Electron Impact Ionization of Adenine: Partial Cross Sections

Absolute partial electron ionisation cross sections, and precursor-specific partial electron ionisation cross sections, for the formation of cations from phosphorus trifluoride (PF3) are reported over the electron energy range 50-200 eV. The absolute values are determined by the measurement of cross sections relative to the formation of PF3+ using 2D ion-ion coincidence time-of-flight mass spectrometry and subsequent scaling using binary encounter-Bethe calculations of the total ionisation cross section. This new dataset significantly augments the partial ionisation cross sections for electron ionization of PF3 found in literature, addressing previous discrepancies in the branching ratios of product ions, and provides the first values for the precursor-specific cross sections. Comparisons to calculated cross sections from the literature are encouraging, although there are discrepancies for individual ions. The coincidence experiments indicate that double and triple ionisation generate approximately 20% of the cationic ionisation products at 200 eV electron energy. One dissociative dication state, dissociating to PF2+ + F+, is clearly identified as the lowest triplet state of PF32+ and five different dications (PF32+, PF22+, PF2+, P2+ and F2+) are detected in the mass spectra. The dication energetics revealed by the experiments are supported by a computational investigation of the dication's electronic structure. The cross sections reported will allow more accurate modelling of the role of the ionization of PF3 in energetic environments. A first investigation of the bimolecular reactivity of metastable states of PF32+ is also reported. In collisions with Ar, O2 and CO dissociative single electron transfer dominates the product ion yield, whereas collision-induced dissociation of the dication is important following collisions with Ne. Consideration of the energetics of these processes indicates that the reactant dication beam contains ions in both the ground singlet state and the first excited triplet state. The deduction regarding the longevity of the triplet state is supported by metastable signals in the coincidence spectra. Weak signals corresponding to the formation of ArF+ are detected following PF32+ collisions with Ar, and experimental and computational considerations indicate this new chemical bond is formed via a collision complex.

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the C2F5 free radical from threshold to 200 eV using the fast-neutral-beam technique. The electron-impact ionization of C2F5 is dominated by two channels, the formation of the CF3+ fragment ion which has the largest partial ionization cross section with a value of 3.5 × 10-16 cm2 at 70 eV and the formation of the C2F5+ parent ion with a cross section of 1.3 × 10-16 cm2 at 70 eV. Both partial cross sections have error margins of about ±15%. All other partial ionization cross sections have maximum cross section values of less than 0.1 × 10-16 cm2. The total single C2F5 ionization cross sections at 70 eV (derived from the two measured partial ionization cross sections and an estimate for the sum of the cross sections for all weaker, unobserved ionization processes) was found to be about 5 × 10-16 cm2. A comparison of the measured total single-ionization cross sections with a calculated cross section, using a modified additivity rule, shows good agreement up to an electron energy of 50 eV, but poor agreement at higher impact energies. This behaviour is somewhat similar to that observed for the CFx (x = 1-3) free radicals, but it is very different from the excellent agreement between the calculated and measured total single-ionization cross sections that was observed for the stable molecules CF4 and C2F6.

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.

Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

Much effort has been recently devoted to the experimental determination of absolute partial and total electron impact ionization cross sections of molecules and radicals due to the ever increasing importance of these cross sections in many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have been recently developed in order to allow the analytic quantitative description of these cross section functions for modeling codes. This review summarizes recent experimental and theoretical progress in this area of electron impact ionization including also a short description of (i) recent advances in the field of electron/ion collisions (i.e., kinetic energy release distributions in dissociative excitation reactions of molecular ions) and (ii) a novel approach to the determination of electron induced appearance energies.

Determination of partial single differential electron ionization cross sections of H2

Dissociative ionization of silane by electron impact

Dissociative electron attachment (DEA) to ethanol has been probed to study fragmentation dynamics using Time-of-Flight (ToF) mass spectrometric technique. Several fragment ions, namely, H-, O-, OH-, C2H3O- and C2H5O- have been observed. Extra effort has been made to detect low mass ions (here, H-). Absolute DEA cross sections for the formation of O- and OH- have been measured for the first time using relative flow technique (RFT). The threshold energy of different dissociation channels has been calculated using density functional theory (DFT) method. By combining the experimental and theoretical data, we found evidence of hydrogen migration in the production of O and C2H3O- ions.

We calculate electron impact partial and total ionization cross sections of R-carvone (C10H14O), 2-butanol (C4H10O), imidazole (C3H4N2), and 2-nitroimidazole (C3H3N3O2). We have used the Binary Encounter Bethe (BEB) model to obtain total electron impact ionization cross sections (TICSs). The modified BEB method in combination with mass spectrum data of the molecules is used to calculate the partial ionization cross section (PICS) of the cationic fragments dissociating from the parent molecule. Our PICS data for R-carvone and 2-butanol are in good agreement with the experimental data for all the cation fragments along with the TICS data. For imidazole and 2-nitroimidazole, the estimates of the PICS are reported for the first time in the present study. We have found that both the modified BEB method and the mass spectrum dependence method work effectively to estimate PICS if we have information about the appearance energies and relative abundance data of the target under investigation.

VOLSCAT2.0: The new version of the package for electron and positron scattering off molecular targets

Absolute partial cross sections for the production of SO2+, SO+, (S+ + O2+ + SO22+), O+, and SO2+ from electron impact ionization of SO2 are reported for electron energies from threshold to 1000 eV. The product ions are analyzed using a time‐of‐flight mass spectrometer and detected with a position‐sensitive detector, making it possible to demonstrate complete collection of all product ions. The overall uncertainty in the absolute cross section values is ±3.5% for SO2+ ions and is slightly higher for fragment and multiply charged ions. While much of the previously published data are in reasonable agreement with the data presented here, the present O+ cross section is higher than earlier measurements by approximately a factor of 3. The cross section for production of SO2+ has not been previously reported. The sum of the partial cross sections is compared to previously measured total cross sections.

A sophisticated R-matrix calculation is performed for the photoionization of the fine-structure levels of the ground state of Fe XVIII. High resolution total and partial cross sections are obtained with the later restricted to the cases of the Fe XVIII ion being left in one of the fine-structure levels corresponding to the energetically lowest 11 LS states of Fe XIX after ionization. Both sets of cross sections are obtained using the Breit-Pauli R-matrix approximation which allows for the possibility of fine structure splitting in both resonances and thresholds. We find extensive resonance structure in the 99 to 113 Ryd photon energy range, a high percentage of which arises from 2p photoionization leaving the ion in the 2s2 2p4 3 P2 state while the background cross section is constructed almost exclusively of 2p and 2s photoionization with the former dominant in both photoionization cases. Relativistic effects are found to be important in obtaining accurate threshold positions as well as being responsible for a significant drop in the background cross section in comparison with various calculations done in LS coupling. We believe that this is the first relativistic calculation for this ion.

We investigate the 5s angular distribution parameter and partial photoionization cross section of atomic Ba in the region of the second Cooper minimum covering a photon energy region from 120 to 260 eV. We observe a strong drop in the Ba 5s β value from 2.0, reaching a minimum of 1.57 ± 0.07 at a photon energy of 150 eV. The β value then slowly rises back towards its nominal value of 2.0 at photon energies beyond the minimum. Our measured 5s partial cross section also shows a pronounced dip around 170 eV due to interchannel coupling with the Ba 4d photoelectrons. After combining our measurements with previous experimental values at lower photon energies, we obtain a consistent data set spanning the photon energy range prior to the onset of the partial cross section maximum and through the cross section minimum. We also calculate the 5s partial cross section under several different levels of approximation. We find that the generalized random-phase approximation with exchange calculation models the shape and position of the combined experimental cross section data set rather well after incorporating experimental ionization energies and a shift in the photon energy scale.

Partial and total cross sections of electron capture processes from H by Be3+ have been calculated for the velocity range from 0.1 to 0.5 au (0.25-6.5 keV amu-1). A molecular-orbital expansion method including electron translation factors has been employed along with classical trajectories. The adiabatic potential energies and wavefunctions of the BeH3+ system were generated using an ab initio molecular-structure calculation method. The cross sections were calculated using ten-channel close-coupling treatments for each of the singlet and triplet states. Calculated total and partial cross sections are highly dependent on the collision energy. The calculations show that the electron is captured preferentially into the Be2+(1s2p) states. Rotational coupling plays an important role in the total cross sections, as well as the partial cross sections, for the electron capture processes.

We report absolute elastic differential cross sections for electron interactions with the C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c-C4F6). The incident electron energy range is 1.5-200 eV, and the scattered electron angular range for the differential measurements varies from 15° to 150°. In all cases the absolute scale of the differential cross section was set using the relative flow technique, with helium as the reference species. Atomic-like behaviour in these scattering systems is shown here for the first time, and is further investigated by comparing the elastic cross sections for the C4F6 isomers with other fluorinated molecules, such as SF6 and CnF6 (n = 2, 3, and 6). We note that for all the six-F containing molecules, the scattering process for electron energies above 30 eV is indistinguishable. Finally, we report results for calculations of elastic differential cross sections for electron scattering from each of these isomers, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements is found to be quite remarkable in all cases.