Attachment of Water and Alcohol Molecules onto Water and Alcohol Clusters

Proton transfer from pinene stabilizes water clusters

Molecular beams experiments and ab initio theory reveal indirect formation of protonated water clusters by ionization of pinene.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

Uptake of methanol on mixed HNO3/H2O clusters: An absolute pickup cross section

The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CH₃OH) on mixed nitric acid-water clusters (HNO₃)_M(H₂O)_N by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO₃)_m(H₂O)_nH⁺ series with m = 0-3 and n = 0-12. In addition, CH₃OH·(HNO₃)_m(H₂O)_nH⁺ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO₃)_m(H₂O)_nH⁺ mass peaks in the pickup spectra originate from the neutral (HNO₃)_M(H₂O)_N clusters which have not picked up any CH₃OH molecule, i.e., methanol has not evaporated upon the ionization. Thus the fraction of the doped clusters can be determined and the mean pickup cross section can be estimated, yielding σ_s¯≈ 20 Ų. This is compared to the lower estimate of the mean geometrical cross section σ_g¯≈ 60 Ų obtained from the theoretical cluster geometries. Thus the "size" of the cluster corresponding to the methanol pickup is at least 3-times smaller than its geometrical size. We have introduced a method which can yield the absolute pickup cross sections relevant to the generation and growth of atmospheric aerosols, as illustrated in the example of methanol and nitric acid clusters.

A gas aggregation source for the production of heterogeneous molecular clusters

We present the design of a versatile gas aggregation source that allows producing molecular beams of charged clusters containing a controlled amount of chosen impurities. Several examples of clusters production using this source characterized by time of flight mass spectrometry are presented here. We demonstrate the source ability to produce homogeneous clusters, such as pure protonated water and alcohol clusters, as well as inhomogeneous ones such as water clusters containing a few units of uracil, glycine, sulfuric acid, or pyrene.