Stabilization of Phenanthrene Anions in Helium Nanodroplets

It has been debated for years if the polycyclic aromatic hydrocarbon phenanthrene exists in its anionic form, or, in other words, if its electron affinity (EA) is positive or negative. In this contribution we confirm that the bare phenanthrene anion Ph- created in a binary collision with an electron at room temperature has a lifetime shorter than microseconds. However, the embedding of neutral phenanthrene molecules in negatively charged helium nanodroplets enables the formation of phenanthrene anions by charge transfer processes and the stabilization of the latter in the ultracold environment. Gentle shrinking of the helium matrix of phenanthrene-doped HNDs by collisions with helium gas makes the bare Ph- visible by high-resolution mass spectrometry. From these and previous measurements we conclude, that the EA of phenanthrene is positive and smaller than 24.55 meV

Hydroperoxyl radical and formic acid formation from common DNA stabilizers upon low energy electron attachment.

2-Amino-2-(hydroxymethyl)-1,3-propanediol (TRIS) and ethylenediaminetetraacetic acid (EDTA) are key components of biological buffers and are frequently used as DNA stabilizers in irradiation studies. Such surface or liquid phase studies are done with the aim to understand the fundamental mechanisms of DNA radiation damage and to improve cancer radiotherapy. When ionizing radiation is used, abundant secondary electrons are formed during the irradiation process, which are able to attach to the molecular compounds present on the surface. In the present study we experimentally investigate low energy electron attachment to TRIS and methyliminodiacetic acid (MIDA), an analogue of EDTA, supported by quantum chemical calculations. The most prominent dissociation channel for TRIS is through hydroperoxyl radical formation, whereas the dissociation of MIDA results in the formation of formic and acetic acid. These compounds are well-known to cause DNA modifications, like strand breaks. The present results indicate that buffer compounds may not have an exclusive protecting effect on DNA as suggested previously

Ion formation upon electron collisions with valine embedded in helium nanodroplets

We report here experimental results for the electron ionization of large superfluid helium nanodroplets with sizes of about 105 atoms that are doped with valine and clusters of valine. Spectra of both cations and anions were monitored with high-resolution time-of-flight mass spectrometry (mass resolution >4000). Clear series of peaks with valine cluster sizes up to at least 40 and spaced by the mass of a valine molecule are visible in both the cation and anion spectra. Ion efficiency curves are presented for selected cations and anions at electron energies up to about 40 eV and these provide insight into the mode of ion formation. The measured onset of 24.59 eV for cations is indicative of valine ionization by He+ whereas broad resonances at 2, 10 and 22 eV (and beyond) in the formation of anions speak to the occurrence of various modes of dissociative electron attachment by collisions with electrons or He*− and the influence of droplet size on the relative importance of these processes. Comparisons are also made with gas phase results and these provide insight into a matrix effect within the superfluid helium nanodroplet

Experimental evidence for the existence of an electronically excited state of the proposed dihydrogen radical cation He-H-H-He

In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J. 2008. 14, 4028) proposed the existence of, a flew class of, radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H(2) molecules we observe various He(x)H(y)(+) cluster ions, including He(2)H(2)(+), which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH(2)(+) + He with a low kinetic energy release of 15 4 meV. Surprisingly, another unimolecular reaction is Observed. into HeH(+) + HeH (or He + H). The probability of this reaction is ail order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited stated they arc consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He(+)

Dissociative Electron Attachment to β-Alanine

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Identification of isomers of nitrotoluene via free electron attachment

Free electron attachment to the three different isomers of mononitrotoluene molecules in the gas phase is studied using a crossed electron−molecule beams technique. In contrast to previous studies for a large number of negative ions, the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO2-, remarkable differences for the three isomers are observed. In almost all fragment anion efficiency curves, the 2-nitrotoluene exhibits pronounced differences from the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other

On the size of ions solvated in helium clusters

Helium nanodroplets are doped with SF(6), C(4)F(8), CCl(4), C(6)H(5)Br, CH(3)I, and I(2). Upon interaction with free electrons a variety of positively and negatively charged cluster ions X(+/-) He(n) are observed where X(+/-) = F(+/-), Cl(+/-), Br(+/-), I(+), I(2)(+), or CH(3)I(+). The yield of these ions versus cluster size n drops at characteristic sizes n(s) that range from n(s)=10.2 +/- 0.6 for F(+) to n(s) = 22.2 +/- 0.2 for Br(-). n(s) values for halide anions are about 70% larger than for the corresponding cations. The steps in the ion yield suggest closure of the first solvation shell. We propose a simple classical model to estimate ionic radii from n(s). Assuming the helium density in the first solvation shell equals the helium bulk density one finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction. In spite of the simplicity of our model, anion radii derived from it agree within approximately 10% with values derived from the mobility of halide anions in superfluid bulk helium, and with values computed by quantum Monte Carlo methods for X(-)He(n) cluster anions

Hydrogen loss in aminobutanoic acid isomers by the sigma* resonance formed in electron capture

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