Delayed ionization from C70 by electron impact excitation

Abstract

Delayed ionization from electron impact excitation of the gas phase C[sub 70] has been observed. The nature of this delayed ionization was studied using a supersonic molecular beam apparatus with a quadrupole mass spectrometer. The results indicate that the field-induced ionization of long-lived high-Rydberg (HR) states in the detector region makes a significant contribution to the delayed ionization. Measured delayed ionization signals as a function of electron impact energy reveal two distinct types of excitation in C[sub 70]. One type of excitation, previously not observed in C[sub 60], is a weak tail in the low-energy range which extends to electron energies below 10 eV. The other type of excitation, similar to the one observed in C[sub 60], shows a prominent peak at about 50 eV. We suggest that the Rydberg states involved may originate from HR states of C[sub 70] with a ground and an excited ion core configuration, respectively. The Rydberg states with an excessive energy in the ion core can dissociate as well as undergo delayed ionization. 26 refs., 6 figs.