He I photoelectron spectroscopy of formic acid isotopomers HCOOH and DCOOD Original Research Article

He I photoelectron spectra (PES) of HCOOH and DCOOD have been measured with an electron kinetic energy resolution of 15 meV. Analysis of observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region provided values for several vibrational modes of the X 2A′ and à 2A″ electronic states of HCOOH+ and DCOOD+. This analysis is based on the implications of structural changes expected to occur on ionization, using literature values for calculated geometries of these states. The vibronic assignments of both PES bands were consistent with isotopic effects on vibrational mode frequencies and were helped by considerations on relations between carbon–oxygen bond lengths and stretch mode frequencies. The analysis of the second PES band indicates that the calculated structure of the à 2A″ state of the ion requires more refined theoretical treatment. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of DCOOD, 11.335±0.002 eV and 12.380±0.002 eV respectively, and the second ionization energy of HCOOH, 12.378±0.002 eV.