Search

The ratios of single, double, and triple ionizations to the total photoionization of the halothane (C

Disulfide bonds (-S-S-) are commonly present in biomolecules and have also been detected in astrophysical environments. In this work, the stability of the disulfide bond towards double ionization is investigated using quantum chemical calculations and photoelectron photoion photoion coincidence (PEPIPICO) spectroscopy measurements on the prototype dimethyl disulfide (CH

Disulfide bonds (–S–S–) are commonly present in biomolecules and have also been detected in astrophysical environments. In this work, the stability of the disulfide bond towards double ionization is investigated using quantum chemical calculations and photoelectron photoion photoion coincidence (PEPIPICO) spectroscopy measurements on the prototype dimethyl disulfide (CH3SSCH3, DMDS) molecule. The experiments were performed using high energy synchrotron radiation photons before (2465.0 eV) and at (2470.9 eV) the first sigma resonance around the S 1s edge. We applied the multivariate normal distribution analysis to identify the most plausible ionic fragmentation mechanisms from the doubly ionized DMDS. By mapping the minimum energy structures on the dicationic C2H6S22+ potential energy surface, we show that disulfide bonds are only present in high-lying isomers, in contrast to their analogous neutral systems. Our results also indicate that the number of fragment ions containing a disulfide bond for both photon energies is negligible. Taken together, our results reveal that the disulfide bond is severely damaged as a consequence of sulfur core–shell ionization processes, due to the lowering of its thermodynamic stability in multiply-charged systems. Universidad de Costa Rica/[]/UCR/Costa Rica Conselho Nacional de Desenvolvimento Científico e Tecnológico/[]/CNPq/Brasil Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro/[]/FAPERJ/Brasil Coordenação de aperfeiçoamento de pessoal de nivel superior/[]/CAPES/Brasil UCR::Vicerrectoría de Docencia::Ingeniería::Facultad de Ingeniería::Escuela de Ingeniería Química

In order to study the stability of the disulfide chemical bond in molecules subjected to a flux of high energy photons, the ionic fragmentation of two molecules containing a disulfide chemical bond, dimethyl disulfide (DMDS) and methyl propyl disulfide (MPDS), has been studied following excitation around the S 1s edge (∼2470 eV). Synchrotron radiation and electron-ion coincidence techniques were used. The core excited states of DMDS and MPDS have also been theoretically investigated at the multiconfigurational self-consistent field (MCSCF) level and multireference perturbation theory. Deep core excitation (S 1s) induces multiple ionization and intense fragmentation of the molecules, as clearly demonstrated by the observation of cations such as S+, S2+ and S3+. Splitting of the S+ peak is observed in the mass spectra of the two molecules when they are excited to a resonant state with antibonding (S–S) character. Although fragments associated with the breakage of the disulfide bond (S+ and CHnS+) play a dominant role in the dissociation of the molecules, a fragment containing the disulfide chemical bond, S2+, survives, nevertheless, at all photon energies. Brazilian Synchrotron Light Facility//LNLS/Brasil Conselho Nacional de Desenvolvimento Científico e Tecnológico//CNPq/Brasil Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro//FAPERJ/Brasil Universidad de Costa Rica//UCR/Brasil UCR::Vicerrectoría de Docencia::Ingeniería::Facultad de Ingeniería::Escuela de Ingeniería Química

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.

We have studied the interaction of an ion beam (17.6 keV F-) with cystine, a dimer formed by the binding of two cysteine residues. Cystine can be considered as an ideal prototype for the study of the relevance of the disulfide (-S-S-) chemical bond in biomolecules. For the sake of comparison, the amino acid cysteine has also been subjected to the same experimental conditions. Characterization of the samples by XPS and NEXAFS shows that both pristine cystine and pristine cysteine are found as a dipolar ion (zwitterion). Following irradiation, the dimer and the amino acid show a tendency to change from the dipole ion form to the normal uncharged form. The largest spectral modification was observed in the high resolution XPS spectra obtained at around the N 1s core level for the two biomolecules. The 2p sulfur edge spectra of cysteine and cystine were much less sensitive to radiation effects. We suggest that the disulfide bond (-S-S-) remains stable before and after irradiation, contributing to the larger radiation stability of cystine as compared to the amino acid cysteine.

Rationale The ion-ion-coincidence mass spectroscopy technique brings useful information about the fragmentation dynamics of doubly and multiply charged ionic species. We advocate the use of a matrix-parameter methodology in order to represent and interpret the entire ion-ion spectra associated with the ionic dissociation of doubly charged molecules. This method makes it possible, among other things, to infer fragmentation processes and to extract information about overlapped ion-ion coincidences. This important piece of information is difficult to obtain from other previously described methodologies. Methods A Wiley-McLaren time-of-flight mass spectrometer was used to discriminate the positively charged fragment ions resulting from the sample ionization by a pulsed 800 eV electron beam. We exemplify the application of this methodology by analyzing the fragmentation and ionic dissociation of the dimethyl disulfide (DMDS) molecule as induced by fast electrons. The doubly charged dissociation was analyzed using the Multivariate Normal Distribution. Results The ion-ion spectrum of the DMDS molecule was obtained at an incident electron energy of 800 eV and was matrix represented using the Multivariate Distribution theory. The proposed methodology allows us to distinguish information among [CHnSHn]+/[CH3]+ (n = 1–3) fragment ions in the ion-ion coincidence spectra using ion-ion coincidence data. Using the momenta balance methodology for the inferred parameters, a secondary decay mechanism is proposed for the [CHS]+ ion formation. As an additional check on the methodology, previously published data on the SiF4 molecule was re-analyzed with the present methodology and the results were shown to be statistically equivalent. Conclusions The use of a Multivariate Normal Distribution allows for the representation of the whole ion-ion mass spectrum of doubly or multiply ionized molecules as a combination of parameters and the extraction of information among overlapped data. We have successfully applied this methodology to the analysis of the fragmentation of the DMDS molecule. Copyright © 2015 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.

We have performed a NEXAFS (S 1s) and mass spectrometry study of solid samples of cysteine, cystine and insulin irradiated with 0.8 keV electrons. The measured mass spectra point out to processes of desulfurization, deamination, decarbonylation and decarboxylation in the irradiated biomolecules. Electron beam irradiation was also conducted at low temperatures in order to evaluate the possible contribution from thermal degradation processes. The NEXAFS spectra of irradiated cysteine and cystine did not show substantial changes when compared to the same spectra obtained using non-irradiated samples. The sulfur K -edge photoabsorption spectrum of irradiated insulin, however, showed clear modifications when compared to the spectrum of the non-irradiated protein, even when the irradiation was conducted at low temperature. Using an empirical combination of the photoabsorption spectra of cysteine and cystine (which are associated respectively with reduced and oxidized forms of sulfur) we have been able to reproduce the absorption spectrum of irradiated insulin.

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.

Photon stimulated ion desorption (PSID) and Near-edge X-ray absorption fine structure (NEXAFS) studies have been performed on poly(vinyl chloride) (PVC) and poly(vinyl dichloride) (PVDC) around the chlorine 1s-edge. Experiments were performed using a synchrotron source operating in the single-bunch mode and a time-of-flight mass spectrometry for ion analysis. Cl + ion yields, as a function of the photon energy, reproduce the photoabsorption spectrum, showing significant increase at the 1s-resonance. Edge-jump ratios, defined as the ratio between edge-jumps (intensity ratio of the yields between above and below the absorption edge) of two different transitions, for Cl + ion yields were much higher than the equivalent electron yields, indicating site-selectivity in C–Cl bond breaking for both polymers, as a result of efficient spectator Auger decay. The expected isotope ratio of 3:1 for chlorine was measured for PVC. The interpretation of the NEXAFS spectrum was assisted by quantum mechanical calculations at a multireference perturbation theory level.

Plant volatiles are emitted by plants in response to several forms of stress, including interaction with energetic photons. In the present work, we discuss the interaction of extreme UV and soft X-ray photons with a plant volatile, vanillin. The single and double (multiple) ionization of the vanillin molecule have been studied for the first time using time-of-flight mass spectrometry and VUV and soft X-ray photons (synchrotron radiation, at 12.0 eV, 21.2 eV, 130 eV, 310 eV, 531 eV, and 550 eV). At 12.0 and 21.2 eV, only singly charged species are observed and the parent ion, C8H8O3 (+), is the dominant species. Energy differences for some selected fragments were calculated theoretically in this energy region. At 130 eV, direct double and triple ionization of the valence electrons may occur. The fragmentation increases and CHO(+) becomes one of the main cations in the mass spectrum. The molecular ion is still the dominant species, but other fragments, such as C6H5O(+), begin to present similar intensities. At 310 eV, C 1s electrons may be ionized and Auger processes give rise to dissociative doubly ionized cations. Ionization around the O 1s edge has been studied both at the 531 eV resonance and above the ionization edge. Resonant and normal Auger processes play a significant role in each case and a large fragmentation of the molecule is observed at both photon energies, with intense fragments such as CHO(+) and CH3 (+) being clearly observed. A near edge X-ray absorption fine structure spectrum of the vanillin molecule was obtained around the O 1s ionization threshold. In addition, the fragmentation of vanillin has also been studied using a fast beam of electrons (800 eV), for the sake of comparison.

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 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)

Dissociation processes of the CCl4 molecule excited in the valence, Cl 2p, and C 1s core levels have been studied using the photoelectron–photoion (PEPICO) coincidence technique combined with synchrotron radiation. Doubly and triply charged fragments not previously reported have been observed. Kinetic energy releases for the fragments have been estimated using the peak width method. In the PEPICO spectra an increasing broadening in the peak widths has been observed as function of the incident photon energy, indicating the repulsive role played by the σ* orbital.

We present data from inner shell excitation and multi-ionization of acetic acid with synchrotron radiation at at energies around C 1s edge. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS) and at center for advanced microstructures & devices (CAMD), employing soft X-ray photons (200–310 eV) from a toroidal grating monochromator (TGM) beamline. The main dissociation route were the secondary decay after deferred charge separation (SDDCS) process and deferred charge separation (DCS) processes. The main dissociation channels were associated to the ions pairs H + + C + , H + + CH + , H + + CH 2 + , H + + O + , H + + CO + and H + + COH + . About 70–80% of the dissociation channels leads to the production of energetic (3–7 eV) protons. The ejection of reactant radical COOH + was seen in about 7–10% of the channels.

Single and multiple photoionization branching ratios of Ar have been measured for photon energies around the K -edge region between 3198 and 3212 eV using synchrotron radiation and time-of-flight mass spectrometry. The present data is normalized to the absolute values using the accurately measured data of Zheng et al. [L. Zheng, M. Cui, Y. Zhao, K. Chen, J. Electron. Spectrosc. Rel. Phenom. 152 (2006) 143–147]. A comparison with other experimental and theoretical data is presented and discrepancies are discussed.

We present data from inner shell excitation and double-ionization of alcohols of interstellar interest, methanol and ethanol at energies around C 1s edge. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons (100–310 eV) from a toroidal grating monochromator (TGM) beamline. The experimental set up consists of a high vacuum chamber with a time-of-flight mass spectrometer TOF-MS. Mass spectra were obtained using coincidence techniques. Mean kinetic energies, abundances and dissociation pathways for each ionic pair in coincidence have been obtained from the analysis of the corresponding peak shapes and slopes observed in photoelectron photoion photoion (PE2PICO) coincidence mass spectra. The main dissociation channels of double ionized methanol were associated to the ions pairs H + + COH + , H + + CO + , H + + CH 2 + and H + + CH + . In the case of double ionized ethanol, the main dissociation channels were associated to the ions pairs CH 3 + + COH + , H + + COH + , H + + CH 2 + and H + + C 2 H 2 + . About 40% and 25% of the dissociation channels of doubly ionized methanol and ethanol, respectively, lead to the production of reactant COH + ion, important for the formation of other organic molecules.

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; We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115–310 nm (10.8–4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin–orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C–Br) ← nBr and σ*(C–Cl) ← nCl transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0–40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20–50 km).

Electron energy-loss spectra, measured as a function of the scattering angle, have been obtained for a carbon disulfide molecule, CS2, in the excitation energy range from 0 to 45 eV. The incident electron energy was 1000 eV. The absolute generalized oscillator strength (GOS) and absolute inelastic differential cross section have been determined as a function of the square of the momentum transfer (K2) for the intense allowed transition 1Σ+u ← 1Σ+g, centred at 6.3 eV. The absolute elastic differential cross section was also determined spanning an angular range from 1.5° to 10°. It is well known that at high incident energies and small scattering angles electrons behave like pseudo photons. The CS2 photoabsorption spectrum was consequently obtained by extrapolation from the measured small angle electron energy-loss spectrum. The result shows a good resemblance to the atmospheric spectrum of Jupiter, following comet impact.

Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC) using synchrotron radiation, covering from valence to core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the TGM beam line from the Brazilian Synchrotron Light Source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOF-MS). A new pulsed system developed uses as a trigger for the TOF-MS experiments the pulsed extraction high voltage applied to the sample. Ionic desorption from PVC and PVDC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied.

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.

Photon stimulated ion desorption (PSID) studies from poly(sulphone) (PS) have been carried out by using high-resolution time-of-flight mass spectrometry. Synchrotron radiation at the Brazilian Synchrotron Light Source (LNLS) operating in a single-bunch mode was used as excitation source. PS was excited at the sulphur 1s-edge and the desorption of small and large fragments has been observed. The results are discussed in terms of the X-ray induced electron stimulated desorption mechanism. The present results contrast with previous ones reported for poly(3-methylthiophene) (PMeT), in which the observation of S + and S 2+ was interpreted solely in terms of an Auger-stimulated ion desorption mechanism.

With the aim of performing time-of-flight studies of photon stimulated ion desorption (PSID) from polymers irradiated with the synchrotron radiation emanating from the Brazilian Synchrotron Light Source (LNLS), normally operated in a multi-bunch mode, a pulsed system has been developed. The new pulsed system uses as a trigger for the experiments the pulsed extraction high voltage applied to the sample. PSID results for poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC) obtained at the TGM beam line (12–310 eV) of LNLS are presented.

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.

Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) using synchrotron radiation, encompassing the valence and core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the Brazilian synchrotron light source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOF-MS). A pulsed high voltage applied to the sample was used as a trigger for the TOF-MS experiments. Ionic desorption from PVC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied.

Photon stimulated ion desorption (PSID) spectra were determined around the Cl 2p edge for poly(vinyl chloride) (PVC), using synchrotron radiation and a time-of-flight mass spectrometer (TOF-MS). Partial ion yields have been determined for the main fragments as a function of the photon energy. A striking demonstration of photon energy dependence is observed for the Cl + ion around the Cl 2p edge.

Photon stimulated ion desorption (PSID) studies have been performed for poly(methyl methacrylate) (PMMA) in the 21.21–300 eV photon energy range, covering the valence region as well as the carbon 1s-edge. The PSID spectra were measured at the Brazilian Synchrotron Light Source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer. Only a single ion ( m / q =29, COH + or C 2 H 5 + ) is observed at 21.21 eV. From 40.8 eV to 300 eV basically the same groups of ions are observed, pointing to the importance of inner-valence processes in the ionic desorption from PMMA.

The absolute generalized oscillator strength and absolute inelastic differential cross section have been determined, as a function of the momentum transfer, for the 1E1u + 1B1u ← 1Ag transition in the benzene molecule. A previously described electron energy-loss spectrometer, featuring a Wien-filter velocity analyser, has been used. Employing 1000 eV incident electron energy and 1.0 eV energy resolution, absolute oscillator strength distribution as a function of electron energy loss was obtained in the 2–8° scattering angle range. The absolute elastic differential cross section was also determined spanning an angular range of 2°–14°. The photoabsorption spectrum converted from the electron energy-loss spectrum was compared with the interstellar extinction curve where we confirm the proportionality between the cross section and extinction.

Electron stimulated ion desorption (ESID) studies have been performed in poly(methyl methacrylate) (PMMA) using a time-of-flight mass spectrometer and a pulsed electron beam source with pulse duration of 0.18 μs (FWHM) and frequency of 3 kHz. The incident electron energy was 350–1200 eV. Several ionic fragments have been observed, H + being the main ionic species desorbed. Partial ion yields have been determined for the main fragments as a function of the electron energy. They show a maximum at 500–600 eV. Ionic desorption from PMMA is tentatively discussed in terms of the Auger stimulated ion desorption (ASID) mechanism. This is, to our knowledge, the first description of ESID on polymers.

Multiple ionization of the SiF 4 molecule has been studied using synchrotron radiation and time-of-flight mass spectrometry in a multi-coincidence mode, in the photon energy range of 70–109.4 eV, which encompasses the Si 2p edge. Photoionization branching ratios have been measured under efficient ion extraction and the results are compared to previously published data. Two electron–ion coincidence techniques (PE2PICO, and PE3PICO) have been used in the elucidation of the fragmentation mechanisms of highly excited molecular ions, formed following the absorption of a high-energy photon. It is shown that the unstable doubly charged parent molecule, SiF 4 2+ , fragments preferentially (90%) via the asymetric mechanism m 2+ → m 1 2+ + m 2 .

The angular dependence of the valence and carbon 1s excitation in the naphthalene molecule has been studied using the electron energy-loss technique. The valence shell spectra were measured at 1.0 keV incident energy, 0–50 eV excitation energy range and in the 1.25–12.0 degrees angular range. The energy resolution was 1.0 eV (full-width at half-maximum (FWHM)). Absolute values were determined, for the first time, for the elastic differential cross section and for the generalized oscillator strength (GOS) and inelastic differential cross section for the 1 B 2u ← 1 A g transition (5.9 eV). The carbon 1s spectrum (270–330 eV) was also measured, at 1.3 keV and at an energy resolution of approximately 1.0 eV (FWHM). Dipole-forbidden processes have been observed, both at the valence and C 1s excitation spectra. In the latter case, a previously unreported, dipole forbidden transition, has been observed at 290.3 eV.

Electron energy-loss spectra for the Xe atom were measured for a given set of scattering angles (1.5°-20.0°), in an energy-loss range from 40 to 180 eV using an incident electron energy of 1045 eV and an energy resolution of 1.0 eV. The absolute generalized oscillator strength (GOS) and cross section have been determined for the intense shape resonance (4d→ef) and for the discrete resonance 4d(1S0)→6p(2D5/2) in the Xe atom. The GOS curve as a function of the square of the momentum transfer for the shape resonance presents a well-defined minimum. In order to obtain the optical oscillator strength for our data normalization, the electron energy-loss spectrum (1.5°) was converted to a photoabsorption spectrum. A broad band has also been observed in the energy-loss spectra, centred at 80.0 eV. According to previous analysis, it has been assigned to double excitations (4d-15p-1).

The fragmentation of the tetrachloromethane molecule following core-shell photoexcitation and photoionization in the neighborhood of the chlorine K-edge has been studied by using time-of-flight mass spectroscopy and monochromatic synchrotron radiation. Branching ratios for ionic dissociation were derived for all the detected ions, which are informative of the decay dynamics and photofragmentation patterns of the core-excited species. In addition, the absorption yield has been measured with a new assignment of the spectral features. The structure that appears above the Cl 1s ionization potential in the photoionization spectrum has been ascribed to the existing connection with electron-CCl4 scattering through experimental data and calculations for low-energy electron-molecule cross sections. In addition, the production of the doubly ionized Cl fragment, Cl2+, as a function of the photon energy has been analysed in the terms of a simple and an appealing physical picture, the half-collision model.

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.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.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 Møller-Plesset perturbation theory (MP2) for the neutral, singly and doubly excited camphor molecule.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 of the silicon tetrafluoride (SiF4) molecule has been studied using the angle-resolved electron energy-loss technique, at 1.0 keV incident electron energy, in the 0-50 eV energy range with an angular range of 1.5°-20.0°. The absolute generalized oscillator strength (GOS) for the 5t2→6a1 electronic transition, located at 13.0 eV, has been determined. A minimum has been observed in the GOS for this transition at K2 = 1.4 au. We have also determined the absolute elastic and inelastic differential cross sections at 1 keV. In order to help in the interpretation of the experimental results, ab initio calculations have been performed for the vertical valence transitions and ionization energies for the SiF4 molecule. Configuration-interaction calculations, including single and double excitations (CI-SD) and the symmetry-adapted-cluster expansion (SAC) were used. The CI-SD approach was also employed to obtain the optical oscillator strength for the 5t2→6a1 transition.

Chlorinated polymers, poly(vinyl chloride) (PVC), poly(vinylidene chloride) (PVDC), and ethylene-chlorotrifluoroethylene copolymer (E-CTFE) have been irradiated with white light from the Brazilian synchrotron light source (LNLS). The volatile products generated by the interaction of the polymers with the photons were analysed by mass spectrometry and their intensities studied as a function of the photon flux. The results show that HCl, the most intensely emitted gas, may be used as an indication of the sensitivity of the polymers to the radiation. Among the polymers investigated, PVDC was found to be the most sensitive to white light.

In the present work we investigated the localized photodoping process of polyaniline-emeraldine base/poly(vinylchloride) blends by high-energy photons from a synchrotron source. The doped blend was characterized using optical and electrical parameters. An abrupt increase of conductivity by four orders of magnitude (10−10 to 10−6 Scm−1) was observed using an initial dose of 500 Jcm−3. Lithographic patterns were recorded with micrometer resolution. Copyright © 2000 John Wiley & Sons, Ltd.

Ab initio calculations have been carried out in order to assign the main bands observed in a recent electron energy-loss study of the methyl methacrylate molecule. We employed the Dunning-Huzinaga double-ζ basis set for the early steps of geometry optimization but for the excited states Rydberg p functions were added to the conjugated heavy atoms. Both isomers, the s-cis and s-trans, had its vertical spectrum calculated in order to evaluate possible conformation effects on the VUV spectrum. SAC (Symmetry Adapted Cluster)-CI calculations pointed to the ethylenic π orbital (8.99 eV) as the HOMO for s-cis while the nσ orbital is predicted around 1 eV more stable. For the nπ orbital, although it was not possible to obtain a SAC-CI estimative, Koopmans’ ionization potentials place it 1.68 eV more stable than the π orbital. The lowest observable transitions have been assigned by SAC-CI calculations as to ethylenic π-π* (7.10 eV) and nπ-π* (7.97 eV) for the cis isomer. For the trans isomer similar values were obtained (7.37 eV and 7.93 eV, respectively). Oscillator strengths have been calculated showing acceptable agreement with the experimental results. Previous assignments have been revised.

Angle-resolved electron energy loss spectra for the methyl methacrylate molecule have been measured at an impact energy of 1 keV. The spectra cover an excitation energy range of 0 to 100 eV and an angular range of 2° to 10°. Within our knowledge, this is the first gas-phase excitation spectrum for MMA in this energy range. At small angles the spectra are dominated by an intense peak at 6.7 eV followed by a broad band centered at about 16 eV. As the scattering angle increases, the intensity maximum in the spectra shifts towards higher energies. The generalized oscillator strength (GOS) for the 6.7 eV peak has been determined in the 0.090 to 1.093 au K 2 range. The absolute elastic differential cross section has also been obtained through normalization of the relative data to the theoretical value, determined by using the independent atom model (IAM) approximation. CI-level theoretical calculations have been also performed in order to assign the low-lying energy states. The experimentally determined value for the optical oscillator strength (0.18) for the 6.7 eV peak showed a good agreement with the theoretical result.

As part of a systematic quantitative study of the angular dependence of valence-shell electron-impact excitation of molecules, we have determined, for the first time, the absolute inelastic differential cross section and the generalized oscillator strength (GOS) for the transition (5.2 eV) in anthracene . A minimum in the GOS has been observed at a value of around 0.2 au. The GOS values were determined from the relative inelastic differential cross section, measured with an electron-impact spectrometer, which uses a Wien-filter velocity analyser. The incident electron energy was 1 keV, the energy resolution was 1.0 eV and the energy-loss spectra were obtained in the range of - . The absolute elastic differential cross section was determined in the - angular range.

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.

International audience; Electronic state spectroscopy of limonene has been investigated using vacuum ultraviolet photoabsorption spectroscopy in the energy range 5.0-10.8 eV. The availability of a high resolution photon beam (~0.075 nm) enabled detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 7.5-8.4 eV region have been studied for the first time. A He(I) photoelectron spectrum has also been recorded from 8.2 to 9.5 eV and compared to previous low resolution works. A new value of 8.521 ± 0.002 eV for the ground ionic state adiabatic ionisation energy is proposed. Absolute photoabsorption cross sections were derived in the 10-26 eV range from electron scattering data. All spectra presented in this paper represent the highest resolution data yet reported for limonene. These experiments are complemented by new ab initio calculations performed for the three most abundant conformational isomers of limonene, which we then used in the assignment of the spectral bands.

Oscillator strengths for the Si 2p, Si 2s, and C 1s excitation of tetramethylsilane, hexamethyldisilane, tetrakis(trimethylsilyl)silane, and dodecamethylcyclohexasilane have been derived from electron energy loss spectra recorded in the dipole-regime. These results are compared to the Si l s and Si 2p photoabsorption spectra of tetramethylsilane, hexamethyldisilane, tris(trimethylsilyl)silane, and tetrakis(trimethylsilyl)silane, recorded using synchrotron radiation. The spectra support the existence of a localised, low-lying state of large Si-Si character in species which contain Si-Si bonds.

We have determined theoretical values for the excitation energy, optical oscillator strength, and generalized oscillator strength (GOS), as well as experimental values for the GOS for the C 1s (1${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$\ensuremath{\rightarrow}1${\mathrm{\ensuremath{\pi}}}_{\mathit{g}}$) and (1${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$\ensuremath{\rightarrow}1${\mathrm{\ensuremath{\pi}}}_{\mathit{g}}$) transitions in the ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ molecule. In order to study the influence of relaxation, correlation, and hole-localization effects on these properties, several calculations with different target wave functions were computed. The experimental results for the GOS were determined using a crossed-beam electron energy-loss spectrometer.

Photoelectron-photoion-photoion coincidence (PEPIPICO) mass spectrometry is applied to Si 2p core ionization. The ion yield spectrum is compared to the spectrum of the tetramethylsilane molecule in order to point out resonances due to the Si-Si chemical bond. Simple coincidence mass spectra are dominated by the SiC 3 H 9 + fragment ion and do not show a strong dependence on photon wavelength. PEPIPICO spectra demonstrate that dissociation dynamics is dominated by stepwise fragmentation of SiC 3 H 9 + and that double ionization always involves Si-Si chemical bond rupture, shown to be faster than the Si-C rupture