Dissociative photoexcitation of CH4 and CD4

Fluorescence emission spectra and ion-yield spectra of CH4 and CD4 have been measured using monochromatic synchrotron radiation. We have studied the X and A ionic states in the 12–30 eV region (400–1000 Å). The visible fluorescence measurements focus upon the Rydberg series converging to vibrational levels of the A state at 22.39 eV; this structure is also reflected in some of the charged fragments including the parent ion indicating that these Rydberg states may autoionize. The character of the Rydberg states is discussed, and a comparison with negative-ion fragment spectra is made.