Partial Ion Yields Research Articles

Ionic Fragmentation of the Halothane Molecule Induced by EUV and Soft X-ray Radiation.

EUV and soft X-ray-induced photofragmentation of the halothane (CF3CHBrCl) molecule has been investigated using time-of-flight mass spectrometry in the coincidence mode (PEPICO) covering the valence region and vicinity of the bromine 3d, chlorine 2p, and carbon 1s edges. Total and partial ion yields have been recorded as a function of photon energy. At lower photon energies, the heavier singly charged molecular fragments predominate in the mass spectra. On the other hand, there is a strong tendency to the atomization of the molecule at higher photon energies. Despite the different chemical environments experienced by the two carbon atoms, weak site-specific fragmentation is observed. In addition, ab initio quantum mechanical calculations at the MP2 level and a series of computations with multiconfigurational self-consistent field have been performed to describe the inner-shell states.

An X-ray spectroscopy study of structural stability of superhydrogenated pyrene derivatives

ABSTRACT The stability of polycyclic aromatic hydrocarbons (PAHs) upon soft X-ray absorption is of crucial relevance for PAH survival in X-ray dominated regions. PAH stability depends on molecular size but also on the degree of hydrogenation that is related to H2 formation in the interstellar medium. In this project, we intend to reveal the changes of electronic structure caused by hydrogenation and the impact of hydrogenation on the stability of the carbon backbone for cationic pyrene and its hydrogenated derivatives by analysis of near C K-edge soft X-ray photoions. In our experiments, the PAH cations were trapped in a cryogenic radiofrequency linear ion trap and exposed to monochromatic X-rays with energies from 279 to 300 eV. The photoproducts were mass-analysed by means of time-of-flight spectroscopy. Partial ion yields were then studied as a function of photon energy. X-ray absorption spectra computed by time-dependent density functional theory (TD-DFT) aided the interpretation of the experimental results. A very good agreement between experimental data and TD-DFT with short-range corrected functionals for all PAH ions was reached. The near-edge X-ray absorption mass spectra exhibit clear peaks due to C 1s transitions to singly occupied molecular orbitals and to low-lying unoccupied molecular orbitals. In contrast to coronene cations, where hydrogen attachment drastically increases photostability of coronene, the influence of hydrogenation on photostability is substantially weaker for pyrene cations. Here, hydrogen attachment even destabilizes the molecular structure. An astrophysical model describes the half-life of PAH ions in interstellar environments.

Ionization and Photofragmentation of Isolated Metalloporphyrin Cations Investigated by VUV Action Spectroscopy*.

We investigated the photoionization and fragmentation of isolated metal protoporphyrin IX cations (MPPIX+ with M=Fe, Co, Zn) by means of vacuum‐ultraviolet (VUV) action spectroscopy in the energy range of 8.5–35 eV. Experiments were carried out in the gas phase by interfacing an electrospray ionization tandem mass spectrometer with a synchrotron beamline. The mass spectra and partial ion yields show that photoexcitation of the precursor ions predominantly leads to .CH2COOH radical side‐chain losses of the macrocycle with additional methyl radical (.CH3) side‐chain losses. Ionization, in contrast, leads to the formation of the intact ionized precursor and various doubly charged fragments which are mostly due to side‐chain cleavages. Although statistical fragmentation dominates, we found evidence for non‐statistical processes such as new fragments involving for example single and double H2O losses, indicating that different relaxation mechanisms are at play upon photoionization compared to photoexcitation. The measured ionization energies were 9.6±0.2 eV, 9.4±0.2 eV and 9.6±0.2 eV for FePPIX+, CoPPIX+ and ZnPPIX+, respectively.

Investigation of the molecular structure and VUV-induced ion dissociation dynamics of 2-azetidinone (C3 H5 NO).

RationaleThe 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 (C3H5NO) molecule, with monochromatic VUV (synchrotron radiation) photons in the 9.5–21.5 eV range, using time‐of‐flight mass spectrometry (TOF‐MS), electron‐ion coincidence (PEPICO), and high‐level theory methods.MethodsA 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.ResultsThe 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.ConclusionsThe 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.

Photoionization and ionic dissociation of the C3 H3 NS molecule induced by soft X-ray near the C1s edge.

Time of flight mass spectrometry, electron-ion coincidence, and ion yield spectroscopy were employed to investigate for the first time the thiazole (C3 H3 NS) 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.

SINGLE AND DOUBLE PHOTOIONIZATION AND PHOTODISSOCIATION OF TOLUENE BY SOFT X-RAYS IN A CIRCUMSTELLAR ENVIRONMENT

ABSTRACT The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives mainly occurs in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 μm, observed in infrared emission spectra of several objects, are attributed C–H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 μm is more intense than that at 3.4 μm. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge using time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. Ab initio calculations based on density functional theory were performed in order to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.

Inner-shell photoionization and core-hole decay of Xe and XeF2.

Photoionization cross sections and partial ion yields of Xe and XeF2 from Xe 3d(5/2), Xe 3d(3/2), and F 1s subshells in the 660-740 eV range are compared to explore effects of the F ligands. The Xe 3d-ϵf continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF2 cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Comparisons of the ion products from the atom and molecule following Xe 3d photoionization show that the charge-state distribution of Xe ions is shifted to lower charge states in the molecule along with production of energetic F(+) and F(2+) ions. This suggests that, in decay of a Xe 3d core hole, charge is redistributed to the F ligands and the system dissociates due to Coulomb repulsion. The ion products from excitation of the F 1s-LUMO resonance are different and show strong increases in the yields of Xe(+) and F(+) ions. The subshell ionization thresholds, the LUMO resonance energies, and their oscillator strengths are calculated by relativistic coupled-cluster methods and agree well with measurements.

Femtosecond laser induced ionization and dissociation of gas-phase protonated leucine enkephalin

We have combined a tandem mass spectrometer with a 780nm fs-laser system to study photoionization and photofragmentation of trapped protonated leucine enkephalin cations for laser intensities between 2×1013W/cm2 and 1×1014W/cm2 and pulse durations of 15fs. In this intensity range, the transition from multiphoton ionization and excitation to tunneling ionization is expected to occur. The observed partial ion yield curves as a function of laser intensity exhibit a power-law dependence, indicating multiphoton absorption to be the dominating mechanism. Pump-probe studies were performed to investigate the time-evolution of the multiphoton ionization process. The partial ion yields of almost all fragmentation channels show a broad but distinct maximum at a delay-time of approximately 750fs. The particularly flat appearance of the pump-probe curves suggests that not a single resonance, but a broad distribution of resonances is involved.

Ionization and Fragmentation of Methane Induced by 40 eV to 480 eV Synchrotron Radiation: From Valence to Beyond Core Electron Ionization

Photoionization and fragmentation of gaseous methane induced by tunable synchrotron radiation were investigated in a wide energy range, from 40 eV up to 480 eV. We report electron-ion coincidence experiments by measuring the relative partial-ion yields and precursor-specific relative yields for individual fragment ions and for ion fragment pairs as a function of photon energy. The fragmentation patterns are discussed with emphasis on the transition behavior of the bond breaking reactions and of the hydrogen rearrangements from valence to core electron ionization. Below the C 1s threshold, a comparison between photon induced dissociation and electron impact data showed that the ionic fragments formation depends for both projectiles on the same final electronic state reached upon ionization.

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C 3H 2N 2O 3) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/ aug-cc-pVTZ level of theory.

Photofragmentation of BF3 on B and F K-shell excitation by partial ion yield spectroscopy

The fragmentation of core-excited BF3 has been studied by means of partial cation and anion yield measurements around the B and F K edges. All detectable ionic fragments are reported, and differences among the fragments are discussed. The observations confirm earlier findings on the dynamics of molecular fragmentation studied by partial ion yields, notably on the influence of Rydberg excitations on fragmentation, the behaviour of anion production near threshold, and the importance of intramolecular rearrangement in the formation of F+2 ions.

Inner-shell photofragmentation of Cl2

We report an extensive study on partial-ion-yield spectroscopy around the Cl 1s and 2p ionization thresholds for Cl2. All positive ion channels, several with the same mass/charge ratio, which could be distinguished by taking the advantage of the 37Cl isotope, have been measured at a photon resolution of nearly 6500. At the Cl 1s ionization threshold, no significant differences are reported between the absorption and the partial-ion yields. In contrast, near the 2p ionization thresholds, we detect large variations in the fragmentation patterns following excitations to the Rydberg series when comparing the atomic fragment ions to the molecular fragment ions. We attribute the different behaviours to the more-or-less diffuse nature of Rydberg states with different angular momenta.

Fragmentation of formic acid following photoexcitation around the carbon K edge

The fragmentation dynamics of core-excited methanoic acid (formic acid) has been studied by means of partial anion- and cation-yield measurements around the C K ionization threshold. All detectable ionic fragments are reported, and significant differences among partial-ion yields are observed. Possible dissociation pathways are discussed. We confirm experimentally the mixed A* molecular/3s Rydberg character of the resonance at 292.1 eV. Atomic rearrangements after photoabsorption leading to the formation of novel cationic fragments and selective fragmentation to the CH2O2+ ion are also reported.

Dissociation of the benzene molecule by ultraviolet and soft X-rays in circumstellar environment

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by ultraviolet (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 UV (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 per cent of the ionized benzene molecules survive to UV dissociation while only about 4 per cent resist to X-rays. Partial ion yields of H+ and small hydrocarbons, such as C2H+2, C3H+3, C4H+2, 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 was obtained.

Dissociative photoionization of the tetramethyl silane molecule, Si(CH 3) 4, around the Si 1s edge

Photoionization of the Si[CH 3] 4 molecule has been studied around the Si 1s edge, using time-of-flight mass spectrometry, photoelectron–photoion coincidence techniques (PEPICO, PE2PICO and PE3PICO) and synchrotron radiation. Partial ion yields have been recorded as a function of the photon energy. Strong fragmentation of the molecule is observed and singly charged species (H +, H 2 +, C +, and CH n +) dominate the spectra, with the H + ion being the most abundant fragment. Although the contribution of stable doubly and triply charged species was not significant in the measured spectra, intense double and triple ionization of the molecule was demonstrated below and above the Si 1s edge, through PE2PICO and PE3PICO measurements.

Photofragmentation of SiF4 upon Si 2p and F 1s core excitation: Cation and anion yield spectroscopy

We have studied the fragmentation dynamics of core-excited SiF4 by means of soft-x-ray photoexcitation and partial positive and negative ion yield measurements around the Si L2,3-shell and F K-shell ionization thresholds. All detectable ionic fragments are reported and we observe significant differences between the various partial ion yields near the Si 2p threshold. The differences are similar to our previous results from CH3Cl showing more extended fragmentation in correspondence to transitions to Rydberg states. At variance with smaller systems, we observe negative ion production in the shape resonance region. This can be related to the possibility in a relatively large system to dissipate positive charge over several channels.

Electronic state dependence in the dissociation of core-ionized methane

Methane and deuteromethane molecules were core ionized using synchrotron radiation, and the ionic fragments from the molecular dissociation were detected in coincidence with the Auger electrons. The electron–ion coincidence spectra are analysed in terms of partial ion yields and ion kinetic energy distributions, both as functions of electron kinetic energy. The dependence of the fragmentation patterns on the electronic character of the Auger final states as well as on the excess energy available for dissociating the molecule is studied. The analysis reveals marked differences in the dissociation of the 2a−21 and 2a−111t−12 states, interpreted as an ‘excess-energy-dependent’ concerted dissociation process for the 2a−21 state and an ‘electronic-state-dependent’ sequential process for the 2a−111t−12 state.

Upper limits for the circular dichroism for the C 1s and O 1s core excitation of methyl oxirane

The circular dichroism (CD) in the total and partial ion yields of methyl-oxirane C3H6O was measured at the C 1s and O 1s edges. The difference of the response of the chiral molecule to circularly polarized light with opposite handedness was found to be less than 0.2% for the total ion yield and less than 0.5% for the partial ion yield. Additionally we tried to find a dipole allowed molecular orientation CD effect by analysing the fragmentation in the forward and backward direction. For this effect we found an upper limit of 1–2% for all abundant ionic fragments.

Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH.H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C[double bond]O/OH) electrons into pi(CO)* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C[double bond]O)-->pi(CO)* transition by approximately 0.2 eV and a redshift of the O1s(OH)-->pi(CO)* by approximately 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD.D+ (protonated HCOOH.H+) cations within the clusters.

Photon stimulated ion desorption from condensed thiophene photoexcited around the S1s-edge

Photon stimulated ion desorption and Auger decay spectra were measured for condensed thiophene around the S1s-edge. Three kinds of ions, H+, S+, and S2+, were observed as main desorbed species. Partial ion yields (PIY) have been determined for these three fragment ions as a function of the photon energy. It was found that the indirect x-ray induced electron stimulated desorption mechanism plays an important role in H+ ion desorption. For S+ ion desorption, on the other hand, a pronounced enhancement at ∼3eV away from the first core-to-valence resonance was observed in the PIY curve. On the basis of the results for the Auger decay spectra, the enhancement of S+ desorption at ∼3eV away from the first resonance is interpreted in terms of the effective screening of positive holes due to the existence of Rydberg electrons.