Production of excited H atoms at the C 1s edge of the methane molecule studied by VUV-photon-photoion and metastable-fragment-photoion coincidence experiments

Coincidences between neutral particles and photoions have been measured at the C 1s edge of the CH4 molecule. These neutral particles are either vacuum ultraviolet (VUV) photons or metastable fragments and their contributions can be separated temporally. Observed VUV-photon-photoion coincidences were mostly triggered by Lyman emission in excited H atoms. Two metastable fragment-photoion channels were observed and assigned to H*-H+ and H*-CHn+ (n = 0-3) coincidences, where H* signifies a metastable H atom. The coincidence yields as a function of photon energy revealed all the core excitations, but were particularly enhanced just above the C 1s ionization threshold and also displayed the vibrational structure related to the core-ionized C 1s(-1) state. The metastable H atoms observed in the present study were produced in highly excited Rydberg states and could be partly field ionized. The approximate kinetic energy distributions of the metastable H* atoms have been derived from the H*-H+ coincidence measurements performed at the C 1s -> 3p and 3d resonances as well as just above the C 1s threshold.