Isotope Effects on Metastable Transitions: C3H8 and C3D8

Abstract

Metastable transitions were studied in the mass spectra of propane and perdeuterated propane in a sector mass spectrometer. The relative abundances of the various metastable ions at 70 eV were determined. One parent‐ion metastable transition was studied carefully: the methane loss at m/e=17.8 in C₃H₈ and at m/e=19.7 in C₃D₈. This transition shows a pronounced isotope effect on the yield. Appearance potentials were determined for the ions involved, by the RPD method and for the metastables themselves by conventional techniques. Reaction rate constants for the metastable transitions were determined by using a pulsed ion source. k=3.2×10⁴ sec⁻¹ for C₃H₈ and k=8.3×10⁴ sec⁻¹ for C₃D₈, at 13 eV. The results are discussed in terms of the quasiequilibrium theory of mass spectra.