Absolute total and partial dissociative cross sections of pyrimidine at electron and proton intermediate impact velocities

Abstract

Absolute total non-dissociative and partial dissociative cross sections of pyrimidine were measured for electron impact energies ranging from 70 to 400 eV and for proton impact energies from 125 up to 2500 keV. MOs ionization induced by coulomb interaction were studied by measuring both ionization and partial dissociative cross sections through time of flight mass spectrometry and by obtaining the branching ratios for fragment formation via a model calculation based on the Born approximation. The partial yields and the absolute cross sections measured as a function of the energy combined with the model calculation proved to be a useful tool to determine the vacancy population of the valence MOs from which several sets of fragment ions are produced. It was also a key point to distinguish the dissociation regimes induced by both particles. A comparison with previous experimental results is also presented.

Ion induced fragmentation cross-sections of DNA constituents

“…The loss of one carbon or nitrogen atom would result in a mass group around 68 amu. As in previously reported studies on electron- [38][39][40], proton- [34] and photon-induced fragmentation [37,41], this group was absent in our spectra.…”

“…Mass 54 amu is assigned to the HCN fragment including the 13 C or 15 N isotope. Our data show a higher ratio than expected from the combined 3.9% isotopic abundance, probably because the contribution from the tail [34], the electron impact mass spectra of Linert (140 eV) [38], NIST [27] and of Wolff (400 eV) [34], and the photo-fragmentation mass spectrum of Vall-llosera (15.7 eV) [37]. of the 53 amu peak was not fully subtracted.…”

“…For 420 keV proton impact, corresponding to about 4 Bohr velocities, more intense fragmentation and hydrogen migration is observed compared to electron impact. For pyrimidine, it was discussed that the energy transferred to some deeper molecular orbitals decreases with the electron energy while it increases for protons, that have their cross section maximum at lower energies (at about 100 keV) [34]. Charge transfer would lead to even stronger fragmentation for multiple charged cation projectiles [16], but for singly charged ions, especially for protons, the charge transfer cross section is low [18,35].…”

“…The relative fragmentation cross sections are shown in Figure 6 for energies between 300 keV and 16.1 MeV. Between 125 keV and 2.5 MeV, fragmentation data for proton impact by Wolff [34] are added to the graph. The cross sections are normalized to the sum of all displayed fragments.…”

“…The data is compared to references [27,37,38]. In order to allow comparison to data by Wolff [34] the mass peak 79 amu from hydrogen loss and the isotopic mass peak 81 amu are added to the mass peak at 80 amu. The relative fragmentation cross sections are shown in Figure 6 for energies between 300 keV and 16.1 MeV.…”