DEUTERIUM-ISOTOPE EFFECTS IN THE PHOTOFRAGMENTATION OF NAPHTHALENE MONOCATIONS

Deuterium isotope effects on the photofragmentation thresholds of naphthalene monocations are studied by photoion mass spectrometry using variable-energy monochromatized synchrotron radiation to photoionize C10H8 and C10D8. Kinetic isotope shifts (d-h) in the range 30-780 meV were observed for seven fragmentation reactions in the 15-20-eV region. The physicochemical factors leading to kinetic isotope effects are discussed. The observed kinetic isotope shifts are rationalized, mainly in terms of the RRKM/QET model and the specific reaction mechanism pathways. An astrophysical implication of these results is the possibility of deuterium fractionation of polycyclic aromatic hydrocarbons in the interstellar medium.