Multiphoton mass spectrometry of metastables: Direct observation of decay in a high-resolution time-of-flight instrument

A time-of-flight mass spectrometer with a laser multiphoton ionization source is employed in studying metastable ions produced in the multiphoton ionization-dissociation process. The use of a reflecting field gives rise to sharp distinguishable mass peaks for the metastable ions. This spectrometer gives short ion pulses (8 ns fwhm) and a very high mass resolution (m/Δm = 3900 at amu 78). With this special mass spectrometer we are able to observe for the first time unimolecular decomposition on a 25-μs time scale in MPI mass spectrometry. Decay rates for hydrogen dissociation from the C₆H₆⁺ ion are measured and the results are compared with the RRKM predictions.