P. Scheier

On the unimolecular fragmentation of C60+ fullerene ions: The comparison of measured and calculated breakdown patterns

The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier‐type ion source/double‐focusing, sector‐field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time‐resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+−C2) of 7.1±0.4 ...

High resolution dissociative electron attachment to gas phase adenine.

The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.

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.

Free electron attachment to C60 and C70

Abstract Free electron attachment to C 60 and C 70 has been studied as a function of electron energy between about 0 and 14 eV. The attachment process yiels copious amounts of C − 60 and C − 70 (with an estimated attachment cross section approaching 10 −12 cm 2 , but neither fragment nor doubly charged anions are observed. The relative attachment cross section curves for C − 60 and C − 70 for energies up to 14 eV consist of broad continuum-type curves superimposed on which appear several resonance-like structures. The resonance at about zero electron energy is attributed to a nuclear-excited Feshbach resonance mechanism, whereas resonances at higher electron energies are attributed to electron-excited Feshbach resonances involving known one-electron π to π * transitions and collective excitations (plasmons). These resonances are broadened due to electron—phonon scattering, i.e. scattered electrons with non-zero energy are caged inside the fullerene. This multiple reaction sequence also accounts for the broad continuum beneath the resonance-like structures and is termed an electron/phonon-excited Feshbach resonance. The electronic structure of C − 60 , derived from the attachment spectrum obtained, is in accordance with theoretical predictions and various electron energy loss studies.

Electron attachment to gas-phase uracil.

We present results about dissociative electron attachment (DEA) to gas-phase uracil (U) for incident electron energies between 0 and 14 eV using a crossed electron/molecule beam apparatus. The most abundant negative ion formed via DEA is (U-H)-, where the resonance with the highest intensity appears at 1.01 eV. The anion yield of (U-H)- shows a number of peaks, which can be explained in part as being due to the formation of different (U-H)- isomers. Our results are compared with high level ab initio calculations using the G2MP2 method. There was no measurable amount of a parent ion U-. We also report the occurrence of 12 other fragments produced by dissociative electron attachment to uracil but with lower cross sections than (U-H)-. In addition we observed a parasitic contaminating process for conditions where uracil was introduced simultaneously with calibrant gases SF6 and CCl4 that leads to a sharp peak in the (U-H)- cross section close to 0 eV. For (U-H)- and all other fragments we determined rough measures for the absolute partial cross section yielding in the case of (U-H)- a peak value of sigma (at 1.01 eV)=3 x 10(-20) m2.

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.

The submersion of sodium clusters in helium nanodroplets: identification of the surface → interior transition.

The submersion of sodium clusters beyond a critical size in helium nanodroplets, which has recently been predicted on theoretical grounds, is demonstrated for the first time. Confirmation of a clear transition from a surface location, which occurs for alkali atoms and small clusters, to full immersion for larger clusters, is provided by identifying the threshold electron energy required to initiate Na(n) cluster ionization. On the basis of these measurements, a lower limit for the cluster size required for submersion, n ≥ 21, has been determined. This finding is consistent with the recent theoretical prediction.

Self‐consistent determination of fullerene binding energies BE (C+n–C2), n=58⋅ ⋅ ⋅44

Using recently measured accurate relative partial ionization cross section functions for production of the C60 fragment ions C+58 through C+44 by electron impact ionization, we have determined the respective binding energies BE(C+n–C2), with n=58,...,44, using a novel self‐consistent procedure. Appearance energies were determined from ionization efficiency curves. Binding energies were calculated from the corresponding appearance energies with the help of the finite heat bath theory. Then using these binding energies we calculated with transition state theory (TST), the corresponding breakdown curves, and compared these calculated ones with the ones derived from the measured cross sections. The good agreement between these breakdown curves proves the consistency of this multistep calculation scheme. As the only free parameter in this procedure is the binding energy C+58–C2, we studied the influence of different transition states chosen in the determination of this binding energy via TST theory and iterati...

Dissociative electron attachment to acetic acid (CH3COOH)

Abstract Dissociative electron attachment (DEA) to acetic acid in the energy range between about 0 and 13 eV generates as much as nine different fragment ions. The dominant products are CH3COO− and CH2O2− which appear from two closely spaced low energy resonances peaking at 0.75 and 1.5 eV. In view of our ab initio calculation we assign these states as single particle shape resonances associated with the first and second virtual MO, respectively. The electronic and geometrical structure of CH2O2− remains under question, the thermodynamics of the associated DEA reaction, however, predict an exceptionally high stability of this anion.

Appearance and ionization energies of singly, doubly and triply charged C60 and its fragment ions produced by electron impact ionization

Electron impact ionization and dissociation of C60 has been studied as a function of electron energy using a crossed molecular beam/electron beam double-focussing mass spectrometer system. Appearance energies have been determined (using a special threshold extrapolation procedure) for singly, doubly and triply charged parent ions Cz+60 and for singly, doubly and triply charged fragment ions Cz+60−2m with m ranging from 1 to 8. The first three appearance energies of C60 with values of 7.6 ± 0.5, 19.0 ± 0.5, and 35.6 ± 1.0 eV, respectively, are in excellent agreement with reliable data reported previously. Further conclusions about the dissociation mechanism are derived from the result that the difference in appearance energies for consecutive fragment ions of C60 is nearly constant and independent of the charge state (yielding an average value 5.7 ± 1.3 eV) and the ionization mechanism.