C.H. Cooper, J.F. Liang, R. Vandenbosch and D.I. Will

The motivation for constructing a gas target grew out of two studies reported last year. In an experimental study¹ of the yields of different RbC_n clusters, even n cluster anions were preferred (over odd n) by one to two orders of magnitude through n=8. Ab initio quantum mechanical calculations performed to elucidate the even n preference also suggested a linear structure for these anions with the Rb at one end. One method of checking this theoretical structure is to fragment the cluster anions to determine the chain lengths of pure C_n cluster fragments. The motivation for studying the fragmentation of C₆₀ arises from conflicting interpretations of fragmentation mechanisms. For example, C₅₀ might be formed by successive emission of C₂ fragments, or by emission of a C₁₀ chain. We explored the latter mechanism in a model calculation² based on an unzipping mechanism suggested by deMuro et al. Our eventual goal is to perform definitive coincidence studies.

Initial calculations indicated a target of 10^-1 to 10^-3 Torr-cm of nitrogen would have the proper thickness. Such a target could consist of ~10 cm of N₂ gas at 10^-2 to 10^-4 Torr. Variation in the target thickness needed for different experiments, uncertainties in the cross sections, and uncertainties in the available pumping speed dictated a design in which the gas canal arms could be changed easily to alter gas loss and target thickness if needed. For ease of construction, 1/4" outside diameter stainless steel tubing of various wall thicknesses and lengths fitting snuggly into a central tee piece provides convenient variability. Endcaps with apertures smaller than tube inside diameter provide some decrease in gas loss while reducing the likelihood that breakup of ions by the tube inner wall will contaminate results. A pressure regulated supply of N₂ bled through a variable leak valve with a numerical turn counter gives repeatable target pressure. Finally a VRC Pirani gauge with capability down to 10^-5 Torr was recently purchased to show actual pressure in the central tee piece of the target.

Our General Ionex model 860 sputter ion source and a new source (see Section 5.1) both located on our injector deck produce anions of up to 300 keV. A 90° electrostatic deflector allows mass analysis of + and - ions after breakup. Our first test of the gas cell was with clusters. Without gas we were able to focus 1/4 of the beam through the gas cell. With N₂ gas we found the optimum pressure for forming C, C₂ and C₃ charged fragments to be 10^-2 Torr to 10^-3 Torr. At these gas pressures the vacuum in the beam line was less than 10^-5 Torr. Fragmentation of at 180 keV gave mostly +1 fragments, although smaller yields of -1 and +2 charge states were observed. We also made a brief attempt to fragment Cs. The transmission through the cell was appreciably poorer and good external beamline pressure more critical. We did observe Cs⁺ and fragments. The latter observation confirms our earlier hypothesis that alkali polycarbide anions are chains with the alkali atom at the end of the chain.

Our principal goal for use of the gas cell is to study the fragmentation of . We were able to transport 150 keV ions through the gas cell. In this initial experiment we found several small-mass fragments with a target pressure of 10^-2 Torr.