J. Harnes

The local structure of small water clusters: imprints on the core-level photoelectron spectrum

We report on an O 1s photoelectron-spectroscopy study of small neutral water clusters produced by adiabatic expansion. The photoelectron spectra were acquired under two different experimental conditions. At intermediate resolution, the cluster signal was characterized by a very broad O 1s peak with a flat top. In the second set of measurements, resolution was significantly increased at the cost of lower count rates. The cluster signal was now partly resolved into a bimodal structure. Extensive theoretical calculations were undertaken to facilitate an interpretation of the spectrum. These results suggest that the bimodal feature may be ascribed to ionization of water molecules in different hydrogen-bonding configurations, more specifically, molecules characterized by donation of either one or both hydrogen atoms in H-bonding.

Size of neutral argon clusters from core-level photoelectron spectroscopy

Theoretical models of lineshapes in Ar2p photoionization spectra have been calculated for free, neutral argon clusters of different sizes. The lineshape models are fitted to experimental spectra and used to estimate the mean cluster size realized in the experiment. The results indicate that size estimators working from stagnation conditions [R. Karnbach, M. Joppien, J. Stapelfeldt, J. Wörmer and T.Möller, Rev. Sci. Instrum., 1993, 64, 2838] may underestimate the mean cluster size.

Size of Free Neutral CO2 Clusters from Carbon 1s Ionization Energies

Free neutral CO(2) clusters were produced by adiabatic expansion and characterized by carbon 1s (C1s) photoelectron spectroscopy using synchrotron radiation. The shift in C1s ionization energy (IE) between the cluster and the monomer, i.e., ΔIE = IE(cluster) - IE(monomer), was found to vary systematically with the experimental conditions. A functional relationship is established between the mean cluster size in the beam, , and ΔIE, in good agreement with theoretical calculations of shifts in ionization energy for model clusters. This makes it possible to use core-level photoelectron spectroscopy to monitor the mean cluster size and also to estimate from expansion conditions.

Structure of neutral nanosized clusters produced by coexpansion of CF4 and CH4.

Mixed CH(4)/CF(4) clusters as well as pure clusters of CF(4) were produced by adiabatic expansion and studied by carbon 1s (C1s) X-ray photoelectron spectroscopy. Evidence is presented that CH(4) and CF(4) do indeed form binary clusters in CH(4)/CF(4) coexpansion experiments and that these clusters exhibit radial structure; i.e., CF(4) is primarily found in the bulk. The interpretation of the photoelectron spectra is supported by calculations of C1s ionization energies based on theoretical clusters models.

Electron attenuation in free, neutral ethane clusters

The electron effective attenuation length (EAL) in free, neutral ethane clusters has been determined at 40 eV kinetic energy by combining carbon 1s x-ray photoelectron spectroscopy and theoretical lineshape modeling. More specifically, theory is employed to form model spectra on a grid in cluster size (N) and EAL (λ), allowing N and λ to be determined by optimizing the goodness-of-fit χ(2)(N, λ) between model and observed spectra. Experimentally, the clusters were produced in an adiabatic-expansion setup using helium as the driving gas, spanning a range of 100-600 molecules in mean cluster size. The effective attenuation length was determined to be 8.4 ± 1.9 Å, in good agreement with an independent estimate of 10 Å formed on the basis of molecular electron-scattering data and Monte Carlo simulations. The aggregation state of the clusters as well as the cluster temperature and its importance to the derived EAL value are discussed in some depth.

Structure of self-assembled free methanol/tetrachloromethane clusters.

The structure of molecular clusters of diameters at or below a nanometer is important both in nucleation phenomena and potentially for the preparation and application of nanoparticles. Little is known about the relationship between the structure and composition of the cluster and about the interplay between cluster composition, size, and temperature. The present project explores how the structure of mixed CH3OH/CCl4 clusters vary with composition and size; implicitly by changing the amount of noncondensing backing gas and thus the capacity to remove heat during cluster condensation, and explicitly through theoretical models. Experimentally, molecular clusters were produced by coexpansion of helium and a vapor of azeotropic methanol/tetrachloromethane composition in a supersonic nozzle flow. The clusters were subsequently characterized by means of carbon 1s photoelectron spectroscopy. Additional information was obtained by molecular-dynamics simulations of clusters at 3 different sizes, 4 different compositions and several temperatures, and using polarizable force fields. Mixed clusters were indeed obtained in the coexpansion experiments. The clusters show an increasing degree of surface coverage by methanol as the backing pressure is lowered, and at the lowest helium pressure the cluster signal from tetrachloromethane has almost vanished. The MD simulations show a gradual change in cluster structure with increasing methanol contents, from that of isolated rings of methanol at the surface of a tetrachloromethane core, to a contiguous methanol cap covering more than half of the cluster surface, to that of subclusters of tetrachloromethane submerged in a methanol environment. Both experimental and computational results support a thermodynamical driving force for methanol to dominate the surface structure of the mixed clusters. At high helium pressure, the growing clusters may cool efficiently, possibly impeding the diffusion of methanol to the surface. At low helium pressure, methanol is completely dominating the outermost few layers of the clusters, possibly in parts caused by preferential loss of tetrachloromethane through evaporative cooling.

Formation and Growth of Clusters of Sulfur Dioxide

Formation and growth of neutral SO2 clusters is investigated in an adiabatic-expansion setup by means of sulfur 2p (S2p) photoelectron spectroscopy and theoretical modeling. The shift in S2p ionization energy between the cluster and a single molecule, i.e., IE(cluster)-IE(monomer), is recorded and used to monitor the mean cluster size over a wide range of expansion conditions. The produced clusters were shown to fall into two different size regimes. Comparison between theoretical simulations and experimental observations suggests that while the smallest clusters belong to the ultrafine particle mode and have a liquid-like structure, the larger clusters belong to the accumulation mode of fine particles and possibly have a frozen cluster core. The transition between the two size/structure regimes occurs over a narrow interval in expansion conditions and may possibly reflect a change in growth mechanism from monomer addition to growth by cluster-cluster collisions. (c) Jarle Harnes, Mahmoud Abu-Samha, Mathias Winkler, and Knut J. Børve