ELECTRON-IMPACT EXCITATION OF DELAYED IONIZATION OF C60 - EVIDENCE FOR RYDBERG STATES

A supersonic molecular beam apparatus with a quadrupole mass spectrometer has been used to study the delayed ionization of C60 by electron impact. Time-of-flight (TOF) analysis through the quadrupole is used to separate the direct ionization of C60 from delayed ionization. The excitation function for this delayed ionization has been measured as a function of electron energy. The threshold for the excitation function is 32 eV, with a peak at 46 eV and a full width at half-maximum of only 15 eV. It is suggested that the excitation function is closely related to the cross section and that the shape of this function indicates a forbidden transition may be excited. The relative magnitudes of the delayed ionization signals are almost independent of the mass setting of the quadrupole which has been used to suggest that most of the delayed ionization signal results from field ionization of a long-lived Rydberg state from C60.