UNIMOLECULAR DISSOCIATION RATE CONSTANTS - CHLOROBENZENE CATIONS REVISITED BY USING A NEW METHOD

Unimolecular dissociation rates of metastable molecular ions formed at the laser focus of a time-of-flight mass spectrometer (TOFMS) have been determined by measuring the relative concentrations of "parent" metastable ions and the "daughter" fragment ions as a function of their retained kinetic energies upon dissociation. This is accomplished by the use of a simple one-stage ion mirror/energy analyzer. For a given reflection potential on the ion mirror, only those ions with a kinetic energy equal to or less than the reflection potential are detected. In this way, the relative concentrations of the parent and daughter ions as a function of their position of dissociation in the time-of-flight (TOF) lens can be measured, and consequently the dissociation lifetime can be derived. This technique has been applied to measuring the unimolecular dissociation rate of chlorobenzene ion (C6H5Cl+) in the three-photon energy range of 13.64-13.92 eV through resonance-enhanced two-photon state-selected ionization of chlorobenzene in a supersonic molecular jet. The method demonstrates a minimum dynamic range of 105 to 2 × 107 s-1 and is in good agreement with previous work. The data suggest that rotational temperature plays a role in the unimolecular dissociation lifetime of the metastable ions. © 1990 American Chemical Society.