Abstract
Threshold-photoelectron photoion coincidence (TPEPICO) spectra of CH4 have been observed with synchrotron radiation at the excitation to the 2A1 (ν1=0−3) ionic states as well as to the 4pt2 Rydberg (ν1=0−4) states. In all the TPEPICO spectra observed, the CH+3 band shape was almost rectangular, which suggests that the translational and internal energy distributions of CH+3 are very narrow. The total kinetic energy releases (KERs) have been estimated from the CH+3 band shape. As a result, it was found that the CH+3 species were in an electronically excited state. There was a narrow distribution of the total KERs and similarity in the TPEPICO CH+3 band shapes between the spectra at the 2A1 ionic state and the 4pt2 Rydberg state excitations, which led to the conclusion that the Rydberg electron is just a spectator and the dissociation of the core ion plays an important role in dissociation through the 4pt2 Rydberg state. Similar results have also been obtained for CH+2 and CH+ productions. However, on the other hand, an H+ fragment has been observed only at the 2A1 state excitation. It showed a band with a long tail in the slower flight time region. The total average KERs and the decay rates have been estimated from band shape simulation. From these results, it has been found that a dissociation limit of the H+ ion exists just below the 2A1 ionic state. The dissociation mechanisms through the 4pt2 Rydberg state have been discussed in detail in comparison with those of the 2A1 ionic state.
Published Version
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