ABSOLUTE CROSS-SECTION FOR THE FORMATION OF C-60(-IMPACT ON C-60() IONS PRODUCED BY ELECTRON)

We employed the fast-neutral-beam technique in a measurement of the ab solute partial cross section for the formation of C-60(+), ions follow ing controlled electron-impact ionization of the fullerene C-60 from t hreshold to 200 eV. Our measurement is the first report of an absolute measurement of this cross section which does not employ any normaliza tion procedure and which does not rely on the use of controversial vap our pressure versus temperature data for C-60 to determine the C60 num ber density in the ion source. All quantities that determine the C-60( +) ionization cross section were measured directly in our apparatus. W e found a peak value for the C-60(+) parent ionization cross section o f 24.4 x 10(-16) cm(2) in the energy range between 70 and 80 eV. This value is in very good agreement (to within 5%) with the cross section reported earlier by Matt et al (1996 J. Chern. Phys. 105 1880) who emp loyed a novel normalization procedure. Our cross section is significan tly lower than the cross sections reported by several other investigat ors who relied on C-60 vapour pressure data in their absolute calibrat ion.