Excited States of Neutral Molecular Fragments from Appearance Potentials by Electron Impact in a Mass Spectrometer

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Abstract
Appearance potentials (A.P.) and ionization efficiency (I.E.) curves have been measured by the retarding potential difference technique for processes of the type AD+ / ABCD where the products at onset are a molecular ion AD+ and a stable molecule BC. The structure in the linear segmented I.E. curves has been correlated with the structure in the I.E. curves for AD+ / AD and the electronic energy levels of the neutral molecule BC. The method provides a means to measure the energy separation between the ground‐state singlet and the first excited state of the molecule BC, assumed to be triplet. Singlet–triplet separations have been measured for H2O at 4.4 eV; C2H6 at 2.4 eV; C2H4 at 3.25 eV; cyclopentene at 3.1 eV; and n‐C6H14 at 2.8 eV.