Absolute Partial Ionization 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.

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.

C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CF3I.

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C-I and C-F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C-I bond cleavage and the CF2I+ products of C-F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C-I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C-F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C-I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved.

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.

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.

Multiple Ionization of Ne in coincidence with partially stripped B2+ ions in the intermediate energy regime

A projectile ion-recoil ion and electron-recoil ion coincidence technique has been used to study the multiple ionization process in collisions of 0.75-4.0MeV B2+ with Ne atoms. Absolute total and partial pure ionization cross sections are reported for the first time at these impact energies with the present setup. The relative contribution of each recoil charge state has been investigated. The experimental data are compared to CDW-EIS calculation and with He2+ projectiles. The data throw further light on the effect of projectile screening in the ionization of multi-electronic targets and provide a support for the validity of the CDW-EIS model in this intermediate-to low velocity regime.

Electron ionization of NF3

Experimental and theoretical study on electron ionization of NF3 is carried out in terms of the absolute partial and total ionization cross sections. The measurements are done in crossed electron beam-molecular beam geometry for electron incident energies ranging from 0 to 500eV using a newly built time of flight mass spectrometer with emphasis on complete collection, transmission and detection without mass and energy discrimination, while maintaining reasonable mass resolution. The cross sections are put on the absolute scale using the relative flow technique. The partial cross sections and the thresholds for the formation of NF3+, NF2+, NF+, F+, and N+ are compared with the previous results. Our cross section values for lower mass ions N+, F+, NF+ are found to be considerably higher as compared to previous measurements. We also have calculated the total scattering and total ionization cross sections using the spherical complex optical potential (SCOP) formalism and compared with existing data. While the total scattering calculations agree with the existing experimental results, the total ionization cross sections do not agree well with our experimental data. The present experimental results agree with those estimated using “Deutsch–Mark formulation with new additivity rules”, which were not in agreement with the existing experimental results for the case of NF3.

Cross sections for ionization and ionic fragmentation of pyrimidine molecules by electron collisions

The electron impact mass spectroscopy and the total ion collection measurements were used to investigate the ionization and ionic fragmentation of the pyrimidine, C4H4N2, molecules in the gas phase. The cation mass spectra were measured in the 10−85 amu range and the observed mass peaks assigned to corresponding ionic fragments. The most abundant cation in the mass spectra is the parent cation, C4H4N \hbox{$_{\mathrm{2}}^{\mathrm{+}}$} + 2 , at 80 amu. The appearance energies of the ionic fragments were determined and the possible fragmentation processes are discussed. The electron impact absolute total and partial ionization cross sections in pyrimidine were obtained over the energy range from the respective ionization thresholds up to 150 eV.

Dissociative-ionization cross sections for 12-keV-electron impact on CO2

The dissociative ionization of a CO${}_{2}$ molecule is studied at an electron energy of 12 keV using the multiple ion coincidence imaging technique. The absolute partial ionization cross sections and the precursor-specific absolute partial ionization cross sections of resulting fragment ions are obtained and reported. It is found that \ensuremath{\sim}75% of single ionization, 22% of double ionization, and \ensuremath{\sim}2% of triple ionization of the parent molecule contribute to the total fragment ion yield; quadruple ionization of CO${}_{2}$ is found to make a negligibly small contribution. Furthermore, the absolute partial ionization cross sections for ion-pair and ion-triple formation are measured for nine dissociative ionization channels of up to a quadruply ionized CO${}_{2}$ molecule. In addition, the branching ratios for single-ion, ion-pair, and ion-triple formation are also determined.

Electron ionization of hydrogen sulfide

Precursor-specific relative partial ionization cross sections, for all the different fragment ions formed by electron ionization of hydrogen sulfide, have been measured using time-of-flight mass spectrometry coupled with a two-dimensional ion coincidence technique. Relative cross sections are reported for ionizing energies from 30 to 200 eV. These cross sections allow the contribution from single, double and triple ionization to the individual fragment ion yields, following ionization of hydrogen sulfide, to be quantified. To compare our data with the literature we reduce our precursor-specific cross sections, by summing them for each fragment ion, to generate relative partial ionization cross sections. Following this data reduction, good agreement is found between our data and one set of recently published absolute partial ionization cross sections, but discrepancies are observed with another set of recently published data. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% at 100 eV. Given the lack of available information on the fate of the excited electronic states of H 2S 2+, we have extracted kinetic energy releases for the various dicationic fragmentation channels from our coincidence data. From these kinetic energy releases, estimates of the energies of the electronic states of H 2S 2+ which are responsible for the different fragmentation channels can be made. These estimates, in comparison with other data on the electronic states of H 2S 2+, reveal the population of excited states of H 2S 2+ at electron energies above 50 eV. At ionizing electron energies above 50 eV, a significant proportion of the major dissociation channels of H 2S 2+ appear to involve the population of excited electronic states.

Studies of the fragmentation of the monocation and dication of methanol

Relative partial ionization cross sections and precursor-specific relative partial ionization cross sections for fragment ions formed by electron ionization of methanol have been measured using time-of-flight mass spectrometry coupled with a two-dimensional ion coincidence technique. Relative cross sections are reported for ionizing energies from 30 to 200 eV. Good agreement is found between our data and one set of recently published absolute partial ionization cross sections. Conversely, discrepancies are observed with another set of recently published data; we attribute these discrepancies to the loss of translationally energetic fragment ions. Our precursor-specific cross sections allow the contribution from single and double ionization to the individual fragment ion yields, following ionization of methanol, to be quantified for the first time. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% between 150 and 200 eV.

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.

Electron ionization of acetylene

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of propene

We present absolute partial electron impact ionization cross sections for propene in the electron energy range between threshold and 1000 eV measured with a two sector field double focussing mass spectrometer. Ion beam profiles have been measured at all electron energies by applying a deflection field method. In combination with a three-dimensional ion trajectory simulation of ions produced in the ion source ion kinetic energy distribution functions and discrimination factors can be derived form these data. Multiplication of these discrimination factors with the measured ion signals leads to relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions exhibiting different electron energy dependencies. Differential cross sections with respect to initial kinetic energy of the ions are given and related to specific ion production channels. The high energetic parts of the initial kinetic energy distributions are assigned to the Coulomb explosion of doubly charged short-lived intermediates. The reaction C 3H 5 2+ → C 2H 2 + + CH 3 + has been studied quantitatively in the ion source, the first field free region and the second field free region of the two sector field mass spectrometer yielding a measured mean kinetic energy release of 4.99 ± 0.5 eV, 4.85 ± 0.5 eV and 5.5 ± 0.5 eV, respectively in good agreement with a theoretical estimation.

Ion chemistries in hexamethyldisiloxane

Dissociative ionization of hexamethyldisiloxane (HMDSO) by electron impact is investigated using Fourier transform mass spectrometry. Absolute total and partial ionization cross sections are measured and the results compared with previous studies. The collision-induced dissociation technique is used to probe the fragmentation pathways of ions, providing insight into the production of neutral products from the electron impact ionization. Ion-molecule reactions of Ar+ and HMDSO ions with the parent gas molecule are studied, and isotope studies are used to evaluate the possible reaction mechanisms.

Electron ionization of N2O

Relative partial ionization cross sections and precursor specific partial ionization cross sections for electron ionization of N2O have been determined using time-of-flight mass spectrometry and ion–ion coincidence techniques. Cross sections are reported for ionizing energies from 30–200 eV. Data are presented for the formation of N2+, O2+, N+, O+, N2O2+, N2+ and NO+ relative to N2O+. Good agreement is found between our data and one set of recently published absolute partial ionization cross sections. However, some discrepancies are observed between the results presented here and two earlier determinations of the absolute partial ionization cross sections. We ascribe these discrepancies to incomplete collection of translationally energetic fragment ions in the earlier studies, providing additional evidence for the incomplete collection of translationally energetic product ions in some determinations of partial ionization cross sections. Our precursor specific cross sections allow the contribution from single, double and triple ionization to the individual fragment ion yields, following ionization of N2O, to be quantified for the first time.