Electron impact fragmentation of thymine: partial ionization cross sections for positive fragments

Abstract

We have measured mass spectra for positive ions for low-energy electron impact on thymine using a reflectron time-of-flight mass spectrometer. Using computer controlled data acquisition, mass spectra have been acquired for electron impact energies up to 100 eV in steps of 0.5 eV. Ion yield curves for most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The ion yield curves have been normalized by comparing the sum of the ion yields to the average of calculated total ionization cross sections. Appearance energies have been determined. The nearly equal appearance energies of 83 u and 55 u observed in the present work strongly indicate that near threshold the 55 u ion is formed directly by the breakage of two bonds in the ring, rather than from a successive loss of HNCO and CO from the parent ion. Likewise 54 u is not formed by CO loss from 82 u. The appearance energies are in a number of cases consistent with the loss of one or more hydrogen atoms from a heavier fragment, but 70 u is not formed by hydrogen loss from 71 u.

Electron impact fragmentation of adenine: partial ionization cross sections for positive fragments

“…A detailed description of the experimental set-up and the data analysis has been given by van der Burgt et al [20] and only a brief overview will be given here. An effusive beam of adenine molecules is produced by heating an oven containing adenine powder (99% purity from Sigma Aldrich) to a temperature of 200…”

Communication: Electron ionization of DNA bases

“…On the experimental side, Burgt et al measured the relative cross sections up to 100 eV and normalized them to absolute values using the average value of various theoretical results at 70 eV. [20][21][22] So far, there exists only one set of absolute cross section measurements. [23][24][25][26] These measurements, carried out up to 200 eV of impact energies, vary from theory by a factor of 0.5-2 depending on the molecule.…”

“…3 shows experimentally determined total ionization cross sections from threshold to 500 eV. The available theoretical [13][14][15][16][17][18][19] and measured [20][21][22][23][24][25][26] cross sections are also shown in the figure. The measurements [23][24][25][26] used a crossed beam method and the absolute cross sections were obtained directly by measuring the total ion current, the target beam density, and the volume overlap between the beams using the geometrical details of the setup.…”

Electron and positron induced processes. POSMOL 2013

“…Finally, we note a set of contributions on electron impact, positron and positronium formation on: (a) "Electron impact fragmentation of thymine: partial ionization cross sections for positive fragments" [31]; (b) "Gas breakdown and secondary electron yields" [32]; (c) "Monte Carlo analysis of ionization effects on spatiotemporal electron swarm development" [33]; (d) "Optimisation of the thickness of the moderator for positron annihilation process study in Ar gas" [34]; (e) "Lowenergy positron scattering from gas-phase uracil" [35]; (f) "Brightness enhancement of a linac-based intense positron beam for total-reflection high-energy positron diffraction (TRHEPD)" [36]; (g) "Interaction of positronium with helium atoms -the classical treatment of the 5-body collision system" [37].…”