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

Peter J. M. van der burgt, Francis Mahon, Gerard Barrett, Marcin, L. Gradziel

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    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.
    Original languageEnglish
    Article number151
    Number of pages9
    JournalThe European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics
    Early online date23 Jun 2014
    Publication statusPublished - Jun 2014


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