Metastable fragment ions in multi-photon time-of-flight mass spectrometry: Decay channels of the benzene cation

The detection of 5 metastable decay channels in the laser-induced multi-photon ionization and fragmentation pathway of C₆H₆ is reported. The mass of the precursor ions and the product ions (C₄H₄⁺, C₄H₃⁺, C₄H₂⁺, C₃H₃⁺) is determined in a reflectron time-of-flight (TOF) mass spectrometer, the latter with the reflecting field acting as an energy analyzer. The metastable decay channels are explained in terms of a switching of photon absorption from the neutral molecule to the parent ions and to the fragment ions (ladder switching) as well as by the heat of formations of the ions under consideration.