Nonthermal secondary electrons with initial kinetic energies below 100 eV are an abundant transient species created in irradiated cells and thermalize within picoseconds through successive multiple energy loss events. Here we show that below 15 eV such low-energy electrons induce single (SSB) and double (DSB) strand breaks in plasmid DNA exclusively via formation and decay of molecular resonances involving DNA components (base, sugar, hydration water, etc.). Furthermore, the strand break quantum yields (per incident electron) due to resonances occur with intensities similar to those that appear between 25 and 100 eV electron energy, where nonresonant mechanisms related to excitation/ionizations/dissociations are shown to dominate the yields, although with some contribution from multiple scattering electron energy loss events. We also present the first measurements of the electron energy dependence of multiple double strand breaks (MDSB) induced in DNA by electrons with energies below 100 eV. Unlike the SSB and DSB yields, which remain relatively constant above 25 eV, the MDSB yields show a strong monotonic increase above 30 eV, however with intensities at least 1 order of magnitude smaller than the combined SSB and DSB yields. The observation of MDSB above 30 eV is attributed to strand break clusters (nano-tracks) involving multiple successive interactions of one single electron at sites that are distant in primary sequence along the DNA double strand, but are in close contact; such regions exist in supercoiled DNA (as well as cellular DNA) where the double helix crosses itself or is in close proximity to another part of the same DNA molecule.

Single, double, and multiple double strand breaks induced in DNA by 3-100 eV electrons.

/pdf/fakultat-fur-physik-und-astronomie-2db3tgabsl.pdf

Fakultät für Physik und Astronomie

The Neutral Gas and Ion Mass Spectrometer (NGIMS) of the Mars Atmosphere and Volatile Evolution Mission (MAVEN) is designed to measure the composition, struc- ture, and variability of the upper atmosphere of Mars. The NGIMS complements two other instrument packages on the MAVEN spacecraft designed to characterize the neutral upper atmosphere and ionosphere of Mars and the solar wind input to this region of the atmo- sphere. The combined measurement set is designed to quantify atmosphere escape rates and provide input to models of the evolution of the martian atmosphere. The NGIMS is designed to measure both surface reactive and inert neutral species and ambient ions along the space- craft track over the 125-500 km altitude region utilizing a dual ion source and a quadrupole analyzer.

/pdf/the-neutral-gas-and-ion-mass-spectrometer-on-the-mars-5e76tm1l09.pdf

The Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution Mission

Electron impact dissociation of oxygen-containing molecules A critical review

The basic ideas of the B-spline R-matrix (BSR) approach are reviewed, and the use of the method is illustrated with a variety of applications to atomic structure, electron?atom collisions and photo-induced processes. Special emphasis is placed on complex, open-shell targets, for which the method has proven very successful in reproducing, for example, a wealth of near-threshold resonance structures. Recent extensions to a fully relativistic framework and intermediate energies have allowed for an accurate treatment of heavy targets as well as a fully nonperturbative scheme for electron-impact ionization. Finally, field-free BSR Hamiltonian and electric dipole matrices can be employed in the time-dependent treatment of intense short-pulse laser?atom interactions.

https://iopscience.iop.org/article/10.1088/0953-4075/46/11/112001/pdf

The B-spline R-matrix method for atomic processes: application to atomic structure, electron collisions and photoionization

Accurate values of the electron-impact ionization cross sections for the rare gases are needed in a variety of contexts. However, despite numerous investigations over many decades, uncertainty as to the correct values has persisted. The pioneering total-cross-section measurements of Rapp and Englander-Golden are generally regarded as the most reliable but no comprehensive study has independently verified their correctness. In this paper, measurements of electron-impact ionization cross sections of helium, neon, argon, krypton, and xenon are reported for energies ranging from the first ionization threshold to 1000 eV. These data confirm the essential correctness of Rapp and Englander-Golden's total measurements and at the same time provide a complete set of consistent absolute partial cross sections.

Determination of the absolute partial and total cross sections for electron-impact ionization of the rare gases

Absolute partial and total cross sections for electron-impact ionization of methanol, ethanol, and 1-propanol are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in most of the absolute cross sections is ±6%–8%. Significant discrepancies are seen between the only prior methanol partial cross section determination and this study but the majority of published total cross section measurements and calculations are in good agreement with this work.

Electron-impact ionization of the simple alcohols

Absolute partial cross sections for electron-impact ionization of SF6 are reported for electron energies from threshold to 1000 eV. The product ions are mass analysed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of SF5+, SF4+, SF3+, SF2+, (SF+ + SF42+), S+, F+, SF32+, SF22+ and SF2+ and for the total cross section, which is obtained as the sum of the partial cross sections. The overall uncertainty in the absolute cross sections for singly charged ions, except S+, is ±5%; that for S+ is ±8%. The uncertainty in the cross sections for doubly charged ions is ±12 to 15%. Data are also presented for formation of (SFn+, F+) ion pairs. Comparison is made with prior experiments and calculations.

Absolute partial cross sections for electron-impact ionization of SF6 from threshold to 1000 eV

Absolute partial and total cross sections for electron-impact ionization of NH3 and ND3 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of (NH3++NH2++NH++N+), H+, H2+, and NH32+ from NH3 and for production of ND3+, ND2+, ND+, N+, D+, D2+, and ND32+ from ND3 and for the total cross sections which are obtained as the sum of these partial cross sections. Data are also presented for formation of (ND+,D+) and (N+,D+) ion pairs. The overall uncertainty in the absolute cross sections for most of the singly charged ions is ±6–8% while that for doubly charged ions is ±20%. It is observed that the isotopic composition of the ammonia target has no discernible effect upon the cross sections and that the part...

Electron-impact ionization of NH3 and ND3

Absolute partial and total cross sections for electron-impact ionization of N2O, H2S, and CS2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for production of N2O+, N2+, NO+, N+, and O+ from N2O; for production of (H2S++HS++S+) and H+ from H2S; and for production of CS2+, S2+, CS+, S+, C+, and CS22+ from CS2. The total cross sections are obtained as the sum of these partial cross sections. The overall uncertainty in the absolute cross sections for most of the singly-charged ions is ±6–8%. For all three targets the cross sections for production of lighter fragment ions are found to be greater than previously reported which is consistent with the presumption of incomplete collection of energetic ions in the earlier ...

R. Rejoub

Papers

Determination of the absolute partial and total cross sections for electron-impact ionization of the rare gases

Electron-impact ionization of the simple alcohols

Absolute partial cross sections for electron-impact ionization of SF6 from threshold to 1000 eV

Electron-impact ionization of NH3 and ND3

Absolute cross sections for electron-impact ionization of N2O, H2S, and CS2 from threshold to 1000 eV