstrates that selective excision of hydrogen atoms at a particular site of the DNA base adenine can be achieved in collisions with electronegative atoms by controlling the impact energy. The result is based on analysing the time-of-flight mass spectra yields of potassium collisions with a series of labeled adenine derivatives. The production of dehydrogenated parent anions is consistent with neutral H loss either from selective breaking of C–H or N–H bonds. These unprecedented results open up a new methodology in charge transfer collisions that can initiate selective reactivity as a key process in chemical reactions that are dominant in different areas of science and technology

Electron stimulated desorption from condensed benzene

Funding Information: This study has partially been supported by the Spanish Ministry of Science and Innovation (Project PID2019-104727RB-C21), A. G. A. and L. A. thank MICIU and the local CAM government, respectively, for their corresponding grants within the “Garantía Juvenil” programmes. A. I. L. and P. L. V. acknowledge the Portuguese National Funding Agency (FCT) through research Grants CEFITEC (UIDB/00068/2020). The experiments were supported by the Natural Science and Engineering Research Council of Canada (RGPIN-2018-14882). Publisher Copyright: © 2024 The Royal Society of Chemistry.The electron induced dissociation of condensed benzene (C6H6) in thin films deposited on a Pt substrate is investigated by electron stimulated desorption (ESD) of anions and cations. The desorbed yields are recorded as a function of incident electron energy in the range of 10 to 950 eV for a fixed film thickness of 2 monolayers (ML) and for a fixed energy of 950 eV, as well as a function of film thickness from 0.5 to 8 monolayers (ML) for anions, and from 0.5 to 12ML for cations. Both energy and thickness dependencies are discussed in terms of the three main mechanisms yielding positively and/or negatively charged fragments: dissociative electron attachment (DEA), dipolar dissociation (DD) and dissociative ionization (DI) processes. At the probed energies, DD is the major mechanism, while DEA is predominantly induced by secondary electrons from the Pt substrate. Desorption of the parent positive ion is strongly suppressed. Similar qualitative behaviours are observed for the energy dependence of both anion and cation ESD yields, while some discrepancies exist in the thickness dependence, including a very significant systematic magnitude difference found between such ions formation. An estimation of the effective DD cross-section including the desorption probability is obtained. Feasible mechanisms behind the observed energy and thickness dependences for anion and cation yields are proposed. These results highlight the need for further investigations to better understand the underlying processes of electron induced dissociation in condensed matter.publishersversionpublishe