Production of excited H at C1s edge of the methane molecule studied by VUV-photon-photion and metastable-fragment-photion coincidence experiments

Core ionization of a molecule just above the 1s ionization potential leads to recapture processes where the photoelectron is pushed back to a high Rydberg orbital of the molecular ion. Upon dissociation highly excited neutral fragments can be produced together with ions. 
Kivimaki A et al.  Phys. Rev. A 88 (2014) 043412

Core ionization of a molecule with a photon energy just above the 1s ionization potential leads to recapture processes where the photoelectron is pushed back to a high Rydberg orbital of the molecular ion. Upon dissociation highly excited neutral fragments can be produced together with ions. Such fragments can also be generated following resonant Auger decay after core–excitation. Metastable-fragment–photoion coincidence spectroscopy was used to examine this particular decay channel. In experiments aimed at the detection of neutral particles, charged particles can be avoided by a suitable choice of potentials in the apparatus. However, special experimental arrangements are usually necessary to study metastable atom formation, because microchannel plate detectors typically used in such studies also detect VUV and soft x-ray photons. Coincidence measurements between neutral particles (photons or metastables) and photoions can be used to separate the two contributions temporally. In addition, an approximate kinetic energy distribution of the metastable fragments can be derived from the time-resolved data.

In this work the metastable-fragment-photoion coincidence technique was used to study the fragmentation of methane near the C1s ionization threshold. Two decay channels, both resulting in the production of metastable hydrogen atoms (H*), were observed and assigned to H^*- H⁺ and H^*- CH⁺_(n=0…3) coincidences. The coincidence yields recorded as a function of photon energy revealed all core excitations, and were particularly enhanced just above the ionization threshold where the vibrational structure related to the core-ionized C 1s⁻¹ state was seen, especially in the stronger H^*- CH⁺_(n=0…3) channel.
At the C 1s → 3pt₂ and 3dt₂ resonances the kinetic energy distribution of H^*atoms displayed a slowly decaying curve, which extended up to 40 eV. The production of such fast fragments could be explained by sequential dissociation in which CH₄⁺ ions produced in the resonant Auger decay first dissociate into CH₃⁺ + H^*, followed by the dissociation of the molecular fragment ion, or by double Auger decay. The increase above threshold can be attributed to recapture processes after Auger emission, leading to the production and fragmentation of highly excited CH₄⁺ ions. The kinetic energy distribution of the H^*atoms in this region of the spectrum shows a structure that extends from 2 eV to above 30 eV, with a maximum around 3.5 eV.