Detection of Non-Emissive CH3 and CH2 Radicals via Low-Lying Electronic States of CH4 by Electron Impact

Abstract

Electron impact dissociation of CH4 into non-emissive neutral fragments, i.e., CH3 and CH2 radicals, has been studied near threshold of the lowest excited states. The neutral fragments produced are discriminated by means of the threshold ionization technique, which is based on the difference in ionization threshold energy between the parent molecule (14.3 eV, CH4 + e → CH3+ + H + 2e) and the produced fragments (9.8 eV, CH3 + e → CH3+ + 2e). Electron impact ionization spectrum of the CH3 radical, which agrees well with the previous measurements, were obtained. The excitation function of CH3 and CH2 radicals by electron impact dissociation of CH4 showed that threshold energies for both CH3 and CH2 radical production were about 8.5 eV. Consideration from the comparison of the threshold energies for CH3 and CH2 radical production with the electron energy loss spectra of CH4, the dissociation channel of the lowest excited states into CH3 radicals play an important role, which is clearly different from the photodissociation.