Isotope effects in the unimolecular decomposition of ethylene by low-energy electron impact

Abstract

In an attempt to resolve certain difficulties noted in previous studies of isotope effects in the metastable spectra of deuterated ethylenes, an electron-impact investigation of the corresponding effects in the normal mass spectra has been undertaken. Quantitative measurements of the relative losses of molecular hydrogen from trans-CHDCHD and C 2D 3H were carried out as a function of ionizing energy, and the appearance potentials of the major ions were determined, using a single-focusing mass spectrometer and the retarding potential difference technique. The isotope effects for metastable ions are very strong in comparison with the values which result from the normal mass spectra. The molecular ions appear to undergo extensive scrambling prior to dissociation. Isotope effects for fragment ions, calculated as a function of electron energy from the quasi-equilibrium theory of mass spectra, are in good agreement with observed values, indicating that the theory provides a satisfactory description of the major processes which determine the normal fragmentation pattern. There are, however, discrepancies in the case of the metastable transitions. An attempt to reconcile these values with a tunneling mechanism is also unsatisfactory.