During a 4-week run in October-November 2006, a pilot experiment was performed at the CERN Proton Synchrotron in preparation for the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment, whose aim is to study the possible influence of cosmic rays on clouds. The purpose of the pilot experiment was firstly to carry out exploratory measurements of the effect of ionising particle radiation on aerosol formation from trace H2SO4 vapour and secondly to provide technical input for the CLOUD design. A total of 44 nucleation bursts were produced and recorded, with formation rates of particles above the 3 nm detection threshold of between 0.1 and 100 cm(-3) s(-1), and growth rates between 2 and 37 nm h(-1). The corresponding H2SO4 concentrations were typically around 10(6) cm(-3) or less. The experimentally-measured formation rates and H2SO4 concentrations are comparable to those found in the atmosphere, supporting the idea that sulphuric acid is involved in the nucleation of atmospheric aerosols. However, sulphuric acid alone is not able to explain the observed rapid growth rates, which suggests the presence of additional trace vapours in the aerosol chamber, whose identity is unknown. By analysing the charged fraction, a few of the aerosol bursts appear to have a contribution from ion-induced nucleation and ion-ion recombination to form neutral clusters. Some indications were also found for the accelerator beam timing and intensity to influence the aerosol particle formation rate at the highest experimental SO2 concentrations of 6 ppb, although none was found at lower concentrations. Overall, the exploratory measurements provide suggestive evidence for ion-induced nucleation or ion-ion recombination as sources of aerosol particles. However in order to quantify the conditions under which ion processes become significant, improvements are needed in controlling the experimental variables and in the reproducibility of the experiments. Finally, concerning technical aspects, the most important lessons for the CLOUD design include the stringent requirement of internal cleanliness of the aerosol chamber, as well as maintenance of extremely stable temperatures (variations below 0.1 degrees C).

Mass-to-charge spectra of ions formed in ionization of freon-12 molecules, CF2Cl2, by protons with energies of E-p = 10 keV and 1 MeV are studied for the first time. Relative probabilities of different dissociation channels of the ionized freon-12 molecule, are obtained and compared with similar data for freon-12 ionization by electrons, positrons and photons. Absolute total and partial experimental cross sections for the formation of different fragment ions are evaluated for the case E-p = 10 keV. Neutral chlorine atom detachment is found to be the main fragmentation channel at both energies studied. Calculations of geometry and binding energies of fragment ions as well as energies required for activating various dissociation channels are carried out. A qualitative explanation is given of the observed fragmentation spectra.

Time-of-flight (TOF) techniques have been used to measure singly and multiply ionized Ar recoils in coincidence with energy-analysed, forward-ejected electrons in 1 MeV H+-Ar collisions. Electron energies up to 2800 eV and charge states 1-4 are covered. It is shown experimentally that the peak of binary-encounter (BE) electrons in the electron energy spectrum is associated almost entirely with single ionization of Ar, whereas the peak of electron-capture-to-the continuum (ECC) electrons is associated with multiple ionization. This shows that the BE electrons originate exclusively from the valence shell of Ar (the M shell) and the ECC electrons predominantly from an inner shell (the L shell).

A STUDY OF THE LINK BETWEEN COSMIC RAYS AND CLOUDS WITH A CLOUD CHAMBER AT THE CERN PS B. Fastrup, E. Pedersen University of Aarhus, Institute of Physics and Astronomy, Aarhus, Denmark E. Lillestol, E. Thorn University of Bergen, Institute of Physics, Bergen, Norway M. Bosteels, A. Gonidec, G. Harigel, J. Kirkby*, S. Mele, P. Minginette, B. Nicquevert, D. Schinzel, W. Seidl CERN, Geneva, Switzerland P. Grundsøe, N. Marsh, J. Polny, H. Svensmark Danish Space Research Institute, Copenhagen, Denmark Y. Viisanen Finnish Meteorological Institute, Helsinki, Finland K. Kurvinen, R. Orava University of Helsinki, Institute of Physics, Helsinki, Finland K. Hämeri, M. Kulmala, L. Laakso, J.M. Mäkelä, C.D. O’Dowd University of Helsinki, Lab. of Aerosol and Environmental Physics, Helsinki, Finland V. Afrosimov, A. Basalaev, M. Panov Ioffe Physical Technical Institute, Dept. of Fusion Technology, St. Petersburg, Russia A. Laaksonen, J. Joutsensaari University of Kuopio, Department of Applied Physics, Kuopio, Finland V. Ermakov, V. Makhmutov, O. Maksumov, P. Pokrevsky, Y. Stozhkov, N. Svirzhevsky Lebedev Physical Institute, Solar and Cosmic Ray Research Laboratory, Moscow, Russia K. Carslaw, Y. Yin University of Leeds, School of the Environment, Leeds, United Kingdom T. Trautmann University of Mainz, Institute for Atmospheric Physics, Mainz, Germany F. Arnold, K.-H. Wohlfrom Max-Planck Institute for Nuclear Physics (MPIK), Atmospheric Physics Division, Heidelberg, Germany D. Hagen, J. Schmitt, P. Whitefield University of Missouri-Rolla, Cloud and Aerosol Sciences Laboratory, Rolla, USA K. Aplin, R.G. Harrison University of Reading, Department of Meteorology, Reading, United Kingdom R. Bingham, F. Close, C. Gibbins, A. Irving, B. Kellett, M. Lockwood Rutherford Appleton Laboratory, Space Science & Particle Physics Depts., Chilton, United Kingdom D. Petersen, W.W. Szymanski, P.E. Wagner, A. Vrtala University of Vienna, Institute for Experimental Physics, Vienna, Austria CLOUD† Collaboration

For single ionization of He atoms in collisions with fast protons we have measured and calculated, as a function of the scattering of the proton, differential reaction cross sections and mean values of the recoil momentum projected onto the transverse direction defined by the collision plane and the plane perpendicular to the proton impact velocity (the mean transverse momentum). The calculations were performed in the continuum distorted-wave eikonal initial-state (CDW-EIS) approximation. The partitioning, at a fixed scattering angle , of the total transverse recoil momentum into two mean transverse momenta, one carried by the free electron and the other by the remaining ion, is almost independent of the impact energy (250, 500 and 1000 keV). At the mean transverse electronic momentum goes to zero, but for the fraction rises smoothly and reaches 80% near 0.4 mrad. The present experimental cross sections and transverse recoil momenta are in good agreement with CDW-EIS results including the internuclear interaction.

A series of ruthenium NH3 synthesis catalysts with selected promoters added have been studied. It is demonstrated that the rate of N2 chemisorption can be strongly enhanced by preadsorbed hydrogen. The promoters La and Ba have a weak influence on the rate of N2 dissociation on the hydrogen free surface, while they cause the rate to increase for a partly hydrogen covered surface. This increase correlates with the effect on the rate of NH3 synthesis. In contrast, Cs was able to increase the rate of N2 dissociation by two to three orders of magnitude with almost no change in the activity for NH3 synthesis.

The main purpose of this paper is to analyze recent nitrogen adsorption and desorption studies of iron-based ammonia synthesis catalysts by Langmuir-type models, to some extent with a linear coverage dependence of the activation energies included. The results are applied to set up a simple microkinetic model for the ammonia synthesis reaction, where due care has been taken to choose the kinetic parameters for the relevant range of coverages. This model is self-consistent in that the predicted coverages are within the range used to determine the input parameters, and the agreement between calculations and experiments is much better than for models based on single-crystal studies. A consequence of the analysis is that chemisorbed atomic nitrogen (N–*) is much more weakly bound at high coverages than is to be expected from low-coverage single-crystal studies. The coverage of N–* is close to 0.5 under synthesis conditions, a result that is in agreement with earlier considerations. The experimentally observed differences in the activity, as well as the pressure dependence of the reaction rate, working with catalysts with and without potassium promoter, can be at least partly explained by a destabilization effect of potassium on N–* or on the hydrogenated species NH–*. The main conclusion is that it is necessary to take surface heterogeneity or adsorbate-adsorbate interactions into account to obtain a model that can compete with the Temkin-type models.

Iron catalysts with and without potassium promoter were studied in a combined temperature programmed reaction and flow micro reactor system connected to a mass spectrometer. The temperature programmed desorption (TPD) studies demonstrated a significant shift of the N-2 peak to lower temperatures with the addition of K to the catalyst. The isobars suggested that this change can be interpreted as a destabilization of N-(*). The N-2 adsorption results were in good agreement with earlier studies of singly promoted catalysts. Thus the rate was found to be slow and activated. The rate constants were not in any disagreement with transition state theory. The addition of K had very little effect on the temperature programmed adsorption (TPA) curves, while for multiply promoted catalysts there was a significant shift to lower temperatures of the main adsorption dip. A microkinetic model for the NH3 synthesis reaction based on isobars for H-2 and N-2 and on the TPD results for N-2 displayed a good agreement with many of the features of the experimental data. An interesting example is that the destabilization of N-(*) and NH-(*) by K can explain the difference in the promoting effect at high and low pressures. However, the conclusion is that a simple Langmuir type of kinetic model cannot result in a perfect fit of a reasonably extended experimental set of data.

Iron catalysts with and without a potassium promoter have been studied in a combined temperature-programmed reaction and flow microreactor system connected to a mass spectrometer. This arrangement has the advantage that sample characterization through temperature-programmed methods can be carried out alternating with measurements of synthesis reaction rates. The desorption studies demonstrate a significant shift of the N2 desorption peak to lower temperatures with the addition of K to the catalyst. This change suggests a destabilization of dissociated nitrogen. The N2 adsorption results are generally in good agreement with earlier studies of singly promoted catalysts. Thus the rate was found to be slow and activated. The addition of K has little effect on the temperature-programmed adsorption (TPA) curves, while for multiply promoted catalysts there is a significant shift of the main adsorption dip to lower temperatures. However, the adsorption was still activated even for this type of catalyst.

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Iron catalysts with and without potassium promoter were investigated in a flow reactor system connected to a mass spectrometer. Temperature-programmed surface reaction (TPSR) of preadsorbed dissociated nitrogen with hydrogen has been studied under various experimental conditions. The results suggest that the total amount of ammonia produced gives a good estimate of the active area of the catalyst. The shape of the TPSR spectrum depends on the promoter content, the initial coverage and the partial pressure of H2 and indicates that the surface is nonuniform. The exact interpretation of the H2 TPD spectra is influenced by several complications. The evaluation of the results is aided by computer simulations.

Once again attention is brought to the sensitivity of iron catalysts to poisoning by sub ppm levels of oxygen and a procedure is suggested to evaluate the purity of the gas supply to a steady state flow reactor setup. It is demonstrated that extremely high gas purities are essential, in particular for studies of alkali promoted samples.

The paper discusses the consequences of using a porous separator which completely occupies the interelectrode space and immobilizes the electrolyte. Transport equations which have been derived for the separator region, assuming planar electrodes, are presented with both rigorous and approximate solutions for long and short periods of time.

Autoionisation in 20-500 keV Li+-He collisions has been studied experimentally and absolute cross sections sigma (Li) and sigma (He) have been obtained. A double hump structure is observed for sigma (Li) against impact velocity, while sigma (He) declines surprisingly rapidly. For the four prominent Li lines: (1s2s2)2S, 1s(2s2p3P)a2P, 1s(2s2p1P)b2P and (1s2p2)2D, angular distributions have been measured to yield relative substate populations. The a2P state has a remarkable alignment structure peaked at an impact energy 250 keV with Q(ML=0)/Q(ML=1)=7.5, not seen in the b2P state. The mechanism of a2P excitation is not yet clear.