A photoion–photoelectron coincidence (PIPECO) study of fragmentation rates and kinetic energy release in energy selected metastable ions

Abstract

The absolute fragmentation rates and kinetic energy releases associated with the unimolecular dissociation reactions of energy selected halo-hydrocarbon ions have been investigated by photoion–photoelectron coincidence (PIPECO) spectroscopy in which nominally zero kinetic energy electrons are detected in coincidence with their corresponding fragment ions. The reactions investigated were HCl loss from C2H5Cl+ and ClCH2CH2Cl+, Cl loss from C3H3Cl+ (propargyl chloride) and Br loss from C3H3Br+ (propargyl bromide). Although the kinetic energy released in the HCl loss from C2H5Cl+ reaction is small, the other three reactions have large energy release values associated with them. We found that X loss from C3H3X+ proceeds via two paths yielding different isomeric forms of C3H3+ with markedly different kinetic energy release values. The most unusual feature in these complex dissociation reactions is that the rate of fragmentation cannot be described by a single exponential decay function but rather must be expressed as the sum of at least two exponentials. Possible explanations for two-component decay rates and their implications for the statistical theory of unimolecular decay are discussed.