Eugen Illenberger

RESONANT DISSOCIATION OF DNA BASES BY SUBIONIZATION ELECTRONS

We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T− and C−. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.

Negative ion formation in CF2Cl2, CF3Cl and CFCl3 following low energy (0—10 eV) impact with near monoenergetic electrons

Abstract Negative ion formation in CF 2 Cl 2 , CF 3 Cl and CFCl 3 under low-energy electron impact has been investigated using a trochoidal monochromat The ions observed are F − , Cl − , FCl − , Cl 2 − , CFCl 2 − from CF 2 Cl 2 ; F − , Cl − , FCl − , CF 2 Cl Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance The extremely high yield of Cl − in CFCl 3 , which is observed at e = 0.0 eV, will be discussed with regard to the lifetime of this molecule i

THE REACTIVITY OF SLOW ELECTRONS WITH MOLECULES AT DIFFERENT DEGREES OF AGGREGATION : GAS PHASE, CLUSTERS AND CONDENSED PHASE

The interaction of free electrons in the energy range between 0 and 15 eV with molecules is reviewed. The studies include beam experiments with gas phase molecules under collision-free conditions and at higher pressures, homogeneous and heterogeneous clusters generated in supersonic beams, and molecules adsorbed and condensed on a cold metallic surface. In single molecules under collision-free conditions the usual relaxation process of a transient negative ion formed by free electron attachment is unimolecular decomposition into stable anionic and neutral fragments (dissociative attachment) with cross-sections exceeding 10−14 cm2. A remarkable exception is C60 which forms metastable anions C60− up to 14 eV electron energy. In clusters, intramolecular stabilization processes lead to the formation of stabilized molecular anions. In addition, intramolecular electron transfer processes can strongly contribute to anion formation in homogeneous and heterogeneous clusters. In condensed and adsorbed molecules, finally, effective desorption of negative fragment ions is observed when repulsive precursor ions are involved. The reactivity of transient anions formed by low energy attachment is generally strongly quenched with the degree of aggregation in favour of associative processes. In the case of core excited resonances, however, the reactivity can be enhanced by the surrounding medium. This effect is explained by the conversion of an open channel resonance in isolated molecules into a Feshbach resonance in clusters and condensed molecules.

Formation of C60- and C70- by free electron capture. Activation energy and effect of the internal energy on lifetime

Abstract Electron attachment to C 60 − and C 70 − yields long-lived (i.e. mass spectrometrically observable) anions C 60 − and C 70 − within a remarkably broad energy region (up to 13–14 eV) as already established in a previous experiment. Inspection of the low energy region reveals, however, that electron capture is characterized by an activation barrier of 240 meV for C 60 close to the results of the previous FALP study and 200 meV for C 70 . Flight time measurements of the anions indicate that the lifetimes of C 60 − and C 70 − decrease exponentially with increasing internal energy in the 8⩽ E * ⩽12 eV. For example, at 10 eV the lifetime is 90 μs for C 60 − and 300 μs for C 70 − . It is also established that the lifetime of both anions is controlled by electron emission (detachment).

Dissociative electron attachment to furan, tetrahydrofuran, and fructose

We study dissociative electron attachment to furan (FN) (C(4)H(4)O), tetrahydrofuran (THF) (C(4)H(8)O), and fructose (FRU) (C(6)H(12)O(6)) using crossed electron/molecular beams experiments with mass spectrometric detection of the anions. We find that FN and THF are weak electron scavengers and subjected to dissociative electron attachment essentially in the energy range above 5.5 eV via core excited resonances. In striking contrast to that, FRU is very sensitive towards low energy electrons generating a variety of fragment ions via a pronounced low energy feature close to 0 eV. These reactions are associated with the degradation of the ring structure and demonstrate that THF cannot be used as surrogate to model deoxyribose in DNA with respect to the attack of electrons at subexcitation energies (<3 eV). The results support the picture that in DNA the sugar moiety itself is an active part in the initial molecular processes leading to single strand breaks.

Associative and dissociative electron attachment by SF6 and SF5Cl

Electron attachment by SF6 and SF5Cl in the energy range 0–20 eV has been studied in a beam experiment at room temperature. At low energies (≊0 eV) electron attachment to SF6 yields the well known parent anion SF−*6 (associative attachment) and SF−5 (dissociative attachment), while other negative ion fragments (F−, F−2 , SF−2 , SF−3 , and SF−4) are generated with comparably low cross sections from various resonances at higher energies. In contrast to that, negative ion formation in SF5Cl is dominated by dissociative channels (F−, Cl−, FCl−, and SF−5 ) and only a weak SF5Cl− signal is observed. A time‐of‐flight analysis of the ionic fragments reveals that the decomposition of all resonances is characterized by a low translational excess energy release indicating effective energy randomization in the parent ion prior to dissociation. The present results are compared with negative ion formation in halogenated hydrocarbons.

Formation of anion fragments from gas-phase glycine by low energy (0–15 eV) electron impact

We have measured the formation of anion fragments in gas phase glycine (H2NCH2COOH) via dissociative electron attachment (DEA) reactions in the 0–15 eV electron energy range, using a monochromatic electron beam and mass spectrometric detection of the negative ions. By far the most intense product observed is the closed shell glycine anion (H2NCH2COO)− which appears from a low-energy resonance with a peak located at 1.4 eV and a cross section in the range 10−16 cm2. The corresponding precursor ion can be characterized by electron attachment into the empty π* orbital of the −COOH group as recently assigned from electron transmission experiments and ab initio self-consistent field calculations [Aflatooni, Hitt, Gallup, and Burrow, J. Chem. Phys. 115, 6489 (2001)]. This precursor state is also observed to decompose (with much lower intensity) yielding a negative ion fragment with 58 amu, which is attributed to anions of the stoichiometric composition H2C2O2− or H4C2NO−. A further prominent DEA peak is observe...

Dissociative electron attachment to gas-phase 5-bromouracil

We report measurements of dissociative electron attachment (DEA) to gaseous 5-bromouracil (BrU) for incident electron energies between 0 and 16 eV. Low energy electron impact on BrU leads not only to the formation of a long lived parent anion BrU−, but also various anion fragments resulting from endo- and exo-cyclic bond ruptures, such as Br−, uracil-yl anions, i.e., (U-yl)−, OCN−, and a 68 amu anion tentatively attributed to H2C3NO−. The incident electron energy dependent signatures of either the Br− and (U-yl)− yields (at 0, 1.4, and 6 eV), or the OCN− and H2C3NO− yields (at 1.6 and 5.0 eV) suggests competing DEA channels for anion fragment formation. The production cross sections, at 0 eV incident electron energy, for BrU−, Br−, and (U-yl)− are estimated to be about 6×10−15, 6×10−14, and 1.0×10−15 cm2, respectively.

Bond selective dissociative electron attachment to thymine

Free-electron attachment to thymine and partially deuterated thymine, where D replaces H at all carbon atoms, is studied in the electron energy range from about 0 to 15 eV. The formation of fragment anions that are formed by the loss of one or two H (D) atoms is analyzed as a function of the incident electron energy using a crossed electron/neutral beam apparatus in combination with a quadrupole mass spectrometer. By using partially deuterated thymine and quantum-chemical calculation a bond selectivity for the loss of one and two hydrogen atoms is observed that is determined only by the kinetic energy of the incident electron.

Dissociative electron attachment to formic acid (HCOOH)

Dissociative electron attachment to formic acid as a fundamental center in enzymatic activity is studied. A prominent resonance is observed peaking at 1.25 eV which decomposes into the formate anion HCOO− and a hydrogen radical. Resonances at higher energy are associated with O− and OH− formation on a considerably smaller intensity scale. On the basis of high level ab initio calculations, the low energy feature arises from different closely spaced single particle shape resonances with no specific valence character. The HCOO− ion yield carries structure which is tentatively ascribed to vibrational excitation in the formate anion.