Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

Absolute total and partial cross-sections for the electron impact ionization of TiCl4

Dissociative ionization of silane by electron impact

Dissociative-ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron-impact excitation of jet-cooled NH3, hydrazine (N2H4), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time-of-flight mass detection to ensure field-free excitation and high collection efficiency for energetic ions. Cross sections for NH3 ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument-dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N2H4 and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å2 (NH3), 3.76 Å2 (N2H4), and 4.20 Å2 (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time-of-flight peak-shape analysis. Mean KE values <εt≳ are reported for all fragment ions studied. For 170-eV excitation of NH3, <εt≳ varied from 0.026 eV (NH2+) to 1.4 eV (H+). The kinetic energies for N2H4 and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH3, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N2H4 and MMH dissociative ionization based on the appearance of ions such as NH3+ and NH4+ (the latter for MMH ionization only) and the magnitude of <εt≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N2H4+.

Determination of absolute partial and total electron impact ionization cross-sections for CF 2Cl 2 from threshold up to 180 eV: an improved experimental method

We report measurements of the absolute cross sections for the electron-impact ionization of SO2 from threshold to 200 eV. Absolute cross sections for the formation of the SO+2 parent ions and of the SO+, S+, O+, and O+2 fragment ions were obtained independently in two different laboratories using two different experimental techniques with uncertainties ranging from ±18% to ±25%. The level of agreement between the absolute cross sections (at 70 eV) obtained by the two techniques ranges from about 10% for SO+2 and SO+ to 20% for (S++O+2) and O+, which in all cases is well within the combined error margins of the two measurements. The high resolution capability of the mass spectrometer employed in one experiment enabled the separation of the S+ and O+2 fragment ions, which are separated by only 0.017 76 atomic mass units (amu), for the first time. The single positive ion formation is the dominant process for all observed product ions. The total single SO2 ionization cross section obtained by the two techniques agreed to within 8%. A comparison of the experimentally determined total SO2 single ionization cross sections with calculated cross sections based on a modified additivity rule revealed agreement to within 20%.

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.

Ultraviolet photodissociation (UVPD) mass spectrometry has gained attention in recent years for its ability to provide high sequence coverage of intact proteins. However, secondary dissociation of fragment ions, in which fragment ions subjected to multiple laser pulses decompose into small products, is a common phenomenon during UVPD that contributes to limited coverage in the midsection of protein sequences. To counter secondary dissociation, a method involving the application of notched waveforms to modulate the trajectories of fragment ions away from the laser beam, termed fragment ion protection (FIP), was previously developed to reduce the probability of secondary dissociation. This, in turn, increased the number of identified large fragment ions. In the present study, FIP was applied to UVPD of large proteins ranging in size from 29 to 55 kDa, enhancing the abundances of large fragment ions. A stepped-FIP strategy was implemented in which UVPD mass spectra were collected using multiple different amplitudes of the FIP waveforms and then the results from the mass spectra were combined. By using stepped-FIP, the number of fragment ions in the midsections of the sequences increased for all proteins. For example, whereas no fragment ions were identified in the middle section of the sequence for glutamate dehydrogenase (55 kDa, 55+ charge state), 10 sequence ions were identified by using UVPD-FIP.

Electron ionization and photoionization of cyclopropylamine

Threshold electron impact ionization studies of uracil

The relativistic distorted-wave and binary-encounter-dipole approximations were employed for calculating the electron-impact single ionization cross sections of the 5s, 4p6, 4s2, 3d10 shells and 4p6 excitation cross section for Rb atom taking into account both configuration interaction and relativistic effects. The capabilities of the most used theoretical approaches in describing the single ionization of Rb atom were considered by comparing the present and other available calculated data with the experimental total ionization and total direct single ionization cross sections over the electron-impact energy range from the 5s threshold to 600 eV. The best agreement within experimental uncertainty was obtained by using the non-relativistic binary-encounter-dipole approximation included in the LANL Atomic Physics Codes package. At present none of the used approximations (plane-wave Born or relativistic distorted wave) can satisfactorily describe the experimental excitation-autoionization cross section in rubidium because the efficient formation of the 4p6 core-excited negative-ion rubidium states at near-threshold impact energies is ignored in calculations.

Differential elastic electron scattering cross sections of trichlorofluoromethane (CCl3F) were measured over a broad energy range spanning 30 to 800 eV in the angular range of 20° to 150°. The experimental results were compared with calculations using the IAM-SCAR+I model. Satisfactory agreements between both data sets were found for electron energies above 200 eV within experimental uncertainties, whereas significant deviations of up to 100% were observed at electron energies below 60 eV. In addition to the measurements of differential elastic-scattering cross sections, total inelastic-scattering cross sections of CCl3F were calculated using the spherical complex optical potential model. These calculations closely match experimental total ionization cross sections available in the literature for energies below 50 eV. The sum of the experimental total elastic-scattering cross sections and the theoretical total inelastic-scattering cross sections aligns very well with the total electron scattering cross sections of CCl3F measured by other groups across the entire energy range (30 to 800 eV), demonstrating the consistency among these three cross sections. Published by the American Physical Society 2025

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.

The dissociative photoionization of methyl vinyl ketone (MVK), an important intermediate in the atmospheric oxidation of isoprene, has been studied by photoelectron photoion coincidence spectroscopy. In the photon energy range of 9.5-13.8 eV, four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C-C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO-loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed herein for gaseous methyl vinyl ketone (ΔfH0K = -94.3 ± 4.8 kJ mol-1; ΔfH298K = -110.5 ± 4.8 kJ mol-1), together with cationic heats of formation and bond dissociation energies.

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time-of-flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C6 H9 (+) , C6 H7 (+) , C5 H7 (+) , C5 H5 (+) , C4 H6 (+) , C4 H5 (+) , C3 H5 (+) and C3 H3 (+) were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X-D/6-31+g(d,p) level. The coupled cluster method, CCSD(T)/cc-pVTZ, was employed to calculate the corresponding energies with the zero-point energy corrections by the ωB97X-D/6-31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro-Cope rearrangement was found to play a crucial role in the formation of C4 H6 (+) , C4 H5 (+) and C3 H5 (+) . Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8- to 15.5-eV photon energy region.

The ionization potentials of methyl ethers of D‐homoequilenine, its 14ß‐isomer, D‐homoestrone and its 8°‐isomer, as well as appearance potentials of certain fragment ions have been determined by means of electron‐impact. The dissociation energies of the bonds which are split in the course of formation of these ions have been calculated.