Excited states of CH4and CD4between 18 and 22 eV

Abstract

Clear structures are reported in the ionisation efficiency curve of CH₄ and CD₄ and in the appearance potential curves of their ionic fragments. They are short-lived Rydberg states converging to the first excited state of CH₄⁺ and CD₄⁺. One resonance and three different excited states involving the excitation of a 2a₁ electron are observed and interpreted.