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International audience; We propose a novel approach for an indirect probing of conjugation and hyperconjugation in coreexcited molecules using resonant Auger spectroscopy. Our work demonstrates that the changes in the electronic structure of thiophene (C 4 H 4 S) and thiazole (C 3 H 3 NS), occurring in the process of resonant sulfur K-shell excitation and Auger decay, affect the stabilisation energy resulting from π−conjugation and hyperconjugation. The variations in the stabilisation energy manifest themselves in the resonant S KL 2,3 L 2,3 Auger spectra of thiophene and thiazole. The comparison of the results obtained for the conjugated molecules and for thiolane (C 4 H 8 S), the saturated analogue of thiophene, has been performed. The experimental observations are interpreted using high-level quantum-mechanical calculations and the natural bond orbital analysis.

6 pags., 5 figs., 1 tab. -- This work is dedicated to Prof. Rafael Notario (IQFR-CSIC)., The gas phase acidities (GA) of 5,5-alkylbarbituric acids have been experimentally determined by electrospray ionization-triple quadrupole (ESI-TQ) mass spectrometry and by using the extended kinetic Cooks method (EKCM). The GAs of C-H (1330.9 ± 10.0 kJ mol-1) and N-H (1361.5 ± 10.5 kJ mol-1) deprotonated sites of bifunctional barbituric acid were determined from the selective production of their corresponding heterodimers. The GA value in the N-H site was confirmed by measuring the GAs of 5,5-dimethyl- and 5,5-diethyl barbituric acids (∼1368 kJ mol-1). The experimental results have been rationalized and discussed with the support of quantum chemical calculations with Gaussian-n (G3 and G4) composite methods, which confirmed the excellent consistency of the results., A.F.L. acknowledges the support from the Brazilian funding agencies CNPq, FAPESP, and CAPES. J.M.O.-E. acknowledges support from the Spanish MICINN, Grant Number CTQ2018-094644-B-C22.

The dissociative photoionization dynamics of the chloroacetone molecule (C3H5OCl) in the gas phase, induced by vacuum ultraviolet (VUV) synchrotron radiation in the range from 10.85 to 21.50 eV, has been investigated by using time-of-flight mass spectrometry in the photoelectron-photoion coincidence mode. The appearance energies for the most relevant cation fragments produced in this energy range have been analyzed, and the fragmentation pathways leading to the formation of the cation species have been proposed and discussed. The mass spectra show that the most dominant VUV photodissociation cation product appears at m/z 43 and has been assigned to the C2H3O+ species. Enthalpies of formation (ΔfH°0K) for the neutral chloroacetone molecule and its molecular cation have been derived and correspond to -207.8 ± 5.8 kJ/mol and 755.1 ± 6.8 kJ/mol, respectively. In addition to the spectral analysis, the structural and energetic parameters for the cations produced have also been examined on the basis of high-level quantum chemical numerical calculations.

The infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are observed in a variety of astrophysical objects, including the circumnuclear medium of active galactic nuclei (AGNs). These are sources of highly energetic photons (0.2 to 10 keV), exposing the PAHs to a harsh environment. In this work, we examined experimentally the photoionization and photostability of naphthalene (C$_{10}$H$_{8}$), anthracene (C$_{14}$H$_{10}$), 2-methyl-anthracene (C$_{14}$H$_{9}$CH$_{3}$) and pyrene (C$_{16}$H$_{10}$) upon interaction with photons of 275, 310 and 2500 eV. The measurements were performed at the Brazilian Synchrotron Light Laboratory using time-of-flight mass-spectrometry (TOF-MS). We determined the absolute photoionization and photodissociation cross sections as a function of the incident photon energy; the production rates of singly, doubly and triply charged ions; and the molecular half-lives in regions surrounding AGNs. Even considering moderate X-ray optical depth values ($\tau = 4.45$) due to attenuation by the dusty torus, the half-lives are not long enough to account for PAH detection. Our results suggest that a more sophisticated interplay between PAHs and dust grains should be present in order to circumvent molecular destruction. We could not see any significant difference in the half-life values by increasing the size of the PAH carbon backbone, N$_C$, from 10 to 16. However, we show that the multiple photoionization rates are significantly greater than the single ones, irrespective of the AGN source. We suggest that an enrichment of multiply charged ions caused by X-rays can occur in AGNs., Comment: 20 pages (appendix: 3 pages), 10 figures, 4 tables. Accepted for publication in the Montly Notices of the Royal Astronomical Society (MNRAS). Accepted 2019 April 4

State-of-the-art argon $KLL$ Auger spectra measured using photon energies of $h\ensuremath{\nu}=3216$ and 3400 eV are presented along with an Ar $[1s]$ photoelectron spectrum (square brackets indicate holes in the respective orbital). The two different photon energies used for measuring the Auger spectra allow distinguishing between the shake transitions during the Auger decay and the Auger transitions of the photoelectron satellites. A complete assignment of satellite transitions is provided, partially based on configuration-interaction calculations. In addition, Ar $[1s3(s,p)]{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}\ensuremath{\rightarrow}[2{p}^{2}{(}^{1}{D}_{2})]$ transitions are observed, which can be explained by knock-down transitions leading to a direct exchange of angular momentum between the excited electron and the Auger electron. The lifetime broadenings of the Ar $[2s]$ single-core-hole state and the $[2{s}^{2}]$ and $[2s2p]$ double-core-hole states are also determined, confirming previously observed trends for double-core-hole states.

8 pags., 7 figs. -- This manuscript is dedicated to Prof. José M. Riveros, We have studied the fragmentation dynamics of the protonated 2-, 3- and 4-hydroxybenzophenones (2H, 3H and 4H, 191 m/z) obtained by CID (“Collision Induced Dissociation”) technique coupled to Mass Spectrometer with electrospray source (MS-ESI). The CID-fragmentation patterns involve the C7OH5 (105 m/z) and C7O2H5 (121 m/z) cations, for which the intensity ratio is typical to the isomer considered. 2H and 3H yield preferably C7O2H5 (121 m/z) and C7OH5 (105 m/z) respectively, while 4H yields both cations in similar quantity. The fragmentation pathways were analyzed and discussed, taking into account the “model of free-proton” and results from computational calculations at B3LYP/6-311++G(d,p) level of theory. CID/MS-ESI fragmentation technique employed to characterize hydroxybenzophenone isomers. Main structures in the spectrum: Parent cation (199 m/z), C7O2H5 (121 m/z), C7OH5 (105 m/z)., This research has been possible thanks to the support of the scholarship program UNI-agreement N◦ 167 - 2015 - Fondecyct UNI Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica, CONCYTEC. A special thanks to Rocío Ramos (JAE-tec) from laboratory of Thermochemistry of IQFR-CSIC, Madrid. A.F.L. acknowledges the support from the brazilian funding agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).

The class of active components of the group of β-lactam antibiotics is very important for several fields and applications, although their stability and radiation reactivity properties are not yet well understood. We have studied the interaction of an important building block species, the 2-azetidinone (CA 2-azetidinine sample was introduced into the UH-vacuum chamber, without purification, through an inlet system for the gas-phase experiments with monochromatic light in the VUV range from the TGM beamline at the Brazilian Synchrotron Facility. A Wiley-McLaren type mass spectrometer in the PEPICO mode was employed to detect and characterize the photoionization and photodissociation products of the 2-azetidinone. The analysis and discussion of the results were supported by high-level density functional theory (DFT) and ab initio methods.The adiabatic ionization energy was determined experimentally in this work as 9.745 ± 0.020 eV, and this was supported by the high level of theory result with good agreement. The heat of formation for the 2-azetininone cation has been derived for the first time as 844.2 ± 1.9 kJ/mol. The dominant ion dissociation channel in the VUV energy range up to 21.5 eV is associated with the cation species at m/z 28.The structural properties, VUV-induced photoionization, and photodissociation dynamics of the 2-azetidinone molecule in the gas phase have been successfully investigated in the energy range of 9.5-21.5 eV. PEPICO mass spectra have been determined for the first time for this molecule at several selected photon energies from which the partial ion yields were determined for all cation species produced from this molecule.

11 pags., 5 figs., 5 tabs., descripción no proporcionada por scopus, This work was mainly financially supported by the Brazilian agenciesCNPq, CAPES and FAPESP. J.Z.D. acknowledges the support of the CSIC (Programa Movilidad). The authors also acknowledge the staff ofthe Brazilian Synchrotron Light Laboratory (LNLS-CNPEM) for their technical support during the experiments.

The Si 1s−1, Si 2s−1, and Si 2p−1 photoelectron spectra of the SiX4 molecules with X = F, Cl, Br, CH3 were measured. From these spectra the Si 1s−1 and Si 2s−1 lifetime broadenings were determined, revealing a significantly larger value for the Si 2s−1 core hole of SiF4 than for the same core hole of the other molecules of the sequence. This finding is in line with the results of the Si 2p−1 core holes of a number of SiX4 molecules, with an exceptionally large broadening for SiF4. For the Si 2s−1 core hole of SiF4 the difference to the other SiX4 molecules can be explained in terms of Interatomic Coulomb Decay (ICD)-like processes. For the Si 2p−1 core hole of SiF4 the estimated values for the sum of the Intraatomic Auger Electron Decay (IAED) and ICD-like processes are too small to explain the observed linewidth. However, the results of the given discussion render for SiF4 significant contributions from Electron Transfer Mediated Decay (ETMD)-like processes at least plausible. On the grounds of our results, some more molecular systems in which similar processes can be observed are identified.

The Si 1s-1, Si 2s-1, and Si 2p-1 photoelectron spectra of the SiX4 molecules with X = F, Cl, Br, CH3 were measured. From these spectra the Si 1s-1 and Si 2s-1 lifetime broadenings were determined, revealing a significantly larger value for the Si 2s-1 core hole of SiF4 than for the same core hole of the other molecules of the sequence. This finding is in line with the results of the Si 2p-1 core holes of a number of SiX4 molecules, with an exceptionally large broadening for SiF4. For the Si 2s-1 core hole of SiF4 the difference to the other SiX4 molecules can be explained in terms of Interatomic Coulomb Decay (ICD)-like processes. For the Si 2p-1 core hole of SiF4 the estimated values for the sum of the Intraatomic Auger Electron Decay (IAED) and ICD-like processes are too small to explain the observed linewidth. However, the results of the given discussion render for SiF4 significant contributions from Electron Transfer Mediated Decay (ETMD)-like processes at least plausible. On the grounds of our results, some more molecular systems in which similar processes can be observed are identified.

A theoretical study on the structural, electronic and spectroscopic properties for the isolated chloroacetone (C3H5OCl) and its molecular cation was performed with ab initio and density functional theory methods. Two conformers were found stable for the neutral molecule, where the structure with the chlorine atom on the edge of a gauche/cis conformation with respect to the methyl group is more stable by 1.0 kcal/mol in comparison with the trans conformation. As for the molecular cation, only the gauche conformation was found stable. Theoretical first adiabatic ionization potential and vibrational frequencies investigated are in very good agreement with the experimental results.

The ratios of single, double, and triple ionizations to the total photoionization of the halothane (C2HBrClF3) molecule have been investigated by a single-photon ionization in the energy range from 21.21 eV to 320 eV. In the valence region, the multiple ionization results can be described by a sum of contributions generated from the shake-off and the two-step one models. At low photon energies (from the threshold of triple ionization up to 100 eV), the triple photoionization dynamics of halothane can be reasonably well described by a model involving a classical electron impact double ionization of the singly ionized parent ion.

5 p., 4 fig., 1 tab., 1 ap., The gas-phase Proton Affinity (PA) and Basicity (GB) of 2-, 3- and 4-hydroxybenzophenone isomers have been experimentally determined by the Extended Kinetic Cooks Method (EKCM), using ESI-TQ Mass Spectrometry. Our results show that 4-hydroxybenzophenone isomer (PA = 912.6 ± 8.4 kJ·mol) is in the order of 10 kJ·mol more basic than the 2- and 3-hydroxybenzophenones isomers. The results have been rationalized and discussed by means of computational DFT calculations at B3LYP/6-311++G(d,p) level of theory., The support of the Peruvian Scholarship program UNI-agreement No167 - 2015 – Fondecyct-UNI, CONCYTEC (Lima, Perú) is gratefully acknowledged. AFL acknowledges the support from the Brazilian funding agencies CNPq, FAPESP and CAPES.

Time of flight mass spectrometry, electron-ion coincidence, and ion yield spectroscopy were employed to investigate for the first time the thiazole (CHNS) molecule in the gas phase excited by synchrotron radiation in the soft X-ray domain. Total ion yield (TIY) and photoelectron-photoion coincidence (PEPICO) spectra were recorded as a function of the photon energy in the vicinity of the carbon K edge (C1s). The C1s resonant transitions as well as the core ionization thresholds have been determined from the profile of TIY spectrum, and the features were discussed. The corresponding partial ion yields were determined from the PEPICO spectra for the cation species produced upon the molecular photodissociation. Additional ab initio calculations have also been performed from where relevant structural and electronic configuration parameters were obtained for this molecule.

We present a comprehensive experimental and theoretical investigation on the LMM, LMN, and LNN normal Auger spectra of xenon, which reveal excellent agreement with theory when core-hole lifetimes of the two-hole final states are taken into account. Generally, the spectra turned out to be highly complex due to a strong overlap of the Auger transitions subsequent to 2s(1/2)(-1), 2p(1/2)(-1), and 2p(3/2)(-1) ionization. This overlap is due to the splitting of the three initial L core holes and the different final M and N core holes being on the same order of magnitude of several hundred eV. The Auger transitions are assigned in detail based on the theoretical results. Most of the MM, MN, and NN final states are described well based on jj coupling. In addition, we present a detailed assignment of the resonant LM45M45 Auger transition subsequent to the 2s -> 6p, 7p and 2p -> 5d, 6d excitations.

We present the hypersatellite Auger spectrum of neon which contains the decay transitions of the ${K}^{\ensuremath{-}2}, {K}^{\ensuremath{-}2}V, {K}^{\ensuremath{-}2}{L}^{\ensuremath{-}1}, {K}^{\ensuremath{-}2}{L}^{\ensuremath{-}1}V$, and ${K}^{\ensuremath{-}1}{L}^{\ensuremath{-}1}V$ states. The Auger decays of these types of core-hole states show different line shapes and linewidths which can be distinguished due to the high experimental resolution available for the present experiments. The individual Auger transitions are assigned based on various approaches. Complementary calculations are performed to assign the ${K}^{\ensuremath{-}2}{L}^{\ensuremath{-}1}\ensuremath{\rightarrow}{K}^{\ensuremath{-}1}{L}^{\ensuremath{-}3}$ and the ${K}^{\ensuremath{-}2}{L}^{\ensuremath{-}1}V\ensuremath{\rightarrow}{K}^{\ensuremath{-}1}{L}^{\ensuremath{-}3}V$ Auger transitions. Based on the obtained results we were able to derive the ratio for the double to single ionization yield.

International audience; A combination of resonant inelastic x-ray scattering and resonant Auger spectroscopy provides complementary information on the dynamic response of resonantly excited molecules. This is exemplified for CH3I, for which we reconstruct the potential energy surface of the dissociative I 3d−2 double-core-hole state and determine its lifetime. The proposed method holds a strong potential for monitoring the hard x-ray induced electron and nuclear dynamic response of core-excited molecules containing heavy elements, where ab initio calculations of potential energy surfaces and lifetimes remain challenging.

International audience; Tuning hard x-ray excitation energy along Cl 1s→σ∗ resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Photoelectron photoion coincidence measurements have been performed for the thiazole (C3H3NS) molecule in gas phase, using time-of-flight mass spectrometry in the electron-ion coincidence mode and vacuum ultraviolet synchrotron radiation. photoelectron photoion coincidence spectra have been recorded as a function of the photon energy covering the valence range from 10 to 21 eV. The resulting photoionization products as well as the dissociation pathways leading to the ionic species were proposed and discussed. We have also performed density functional theory and ab initio calculations for the neutral molecule, its cation and the ion fragments produced in order to determine their electronic and structural parameters. Copyright © 2014 John Wiley & Sons, Ltd.

RATIONALE An interesting class of volatile compounds, the monoterpenes, is present in some plants although their functions are not yet fully understood. We have studied the interaction of the camphor molecule with monochromatic high-energy photons (synchrotron radiation) using time-of-flight mass spectrometry and coincidence techniques. METHODS A commercial sample of S-camphor was admitted into the vacuum chamber, without purification, through an inlet system. Monochromatic light with energy around the C 1s edge was generated by the TGM beamline at the Brazilian Synchrotron Facility. A Wiley-McLaren mass spectrometer was used to characterize and detect the ions formed by the camphor photoionization. The data analysis was supported by energy calculations. RESULTS Although the fragmentation patterns were basically the same at 270 eV and 330 eV, it was observed that above the C 1s edge the contribution to the spectrum from lower mass/charge fragment ions increased, pointing to a higher degree of dissociation of the molecule. Projections of the PEPIPICO spectra demonstrated the existence of unstable doubly charged species. The Gibbs free energy was calculated using the Moller-Plesset perturbation theory (MP2) for the neutral, singly and doubly excited camphor molecule. CONCLUSIONS Our PEPIPICO spectrum clearly demonstrated the formation of doubly ionic dissociative species. From a slope analysis, we propose a secondary decay after a deferred charge separation mechanism in which, after a few steps, the camphor dication dissociates into C2 H3 (+) and C3 H5 (+) . This is the main relaxation route observed at 270 eV and 330 eV. The large energy difference between the mono and the dication (of the order of 258.2 kcal/mol) may explain the experimentally observed absence of stable dications in the spectra, because their formation is disadvantaged energetically.

The electronic excitation and associated ionic dissociation of the carvone molecule have been studied around the oxygen 1s edge, using synchrotron radiation and time-of-flight techniques. Photoabsorption spectrum (total ion yield) and mass spectra have been obtained in the range between 520 and 545 eV. For the sake of comparison, carvone mass spectra have also been obtained following valence (21.21 eV) and core (carbon 1s) ionization. Fragmentation of the molecule is seen to be greatly enhanced following core excitation. Around the oxygen 1s edge, we observe an extensive fragmentation of the molecular skeleton, as exemplified by the appearance of several previously unreported ions: H + , H 2 + , CH + , CH 2 + and CH 3 + , which are not formed at low energies. A maximum is observed at 536 eV photon energy in the relative intensity of the oxygen-containing ions O + , O 2+ and OH + , as an evidence for the existence of site-selective fragmentation of the carvone molecule excited around the O 1s edge. Absolute values for the photoionization and photodissociation cross sections were estimated using the molecular additive rule.

Dissociative photoionization of trifluoroethyl iodide (C2H2F3I) molecule has been investigated by electron-ion coincidence technique (PEPICO) and VUV synchrotron radiation. Mass spectra have been recorded for this molecule in the photon energy range of 10–22 eV. The molecular ion as well as cationic fragments have been detected and analyzed as a function of photon energy and the main dissociation pathways are proposed. We also performed ab initio calculations for the neutral molecule, its cation and the ion fragments in order to determine electronic and structural parameters.

Using synchrotron radiation and high-resolution electron spectroscopy, we have directly observed and identified specific photoelectrons from K^{-2}V states in neon corresponding to simultaneous 1s ionization and 1s→valence excitation. The natural lifetime broadening of the K^{-2}V states and the relative intensities of different types of shakeup channels have been determined experimentally and compared to ab initio calculations. Moreover, the high-energy Auger spectrum resulting from the decay of Ne^{2+}K^{-2} and Ne^{+}K^{-2}V states as well as from participator Auger decay from Ne^{+}K^{-1}L^{-1}V states, has been measured and assigned in detail utilizing the characteristic differences in lifetime broadenings of these core hole states. Furthermore, post collision interaction broadening of Auger peaks is clearly observed only in the hypersatellite spectrum from K^{-2} states, due to the energy sharing between the two 1s photoelectrons which favors the emission of one slow and one fast electron.

The anionic and cationic photodissociation from deep-core excited chloroform molecule has been investigated in the vicinity of the Cl 1s excitation edge. We used synchrotron radiation, time of flight mass spectrometry and ion yield spectroscopy. From our partial anion yield results we were able to observe for the first time the formation of the following anionic species: Cl − , H − and C − . From our TOF spectra we determined the cations produced in order to discuss the photoionization and dissociation dynamics for this core excited molecule. It provided evidences of the complex dynamics of negative and positive ion formation from CHCl 3 upon photo-excitation in the vicinity of the Cl 1s edge.

An experimental photochemistry study involving gas- and solid-phase amino acids (glycine, DL-valine, DL-proline) and nucleobases (adenine and uracil) under soft X-rays was performed. The aim was to test the molecular stabilities of essential biomolecules against ionizing photon fields inside dense molecular clouds and protostellar discs analogs. In these environments, the main energy sources are the cosmic rays and soft X-rays. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing 150-eV photons. In situ sample analysis was performed by time-of-flight mass spectrometer (TOF-MS) and Fourier transform infrared (FTIR) spectrometer, for gas- and solid-phase analysis, respectively. The half-life of solid-phase amino acids, assumed to be present at grain mantles, is at least 3 × 10 5 and 3 × 10 8 yr inside dense molecular clouds and protoplanetary discs, respectively. We estimate that for gas-phase compounds these values increase 1 order of magnitude since the dissociation cross-section of glycine is lower in gas phase than in solid phase for the same photon energy. The half-life of solid-phase nucleobases is about 2–3 orders of magnitude longer than found for amino acids. The results indicate that nucleobases are much more resistant to ionizing radiation than amino acids. We consider these implications for the survival and transfer of biomolecules in space environments.

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by UV and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time of flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50% of the ionized benzene molecules survive to UV dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained., The paper contains 8 pages, 9 figures and 4 tables. Accepted to be published on MNRAS on 2008 November 26

We have investigated the thermochemistry and ion energetics of the oxybenzone (2-hydroxy-4-methoxy-benzophenone, C14H12O3, 1H) molecule. The following parameters have been determined for this species: gas-phase enthalpy for the of neutral molecule at 298.15K, (Delta(f)H0(m)(g) = -303.5 +/- 5.1 kJ x mol-1), the intrinsic (gas-phase) acidity (GA(1H) = 1402.1 +/- 8.4 kJ x mol-1), enthalpy of formation for the oxybenzone anion (Delta(f)H0(m)(1-,g) = -402.3 +/- 9.8 kJ x mol-1). We also have obtained the enthalpy of formation of, 4-hydroxy-4'-methoxybenzophenone (Delta(f)H0(m)(g) = -275.4 +/- 10 kJ x mol-1) and 3-methoxyphenol anion (Delta(f)H0(m)(C7H7O2-,g) = -317.7 +/- 8.7 kJ x mol-1). A reliable experimental estimation of enthalpy related to intramolecular hydrogen bonding in oxybenzone has also been obtained (30.1 +/- 6.3 kJ x mol-1) and compared with our theoretical calculations at the B3LYP/6-311++G** level of theory, by means of an isodesmic reaction scheme. In addition, heat capacities, temperature, and enthalpy of fusion have been determined for this molecule by differential scanning calorimetry.

We present experimental studies on the photoionization and photodissociation processes (photodestruction) of gaseous amino acids and nucleobases in interstellar and interplanetary radiation conditions analogs. The measurements have been undertaken at the Brazilian Synchrotron Light Laboratory (LNLS), employing vacuum ultraviolet (VUV) and soft X-ray photons. The experimental set up basically consists of a time-of-flight mass spectrometer kept under high vacuum conditions. Mass spectra were obtained using photoelectron photoion coincidence technique. We have shown that the amino acids are effectively more destroyed (up to 70-80%) by the stellar radiation than the nucleobases, mainly in the VUV. Since polycyclic aromatic hydrocarbons have the same survival capability and seem to be ubiquitous in the ISM, it is not unreasonable to predict that nucleobases could survive in the interstellar medium and/or in comets, even as a stable cation., 4 pages, 2 figures. To be published in the Proceedings of the IAU-251 symposium - Organic Mater in Space, Hong Kong, China (2008)

The p-aminobenzoic acid (C7H7NO2) molecule has been investigated at different levels of theory. DFT methods (B3LYP and PBE1PBE), second order Moller–Plesset perturbation theory (MP2) and composite ab initio methods (G3MP2 and CBS) have been employed, in conjunction with large basis sets. Important informations on the electronic structure and thermochemistry of this molecule have been extracted, and the performance of the density functional and ab initio methods has been evaluated, based on the comparison of the calculated and the available experimental data.

Electron-ion coincidence technique (PEPICO) and synchrotron radiation have been employed in the study of the ionic fragmentation of epichlorohydrin (C3H5ClO) molecule. Total ion yield and mass spectra have been recorded for the first time for this molecule in the vicinity of the C 1s, O 1s and Cl 1s core excitation edges. We have found evidences of site selective fragmentation upon core excitation, as well as indication of molecular quasi-alignment in the photoexcitation and dissociation at the Cl 1s resonance.

Vacuum ultraviolet beamlines based on grating monochromators share the problem of high-order harmonics contamination. In the low-photon energy range up to 25 eV, this problem may be overcome by letting the light beam pass through a high-pressure gas region. Here the photons with energies higher than the gas ionization threshold are absorbed and dumped. We have recently proposed and demonstrated a simple and inexpensive type of mixed gas metallic film filter. We present here the design and results of a complete gas-phase harmonics filter based on a very efficient and affordable differential pumping system which currently works at a bending magnet beamline at LNLS. Besides the efficient depletion of high-order harmonics, a noble gas harmonic filter has additional advantages. It can also provide a precise photon energy calibration, as well as a means for determining the resolving power. The use of such a gas filter is very important for photochemistry and molecular dynamics where even a very small portion of high-energy photons can mask the fragments generated by the first order photons.

We present results on the valence level excitation, ionization and dissociation of adenine, using time-of-flight mass spectrometry and synchrotron radiation, in the vacuum ultraviolet (VUV) range of 12-21 eV. The measurements were performed using a gas-phase (Ne) harmonics filter in order to eliminate contributions from higher-order harmonics. Mass spectra were obtained using the photoelectron-photoion coincidence technique (PEPICO). The relative abundances for each ionic fragment and their mean kinetic energy release have been determined from the analysis of the corresponding peak shapes in the mass spectra. Comparison with the available photoelectron spectra and previous measurements allowed the assignment of the main features in the spectra. A discussion on the dissociative photoionization channels of this molecule has also been included. Due to our harmonics-free incident photon beam we were able to propose new appearance energy (AE) for the most important ionic channels in this energy range. The precursor ion, C(5)H(5)N(5)+, is the most abundant species (40% at 15 eV and 20% at 20 eV), which confirms the high stability of adenine upon absorption of VUV photons. We have observed other intense fragment ions such as: C(4)H(4)N(4)+, C(3)H(3)N(3) (+), C(2)H(2)N(2)+ and HCNH+. The production of the neutral HCN fragment represents up to 40% of the dissociative channels for this molecule as induced by VUV photons.

International audience; In recent years double core-hole states are intensively studied since their chemical shifts provide detailed information about initial-state and relaxation effects in a molecule. We derived the Si 1s−1, 2s−1, and 2p−1 binding energies as well as the Si 2s−2, 2s−1, 2p−1, and 2p−2 double-core hole binding energies of different SiX4 systems in order to derive the chemical shifts. Based on these results we created Wagner plots, which give insight in the initial state and the relaxation effects in the different molecules.

We have investigated resonant Auger decay of xenon following photoexcitation of each of the three $L$ edges under resonant-Raman conditions, which allowed us to characterize several higher Rydberg transitions. Relative intensities for spectator final states reached after ${L}_{1}\text{\ensuremath{-}}$, ${L}_{2}\text{\ensuremath{-}}$, and ${L}_{3}$-edge excitations are studied in detail. Thanks to state-of-the-art experimental arrangements, our results not only reproduce the previously calculated $3{d}^{\ensuremath{-}2}5d$ and $nd\phantom{\rule{0.28em}{0ex}}(ng5)$ state cross sections after ${L}_{3}$ excitation, but also allow extracting the $3{d}^{\ensuremath{-}2}6d$ spectator state energy position and revealing its resonant behavior, blurred by the insufficient experimental resolution in previous data sets. The $3{d}^{\ensuremath{-}2}6p$ and $3{d}^{\ensuremath{-}2}7p$ states reached after ${L}_{1}$ excitation as well as the $3{d}^{\ensuremath{-}2}5d$ and $3{d}^{\ensuremath{-}2}6d$ states reached after ${L}_{2}$ excitation are also investigated and their relative intensities are reported and compared to ab initio Dirac-Hartree-Fock configuration-interaction calculations. We found the signature of electronic-state-lifetime interference effects between several coherently excited intermediate states, due to large lifetime broadening. Electron recapture processes are also identified above all three photoionization thresholds.

The cationic and anionic fragmentation of dichloromethane (CH 2 Cl 2 ) molecule have been investigated in the energy range of the Cl K shell by using synchrotron radiation, ion yield spectroscopy, and electron-ion coincidence spectroscopy. Total and partial ion-yield and mass spectra have been recorded as a function of the photon energy. We were able to identify several singly and multiply charged cationic fragments and the following anionic species: H - ; C - ; Cl - . The present results provide the first experimental report of negative ion formation from a molecule excited at the Cl Is edge. In addition, our electron-ion coincidence data provide strong evidence of the preservation of molecular alignment for the photodissociation of CH 2 Cl 2 after deep core-electron resonant excitation.

We report on new soft X-ray photoionization studies of the amino acids l -alanine and l -proline. Total ion yield and mass spectra have been obtained in gas phase, around the N 1s and O 1s edges, using Time-of-Flight Mass Spectrometry and synchrotron radiation as excitation source. No site selectivity was observed in the ionic dissociation of the molecules. The ionic species have been assigned and a qualitative fragmentation mechanism has been proposed.

Threshold photoelectron photoion coincidence (TPEPICO) technique has been employed to determine accurate dissociative photoionization onsets for chloroform and 1,1,2,2-tetrachloroethane molecules. Our 0 K onsets for these reactions are CHCl 3 + hv → CHCl 2 + + Cl (11.500 ± 0.007 eV); C 2 H 2 Cl 4 + hv → C 2 H 2 Cl 3 + + Cl (11.565 ± 0.010 eV) and C 2 H 2 Cl 4 + hv → CHCl 2 + + CHCl 2 (11.669 ± 0.030 eV). The well established heat of formation of CHCl 3 was used as an anchor and the measured dissociation energies were used to determine the heats of formation, Δ f H 298 K o in kJ/mol, of the following species: CHCl 2 + (888.9 ± 1.3), CHCl 2 (87.1 ± 1.6), C 2 H 2 Cl 4 (−152.7 ± 3.5), and C 2 H 2 Cl 4 + (918.3 ± 5.9). Our results also made possible the determination of reliable 298 K bond enthalpies for: CHCl 2 Cl (311.1 ± 2.0 kJ/mol), CHCl 2 H (400.6 ± 2.0 kJ/mol), and CHCl 2 CHCl 2 (326.9 ± 4.1 kJ/mol).

The liquid phase equilibrium reaction of dihalomethanes (2CH2BrI ⇄ CH2Br2 + CH2I2) has been investigated by NMR spectroscopy, as a function of the temperature and initial concentration of the reactants. The equilibrium constants have been experimentally determined for this reaction from the profile of the NMR spectra. Heat capacity measurements were carried out in the temperature range from 293.15 to 353.15 K by differential scanning calorimetry. The results relate the heats of formation of the three compounds and confirm the recently determined heat of formation of CH2I2 of 107.5 kJ mol−1.

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.

Ionic fragmentation of the sublimated α-amino acids glycine, l -alanine, l -proline and l -valine has been studied using a time-of-flight mass spectrometer coupled to a He I lamp (hν = 21.21 eV). Partial ion yields (branching ratios) and kinetic energy releases for the fragments have been determined. The spectra basically resemble available mass spectra obtained through electron impact (70 eV) demonstrating that the ionic dissociation of these compounds, induced by energetic electrons, is also dominated by processes involving low-energy valence-shell orbitals. A previously unreported fragment has been observed at m/z = 32 ( O 2 + ) in the spectra of valine and proline.

The ionic fragmentation of the chloroform (CHCl 3 ) molecule has been studied along the Cl1s edge, using synchrotron radiation and a magnetic mass spectrometer. The following cations were observed: Cl + , Cl 2+ , CCl + , CCl 2 + , CHCl + , Cl 3+ , H + and C + . While all ions are seen to be formed at the intense resonance which dominates the photoabsorption spectrum below the chlorine 1s edge (taken to occur at 2828.6 eV), the probability of observation of the CHCl + , C + and Cl 3+ cations is seen to significantly decrease above the ionization edge.

We present a series of photoabsorption and partial-ion-yield experiments on thionyl chloride, SOCl(2), at both the sulfur and chlorine K edges. The photoabsorption results exhibit better resolution than previously published data, leading to alternate spectral assignments for some of the features, particularly in the Rydberg-series region. Based on measured fragmentation patterns, we suggest the LUMO, of a(') character, is delocalized over the entire molecular skeleton. Unusual behavior of the S(2 +) fragment hints at a relatively localized bond rupture (the S-O bond below the S K edge and the S-Cl bonds below the Cl K edge) following excitation to some of the higher lying intermediate states.

Two special dynamical transitions of universal character have been recently observed in macromolecules at $T_{D}\sim 180 - 220$ K and $T^{*}\sim 100$ K. Despite their relevance, a complete understanding of the nature of these transitions and their consequences for the bio-activity of the macromolecule is still lacking. Our results and analysis concerning the temperature dependence of structural, vibrational and thermodynamical properties of the orthorhombic polymorph of the amino acid L-cysteine (at a hydration level of 3.5%) indicated that the two referred temperatures define the triggering of very simple and specific events that govern all the biochemical interactions of the biomolecule: activation of rigid rotors ($TT_{D}$)., Comment: 4 pages, 4 figures, submitted to Physical Review Letters

We have carried out a study of the energetics, structural, and physical properties of o-, m-, and p-hydroxybenzophenone neutral molecules, C(13)H(10)O(2), and their corresponding anions. In particular, the standard enthalpies of formation in the gas phase at 298.15 K for all of these species were determined. A reliable experimental estimation of the enthalpy associated with intramolecular hydrogen bonding in chelated species was experimentally obtained. The gas-phase acidities (GA) of benzophenones, substituted phenols, and several aliphatic alcohols are compared with the corresponding aqueous acidities (pK(a)), covering a range of 278 kJ.mol(-1) in GA and 11.4 in pK(a). A computational study of the various species shed light on structural effects and further confirmed the self-consistency of the experimental results.

Recent advances in threshold photoelectron photoion coincidence (TPEPICO) make possible the analysis of several parallel and sequential dissociations of energy selected ions. The use of velocity focusing optics for the simultaneous collection of threshold and energetic electrons not only improves the resolution, but also permits subtraction of coincidences associated with “hot” electrons, thereby yielding TPEPICO data with no contamination from “hot” electrons. The data analysis takes into account the thermal energy distribution of the sample and uses statistical theory rate constants and energy partitioning in dissociation reactions to model the time of flight distributions and the breakdown diagram. Examples include CH2BrCl and P(C2H5)3. Of particular interest is the ability to extract error limits for rate constants and dissociation energies.

An experimental study of the electronic excitation and ionic dissociation of two important classes of biomolecules–natural products (biogenic volatile organic compounds, VOCs, and volatile components of essential oils) and DNA and RNA constituents (aminoacids and bases) is here exemplified with recent results on the fragmentation of thymine and isoprene as induced by synchrotron radiation and fast electrons. Fragmentation of the thymine molecule was seen to dramatically increase as the photon energy increased from 21 to 300 eV and 450 eV. At the highest photon energy, simply and doubly charged N and O atoms were observed. The parent ion (m/z = 126) could be observed at all photon energies. The fragmentation pattern observed in the 1.0 keV electron impact mass spectrum of thymine resembled more closely the fragmentation observed with 21 eV photons. In isoprene, the dominant fragments observed at 21 eV and 310 eV photon energy as well as in the 1.0 keV electron impact mass spectrum were C5H7+(m/z = 67), C4H5+(m/z = 53), C3H3+(m/z = 39) and C2H3+(m/z = 27). Previously unreported fragments, namely H+, C+, CH+, CH2+, and CH3+ were observed at the high photon energies and at the electron impact mass spectrum.

Ionic fragmentation of the sublimated amino acid DL-proline has been studied using time-of-flight mass spectrometry and synchrotron radiation. Total ion yield and mass spectra were recorded in the 13 to 21.6 eV energy range. Partial ion yields have been calculated for the produced fragments and the results analyzed in a comparative way. Mass spectrum of proline previously obtained at 21.21 eV using photons from a discharge lamp (He I), was used as reference in the comparison to the synchrotron radiation based spectra. The loss of the COOH fragment represents the most probable dissociation pathway following the photoionization of DL-proline in the valence region. These are the first results of total and partial ion yields spectra for this molecule in its gas phase in the valence region using time-of-flight spectrometry.