Photoion Coincidence Technique Research Articles

Competitive Dehydrogenation and Backbone Fragmentation of Superhydrogenated PAHs: A Laboratory Study

Abstract Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) have been suggested to catalyze the formation of H2 in certain regions of space, but it remains unclear under which circumstances this mechanism is viable given the reduced carbon backbone stability of superhydrogenated PAHs. We report a laboratory study on the stability of the smallest pericondensed PAH, pyrene (C16H10+N , with N = 4, 6, and 16 additional H atoms), against photodestruction by single vacuum ultraviolet photons using the photoelectron–photoion coincidence technique. For N = 4, we observe a protective effect of hydrogenation against the loss of native hydrogens, in the form of an increase in the appearance energies of the and C16H8 + daughter ions compared to those reported for pristine pyrene (C16H10). No such effect is seen for N = 6 or 16, where the weakening effect of replacing aromatic bonds with aliphatic ones outweighs the buffering effect of the additional hydrogen atoms. The onset of fragmentation occurs at similar internal energies for N = 4 and 6, but is significantly lower for N = 16. In all three cases, H-loss and C m H n -loss (m ≥ 1, carbon backbone fragmentation) channels open at approximately the same energy. The branching fractions of the primary channels favor H-loss for N = 4, C m H n -loss for N = 16, and are roughly equal for the intermediate N = 6. We conclude that superhydrogenated pyrene is probably too small to support catalytic H2-formation, while trends in the current and previously reported data suggest that larger PAHs may serve as catalysts up to a certain level of hydrogenation.

The Underlying Chemistry to the Formation of PO2 Radicals from Organophosphorus Compounds: A Missing Puzzle Piece in Flame Chemistry.

Reactive species, such as . PO2 and HOPO, are considered of upmost importance in flame inhibition and catalytic combustion processes of fuels. However, the underlying chemistry of their formation remains speculative due to the unavailability of suitable analytical techniques that can be used to identify the transient species which lead to their formation. This study elucidates the reaction mechanisms of the formation of phosphoryl species from dimethyl methyl phosphonate (DMMP) and dimethyl methyl phosphoramidate (DMPR) under well-defined oxidative conditions. Photoelectron photoion coincidence techniques that utilized vacuum ultraviolet synchrotron radiation were applied to isomer-selectively detect the elusive key intermediates and stable products. With the help of in situ recorded spectral fingerprints, different transient species, such as PO2 and triplet O radicals, have been exclusively identified from their isomeric components, which has helped to piece together the formation mechanisms of phosphoryl species under various conditions. It was found that . PO2 formation required oxidative conditions above 1070 K. The combined presence of O2 and H2 led to significant changes in the decomposition chemistry of both model phosphorus compounds, leading to the formation of . PO2 . The reaction . PO+O2 →. PO2 +O: was identified as the key step in the formation of . PO2 . Interestingly, the presence of O2 in DMPR thermolysis suppresses the formation of PN-containing species. In a previous study, PN species were identified as the major species formed during the pyrolysis of DMPR. Thus, the findings of this study has shed light onto the decomposition pathways of organophosphorus compounds, which are beneficial for their fuel additive and fire suppressant applications.

C-F and C-H bond cleavage mechanisms of trifluoromethane ions in low-lying electronic states: threshold photoelectron-photoion coincidence imaging and theoretical investigations.

Dissociative ionization of trifluoromethane (CHF3) is investigated in the 13.9-18.0 eV energy range using the threshold photoelectron-photoion coincidence (TPEPICO) technique coupled to a vacuum ultraviolet synchrotron radiation source. Four electronic states of CHF3+, i.e., the X2A1, A2A2, B2E, and C2E states, are populated upon ionization. In this energy range, the parent CHF3+ ions fully dissociate. For the CHF3+ ions in the ground state, the analysis of the time-of-flight profile of the unique CF3+ fragment ions suggests statistical dissociation. For the electronically excited CHF3+ ions, the C-F bond cleavage preferentially occurs to predominantly produce CHF2+ + F. Moreover, all TPEPICO images of the CHF2+ ions exhibit identical patterns, with a weak central spot revealing a previously unobserved statistical decomposition pathway, and the predominant ring in the images documents a fast nonstatistical dissociation channel. The unimolecular decomposition mechanisms of the CHF3+ ions are illuminated with the aid of the one-dimensional potential energy curves along the C-H and C-F coordinates calculated using the time-dependent density-functional theory. Moreover, a comparison of the dissociation dynamics of CHF3+ in these low-lying states with those of CF3Cl+ strongly suggests a substituent effect of chlorine atoms on the binding structure.

New insights onto dissociation of state-selected O2+ ions investigated by double imaging photoelectron photoion coincidence: The superimposed 32Πu and c4Σu- inner-valence states.

The double imaging photoelectron photoion coincidence technique has been applied to investigate the dissociation of state-selected O2+(32Πu, c4Σu-) ions at hv = 25.4 eV. It is shown that the O2+ ions prepared in the 32Πu inner-valence state can dissociate toward two channels where a distinct changeover from the third dissociation limit O(3P) + O+(2D) to the fifth limit O(3P) + O+(2P) is observed at a binding energy of ∼23.8 eV. Up to four dissociation limits have been identified in the dissociation of the v = 0 vibrational level of the c4Σu- electronic state, and in particular, the products toward the third limit with a very weak intensity are definitely identified and assigned. In addition, the individual contributions of the two superimposed 32Πu and c4Σu- states to the fifth dissociation limit are clearly separated and determined for the first time. The anisotropy parameters of the O+ fragment ion toward different dissociation limits have also been measured and exhibit specific state-selected characteristics. The dissociation mechanisms of the 32Πu and c4Σu- states including predissociation and tunneling motion are proposed based on the present experimental results and the existing theoretical potential energy curves.

Photofragmentation of Serine Following C 1s Core Ionization-Comparison with Cysteine.

Photofragmentation pathways of doubly ionized serine molecules are investigated and compared with those of cysteine. The main motivation for the study is to investigate if an atomic substitution within the same group of elements, namely, replacing sulfur (in cysteine) with oxygen (in serine), causes a major change in the C 1s core ionization induced dissociation pattern in the molecules of an otherwise identical structure. The results show that in serine there is a single completely dominant fragmentation channel producing the (CNH2–4+, COH1–3+) pairs, whereas in cysteine there are also many other fragmentation channels. The employed experimental method was the photoelectron–photoion–photoion coincidence (PEPIPICO) technique combined with synchrotron radiation tuned to ionize desired core levels. Molecular dynamics calculations were also carried out in order to extract information on the fragmentation and the neutral final fragments.

Ionization and Fragmentation of Formamide Induced by Synchrotron Radiation in the Valence Region via Photoelectron Photoion Coincidence Measurements and Density Functional Theory Calculations.

We have performed a theoretical and experimental study of the formamide (HCONH2) photofragmentation and photoionization processes in the gas phase. The experiment was perfomed by using a time-of-flight mass spectrometer using the photoelectron photoion coincidence (PEPICO) technique in the valence region, from photons with energy between 10 and 20 eV. We have obtained both mass and partial ion yield spectra, identified by the mass-to-charge ratio as a function of the photon energy. With this setup, we could ascertain the threshold energy for the production of formamide cation and its cationic fragments. The theoretical analysis of the formamide photofragmentation channels are fulfilled by the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The theoretical analysis allowed us to estimate, for example, which atoms are lost during the photofragmentation. We have also developed a theoretical-experimental analysis of the main fragments produced in the dissociation: m/q = 45 (HCONH2+), m/q = 44 (CONH2+), m/q = 29 (HCO+), m/q = 17 (NH3+), and m/q = 16 (NH2+).

Ionization of Kr.CF4 and Xe.CF4 van der Waals clusters: from face to vertex geometry

SynopsisWe report on photoionization studies of Xe.CF4 and Kr.CF4 van-der-Waals (vdW) clusters produced in a supersonic expansion and detected using synchrotron radiation and photoelectron- photoion coincidence techniques. Complementary ab initio calculations of the potential energy surfaces for the ground state and the first ionized state are performed and show generally good agreement with the experimental results. In particular, the calculation reveals a change in the equilibrium configuration from face to vertex geometry upon ionization. This observation is fully in line with the lineshapes of the Xe.CF4 and Kr.CF4 clusters in the photoelectron spectra.

VUV Photofragmentation of CH2I2: The [CH2I-I](•+) Iso-diiodomethane Intermediate in the I-Loss Channel from [CH2I2](•.).

Diiodomethane is an important halocarbon responsible for several atmospheric processes like ozone depletion and aerosol particle formation. Despite this, the thermochemical data and a detailed analysis of the pathways for the decomposition of this halomethane and its molecular ion [CH2I2](•+) are scarce. In this paper an investigation of the photodissociation dynamics of the CH2I2 molecule focused on the I-loss channel by the photoelectron-photoion coincidence (PEPICO) technique and computational methods is reported. The experimental results show that upon VUV irradiation the dissociation of the lower electronic ionic states of diiodomethane leads only to the CH2I(+) ion and the I atom. The theoretical calculations point out that isomerization of [CH2I2](•+) into iso-diiodomethane [CH2I-I](•+) may play an important role in the emission of iodine atom as compared to direct C-I bond breaking.

Theoretical and Experimental Photoelectron Spectroscopy Characterization of the Ground State of Thymine Cation.

We report on the vibronic structure of the ground state X̃(2)A″ of the thymine cation, which has been measured using a threshold photoelectron photoion coincidence technique and vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum, recorded over ∼0.7 eV above the ionization potential (i.e., covering the whole ground state of the cation) shows rich vibrational structure that has been assigned with the help of calculated anharmonic modes of the ground electronic cation state at the PBE0/aug-cc-pVDZ level of theory. The adiabatic ionization energy has been experimentally determined as AIE = 8.913 ± 0.005 eV, in very good agreement with previous high resolution results. The corresponding theoretical value of AIE = 8.917 eV has been calculated in this work with the explicitly correlated method/basis set (R)CCSD(T)-F12/cc-pVTZ-F12, which validates the theoretical approach and benchmarks its accuracy for future studies of medium-sized biological molecules.

State selective fragmentation kinematics of OCS2+ following S([formula omitted]) Auger decay

Kinematically complete study of dissociation of OCS due to various Auger decays upon excitation at S(2p1/2) ionization threshold is presented. Ion momentum spectrometry and energy selected Auger electron–photoion–photoion coincidence techniques are employed to perform these studies. Fragmentation mechanisms show marked differences (both kinematic and cross sectional) depending on the final Auger state attained. The kinematic features for S(2p) ionization are significantly different from that of valence photo double ionization of OCS.

Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy

In this paper we describe femtosecond photoionization and the imaging photoelectron photoion coincidence spectroscopy of tetrathiafulvene, TTF. Femtosecond photoionization of TTF results in the absorption of up to twelve 808 nm photons leading to ion internal energies up to 12.1 eV as deduced from the photoelectron spectrum. Within this internal energy a variety of dissociation channels are accessible. In order to disentangle the complex ionic dissociation, we utilized the imaging photoelectron photoion coincidence (iPEPICO) technique. Above the dissociation threshold, iPEPICO results show that the molecular ion (m/z = 204) dissociates into seven product ions, six of which compete in a 1.0 eV internal energy window and are formed with the same appearance energy. Ab initio calculations are reported on the possible fragment ion structures of five dissociation channels as well as trajectories showing the loss of C2H2 and C2H2S from high internal energy TTF cations. A three-channel dissociation model is used to fit the PEPICO data in which two dissociation channels are treated as simple dissociations (one with a reverse barrier), while the rest involve a shared barrier. The two lower energy dissociation channels, m/z = 146 and the channel leading to m/z = 178, 171, 159, 140, and 127, have E0 values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by ΔS(‡)(600 K) values of -9 ± 6 and 1 ± 6 J K(-1) mol(-1), respectively. Competing with them at higher internal energy is the cleavage of the central bond to form the m/z = 102 fragment ion, with an E0 value of 3.65 ± 0.10 eV and ΔS(‡)(600 K) = 83 ± 10 J K(-1) mol(-1).

Fluorescence decay processes following resonant 2p photoexcitation of Ar atoms and clusters studied using a time-resolved fluorescence and photoion coincidence technique

The novel spectroscopic technique of time-resolved fluorescence–photoion coincidence spectroscopy (TFPICO) has been applied to the investigation of the decay processes of 2p inner-shell excited Ar atoms and clusters. For the Ar atom, only that fluorescence accompanying the production of Ar+ showed a strong dependence on excitation energy. This dependence is discussed in terms of competing Auger decay processes. For Ar clusters, the TFPICO spectra for dimer ions (Ar2)+ revealed long-lifetime fluorescence components which can be attributed to the ‘third excimer continuum’. With this work we demonstrate the usefulness of this technique for investigating the decay processes of inner-shell excited atoms and clusters.

Photofragmentation of the K-shell excited perfluorocyclobutane probed by a multiple-ion coincidence technique

Dissociation of perfluorocyclobutane into cation pairs and neutral fragments following the C and F K-shell excitation was investigated using the photoelectron–photoion–photoion coincidence (PEPIPICO) technique. Based on the analysis of islands in the PEPIPICO spectra, fragmentation pathways of the molecular dication were determined. Two characteristic series of the pathways, i.e., fission of F atom(s) followed by charge separation and elimination of CF2 or CF3 followed by charge separation, were identified. The symmetric fission of the molecular dication was found only for the C 1s excitation.

A dissociative photoionization study of the c4Σu− state in O+2 using the TPEPICO technique

In the dissociative photoionization (DPI) process, hν + O2 → O + O+ + e−, ionisation and dissociation both occur (either as a direct or indirect process) following photoabsorption. The Threshold Photoelectron Photoion Coincidence (TPEPICO) technique, i.e. measuring the coincidence yield between threshold photoelec-trons and photoions is a powerful way of studying the dynamics involved. The c4Σu− state in O+2 at ~ 24.56 eV has a shallow minimum in its potential that supports two distinct quasi-bound vibrational levels (v = 0, 1). We have investigated the angular distributions of 2 eV O+(4S) ions produced from DPI of O +2 c4Σu− (v = 0, 1) using the TPEPICO technique.

Ionization photophysics and Rydberg spectroscopy of diacetylene

Photoionization of diacetylene was studied using synchrotron radiation over the range 8–24 eV, with photoelectron-photoion coincidence (PEPICO) and threshold photoelectron–photoion coincidence (TPEPICO) techniques. Mass spectra, ion yields, total and partial ionization cross-sections were measured. The adiabatic ionization energy of diacetylene was determined as IEad = (10.17 ± 0.01) eV, and the appearance energy of the principal fragment ion C4H+ as AE = (16.15 ± 0.03) eV. Calculated appearance energies of other fragment ions were used to infer aspects of dissociation pathways forming the weaker fragment ions , C3H+, and C2H+. Structured autoionization features observed in the PEPICO spectrum of diacetylene in the 11–13 eV region were assigned to vibrational components of three new Rydberg series, R1(nsσg, n = 4–11), R2(ndσg, n = 4–7) and R3(ndδg, n = 4–6) converging to the A2Πu state of the cation, and to a new series R′1(nsσg, n = 3) converging to the state of the cation. The autoionization mechanisms and their consistence with specific selection rules are discussed.

O 1s excitation and ionization processes in the CO2 molecule studied via detection of low-energy fluorescence emission

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon–photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core–valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300–650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400–500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon–photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2 +. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV–O+ coincidence peak in the mass spectrum carries some information on the initial core excitation.

Resonant Auger electron–photoion coincidence study of the fragmentation dynamics of an acrylonitrile molecule

Monochromatic synchrotron radiation was used to promote K-shell electrons of nitrogen and carbon from the cyano group (C ≡ N) of gaseous acrylonitrile (C2H3–CN) to the unoccupied antibonding π*C ≡ N orbital. Photofragmentation of acrylonitrile molecules following selective resonant core excitations of carbon and nitrogen core electrons to the π*C ≡ N orbital was investigated using the electron-energy-resolved photoelecton–photoion coincidence technique. The fragment ion mass spectra were recorded in coincidence with the resonant Auger electrons, emitted in the decay process of the core-excited states. Singly and triply deuterated samples were used for fragment identification. The results showed the initial core-hole localization to be of minor importance in determining the dissociation pattern of the molecular cation. The participator and spectator Auger transitions produce entirely different fragmentation patterns and the latter indicates that complex nuclear rearrangements take place. It is suggested that the calculated kinetic energy releases are caused by the existence of metastable states, which appear with the opening of the spectator Auger channels.

VUV-induced dissociation of methylchlorosilanes, studied by electron–ion coincidence spectroscopy

Fragmentation patterns of gas-phase trimethylchlorosilane and methyltrichlorosilane molecules were investigated using the electron-energy-resolved photoelectron–photoion coincidence technique. The production of parent ions and the dominant fragments by 30 eV synchrotron radiation was followed as a function of the photoelectron binding energy. Ion appearance energies were determined from these coincident ion yield curves. The fragmentation patterns and their development are discussed in terms of available internal excess energy as well as the molecular orbital character of the parent molecules. A common dissociation scenario is proposed for both molecules, producing methyl- and chlorine-loss fragments at lower excess energies and smaller fragments through second-step processes at sufficiently high energies.

Photoion mass spectroscopy and valence photoionization of hypoxanthine, xanthine and caffeine

Photoionization mass spectra of hypoxanthine, xanthine and caffeine were measured using the photoelectron–photoion coincidence technique and noble gas resonance radiation at energies from 8.4 to 21.2 eV for ionization. The fragmentation patterns for these compounds show that hydrogen cyanide is the main neutral loss species at higher photon energies, while photoionization below 16.67 eV led predominantly to the parent ion. The valence photoelectron spectra of this family of molecules were measured over an extended energy range, including the inner C, N and O 2s valence orbitals. The observed ion fragments were related to ionization of the valence orbitals.

Electronic state resolved PEPICO spectroscopy of pyrimidine

Photoionization techniques, such as photoelectron spectroscopy (PES) and photoionization mass spectrometry (PIMS), are well-established and powerful methods for studying the spectroscopy of isolated bio-organic molecules and their fate under vacuum ultraviolet (VUV) irradiation. Measuring the energy selected electron leaving a molecular ion in coincidence with other particles, such as ions, can provide even deeper insights into the mechanisms of the interaction of molecules with ionizing radiation. We have thus implemented the electronic state resolved photoelectron photoIon coincidence (ER-PEPICO) technique in our laboratory. Here, we report our newly constructed apparatus, and its application for characterizing fragmentation processes occurring in pyrimidine. Ionization of the two highest molecular orbitals (MOs) of the valence band does not lead to fragmentation of the resulting ion. The third band observed in photoemission is due to the ionization of two MOs, and leads mainly to the formation of the parent ion. The next three electronic states are not resolved experimentally and appear as a single band; their ionization leads to fragments of mass to charge ratio m/e= 53 (C3H3N+), while ionization of deeper lying MOs leads mostly to m/e= 26 (C2H2+). We compare our data with previous non-coincidence photoionization results and describe the problems encountered and their solutions.