Clusters in molecular beam experiments can mimic aerosol nanoclusters and provide molecular-level details for various processes relevant to atmospheric aerosol research. Aerosol nanoclusters, particles of sizes below 10 nm, are difficult to investigate in ambient atmosphere and thus represent a gap in our understanding of the new particle formation process. Recent field measurements and laboratory experiments are closing this gap; however, experiments with clusters in molecular beams are rarely involved. Yet, they can offer an unprecedented detailed insight into the processes including particles in this size range. In this Perspective, we discuss several up-to-date molecular beam experiments with clusters and demonstrate that the investigated clusters approach aerosol nanoclusters in terms of their complexity and chemistry. We examine remaining gaps between atmospheric aerosols and clusters in molecular beams and speculate about future experiments bridging these gaps.

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• časopisecké články MeSH
• přehledy MeSH

The dominant pathway of radiation damage begins with the ionization of water. Thus far, however, the underlying primary processes could not be conclusively elucidated. Here, we directly study the earliest steps of extreme ultraviolet (XUV)-induced water radiolysis through one-photon excitation of large water clusters using time-resolved photoelectron imaging. Results are presented for H2O and D2O clusters using femtosecond pump pulses centered at 133 or 80 nm. In both excitation schemes, hydrogen or proton transfer is observed to yield a prehydrated electron within 30 to 60 fs, followed by its solvation in 0.3 to 1.0 ps and its decay through geminate recombination on a ∼10-ps time scale. These results are interpreted by comparison with detailed multiconfigurational non-adiabatic ab-initio molecular dynamics calculations. Our results provide the first comprehensive picture of the primary steps of radiation chemistry and radiation damage and demonstrate new approaches for their study with unprecedented time resolution.

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• časopisecké články MeSH
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Polar stratospheric clouds, which consist mainly of nitric acid containing ice particles, play a pivotal role in stratospheric chemistry. We investigate mixed nitric acid-water clusters (HNO3)m(H2O)n, m ≈ 1-6, n ≈ 1-15, in a laboratory molecular beam experiment using electron attachment and mass spectrometry and interpret our experiments using DFT calculations. The reactions are triggered by the attachment of free electrons (0-14 eV) which leads to subsequent intracluster ion-molecule reactions. In these reactions, the nitrate anion NO3- turns out to play the central role. This contradicts the electron attachment to the gas-phase HNO3 molecule, which leads almost exclusively to NO2-. The nitrate containing clusters are formed through at least three different reaction pathways and represent terminal product ions in the reaction cascade initiated by the electron attachment. Besides, the complex reaction pathways represent a new hitherto unrecognized source of atmospherically important OH and HONO molecules.

• Publikační typ
• časopisecké články 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.

• Klíčová slova
• aerosols, atmospheric chemistry, molecular beams, molecular dynamics, photochemistry, photodissociation, water clusters,
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH