Threshold photoelectron spectra of atmospheric molecules. I. Description of method and application to H2, D2, and N2

Threshold photoelectron spectra (TPES) are presented for H2, D2, and N2 with a resolution of 28 meV full width at half maximum. The results are compared with Franck--Condon transition probabilities, previous TPES and electron attachment threshold photoelectron spectra. Peaks in the TPES not predicted by Franck--Condon calculations are explained by autoionization processes. These processes in N2 make possible the observation of vibrational levels of the ions not visible spectroscopically or in photoelectron spectra. A shift of the TPES peaks in N2 toward lower energy is explained by conversion of rotational energy in the ionization transition. Peaks between the v=O and v=1 vibrational levels of the X 2Σ/sub g/+ state of N2+ consist of near monoenergetic electrons implying little or no change in the rotational quantum number in the autoionization process