Absolute Partial Cross Sections Research Articles

Unveiling the electron-induced ionization cross sections and fragmentation mechanisms of 3,4-dihydro-2H-pyran.

The interactions of electrons with molecular systems under various conditions are essential to interdisciplinary research fields extending over the fundamental and applied sciences. In particular, investigating electron-induced ionization and dissociation of molecules may shed light on the radiation damage to living cells, the physicochemical processes in interstellar environments, and reaction mechanisms occurring in combustion or plasma. We have, therefore, studied electron-induced ionization and dissociation of the gas phase 3,4-dihydro-2H-pyran (DHP), a cyclic ether appearing to be a viable moiety for developing efficient clinical pharmacokinetics and revealing the mechanisms of biofuel combustion. The mass spectra in the m/z = 10-90 mass range were measured at several different energies of the ionizing electron beam using mass spectrometry. The mass spectra of DHP at the same energies were simulated using on-the-fly semi-classical molecular dynamics (MD) within the framework of the QCxMS formalism. The MD settings were suitably adjusted until a good agreement with the experimental mass spectra intensities was achieved, thus enabling a reliable assignment of cations and unraveling the plausible fragmentation channels. Based on the measurement of the absolute total ionization cross section of DHP (18.1 ± 0.9) × 10-16cm2 at 100eV energy, the absolute total and partial ionization cross sections of DHP were determined in the 5-140eV electron energy. Moreover, a machine learning algorithm that was trained with measured cross sections from 25 different molecules was used to predict the total ionization cross section for DHP. Comparison of the machine learning simulation with the measured data showed acceptable agreement, similar to that achieved in past predictions of the algorithm.

Absolute partial and total ionization cross sections of carbon monoxide with electron collision from 350 eV to 8000 eV

The absolute partial and total cross sections for electron impact ionization of carbon monoxide are reported for electron energies from 350 eV to 8000 eV. The product ions (CO+, C+, O+, CO2+, C2+, and O2+) are measured by employing an ion imaging mass spectrometer and two ion-pair dissociation channels (C+ + O+ and C2+ + O+) are identified. The absolute cross sections for producing individual ions and their total, as well as for the ion-pair dissociation channels are obtained by normalizing the data of CO+ to that of Ar+ from CO–Ar mixture target with a fixed 1:1 ratio. The overall errors are evaluated by considering various kinds of uncertainties. A comprehensive comparison is made with the available data, which shows a good agreement with each other over the energy ranges that are overlapped. This work presents new cross-section data with electron energies above 1000 eV.

Electron Impact Ionization of Adenine: Partial Cross Sections

Electron ionization of a genetically important nucleobase, adenine, was investigated from threshold to 500 eV using crossed electron beam–effusive molecular beam geometry and time-of-flight mass spectrometry. We measured the complete set of absolute partial cross sections for adenine using the relative flow technique (RFT) up to an electron energy of 500 eV. Normalization to absolute values was performed using electron ionization cross sections for argon and the vapor pressure data of adenine. The total cross sections obtained by summing the partial cross sections were compared with the existing theoretical and experimental data. The appearance energies of various fragment ions were also measured and compared with the reported data. The prominence of ions with mass (HCN)n+ (n = 1 to 5) indicated a possible pathway to form adenine in the interstellar medium through aggregation of HCN units. Analysis of the partial cross sections for various groups of fragment ions as a function of electron energy was found to give insights into their composition.

Electron ionisation of cyanoacetylene: ionisation cross sections and dication formation

Cyanoacetylene (HC3N) is an important trace species in the atmosphere of Titan. We report, for the first time, absolute partial electron ionisation cross sections and absolute precursor-specific partial electron ionisation cross sections for cyanoacetylene, following an experimental and computational investigation. Our methodology involves using 2D ion–ion coincidence mass spectrometry to generate relative cross sections, over the electron energy range 50–200 eV. These relative values are then normalised to an absolute scale, using a binary encounter-Bethe (BEB) calculation of the total ionisation cross section. The BEB calculation agrees well with previous determinations in the literature. The mass spectrometric observations of HC2N+ and HCN+, ions with a connectivity markedly different to that of the neutral molecule, point towards a rich cationic energy landscape possessing several local minima. Indeed, [HC3N]2+ minima involving a variety of cyclic configurations are revealed by a preliminary computational investigation, along with two minima with linear and bent geometries involving H atom migration (CCCNH2+). Determination of the energy of a transition state between these local minima indicates that the dication is able to explore the majority of this rich conformational landscape at our experimental energies. This investigation of the energetics also determines an adiabatic double ionisation energy of 30.3 eV for the lowest lying singlet state of HCCCN2+, and 30.1 eV for the lowest-lying triplet state. The bulk of the cation pairs detected in the coincidence experiment appear to originate from markedly excited dication states, not the ground state. We observe 5 two-body dissociations of HCCCN2+, and subsequent decay of one of the ions generated in such two-body processes accounts for the majority of three-body dissociations we observe.

Photoionisation of the CH radical using the B-spline R-matrix method

Aims. The primary motivation for this paper is to provide accurate data for the photoionisation of the CH radical, including the absolute total photoionisation cross-section, partial cross-sections, and photoelectron angular distribution. In addition, the near threshold features in the photoionisation curve (which are absent in previous studies) are produced with high precision. Methods. A multichannel wavefunction based on the R-matrix approach, which uses the configuration interaction (CI) method to describe electronic correlation, is carried out in the present calculations. A set of B-spline orbitals is employed to represent the accurate continuum. The distinctive feature of the present calculations allows us to generate a more accurate description of the bound and continuum states than those employed before. Results. Total photoionisation cross-sections from the ground state of CH radicals and partial cross-sections corresponding to 1π, 3σ, and 2σ states of CH+ ions are presented for photon energies ranging from threshold to 80 eV. Extensive resonance structures, which are absent in previous studies, are observed for the first time near the ionisation threshold. The cross-section dataset obtained from the present calculations is expected to be sufficiently accurate and comprehensive for most current modelling applications involving the photon and CH radical scattering system.

Role of L -shell single and double core-hole production and decay in m -fold ( 1≤m≤6 ) photoionization of the Ar+ ion

Multiple ionization of the ${\mathrm{Ar}}^{+}(3{s}^{2}3{p}^{5})$ ion by a single photon has been investigated in the photon-energy range 250--1800 eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all ${\mathrm{Ar}}^{(1+m)+}$ product-ion channels with $1\ensuremath{\le}m\ensuremath{\le}6$ covering a size range from several tens of Mb down to a few b. Narrow $2p$-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52 meV. Double excitations involving a $2p$ and a $3s$ or $3p$ electron were also studied at high resolution and the measurements of the broad $2s$ excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the $2s$ subshell being mainly responsible for the production of ${\mathrm{Ar}}^{7+}$. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionization mechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net $m$-fold ionization with $m\ensuremath{\ge}4$.

Absolute partial ionization cross sections for electron impact of R-carvone from threshold to 100 eV

Partial ionization cross section (PICS) measurements, of the cations created through electron ionization from threshold to 100 eV, have been taken for 78 cationic fragments of R-carvone, using a HIDEN quadrupole mass spectrometer (EPIC 300) with a mass resolution of 1 amu. These PICSs correspond to the cationic fragments with the highest intensity signal registered in our R-carvone mass spectrum reported in Lopes et al. (Int. J. Mass Spectrom. 456, 116395 (2020)) and, to the best of our knowledge, are being presented here for the first time to the scientific community. We also associate the profile observed in the PICSs with some ionic fragmentation mechanisms for R-carvone, based on the appearance energy (AE) values found in Lopes et al.

Photoabsorption and Photoionization Cross Sections of Pyridine in the Vacuum-Ultraviolet Energy Range.

We have performed an experimental investigation into the interaction of vacuum-ultraviolet synchrotron radiation with pyridine molecules in the gas phase. Specifically, a double-ion chamber spectrometer was used to measure the absolute photoabsorption cross sections and the photoionization quantum yields from the ionization threshold to 21.5 eV. Moreover, photoionization and neutral-decay cross sections in absolute scale were derived from these data. In addition, the fragmentation pattern was investigated as a function of the photon energy by using a time-of-flight mass spectrometer and the photoelectron-photoion coincidence technique. Thus, the absolute partial ionization cross sections for each ionic fragment were obtained. Comparisons are made with experimental data available in the literature.

Absolute total, partial, and differential cross sections for photodetachment of O−

Absolute total, partial, and differential cross sections for the photodetachment of the oxygen anion are reported for photon energies ranging from threshold (1.46 eV) to 5.5 eV. The total cross section was measured using the animated-crossed-beam technique while partial and differential cross sections were obtained using velocity map imaging. The measured values are in good agreement with theoretical results obtained from an $R$-matrix calculation using polarized pseudostates.

Communication: Electron ionization of DNA bases.

No reliable experimental data exist for the partial and total electron ionization cross sections for DNA bases, which are very crucial for modeling radiation damage in genetic material of living cell. We have measured a complete set of absolute partial electron ionization cross sections up to 500 eV for DNA bases for the first time by using the relative flow technique. These partial cross sections are summed to obtain total ion cross sections for all the four bases and are compared with the existing theoretical calculations and the only set of measured absolute cross sections. Our measurements clearly resolve the existing discrepancy between the theoretical and experimental results, thereby providing for the first time reliable numbers for partial and total ion cross sections for these molecules. The results on fragmentation analysis of adenine supports the theory of its formation in space.

Ion chemistries in triethylborane

The absolute total and partial cross sections of electron ionization on triethylborane (TEB) are measured in an electron energy range of 6.6–200eV. The total cross section reaches a maximum of 2.2×10−15cm2 in 65–85eV energy region. Product ions from the electron ionization include C2H2–5+, BCH4+, BC2H2,4,6+, BC3H4,6+, BC4H8,10+ and BC6H15+, among which BC6H15+, BC4H10+ and BC2H6+ are the most abundant ions. Gas-phase reactions between TEB and certain major ions from the electron ionization of TEB are studied. Common products from the ion-molecule reactions are BC4H10+ and BC2H6+, as well as association products for certain heavy-ion reactions. The mechanisms of forming BC4H10+ and BC2H6+ in ion-molecule reactions are believed to be simple ethide transfer and ethide transfer followed by dissociation (loss of C2H4), respectively.

Absolute partial and total electron ionization cross sections of uracil

Absolute partial and total electron ionization cross sections for uracil in the gas phase as a function of energy up to 500eV are measured using the well-known relative flow technique (RFT). These measurements have significant consequences for the study of radiation damage, tests of several theoretical models and application of the RFT for molecules existing in solid form at room temperature. Also presented are the thresholds for the formation of various fragment ions from uracil and the comparison of total ionization cross section with the existing results.

Electron Ionization of Adenine

SynopsisWe present the measurements on absolute total and partial ionization cross sections and fragmentation studies for DNA base Adenine on electron impact using a newly designed experimental setup which uses Time of Flight Mass Spectrometer and the relative flow technique.

Ionization and fragmentation of furan molecules by electron collisions

Cation mass spectra obtained by electron impact were measured to investigate the ionization and ionic fragmentation of furan, C4H4O, molecules in the gas phase. The most abundant cations, with comparable intensities in the mass spectra, were the parent, C4H4O+, cation (68 amu) and the C3H3+ fragment (39 amu). The appearance energies of most of the observed ionic fragments were determined and the possible fragmentation processes are discussed. The electron-impact absolute total and partial ionization cross sections in furan were measured using the total ion collection technique over the energy range from the respective ionization thresholds to 150 eV.

A study of the valence shell electronic structure and photoionisation dynamics of s-triazine

A joint experimental and theoretical approach has been used to study the valence shell electronic structure and photoionisation dynamics of s-triazine (1,3,5-triazine). Synchrotron radiation has been employed to record angle resolved photoelectron spectra of the complete valence shell for photon energies between 17.5 and 100eV, thereby allowing photoelectron anisotropy parameters and branching ratios to be determined. Absolute photoionisation partial cross sections have been estimated as the product of these branching ratios and the absolute photoabsorption cross section. The Kohn–Sham and the time-dependent version of density functional theory methods have been used to calculate photoelectron anisotropy parameters and photoionisation partial cross sections, and these have been compared with the corresponding experimental data. The calculations predict that shape resonances affect the photoionisation dynamics of several of the molecular orbitals. The angle resolved photoelectron spectra suggest that the 1a2″(π) orbital is more tightly bound than the 5e′(σ) orbital, and that the 4a1′(σ) orbital is more tightly bound than the 1a2′(σ) orbital, in agreement with the predicted molecular orbital sequence. For the outer valence orbitals the single-particle picture of ionisation holds but electron correlation becomes increasingly important for the inner valence 4e′, 3e′ and 3a1′ orbitals and leads to a redistribution of intensity. Instead of a readily distinguishable main-line, associated with each of these orbitals, the photoelectron intensity is spread over numerous satellite states and the resulting band is broad and featureless.

Electron-impact dissociative ionization of the molecular ionHDO+: A global view

We present a combined experimental and theoretical study of the fragmentation of HDO2+ molecular ions produced by electron-impact ionization of HDO+ in the collision energy range 20–2500 eV. Experimental absolute partial inclusive cross sections for the production of OD+, OH+, and O+ are reported and compared successfully to theoretical predictions. Ab initio methods are used to calculate the electron-impact ionization cross sections of the cationic ground state and first excited state leading to the first seven dicationic states. Dissociation probabilities of each channel are obtained by performing classical molecular dynamics on fitted dicationic potential energy surfaces. The predictive character of the theoretical modeling allows us to estimate that the nonmeasured dissociation channel giving a neutral oxygen atom contributes to 30% of the total ionization cross section. The isotopic ratio OD+/OH+ educed from the experiment is (3.1 ± 0.2) on average, constant in the 30–2500 eV energy range. The calculated isotopic ratio is found to be strongly dependent on the vibrational excitation of the target. Good agreement with the experimental value is obtained for a vibrational excitation corresponding to a temperature of about 2500 K, which is compatible with typical characteristics of electron cyclotron resonance (ECR) ion sources.

Vibrationally resolved C 1s photoionization cross section of CF4

The differential photoionization cross section ratio (ν = 1)/(ν = 0) for the symmetric stretching mode in the C 1s photoionization of CF4 was studied both theoretically and experimentally. We observed this ratio to differ from the Franck–Condon ratio and to be strongly dependent on the photon energy, even far from the photoionization threshold. The density-functional theory computations show that the ratio is significantly modulated by the diffraction of the photoelectrons by the neighbouring atoms at high photon energies. At lower energies, the interpretation of the first very strong maximum observed about 60 eV above the photoionization threshold required detailed calculations of the absolute partial cross sections, which revealed that the absolute cross section has two maxima at lower energies, which turn into one maximum in the cross section ratio because the maxima appear at slightly different energies in ν = 1 and ν = 0 cross sections. These two strong, low-energy continuum resonances originate from the trapping of the continuum wavefunction in the molecular potential of the surrounding fluorine atoms and from the outgoing electron scattering by them.

Measurements of Partial γ-ray Cross Sections in 60Ni([formula omitted]) Reactions

Absolute partial cross sections for production of discrete γ-rays using 60Ni(n,xnypzαγ) reactions with x≤3, y≤2 and z≤4 in a total of 20 reaction channels were measured. The data were taken using the GEANIE spectrometer comprised of 20 high-purity Ge detectors with BGO escape-suppression shields. The broad-spectrum pulsed neutron beam of the Los Alamos Neutron Science Center's (LANSCE) WNR facility provided neutrons in the energy range from 0.2 to 300 MeV. The time-of-flight technique was used to determine the incident neutron energies. Partial γ-ray cross sections have been measured for a total of 59 transitions and for neutron energies 1 MeV <En< 300 MeV. Hauser-Feshbach theoretical calculations with an improved cluster emission model in the pre-equilibrium process were performed up to En = 30 MeV and are compared to the experimental results.

Absolute total and partial dissociative cross sections of pyrimidine at electron and proton intermediate impact velocities

Absolute total non-dissociative and partial dissociative cross sections of pyrimidine were measured for electron impact energies ranging from 70 to 400 eV and for proton impact energies from 125 up to 2500 keV. MOs ionization induced by coulomb interaction were studied by measuring both ionization and partial dissociative cross sections through time of flight mass spectrometry and by obtaining the branching ratios for fragment formation via a model calculation based on the Born approximation. The partial yields and the absolute cross sections measured as a function of the energy combined with the model calculation proved to be a useful tool to determine the vacancy population of the valence MOs from which several sets of fragment ions are produced. It was also a key point to distinguish the dissociation regimes induced by both particles. A comparison with previous experimental results is also presented.

Absolute photoionization cross-sections of selected furanic and lactonic potential biofuels

Absolute photoionization cross sections of the molecules γ-butyrolactone (GBL), γ-valerolactone (GVL), α-angelicalactone (AAL), and γ-methylene-γ-butyrolactone (GMGB), including partial ionization cross sections for dissociative ionization, are measured. The experiments are carried out using photoionization mass spectrometry (PIMS) at the Advanced Light Source, and an orthogonal extraction time-of-flight spectrometer is used for mass analysis. Photoionization of furan, 2-methyl furan (2-MF), and 2,5-dimethyl furan (2,5-DMF) is also investigated to confirm the accuracy of our measurements, which show a good agreement of the absolute partial photoionization cross sections with literature values. CBS-QB3 calculations of adiabatic ionization energies (AIE) and appearance energies (AE) agree well with the experimental results.