We probe the stability of water clusters by means of their metastable decay probability extracted from two-dimensional reflectron time-of-flight mass spectra. Two different methods are used to ionize and potentially excite the clusters and trigger the evaporation: (i) attachment of electrons with near-zero energies, producing negatively charged (H2O)n- clusters, and (ii) electron impact ionization, producing protonated (H2O)nH+ clusters. The electron attachment is a soft ionization and therefore provides information about the size distribution of the neutral clusters in the beam due to a very limited amount of post-ionization loss of water molecules. A dependence of metastable fractions on the conditions of neutral clusters production prior to the electron attachment is reported. For the cations, the higher energy electron impact ionization leads to a more extensive metastable loss of water molecules. The results are discussed in the light of neutral cluster excitation energy distributions and, for negative clusters, also in terms of binding energies. The experiments demonstrate clearly the role of the excess electron vs the excess proton in the two different charge states of the clusters around sizes N = 50-55, for which binding energies of the anions are derived from the data.

• Publication type
• Journal Article MeSH

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH3)N·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH3)n-1NH2- ions are produced via dissociative electron attachment (DEA) to NH3 molecules. (NH3)n- ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH3)n- ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH3)n·Bz- cluster ions exhibit essentially the same energy dependence. The (NH3)n·Bz- ions are metastable and evaporate NH3 molecule(s), while pure (NH3)n- ions are stable. The lifetime for NH3 molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz- ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH3 molecule(s).

• Publication type
• Journal Article MeSH

Metronidazole belongs to the class of nitroimidazole molecules and has been considered as a potential radiosensitizer for radiation therapy. During the irradiation of biological tissue, secondary electrons are released that may interact with molecules of the surrounding environment. Here, we present a study of electron attachment to metronidazole that aims to investigate possible reactions in the molecule upon anion formation. Another purpose is to elucidate the effect of microhydration on electron-induced reactions in metronidazole. We use two crossed electron/molecular beam devices with the mass-spectrometric analysis of formed anions. The experiments are supported by quantum chemical calculations on thermodynamic properties such as electron affinities and thresholds of anion formation. For the single molecule, as well as the microhydrated condition, we observe the parent radical anion as the most abundant product anion upon electron attachment. A variety of fragment anions are observed for the isolated molecule, with NO2- as the most abundant fragment species. NO2- and all other fragment anions except weakly abundant OH- are quenched upon microhydration. The relative abundances suggest the parent radical anion of metronidazole as a biologically relevant species after the physicochemical stage of radiation damage. We also conclude from the present results that metronidazole is highly susceptible to low-energy electrons.

• Keywords
• clusters, electron attachment, hydration, low-energy electron, metronidazole, radiosensitizer, reduction,
• Publication type
• Journal Article MeSH

We study the reactivity of misonidazole with low-energy electrons in a water environment combining experiment and theoretical modelling. The environment is modelled by sequential hydration of misonidazole clusters in vacuum. The well-defined experimental conditions enable computational modeling of the observed reactions. While the NO 2 - dissociative electron attachment channel is suppressed, as also observed previously for other molecules, the OH - channel remains open. Such behavior is enabled by the high hydration energy of OH - and ring formation in the neutral radical co-fragment. These observations help to understand the mechanism of bio-reductive drug action. Electron-induced formation of covalent bonds is then important not only for biological processes but may find applications also in technology.

• Keywords
• bond formation, clusters, electron attachment, low-energy electron, misonidazole,
• MeSH
• Electrons * MeSH
• Misonidazole chemistry   MeSH
• Models, Molecular MeSH
• Molecular Structure MeSH
• Solvents MeSH
• Spectrum Analysis MeSH
• Models, Theoretical MeSH
• Water MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Misonidazole MeSH
• Solvents MeSH
• Water MeSH

While matter is irradiated with highly-energetic particles, it may become chemically modified. Thereby, the reactions of free low-energy electrons (LEEs) formed as secondary particles play an important role. It is unknown to what degree and by which mechanism LEEs contribute to the action of electron-affinic radiosensitisers applied in radiotherapy of hypoxic tumours. Here we show that LEEs effectively cause the reduction of the radiosensitiser nimorazole via associative electron attachment with the cross-section exceeding most of known molecules. This supports the hypothesis that nimorazole is selectively cytotoxic to tumour cells due to reduction of the molecule as prerequisite for accumulation in the cell. In contrast, dissociative electron attachment, commonly believed to be the source of chemical activity of LEEs, represents only a minor reaction channel which is further suppressed upon hydration. Our results show that LEEs may strongly contribute to the radiosensitising effect of nimorazole via associative electron attachment.

• MeSH
• Chemoradiotherapy * MeSH
• Electrons * MeSH
• Humans MeSH
• Neoplasms therapy   MeSH
• Nimorazole chemistry   therapeutic use   MeSH
• Oxidation-Reduction MeSH
• Radiation-Sensitizing Agents chemistry   therapeutic use   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Nimorazole MeSH
• Radiation-Sensitizing Agents MeSH

THIS REVIEW SUMMARIZES SOME RECENT EXPERIMENTS WITH ICE NANOPARTICLES (LARGE WATER CLUSTERS) IN MOLECULAR BEAMS AND OUTLINES THEIR ATMOSPHERIC RELEVANCE: (1) Investigation of mixed water-nitric acid particles by means of the electron ionization and sodium doping combined with photoionization revealed the prominent role of HNO3 molecule as the condensation nuclei. (2) The uptake of atmospheric molecules by water ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. (3) Photodissociation of hydrogen halides on water ice particles has been shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H3O dissociation. The photodissociation of CF2Cl2 molecules in clusters is also mentioned. Possible atmospheric consequences of all these results are briefly discussed.

• Keywords
• aerosols, atmospheric chemistry, molecular beams, molecular dynamics, photochemistry, photodissociation, water clusters,
• Publication type
• Journal Article MeSH
• Review MeSH