Ion-pair formation in gaseous nitromethane (CH 3 NO 2 ) induced by electron transfer has been studied by investigating the products of collisions between fast potassium atoms and nitromethane molecules using a crossed molecular-beam technique. The negative ions formed in such collisions were analysed using time-of-flight mass spectroscopy.

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The ATLAS Roman Pot system is designed to determine the total proton-proton crosssection as well as the luminosity at the Large Hadron Collider (LHC) by measuring elastic proton scattering at very small angles. The system is made of four Roman Pot stations, located in the LHC tunnel in a distance of about 240 m at both sides of the ATLAS interaction point. Each station is equipped with tracking detectors, inserted in Roman Pots which approach the LHC beams vertically. The tracking detectors consist of multi-layer scintillating fibre structures readout by Multi-Anode-Photo-Multipliers.

Dissociative ion-pair formation in collisions of fast potassium atoms with benzene and fluorobenzeneThe reaction K + ACl,----K+ + (A -Cl.r· with A = Sn and C was examined as a function of the collision energy from threshold up to about 40 eV in the c.m. system. Total cross sections of the mass-selected negative ions and doubly differential cross sections (energy and angle) of the K+ ions have been determined. Electron affinities, bond energies, and electronic excitation were calculated from the appearance potentials. In addition, the total cross sections for SnCI. were measured and are contrasted with the earlier results of CCI, from Dispert and Lacmann. Although both parent molecules have the same electron affinity within their error limits (2.2 eV for SnCI. and 2.0 eV for CCI.) and the same dissociation energy for the negative ions of 1.4±0.2 eV, the product ion yields differ drastically. The main negative ion yield in K + SnCI. results from SnCl.formation (over 80%). Its lowest dissociation channel leads to SnCI; formation, while Cl-ions are the main ions produced (90%) from CCl" with only 7% leading to CCl; + CI formation at higher energies. These results support orbital energy considerations of electron addition to SnCl, and CCI. as applied to the results of reactive colJisions of these molecules. The electron affinity and an electronically excited state of SnCl 3 have been also determined. Morse potentials of CCI. and SnCl. were fitted to the experimental results of energy loss measurements from this work. The vertical electron affinities thus derived are 1.15 eV for SnCI. and -1.0 eV for CCl •.

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