Penning ionization of C60 and C70 Original Research Article

Using crossed beams and mass spectrometric ion detection we have measured the cross-sections for Penning ionization of the fullerene molecules C60 and C70 in thermal energy collisions with metastable He*, Ne*, Ar*, and Kr* atoms and with laser-excited Ar*(4p 3D3) atoms. For both C60 and C70, the Penning ionization cross-sections are found to increase exponentially by about a factor of 100 with rising excitation energy (from 10 to 20 eV), in contrast to the geometrical collision cross-sections which are nearly the same (∼200×10−20 m2) for all studied collision pairs. For He*+C60, we determine the absolute value of the Penning ionization cross-section as 77×10−20 m2 with reference to the He I (21.22 eV) photoionization cross-section of C60 which we estimate to be 8.3×10−20 m2. The findings are discussed in terms of statistical processes in the electron system on the sub-ps time scale.