Fullerene Endohedrals of Praseodymium
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FULLERENE ENDOHEDRALS OF PRASEODYMIUM R. S. RUOFF, R. MALHOTRA, L. MORO, C. H. BECKER, AND D. C. LORENTS Molecular Physics Laboratory, SRI International, Menlo Park, CA. 94025 INTRODUCTION Endohedral fullerenes were first observed in molecular beams produced in laser vaporizations sources.[1] Since the discovery of the arc method [2] of producing large quantities of fullerenes several laboratories have been searching for methods to produce and isolate sufficient quantities of endohedral fullerenes to permit the study of their unique material properties. PRODUCTION Endohedral praseodymium fullerenes were produced in a carbon arc soot generator using 8 mm diameter, 30-cm-long graphite rods axially drilled to a depth of 15 cm with holes of various diameters. These hollow graphite rods were packed with praseodymium oxide (Pr6 01 1)powder and vaporized in a high current DC arc discharge system. The arc system was operated in He at -150 Torr with a discharge gap of -5 mm and at a current of -125 A. The discharge is maintained between the ends of a stationary 12 mm rod and the Pr filled rod which is rotated and translated inward as the rod is consumed. The discharge region is enclosed in a water cooled copper cylinder transverse to the rod axis. He flowing along the axis of the cylinder at -0.1 lit/min carries a portion of the soot into a collection chamber fitted with filter through which the He is pumped. Soot remaining on the walls of the copper cylinder is swept into the collection chamber at the end of a run with a high pressure gas jet. The arc current is more stable with these rods than with ordinary graphite rods perhaps due to the metal cations. The typical bum rate of these rods was -3 mm/min. MASS ANALYSIS OF SOOT. Samples of Pr containing soot and solvent extract were directly mass analyzed in the SRI Surface Analysis by Laser Ionization (SALI) facility. This post-ionization TOF mass spectrometer utilizes a 118 nm laser beam to photoionize the neutral molecules desorbing from a surface. The desorption can be affected by thermal heating, laser heating, or ion beam sputtering. In this work either 1060 nm or 532 nm laser heating was used for desorption. The direct sensitive detection of the desorbed neutrals considerably reduces the problems of matrix and surface chemical effects experienced in laser ion desorption (LID) and secondary ion (SIMS) mass spectrometry. SALI has become an indispensable analytical tool in our fullerene research efforts due to its ability to provide immediate and reliable relative concentrations of fullerene samples. Used together with HPLC analysis of common samples it is possible to obtain absolute fullerene yields in various samples. Primary soot samples from several runs with different ratios of Pr/C were analyzed with SALI. The dominant peaks in these samples are the normal C60 and C70 that occur in the soot generated from pure graphite rods. However the ratio of higher fullerenes to C60 appears to be larger in the samples from Pr containing rods. It may be expected that the oxygen
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