Multiphoton ionization in a reflectron time-of-flight mass spectrometer: Individual rates of competing dissociation channels in energy-selected benzene cations

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Abstract
Decay rate measurements of internal energy selected ions were performed with a novel technique employing highly selective two-photon laser ionization in a reflectron time-of-flight mass spectrometer. Resonantly enhanced two-photon ionization leads to vibrational state selected C6H+6 cations. These are further excited with a second laser pulse of variable frequency beyond the dissociation threshold for the four decay channels of lowest energy. The excitation results in a metastable decay of C6H+6 with typical decay rate constants of some 106 s−1. Decay rate constants for two C-loss channels and for the first time also for the two H-loss channels are measured in the energy range from 5.1 to 5.5 eV. It is shown that H-loss and C-loss dissociation occurs from a common electronic state and is competing. From the total decay rate constants and the measured branching ratios individual decay rate constants for all decay channels are obtained and found to be in good agreement with RRKM calculations. This confirms the statistical character of the C6H+6 ion dissociation.